An Introduction to the Human Body

Before you study any one organ, you need two things: a map and a compass. This chapter gives you both. First, you will see how the body is built in nested levels, from tiny atoms up to a whole person. Next, you will meet the eleven organ systems that share the work of staying alive. Then you will learn the one big idea under all of body science. It is homeostasis, the body's constant push to keep things steady inside. Last, you will learn the exact language used to say where body parts are. After that, words like "left," "above," and "in front" will always mean the right thing.

Section 1The levels of organization

Two questions, always asked together

This whole book grows out of two words. Anatomy is the study of body parts. It asks what is it, and where is it? It looks at a part's shape, pieces, and place. Physiology is the study of how the body works. It asks how does it work? It looks at the job a part does and the steps it takes.

You can't really split the two, because in the body structure always fits function. In other words, how a part is built matches the job it does. The wall of a tiny air sac in your lung is as thin as tissue paper. That lets oxygen cross it in a split second. The wall of the heart's main pumping chamber is thick and full of muscle. It has to be strong to push blood to your whole body. Change how a part is built, and you change what it can do. You will see this rule again and again.

Small parts build bigger parts

The body is set up like a set of nesting dolls. Each level is built from the one below it. There are six levels, and it helps to know them in order:

You can read this ladder in either direction. Going up, tiny parts join into bigger and more useful parts. In the end, they add up to a living human. Going down, you can trace any disease to a lower level. Take a heart attack. It is an organ problem. But it starts with cells that don't get enough oxygen. That happens because a molecule (cholesterol) built up in a blood vessel wall. Moving up and down this ladder is one of the most useful habits you can build.

Zoom in and out through all six levels in the interactive below.

Section 2The organ systems

Eleven teams, one goal

Keeping a person alive is too big a job for one organ. So the work is split among eleven organ systems. They are not really separate. They overlap and depend on each other all the time. But sorting them by their main job makes the body easier to learn.

A handy trick is to group the systems by the kind of job they do. Some cover and support the body. Some control it and carry messages. Some move things around. And some make new people.

No system works alone

The real magic is in how the systems team up. Take one breath and follow it. The respiratory system brings in oxygen. But the cardiovascular system carries that oxygen to your tissues. The muscular system (the diaphragm) did the pulling. The nervous system set how fast you breathe. And the skeletal system (the rib cage) gave the lungs a frame to grow inside. When you learn about a disease later on, ask which systems it touches. The answer is almost never just one.

Tap each system in the interactive to watch its organs light up on the body.

Section 3Homeostasis & feedback

The single big idea of physiology

If you remember just one word from this book, make it homeostasis. It means keeping the inside of the body steady, even when the world outside changes. Your body temperature stays near 37 °C, whether you are in a snowstorm or a sauna. Your blood sugar (glucose), blood pH, and fluid amount all stay inside narrow ranges too.

These values don't stay perfectly still. They wobble a little around a set point, the target value the body aims for. Think of a person on a tightrope, always making tiny fixes to stay balanced. That moving balance is health. If a value moves too far out of range, cells stop working. If it moves even further, the person dies. Almost every disease in this book comes down to a failure of homeostasis.

The negative feedback loop: three parts

The body keeps values steady mostly through negative feedback. This is a loop that undoes a change. Every negative feedback loop has the same three parts. Learn them once, and you can follow any loop in the body:

• A receptor (a sensor) notices the change and reports it.
• A control center (often the brain or a hormone gland) compares the reading to the set point. Then it decides what to do.
• An effector (a muscle or gland) carries out the response. This pushes the value back toward normal.

It is called negative feedback because the response works against the first change. If the value goes up, the loop brings it down. If it goes down, the loop brings it up. Body temperature is the classic example. Say you step out into the cold. Temperature receptors in your skin send signals. The control center in your brain (the hypothalamus) reads "too cold." It then turns on the effectors. Blood vessels in your skin get narrower to hold heat in. Your muscles shiver to make heat. Your temperature climbs back to the set point, and the loop calms down. A home thermostat works the same way. It senses, compares, and fixes.

Positive feedback: the rare amplifier

Once in a while, the body wants the opposite. It needs a signal that builds on itself until a job is done. This is positive feedback. It is rare because it can quickly get out of hand.

Childbirth is one example. The baby's head stretches the cervix, the opening of the womb. This stretch causes the release of the hormone oxytocin. Oxytocin makes contractions stronger, which stretches the cervix even more. The cycle snowballs until the baby is born, which ends it. Blood clotting works the same way. The first platelets that stick to a cut give off chemicals. These call in more platelets, which call in still more. This seals the wound fast. Positive feedback always needs a built-in stop, like a birth or a finished clot. Without one, it would run out of control.

Knock a value off its set point in the interactive and watch a negative feedback loop pull it home.

Section 4The language of position

Start from the anatomical position

To say where things are without mix-ups, everyone starts from the same pose. It is called the anatomical position. The body stands upright and faces forward. The feet are a little apart. The arms hang at the sides with the palms facing forward. Every direction word is based on this pose. So a part's description never changes, even if the real patient is lying down or upside down.

One more rule trips up a lot of people. "Right" and "left" always mean the patient's own right and left. They do not mean your right and left as you look at the patient.

The directional pairs

Direction words come in opposite pairs. That makes them easy to learn two at a time:

Two of these pairs need extra care. Proximal and distal are used mostly for the arms and legs. They are always measured from where the limb joins the trunk. So the shoulder is proximal, and the fingertips are distal. Medial and lateral are measured from the midline, an imaginary line down the center of the body. Parts near the center are medial. Parts out toward the sides are lateral.

See each pair drawn on a body in anatomical position below.

Section 5Planes & sections

Three ways to slice

To see inside the body, we picture cutting it along flat surfaces called planes. This might be on a body in a lab or on a CT scan. There are three planes, and each one is at a right angle to the others:

• The sagittal plane splits the body into left and right. A cut right down the midline is a midsagittal section. A cut off to one side is parasagittal.
• The frontal (coronal) plane splits the body into front and back (anterior and posterior).
• The transverse (horizontal) plane splits the body into top and bottom (superior and inferior). This is the "slice" view you see on a CT scan.

Why does this matter to a nurse or other health worker? Medical images are always taken in one of these planes. To read a scan, you have to know which slice you are looking at. A transverse CT of the belly shows the organs cut straight across. A sagittal MRI of the spine is seen from the side. It shows the bones of the spine stacked from top to bottom (superior to inferior). Naming the plane is always the first step in making sense of an image.

Watch each plane sweep through the body in the interactive below.

Section 6Cavities & regions

Where the organs live

Your organs are not loose inside you. They sit in closed spaces called body cavities. These spaces protect the organs and give them room to move. There are two main groups.

The dorsal (posterior) cavity runs along the back. It holds the nervous system. It has two parts: the cranial cavity, which holds the brain, and the vertebral cavity, which holds the spinal cord. The bigger ventral (anterior) cavity holds most of the other organs. A sheet of muscle called the diaphragm splits it in two. Above it is the thoracic cavity, or chest, with the heart and lungs. Below it is the abdominopelvic cavity, or belly and hip area. It holds the digestive organs, with the pelvic organs lower down.

Serous membranes: the slippery linings

Organs in the ventral cavity are wrapped in thin serous membranes. These are two-layered linings that give off a slippery fluid. The fluid lets organs slide past each other without rubbing. Each membrane has two layers with paired names. The parietal layer lines the wall of the cavity. The visceral layer hugs the organ itself.

There are three of these membranes. The parietal and visceral pleura wrap the lungs. The parietal and visceral pericardium wrap the heart. The parietal and visceral peritoneum wrap the belly organs. When one of them gets inflamed (red and swollen), you get an "-itis" disease. These are pleuritis, pericarditis, and peritonitis. The rubbing or fluid buildup can cause real trouble.

Mapping the abdomen: regions and quadrants

A lot can go wrong in the belly. So health workers split its surface into a grid. This lets them say exactly where a pain or a lump is. The simple version uses four quadrants: right and left, upper and lower. These are called RUQ, LUQ, RLQ, and LLQ. The more detailed version uses nine regions, like a tic-tac-toe grid. The umbilical region is in the center, around the belly button. The epigastric region is above it, and the hypogastric region is below it. More regions line up on each side. Saying "RLQ pain" quickly narrows down the cause, since the appendix sits there.

Explore the cavities, membranes, and the region/quadrant grids in the interactive.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back any time a word trips you up.

Anatomical position

The standard pose used to describe the body. You stand upright and face forward, with arms at your sides and palms forward. All direction words are based on it.

Anatomy

The study of the body's parts and how they sit next to each other. It asks what a part is and where it is.

Anterior (ventral)

Toward the front of the body.

Control center

The part of a feedback loop that compares a reading to the set point. It then decides on a response. It is often the brain or a hormone (endocrine) gland.

Distal

Farther from where a limb joins the trunk. The fingers are distal to the elbow.

Effector

The muscle or gland in a feedback loop that carries out the fix.

Frontal (coronal) plane

An up-and-down plane that splits the body into front (anterior) and back (posterior) parts.

Homeostasis

Keeping the inside of the body fairly steady, even when things outside change. Feedback keeps it near a set point.

Medial

Toward the midline of the body.

Negative feedback

A loop whose response works against the first change and reverses it. This brings the value back to its set point. It is the body's main way of keeping things steady.

Organ

A body part made of two or more kinds of tissue that does a certain job. The heart and stomach are organs.

Physiology

The study of how the body's parts work and the processes that keep you alive.

Positive feedback

A loop whose response makes the first change bigger. It keeps going until an event stops it, such as childbirth or a finished blood clot.

Proximal

Closer to where a limb joins the trunk. The elbow is proximal to the wrist.

Sagittal plane

An up-and-down plane that splits the body into left and right parts. A cut right down the midline is midsagittal.

Serous membrane

A thin, two-layered lining of a ventral cavity, such as the pleura, pericardium, or peritoneum. It gives off fluid so organs don't rub.

Set point

The target value the body tries to hold something near. For body temperature, it is about 37 °C.

Superior

Toward the head or the upper part of the body.

Transverse (horizontal) plane

A plane that splits the body into top (superior) and bottom (inferior) parts. It gives a cross-section, or slice, view.

The Chemical Level of Organization

Every heartbeat, every thought, and every drop of sweat starts as chemistry. Before a cell can divide or a nerve can fire, atoms must join, reactions must run, and water must do its quiet but vital work. This chapter takes you down to the chemical level. It is the base that the rest of this book is built on.

First you will meet the atom and the few elements that make up almost all of you. You will learn the three bonds that hold molecules together. You will watch reactions rearrange matter and see how enzymes speed them up. You will find out why water is perfect for life. You will learn to read pH and meet the buffers that guard your blood. Last, you will meet the four big families of molecules that build your body. Take your time here. Everything else in the body rests on this floor.

Section 1Atoms & the elements

Matter, elements, and the atom

Anything that takes up space and has mass is matter. Your bones, your blood, and the air in your lungs are all matter. All matter is made of elements. An element is a pure substance that normal chemistry can't break into anything simpler. There are 118 known elements. But life uses only a few of them. Only about two dozen show up in the body. Just six of them make up about 99% of your mass. They are oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus.

The smallest bit of an element that still acts like that element is an atom. If you split an atom apart, you no longer have the element. You just have its raw parts.

Inside the atom: three particles

An atom has a packed center called the nucleus. A cloud of moving electrons surrounds it. Only three tiny particles matter for this book. Protons have a positive charge and sit in the nucleus. Neutrons have no charge. They share the nucleus with the protons and add weight but no charge. Electrons have a negative charge and weigh almost nothing. They zip around the nucleus in layers called shells (energy levels).

Protons and electrons have equal but opposite charges. So a neutral atom has just as many electrons as protons. The charges cancel out, and the atom has no overall charge.

Atomic number, isotopes, and the outer shell

One number is like an atom's fingerprint. It is the atomic number, the number of protons in the nucleus. Change it, and you change the element itself. An atom with 1 proton is always hydrogen. One with 6 is always carbon, and one with 8 is always oxygen. The mass number is the protons and neutrons added together.

Atoms of the same element can have different numbers of neutrons. These are called isotopes. Carbon-12 and carbon-14 are both carbon, with 6 protons each. But carbon-14 has two extra neutrons, and it is radioactive. That is why radioactive isotopes are so useful for medical scans and for dating old things.

Still, an atom's chemistry doesn't come from its nucleus. It comes from its outermost electrons, in the valence shell. Atoms are most stable, or "happiest," when that outer shell is full. For most atoms in the body, full means eight electrons. This is the octet rule. An atom with a full outer shell doesn't react. An atom missing a few electrons is reactive and ready to bond.

Pick an element and watch its atom take shape in the interactive below. You will see the nucleus in the center and the electrons filling their shells.

Section 2Chemical bonds

Why atoms bond at all

Atoms bond for one reason: to get a stable, full outer shell. An atom with a half-empty outer shell will do almost anything to fill it. It may give electrons away, take them, or share them. Each of these ways makes a different kind of bond. The noble gases, like helium, neon, and argon, already have full shells. So they drift around and don't react. Every other atom is looking for a partner. There are three main bonds: ionic, covalent, and hydrogen. Once you know them, you can predict how almost every molecule in the body behaves.

Ionic bonds: give and take

Sometimes the easiest way to fill a shell is to hand an electron over for good. One atom gives an electron, and another takes it. Then both end up with a charge. The giver becomes a positive ion called a cation. The taker becomes a negative ion called an anion. Opposite charges attract, and that pull is the ionic bond.

Table salt is the classic example. Sodium hands its one outer electron to chlorine and becomes Na⁺. Chlorine becomes Cl⁻. The two cling together as NaCl. In water, ionic compounds break apart into separate ions. These charged bits floating in body fluids are your electrolytes, such as Na⁺, K⁺, Ca⁺⁺, and Cl⁻. They run nerve signals, muscle contractions, and fluid balance.

Covalent bonds: sharing, polar and nonpolar

More often, two atoms just share a pair of electrons. Both atoms count the pair toward a full shell. This is a covalent bond. It is the strongest and most common bond in the body. Carbon builds all of life with covalent bonds.

But sharing is not always fair. When both atoms pull the same amount, the shared electrons sit in the middle. The bond is nonpolar, as in O₂ or the carbon–hydrogen bonds of fats. When one atom pulls harder, it hogs the electrons. The bond becomes polar. One end is slightly negative, and the other end is slightly positive. Water is the great polar molecule. Its oxygen pulls the shared electrons away from its two hydrogens. So the oxygen has a partial negative charge, and the hydrogens have a partial positive charge. As you will see, this lopsided charge makes life possible.

Hydrogen bonds: weak but mighty

A hydrogen bond is not a bond inside a molecule. It is a gentle pull between molecules, or between far-apart parts of one big molecule. It forms when the slightly positive hydrogen of one polar molecule is drawn to the slightly negative oxygen or nitrogen of another.

One hydrogen bond on its own is weak, only a small part of the strength of a covalent bond. But millions of them together are mighty. Hydrogen bonds hold liquid water together. They zip the two strands of DNA into a double helix, like the teeth of a zipper. They also fold proteins into their working shapes. They are strong enough to give shape, but weak enough to break and form again. That is just what living molecules need.

Flip between ionic, covalent, and hydrogen bonds and watch how each one forms in the interactive.

Section 3Chemical reactions & enzymes

Reactants, products, and three ways to rearrange atoms

A chemical reaction is just the making or breaking of bonds. Atoms let go of old partners and grab new ones. The starting materials are the reactants. What you end up with are the products. No atoms are made or destroyed. They are only moved around.

Almost every reaction in the body fits one of three patterns. Synthesis reactions join small parts into a bigger molecule (A + B → AB). These are the building, or anabolic, reactions. Decomposition reactions break a big molecule into smaller pieces (AB → A + B). These are the breaking-down, or catabolic, reactions, like digesting a meal. Exchange reactions do both at once and swap partners (AB + CD → AC + BD). Many reactions are also reversible. They can run either way, depending on conditions.

Energy: the hill every reaction must climb

Chemical bonds store energy, so moving them around always changes energy. Reactions that give off energy, often as heat, are exergonic. These power the body, like breaking down glucose. Reactions that take in energy are endergonic. Building big molecules costs energy.

But here is the catch. Even a reaction that gives off energy must first get over a small hump. This hump is the activation energy, the energy needed to get a reaction started. Picture a boulder resting in a small dip near the top of a hill. It will roll down and release lots of energy. But first, something has to lift it over the lip of the dip. That lip is the activation energy. At body temperature, most reactions would sit stuck behind it for hours or even years.

Enzymes: shrinking the hill

The body can't wait hours. It can't just turn up the heat, either. So it uses enzymes. An enzyme is a catalyst, a helper that speeds up a reaction. Enzymes are almost always proteins. They lower the activation energy. This lets reactions run thousands or millions of times faster at a safe 37 °C. And an enzyme is not used up. It does its job and is free to do it again.

Each enzyme is very specific. It has an active site shaped to fit one certain molecule, its substrate. Think of a key in a lock. A better picture is a glove that shapes itself around a hand. Since enzymes are proteins, anything that unfolds a protein can harm them. A high fever or the wrong pH can do this. That is one reason your body guards its temperature and pH so closely.

Watch a reaction climb over its energy hill in the interactive below. Then switch on an enzyme and see the hill shrink.

Section 4Water, the solvent of life

Polarity: the source of all water's tricks

You are about 60% water, and that is no accident. Almost everything special about water comes from one fact: it is polar. Recall from Section 2 that oxygen hogs the shared electrons in each O–H bond. So a water molecule (H₂O) is slightly negative at the oxygen end. It is slightly positive at the two hydrogen ends. Opposite charges attract. So each water molecule clings to the ones around it by hydrogen bonds. This shifting web of hydrogen bonds is the hidden reason water can be a solvent, a heat buffer, and a lubricant.

The universal solvent

Water is called the universal solvent. Its charged ends surround other charged or polar substances and pull them apart. They spread out evenly to form a solution. The liquid that does the dissolving is the solvent. The substance that gets dissolved is the solute.

Molecules that mix into water, like salts, sugars, and amino acids, are hydrophilic ("water-loving"). Nonpolar molecules, like fats, are hydrophobic ("water-fearing"). They won't dissolve in water. This split does real work in the body. It is why cell membranes, made of hydrophobic lipids, can form watertight walls in your watery body. So much of the body's chemistry happens in water. Nutrients travel in it, wastes dissolve in it, and reactions happen in it. Blood plasma is really just water carrying dissolved cargo.

Heat, cushioning, and chemistry

Water's hydrogen bonds also give it a very high heat capacity. It takes a lot of energy to warm water up. So your water-filled body resists sudden changes in temperature and stays near 37 °C. And when water evaporates, it carries away a lot of heat. That is why sweating cools you so well on a hot day.

Water molecules also stick to each other. This lets water form slippery films that protect and cushion. The fluid in serous membranes, the fluid in your joints (synovial fluid), and the mucus lining your airways all depend on it. Last, water doesn't just sit by during chemistry. It takes part. The body builds big molecules by taking water out (dehydration synthesis). It breaks them down by adding water in (hydrolysis). You will see these two reactions again in Section 6.

Explore what water's lopsided charge does for you in the interactive. See how it dissolves things, sticks together, and soaks up heat.

Section 5pH, acids, bases & buffers

What pH actually measures

pH measures how many free hydrogen ions (H⁺) are floating loose in a fluid. The more H⁺ there is, the more acidic the fluid. The less there is, the more basic (alkaline) it is. The scale runs from 0 to 14. 7 is neutral, like pure water. Below 7 is acidic, and above 7 is basic.

There is one twist you must keep in mind. The scale is logarithmic. That means each whole step is a tenfold change in H⁺. A fluid at pH 4 is not just a bit more acidic than one at pH 5. It holds ten times more hydrogen ions. That is why a shift of even a few tenths of a pH unit in the blood is a real emergency.

Acids, bases, and the number that matters most

An acid is a substance that gives off H⁺ into a fluid. This pushes the pH down. Stomach acid is near pH 2. A base takes up H⁺ (or gives off OH⁻). This pushes the pH up. Household bleach and the juices from the pancreas are basic.

The most important pH in the body is the pH of blood. It must stay in the very narrow range of 7.35 to 7.45, or about 7.4. If it drops below 7.35, that is acidosis. If it rises above 7.45, that is alkalosis. If it goes past about 6.8 or 7.8, proteins lose their shape and enzymes fail. Then life is in danger. The pH of the world outside swings a lot. But the body keeps its blood almost perfectly steady.

Buffers: holding the line at 7.4

Acids pour into the blood from every meal and every working muscle. So how does blood keep its pH so steady? The answer is buffers. A buffer is a team of chemicals that soaks up extra H⁺ when a fluid gets too acidic. It gives off H⁺ when the fluid gets too basic.

The body's first line of defense is the bicarbonate system. It is made of carbonic acid (H₂CO₃) and the bicarbonate ion (HCO₃⁻). The two trade hydrogen ions back and forth to soften any swing. Buffers don't make acid go away. They soften the blow and buy time. Then the lungs and kidneys fix the real balance. The lungs breathe out CO₂, and the kidneys get rid of H⁺ in urine. This is homeostasis at the level of molecules. It is also a sneak peek at the acid–base chapters later in the book.

Push the pH up and down in the interactive and watch buffers fight to pull it back toward neutral.

Section 6The molecules of life

Organic molecules and the carbon backbone

Now let's move from small inorganic molecules, like water, salts, and gases, to the big organic ones. These are the true building blocks of the body. Organic molecules are built on a frame of carbon. Carbon is the perfect building atom. It can form four covalent bonds at once. This lets it link into long chains, branch out, and close into rings.

The biggest of these molecules are called macromolecules. Most are polymers, long chains snapped together from small repeating units called monomers. Think of beads on a string. The body builds a polymer by dehydration synthesis. It takes out a water molecule to join two monomers. It breaks a polymer apart by hydrolysis. It adds water to split the bond. Almost everything you are made of belongs to just four families. There is also one special molecule that powers it all.

The four families

Carbohydrates are sugars and chains of sugars. Their monomers are called monosaccharides, such as glucose. These link up into polysaccharides, such as glycogen and starch. Carbohydrates are the body's quick, favorite fuel.

Lipids are the oily molecules that don't mix with water. Triglycerides store energy and keep you warm. Phospholipids build every cell membrane. Steroids, like cholesterol, are used to make hormones. Lipids avoid water, and that is exactly what makes membranes work.

Proteins are the body's workhorses. They are built from twenty kinds of amino acids linked in a chain. The chain then folds into an exact 3-D shape. A protein's shape decides its job. So if a protein unfolds (denatures) from heat or the wrong pH, it stops working. Think of egg white turning solid in a hot pan. Proteins act as enzymes, building fibers, carriers, and messengers.

Nucleic acids, DNA and RNA, are chains of nucleotides. They store and carry the genetic instructions for building every protein in your body.

ATP: the cell's energy currency

One small molecule gets its own spotlight. It is adenosine triphosphate, or ATP. ATP is the energy "money" that every cell spends. It has a tail of three phosphate groups. The bond to the last phosphate is like a loaded spring.

When a cell needs power, it snaps off that last phosphate (ATP → ADP + phosphate). The energy that comes out does the work. This might be making a muscle contract, pumping an ion, or building a molecule. Then the cell uses energy from food to turn ADP back into ATP. In a real sense, you remake about your own body weight in ATP every day. Your body spends and rebuilds it billions of times a second.

Pick a molecule family in the interactive below and watch it snap together from its monomers.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back any time a word trips you up.

Acid

A substance that gives off hydrogen ions (H⁺) into a fluid. It lowers the pH below 7.

Activation energy

The first energy hump a reaction must get over before it can go. Enzymes lower it.

Atom

The smallest unit of an element that still acts like that element. It has a nucleus of protons and neutrons, with electrons around it.

Atomic number

The number of protons in an atom's nucleus. It tells you which element the atom is.

ATP (adenosine triphosphate)

The cell's energy currency. It gives off usable energy when its last phosphate is removed, which turns it into ADP.

Base

A substance that takes up hydrogen ions (or gives off OH⁻). It raises the pH above 7.

Buffer

A set of chemicals that keeps pH from changing by soaking up or giving off H⁺. The bicarbonate buffer in blood is one example.

Carbohydrate

A family of organic molecules made of sugars and chains of sugars, like glucose and glycogen. They are used mainly for quick energy.

Covalent bond

A bond in which atoms share electrons. Equal sharing is nonpolar. Unequal sharing is polar.

Dehydration synthesis

Building a polymer by taking out a water molecule to join two monomers.

Electron

A tiny, almost weightless particle with a negative charge. It moves around the nucleus in shells and drives chemical bonding.

Element

A pure substance that can't be broken down by chemistry. The body is built mostly from six of them.

Enzyme

A helper molecule, usually a protein, that speeds up a reaction by lowering its activation energy. It is not used up.

Hydrogen bond

A weak pull between the slightly positive hydrogen of one polar molecule and a slightly negative atom of another.

Hydrolysis

Breaking a polymer apart by adding a water molecule across a bond.

Ionic bond

A bond that forms when one atom gives an electron to another. This makes two ions with opposite charges that attract.

Isotope

An atom of an element with the usual number of protons but a different number of neutrons.

Lipid

A family of organic molecules that avoid water (hydrophobic). It includes triglycerides, phospholipids, and steroids. They are used for energy storage, membranes, and hormones.

Nucleic acid

A chain of nucleotides, either DNA or RNA. It stores and uses genetic information.

pH

A logarithmic measure of how many free hydrogen ions are in a fluid. 7 is neutral, lower is acidic, and higher is basic.

Polar molecule

A molecule with an uneven charge, such as water. One end is slightly negative, and the other end is slightly positive.

Protein

A chain of amino acids folded into an exact shape. Its shape sets its job, such as an enzyme, a building block, a carrier, or a messenger.

The Cellular Level of Organization

The last chapter looked at the whole body. This one zooms all the way in, to the smallest thing that is still alive. The cell is where the body's work really happens. It is where oxygen is used to make energy. It is where genes are read to make proteins. It is where the choice to grow or divide is made. In this chapter you will meet the membrane, the thin wrap around every cell that picks what gets in and out. You will tour the organelles, the tiny parts that share the work inside. You will follow a gene, a set of instructions, as it becomes a working protein. Then you will watch one cell become two. Once you know the cell, the rest of the body makes more sense. Every tissue, organ, and system is just cells working together.

Section 1The cell membrane

A smart wall

Every cell in your body is wrapped in a plasma membrane. This is the thin outer skin of the cell. It is so thin that you cannot see it with a normal light microscope. Yet it is smart enough to decide what comes in and what goes out. Without it, a cell would just fall apart into the fluid around it.

The membrane does three big jobs at once. First, it separates the inside of the cell from the outside. Second, it controls traffic across that border. It is selectively permeable, which means it lets some things through and blocks others. Third, it communicates. It carries receptors and ID tags, so the cell can sense signals and be known by other cells. Everything else in this chapter depends on this one wall doing its job.

The fluid mosaic model

Scientists picture the membrane with the fluid mosaic model. Both words matter. Mosaic means it is a patchwork of many pieces. Fats, proteins, and sugars sit side by side like tiles. Fluid means the pieces are not glued down. They drift and wiggle sideways in the sheet. So the membrane acts more like a thin film of oil than a hard wall.

The base of the membrane is the phospholipid bilayer. It is two layers of fat molecules called phospholipids, lined up tail to tail. Each phospholipid is amphipathic, which means it has two very different ends. It has a hydrophilic ("water-loving") head and two hydrophobic ("water-fearing") fatty tails. Put these molecules in water and they line up on their own. The heads face the watery fluid on both sides. The oily tails hide in the middle, away from water.

That oily middle is the secret to how picky the membrane is. Small molecules that dissolve in fat slip right through it. Ions and other water-loving molecules get turned away. They need a protein doorway to get across.

Set into this fat sheet are the membrane proteins, and they do most of the work. Integral (transmembrane) proteins pass all the way through the membrane. They form the channels, carriers, and pumps that move things across. Peripheral proteins stick to just one surface. They often act as enzymes, which speed up reactions, or as anchors. Some proteins are receptors that catch hormones and other signals. Others are identity markers that label the cell as "self," part of your own body.

Tucked between the fats are molecules of cholesterol. Cholesterol keeps the membrane just right. It stops the membrane from getting too stiff when cold or too runny when warm. Last, short sugar chains stick to the outside of some proteins and fats. Together they form the glycocalyx, a fuzzy sugar coat on the cell's outer face. It helps cells know each other, stick together, and tell "self" from "not self."

Tap each part of the membrane below to see it up close.

Section 2Moving things across

The one question to ask: does it cost energy?

A cell must always bring in food, push out waste, and keep the right mix of salts and water. It has many ways to move things across its membrane. Every one of them falls into one of two groups. The dividing line is energy.

In passive transport, things move down their concentration gradient. That means they go from where there is a lot of them to where there is less. This costs the cell no ATP, because it just goes with the natural flow. In active transport, the cell moves things up a gradient. They go against the flow, from low amounts to high amounts. That always takes energy, most often ATP. So before you learn the names, ask one thing. Is it rolling downhill for free (passive)? Or is it being pushed uphill at a cost (active)?

Passive transport: rolling downhill

The simplest kind is simple diffusion. Molecules are always moving around at random. Over time, they spread out from crowded places to less crowded places, until they are evenly mixed. Small molecules with no charge, like oxygen and carbon dioxide, slip right through the fat layer. That is how O₂ gets into every cell and CO₂ gets out.

Some molecules are too big or too charged to cross the fat layer alone. They use facilitated diffusion. They still move downhill for free. But they go through a protein channel or carrier that gives them a way in. Glucose, a sugar, getting into a cell is the classic example.

One special and vital kind of passive transport is osmosis. Osmosis is the diffusion of water across a selectively permeable membrane. It is so important that it gets its own section at the end of this chapter.

Active transport: pushing uphill

Sometimes a cell needs more of a substance on one side than the other. That means moving it against its gradient, which takes energy. In primary active transport, a pump uses ATP directly. The best known is the sodium–potassium pump (Na⁺/K⁺ pump). For every ATP it uses, it pushes 3 Na⁺ out of the cell and brings 2 K⁺ in. This pump runs in every cell you have. It sets up the electrical charge that lets nerves send signals and muscles squeeze.

In secondary active transport, the cell uses the gradient that a primary pump already built. One ion rushes downhill, and it drags another substance uphill along with it. It is a bit like a water wheel that uses falling water to lift something else.

Some cargo is far too big for any channel, like whole droplets or bits of matter. For these the cell uses bulk transport. It wraps the cargo in a piece of membrane. Endocytosis brings cargo in. This includes phagocytosis, "cell eating," and pinocytosis, "cell drinking." Exocytosis sends cargo out.

Pick a transport method below and watch the molecules cross.

Section 3Inside the cell: the organelles

A factory with departments

Look inside the membrane and you will not find an empty bag. You will find a busy, well-run workshop. The cell is filled with a jelly-like fluid called the cytosol. Floating in it are the organelles, which means "little organs." Each one is a department with its own job.

Splitting up the work lets the cell run many chemical jobs at once without them getting in each other's way. It is like the walls that separate the rooms of a factory. Many organelles have their own membrane that seals off what happens inside. A few, like ribosomes, do not. To learn the cell, you learn who does what.

The control room and the assembly line

The largest organelle is usually the nucleus, the cell's control center. It is wrapped in a double membrane called the nuclear envelope. The envelope has small holes, or pores, that let messages in and out. The nucleus holds the cell's DNA. DNA is the master set of instructions for making every protein.

Most of the time, DNA is spread out as a loose tangle called chromatin. When the cell gets ready to divide, the DNA packs tightly into chromosomes that you can see. Inside the nucleus is a dark spot called the nucleolus. This is where ribosomes are built.

Ribosomes are the machines that build proteins. Some float free in the cytosol. Others dot the rough endoplasmic reticulum (rough ER). The rough ER is a folded network of membrane. It makes and packs proteins that will be sent out of the cell or placed in the membrane. Next to it is the smooth ER, which has no ribosomes. The smooth ER makes fats, breaks down harmful substances, and stores calcium.

Proteins and fats then move to the Golgi apparatus. The Golgi is the cell's shipping center. It changes, sorts, and packs them into small bubbles called vesicles. Each vesicle is sent to where it is needed.

Power, cleanup, and shape

All this work needs energy. That energy comes from the mitochondria, the "powerhouses" of the cell. Here food and oxygen are turned into ATP, the energy the cell can spend. Cells that work hard, like muscle and liver cells, are packed with mitochondria.

Two organelles handle cleanup. Lysosomes are sacs full of digestive enzymes. They break down worn-out parts, germs, and junk. Peroxisomes make harmful by-products safe.

The cytoskeleton holds the whole cell together. It is a frame of protein threads. It gives the cell its shape, holds organelles in place, and helps things move. The centrosome, with its pair of centrioles, organizes this frame. It also builds the fibers that pull chromosomes apart when the cell divides.

Tap an organelle to light it up, or press Tour to walk the whole factory floor.

Section 4From gene to protein

The recipe and the problem

Your DNA is like a recipe book. Each gene is one recipe for one protein. But there is a problem. The DNA is locked inside the nucleus. Proteins are built by ribosomes out in the cytoplasm, the space outside the nucleus. The master book can never leave the library.

The fix is to make a throwaway copy of just the one recipe you need. That copy is carried out to the workshop. This is the heart of protein synthesis, and it takes two steps. Transcription copies the gene. Translation reads the copy and builds the protein. This flow of information, from DNA to RNA to protein, is so basic that it is called the central dogma of biology.

Transcription: copying the gene

Transcription happens inside the nucleus. An enzyme called RNA polymerase unzips the DNA at the gene it needs. It reads one strand and builds a matching strand of messenger RNA (mRNA). RNA is a lot like DNA. But it has only one strand, and some of its parts are a little different. For example, it uses the base uracil (U) in every place where DNA would use thymine.

The matching rules are strict. DNA's C pairs with G. DNA's A calls for a U in the RNA. The finished mRNA is a copy of the gene you can carry around. It is the working copy. It slips out through a pore in the nucleus and goes off to find a ribosome.

Translation: reading the copy

Translation happens at a ribosome. Here the mRNA is read three bases at a time. Each group of three is called a codon. Each codon stands for one amino acid, a building block of proteins.

The amino acids are brought in by transfer RNA (tRNA). Each tRNA carries one kind of amino acid. It also has a three-base anticodon that matches one codon. The ribosome moves along the mRNA one codon at a time. The tRNAs line up in order, and the ribosome links their amino acids into a growing chain.

When the ribosome reaches a "stop" codon, it lets the finished chain go. The chain folds into its working shape and becomes a protein. The whole path is DNA → mRNA → protein. It turns a stored instruction into a real molecule that does a job.

Step through the whole trip from gene to protein below.

Section 5The cell cycle & mitosis

Living, then dividing

Your body makes billions of new cells every day. It needs them to grow, to heal a cut, and to replace skin and blood cells that wear out. Every new cell comes from an old cell that grew and split in two. The steps a cell follows to do this are called the cell cycle.

The cell cycle has two very unequal parts. The long part is interphase. This is the cell's normal working life. It grows, does its job, and quietly gets ready to divide. The short, dramatic ending is mitosis, when the nucleus actually splits. A cell spends most of its life in interphase. Mitosis is over quickly.

Interphase: grow, copy, get ready

Interphase has three stages. In G₁ (first gap), the cell grows, makes proteins, and does its normal work. In S phase (synthesis), the cell copies all of its DNA. That way, each new cell will get a full set. This is the most important step in getting ready to divide. In G₂ (second gap), the cell finishes growing. It builds the organelles it will need and checks the copied DNA for mistakes.

Only after all this does the cell commit to dividing. Checkpoints guard each step. They are quality-control stops, like a safety check before a plane takes off. At each one, the cell makes sure everything is in order before it moves on.

Mitosis: splitting the nucleus, then the cell

Mitosis splits the copied DNA evenly into two nuclei. It has four stages. You can remember them as PMAT.

In prophase, the copied chromatin packs into chromosomes you can see, and spindle fibers start to form. In metaphase, the chromosomes line up single file across the middle of the cell. In anaphase, the spindle fibers pull the two copies of each chromosome apart, toward opposite ends. In telophase, a new nuclear envelope forms around each of the two sets.

Mitosis splits the nucleus. A separate step, cytokinesis, pinches the rest of the cell in two. The result is two genetically identical daughter cells. Each has the same DNA, and each is ready to start its own interphase.

Step through interphase and the four phases of mitosis below.

Section 6Osmosis & tonicity

Water chases salt

We saved the kind of transport that matters most in hospitals for last. Osmosis is the movement of water across a selectively permeable membrane. It follows one simple rule. Water moves toward the side with more dissolved particles. We call that side the one with more solute, or the "saltier" side.

The membrane keeps the solute from moving across to even things out. So water moves instead. It flows toward the saltier side until the two sides are balanced. A cell's membrane lets water pass freely but holds many solutes in place. So the amount of solute outside the cell decides which way water flows. That decides whether the cell gains or loses water.

Tonicity: three fluids, three results

Tonicity tells you how a fluid will change a cell's water balance. It compares the solute outside the cell to the solute inside. There are three cases.

An isotonic fluid has the same amount of solute as the inside of the cell. Water moves in and out equally, so the cell stays the same. This is its normal, healthy state.

A hypotonic fluid is more watery than the cell, with less solute outside. So water rushes in. The cell swells. If enough water gets in, the cell can burst. This is called lysis. In red blood cells it is called hemolysis.

A hypertonic fluid is saltier than the cell, with more solute outside. So water is pulled out. The cell shrivels. This shrinking is called crenation. Here is the trick to keep them straight. Water always follows the solute. Find the saltier side, and water goes there.

Drop a cell into each fluid below and see what happens to it.

ReferenceWords worth knowing

Here are the key words from this chapter. Skim them now, or come back any time a word trips you up.

Active transport

Moving a substance up its concentration gradient, from low to high. It needs energy, such as ATP.

Codon

A group of three mRNA bases. It stands for one amino acid during translation.

Crenation

The shriveling of a cell that loses water in a hypertonic fluid.

Cytokinesis

The splitting of the cytoplasm after mitosis. It turns one cell into two.

Cytoskeleton

The inside frame of protein threads. It gives a cell shape and support and moves things around.

Diffusion

The free spreading of molecules from where there are many to where there are few, until they are evenly mixed.

Fluid mosaic model

The picture of the cell membrane as a fluid phospholipid bilayer. Proteins, cholesterol, and sugars sit in it like tiles.

Golgi apparatus

The organelle that changes, sorts, and packs proteins and fats into vesicles for delivery.

Hypertonic

A fluid with more solute than the cell. Water leaves the cell, and the cell shrivels.

Hypotonic

A fluid with less solute than the cell. Water enters the cell, and the cell swells or bursts.

Interphase

The long part of the cell cycle (G₁, S, G₂). The cell grows, copies its DNA, and gets ready to divide.

Isotonic

A fluid with the same amount of solute as the cell. Water does not move in or out overall, so the cell keeps its normal shape.

Lysosome

A sac of digestive enzymes wrapped in membrane. It breaks down junk, worn-out organelles, and things the cell takes in.

Mitochondrion

The organelle that makes ATP from food and oxygen. It is the cell's powerhouse.

Mitosis

The splitting of the nucleus into two nuclei with identical DNA. It goes through prophase, metaphase, anaphase, and telophase.

Nucleus

The control center of the cell, wrapped in membrane. It stores the cell's DNA and directs what the cell does.

Osmosis

The diffusion of water across a selectively permeable membrane. Water moves toward the side with more solute.

Passive transport

Moving a substance down its concentration gradient. It needs no energy from the cell.

Phospholipid bilayer

The double layer of amphipathic phospholipids that forms the core of the membrane. The water-loving heads face out, and the water-fearing tails face in.

Sodium–potassium pump

A primary active transport protein. For each ATP, it pushes 3 Na⁺ out and brings 2 K⁺ in. This keeps the cell's ion gradients in place.

Transcription

Copying a gene from DNA into messenger RNA. It happens inside the nucleus.

Translation

Reading mRNA codons at a ribosome to join amino acids into a protein.

The Tissue Level of Organization

In the last chapter we zoomed in on a single cell. Now we take one step back up the ladder. Cells rarely work alone. They gather into tissues, which are teams of similar cells that share a job. Together, they do things no single cell could do. The amazing part is how few kinds there are. Your whole body, and every organ in it, is built from just four primary tissues. This chapter introduces all four. You will see how each looks under the microscope, where it lives, and what it does. The chapter ends with a story every tissue must tell sooner or later: how it heals when it gets hurt.

Section 1The four tissue types

A team of cells

A tissue is a group of similar cells that work together to do a shared job. That is the whole definition. But the word "together" matters a lot. The cells in a tissue are not just neighbors that look alike. They work as a team. They are held in place by links between them, or by material they make around themselves. And each tissue is built for one kind of job.

The study of tissues under the microscope is called histology. It is really anatomy at very high zoom. Instead of asking what an organ looks like to the eye, we ask what it is made of.

Four families do all the work

Here is what makes this subject easy to learn. No matter how complex an organ looks, it is built from only four primary tissue types. Learn these four, and you have the words to describe any part of the body. Each family meets a different basic need. A body must cover and line its surfaces. It must bind and support its parts. It must move. And it must communicate.

Notice how different the four are in how much room the cells give each other. Epithelial cells are packed so tightly that there is almost no space between them. That is what makes them a good barrier. Connective tissue is the opposite. Its cells are few and far apart. They float in a sea of non-living material that they make. This one contrast is one of the fastest ways to tell the first two tissues apart on a slide. Look for crowded cells or cells spread out in a matrix.

From tissues to organs and membranes

An organ is two or more tissue types layered together. The whole organ can do something none of its tissues could do alone. The stomach is the clearest example. It has an epithelial lining that makes acid and mucus. It has a connective tissue layer that carries its blood vessels and nerves. It has smooth muscle that churns food. And it has nervous tissue that times the churning. That is four tissues in one organ.

The simplest of these teams is a body membrane. This is a thin sheet that covers or lines a surface. There are only four kinds.

The cutaneous membrane is the skin itself. It is stratified squamous epithelium on top of dense irregular connective tissue. It is the only membrane that is dry. A mucous membrane lines every space that opens to the outside. This includes the airway, the gut, and the urinary and reproductive tracts. It pairs an epithelium with a loose connective layer under it, called the lamina propria.

A serous membrane lines the closed body spaces in the front of the trunk and wraps the organs inside them. Examples are the pleura, pericardium, and peritoneum. It joins a simple squamous epithelium called mesothelium to a thin areolar layer. It makes a watery fluid so organs can slide past each other. The synovial membrane is the odd one out. It lines the spaces inside joints. It is made of connective tissue only, with no epithelium at all.

Meet all four tissues side by side in the interactive below.

Section 2Naming an epithelium

What every epithelium has in common

Epithelial tissue covers every open surface of the body. It lines every space, tube, and duct. Think of your outer skin, the inside of your gut, your airways, and your blood vessels. Because it sits at every border, it has a set of features built for that role.

Its cells are tightly packed, with very little material between them. So almost nothing gets across without going through a cell. Every epithelium has polarity, which means it has a top and a bottom. The apical surface is the top. It faces the open space and often has tiny fingers (microvilli) or hairs (cilia). The basal surface is the bottom. It is anchored to a thin basement membrane that glues it to the connective tissue below.

Epithelium is avascular. This means it has no blood vessels of its own. It gets its food by diffusion from the tissue underneath. Yet it has plenty of nerves. It also regenerates, or regrows, quickly. It has to, because being a surface means cells are always wearing away.

The two-word naming rule

Here is the big reward of this section. Every epithelium is named with the same simple formula. Once you learn the rule, you can name a tissue you have never seen. The name has two parts. First comes the number of layers. Then comes the shape of the cells at the surface.

• Layers. One layer of cells is simple. Two or more stacked layers is stratified.
• Shape. Flat cells, like fish scales, are squamous. Cells shaped like cubes are cuboidal. Tall cells shaped like columns are columnar.

Snap the two halves together and you have the name. One layer of flat cells is simple squamous. Many stacked layers of flat cells make stratified squamous. One row of tall cells is simple columnar.

The build even tells you the job. Thin simple epithelia are best where things must cross quickly. Simple squamous lines the air sacs of the lungs and the walls of capillaries. Oxygen and nutrients slip across it in an instant. Thick stratified epithelia are built for protection. Stratified squamous forms the surface of your skin and lines your mouth and esophagus. You can afford to lose and rebuild these layers.

Two rows in that table are the famous exceptions. Pseudostratified columnar epithelium is really just one layer. Every cell touches the basement membrane. But the nuclei sit at different heights, so it looks stacked. That is why it is called "pseudo" (false) stratified. Transitional epithelium is the shape-shifter of the urinary tract. Its cells slide over each other and flatten as the bladder fills. Then they bunch back up as it empties.

Build an epithelium's name box by box and watch it appear in the interactive.

Section 3Connective tissue

One recipe, many results

Connective tissue is the most common tissue in the body. It is also the most varied. At first it can seem like a random mix. Bone, fat, tendon, cartilage, and even blood all belong to it. What links them is one shared plan.

In epithelium, the cells are everything. Connective tissue is different. It is mostly extracellular matrix. This is non-living material that the cells make and then sit inside. The cells are few and spread far apart. The feel of the tissue comes almost entirely from the matrix around them.

This is the key idea of the whole section: change the matrix and you change the tissue. Make the matrix a firm, rubbery gel, and you get cartilage. Harden it with calcium salts, and you get bone. Make it liquid, and you get blood. Adipose tissue, or fat, is the useful exception. Its cells store fat inside themselves. Each cell swells up with one big droplet. So the matrix gets squeezed down to almost nothing. Fill the cells instead of the matrix, and you get fat.

The matrix: fibers and ground substance

The extracellular matrix always has two parts. The first part is protein fibers. There are three kinds. Collagen fibers are strong, like ropes, and resist pulling. Elastic fibers stretch and snap back into shape, like rubber bands. Reticular fibers are thin and form a fine mesh that holds things up.

The second part is the ground substance. It fills the space between the fibers and cells. Depending on the tissue, it can be watery, like jelly, or rock hard.

The cells often come in two kinds, named by a simple rule. A name that ends in -blast is a young, busy builder cell that makes matrix. Examples are the fibroblast, osteoblast, and chondroblast. A name that ends in -cyte is a grown-up cell that keeps the matrix in good shape. Examples are the fibrocyte, osteocyte, and chondrocyte.

Read that table from top to bottom. You can watch one idea stretch across every level of stiffness. It goes from a soft cushion, to a fat pad, to a tough cable, to a rubbery shock absorber, to solid mineral, and all the way to a flowing liquid. It is the same recipe with very different results. What changes is what the cells put into the matrix.

Rebuild the matrix in the interactive and watch one tissue turn into the next.

Section 4Muscle tissue

Cells built to shorten

Muscle tissue has one skill that no other tissue has. It can contract. That means it can actively shorten to pull on something. Its cells are packed with two protein threads, actin and myosin. These threads slide past each other to shorten the cell. Everything about a muscle cell's shape helps with that pull.

Sometimes these threads line up in neat, repeating rows. Then they catch the light in bands, and the tissue looks striated, or striped. When the threads are less orderly, the tissue looks smooth. Contraction does more than move the body. It also makes much of your body heat. That is why shivering warms you up.

The three muscle tissues

There are exactly three muscle tissues. You can tell them apart with three questions. Is it striped? Can you control it on purpose? What shape are its cells?

Skeletal muscle is the meat that moves your bones. It is striated and voluntary, which means you control it on purpose. It is made of long, tube-shaped fibers, each with many nuclei.

Cardiac muscle is found only in the wall of the heart. It is striated but involuntary, so it works without you thinking about it. Its short, branching cells are joined end to end by intercalated discs. These let the whole heart squeeze as one team.

Smooth muscle lines the walls of hollow organs like the gut, blood vessels, and bladder. It is non-striated and involuntary. It is made of small, tapered cells shaped like spindles, thick in the middle and pointed at the ends. They squeeze those organs slowly and without tiring.

Here is a handy shortcut. The two striated muscles, skeletal and cardiac, are the fast, strong ones. The one smooth muscle is the slow, steady one. And only one of the three, skeletal, does what you tell it. The heart and the gut run on their own.

Watch each of the three muscles contract in the interactive below.

Section 5Nervous tissue

The neuron: a cell shaped like a message

Nervous tissue senses changes, makes decisions, and sends electrical signals. It does this faster than any other tissue in the body. Its star cell is the neuron, or nerve cell. The neuron's odd shape is really a map of how a signal flows through it. The signal goes in one end and out the other.

Signals arrive at the bushy dendrites. These are short, branching arms that pick up messages from other cells. The signals flow into the cell body (or soma). The cell body holds the nucleus and keeps the cell running. If the incoming signal is strong enough, the neuron fires an electrical pulse. The pulse races away down the axon, one long fiber that carries the message to the next cell.

Many axons are wrapped in a fatty myelin sheath. It works like the plastic coating on a wire. It lets the signal jump along much faster. The traffic only goes one way: dendrite → cell body → axon, every time.

Neuroglia: the support crew

Neurons get the fame. But there are at least as many neuroglia, also called glial cells. These are the support cells of nervous tissue. Careful counts show about one glial cell for each neuron across the whole brain. Glia outnumber neurons in the cerebral cortex, the brain's wrinkled outer layer. Neurons far outnumber glia in the cerebellum, at the back of the brain.

Glia do not carry the signal themselves. Instead, they feed neurons and wrap their axons in myelin. They hold the tissue together and defend it from germs. Two features define what nervous tissue can do. Excitability is the ability to respond to a trigger by making an electrical signal. Conductivity is the ability to carry that signal from one place to another. Together, they make this tissue the body's high-speed communication network.

Tap a part of the neuron and watch a signal fire in the interactive.

Section 6Tissue repair

Two ways to fix a wound

When a tissue is hurt, the body can fix it in one of two ways. The better result is regeneration. The damaged cells are replaced with new cells of the very same type. The tissue goes back to normal, as if the injury never happened.

The backup plan is fibrosis. Here the gap is filled with thick, tough scar tissue instead of the original cells. Scar tissue plugs the hole and makes the area strong again. But it cannot do the original job. Scar in the heart cannot beat. Scar in the skin has no hair or sweat glands.

Which path a tissue takes depends mostly on how well its cells can divide. Tissues whose cells divide fast, like skin and gut, regenerate very well. Tissues whose cells barely divide, like heart muscle and neurons, tend to scar.

The healing playbook

For any tissue, the body runs the same four-step repair plan, in order. Think of it as a building project on a rush schedule. First stop the flooding. Then clear the site. Then rebuild the frame. Then finish the surfaces.

The signs you can see in stage two are redness, heat, swelling, and pain. These are the classic marks of inflammation. They feel like the problem, but they are really the body doing its job. The vessels widen and leak. That causes the redness and swelling. It also brings in the clotting factors, germ fighters, and cleanup crews the wound needs. Inflammation is not the injury. It is the repair getting started.

Step through a healing wound from clot to scar in the interactive below.

ReferenceWords worth knowing

Here are the key words from this chapter. Skim them now, or come back any time a word trips you up.

Apical surface

The free top surface of an epithelial cell. It faces the open space, cavity, or lumen, and often has microvilli or cilia.

Areolar tissue

The most common loose connective tissue. It is soft, all-purpose packing that lies under epithelia and cushions organs.

Axon

The one long fiber of a neuron. It carries the electrical signal away from the cell body toward the next cell.

Basement membrane

A thin layer that anchors the bottom (basal) surface of an epithelium to the connective tissue below it.

Cardiac muscle

Striated, involuntary muscle found only in the heart wall. Its branched cells are joined by intercalated discs, so the heart squeezes as one.

Collagen fiber

The strong, rope-like protein fiber of connective tissue. It resists pulling.

Connective tissue

The varied tissue that binds, supports, protects, and carries things. It is made of spread-out cells in an extracellular matrix.

Dendrite

A short, branching arm of a neuron. It picks up signals from other cells.

Epithelial tissue

Tightly packed tissue that covers body surfaces, lines spaces and tubes, and forms glands.

Extracellular matrix

The non-living material, fibers plus ground substance, that connective tissue cells make and sit inside.

Fibrosis

Repair by laying down tough scar tissue in place of the original cells. It brings back strength but not full function.

Ground substance

The part of the extracellular matrix that fills the space between cells and fibers. It can be watery, like jelly, or solid.

Histology

The study of tissues under the microscope. It is anatomy at high zoom.

Muscle tissue

Tissue built to contract using actin and myosin threads. It makes movement, force, and heat.

Nervous tissue

Tissue built to sense, sort out, and quickly send electrical signals. It is made of neurons and neuroglia.

Neuroglia

The support cells of nervous tissue. They feed, wrap, hold together, and protect neurons.

Neuron

The signal-carrying cell of nervous tissue. It has dendrites, a cell body, and an axon.

Regeneration

Repair in which damaged cells are replaced by new cells of the same type. Normal structure and function come back.

Simple epithelium

An epithelium one cell layer thick. It is suited to diffusion, absorbing, and releasing substances.

Squamous

Describes flat, scale-like epithelial cells. They are ideal where substances must cross quickly.

Stratified epithelium

An epithelium with two or more stacked layers of cells. It is built to protect against rubbing and scraping.

Tissue

A group of similar cells that work together to do a shared job.

The Integumentary System

Spread flat, your skin would cover about two square meters. It weighs more than any other organ you have. Yet most of us never think of it as an organ at all. This chapter will change that.

You will build the skin from the top down, one layer at a time. Then you will follow a single cell on its month-long trip. It starts as a busy, living cell and ends as a dead flake on the surface. You will learn where skin color comes from. You will meet the hair, nails, and glands that grow out of the skin. You will also see the five quiet jobs your skin does every second. We end with burns and skin cancer, the two main ways this amazing barrier can fail.

Section 1Layers of the skin

An organ you can see

The integumentary system is your skin plus everything that grows out of it. That means hair, nails, and glands. The word integument just means "a covering." That is the simplest way to picture skin. It is a wrapper. It keeps the wet, carefully controlled world inside you apart from the dry, risky world outside.

But "wrapper" sells it short. The skin is a true organ, a body part built from several tissue types working together. In fact, it uses all four types. It is the largest organ in the body, both by weight and by surface area. One fact holds this whole chapter together. The skin is two layers stacked on top of a third, supporting layer. And that third layer is not really skin at all.

Two layers of skin, one layer beneath

Read the skin from the surface down, and you pass through three named layers. The top two are the skin itself. The third one holds the skin to the body.

The epidermis is the top layer. It is made only of epithelium, a sheet of tightly packed cells. It is also avascular, which means it has no blood vessels of its own. Its living cells get oxygen and food that seep up from the dermis below. The outer part of the epidermis is too far from this supply. So those outer cells are all dead.

The dermis is where the action is. It is a thick mesh of collagen and stretchy fibers. It is packed with blood vessels, nerves, hair follicles, and glands. These are the parts that let skin do its jobs. The dermis has two smaller layers. On top is a thin, bumpy papillary layer. Its bumps, the dermal papillae, make your fingerprints. Below it is a thick reticular layer that holds most of the collagen.

Under the dermis lies the hypodermis. It is also called the subcutaneous layer or superficial fascia. It is mostly fat. It pads and warms the body. It also ties the skin loosely to the parts underneath.

Thick skin and thin skin

Not all skin is the same thickness. Thick skin covers the palms of your hands and the soles of your feet. These are the spots that get rubbed the most. Thick skin has all five layers of epidermis, including an extra clear one.

Thin skin covers the rest of your body. It is more delicate and has only four layers of epidermis. It is also the only skin that grows hair. Watch out for one common mix-up. The names "thick" and "thin" describe the epidermis only. They do not describe how thick the whole skin is.

Section 2The five epidermal strata

A conveyor belt of cells

The epidermis is like a factory that never stops. Its main cell is the keratinocyte. It is born at the very bottom. New cells form below it and push it upward. As it rises, it fills with a tough protein called keratin. It dies along the way. At last it flakes off the top.

The whole trip takes about four weeks, or roughly 25 to 45 days. So the outer surface of your body is fully replaced about once a month. This slow climb is called keratinization. It is the most important process in the epidermis.

The five layers, bottom to top

In thick skin, the keratinocyte passes through five named layers, or strata, on its way up. Learn them in order. Start at the deepest layer, where cells are born. End at the surface, where they are shed:

A memory trick helps. Read from the base up: Basale, Spinosum, Granulosum, Lucidum, Corneum. Think "Bad Students Get Lousy Classes." The bottom two layers, basale and spinosum, are alive. They are sometimes grouped together as the stratum germinativum, the growth zone. From the granulosum up, the cells are dying or dead.

By the time a cell reaches the stratum corneum, it is just a flat bag of keratin. It is glued to its neighbors and coated in fat. That dead, waterproof crust is what makes skin such a good barrier. You shed about a million of these cells every day.

Section 3Skin color & melanin

Three pigments, one main player

Skin color comes from three pigments mixed together. A pigment is a substance that gives color. Melanin is the main one. It is a brown-to-black (or red-to-yellow) pigment made in the epidermis. Carotene is a yellow-orange pigment that comes from food, such as carrots. It collects in the stratum corneum and in the fat of the hypodermis.

Hemoglobin is the red pigment in blood. It shows through where the epidermis is thin. It gives lighter skin its pink glow. Of the three, melanin matters most. It is also the one that varies most from person to person.

Melanocytes hand off their pigment

Melanin is made by a special cell called the melanocyte. It sits down in the stratum basale. Here is the surprising part. Melanocytes do not keep their pigment. They pack melanin into tiny sacs called melanosomes. Then they hand these sacs to the keratinocytes around them.

Inside a keratinocyte, the melanin moves to the sunny side of the cell. It forms a small cap over the top of the nucleus. It is like a tiny umbrella held over the cell's DNA. There it soaks up incoming ultraviolet (UV) radiation, the invisible rays in sunlight. It catches the UV before it can reach and harm the DNA. That is the whole point of skin pigment. Melanin is sunscreen your cells make for themselves.

When pigment goes wrong, and reading skin color in the clinic

Two conditions show what happens at the extremes. In albinism, an inherited condition, melanocytes are present but cannot make melanin. So the skin, hair, and eyes have little or no color, and they burn easily. In vitiligo, the immune system destroys melanocytes in patches. This leaves pale blotches on the skin.

Skin color also gives some of the fastest clues to illness. Health workers can see it right at the bedside. That is because skin color reflects what is happening in the blood underneath.

Section 4Hair, nails & glands

Hair: keratin threads with a living root

The skin's extra parts, like hair and glands, are called accessory structures. Each one grows from the epidermis. But each one is tucked down into the dermis, where the blood supply is.

Hair is a bendy strand of dead, hard keratin. The part you see above the skin is the shaft. The part below is the root. The root sits inside a tube called the hair follicle. At the very bottom, the follicle swells into a bulb. Inside the bulb is a small knot of connective tissue called the papilla. It brings in the blood vessels that feed the growing hair.

Just above the papilla is the matrix. It is the only living, dividing part of a hair. That is why a haircut does not hurt but plucking a hair does. A tiny muscle called the arrector pili is attached to each follicle. When you are cold or scared, it tightens. It pulls the hair upright and dimples the skin into "goosebumps."

Nails: a window over the fingertip

A nail is a plate of hard keratin. It protects the tip of each finger and toe. It also gives you a tool for scratching and gripping. The part you see is the nail body. It sits on the nail bed. The pale half-moon at its base is the lunula. It marks the nail matrix underneath, where the nail actually grows. The cuticle (eponychium) is the fold of skin that seals the base against dirt and germs.

Nails are a useful window on health. They turn blue when the blood is low in oxygen. They become curved and rounded, called clubbed, in long-term heart or lung disease. They curve inward like a spoon in iron-deficiency anemia, when the body is short on iron.

Glands: oil and sweat

The skin holds two big families of glands. Sebaceous (oil) glands usually empty into a hair follicle. They release an oily mix called sebum. Sebum keeps the hair and skin soft and slows the growth of bacteria. When a follicle clogs with sebum and dead cells, the result is acne.

Sweat (sudoriferous) glands come in two types. It is worth keeping them straight:

Section 5What the skin does

One organ, five jobs

It is easy to think the skin just covers you. But covering you is only the first of five real jobs it does every moment. Hold on to all five. They come up again and again in later chapters.

The barrier you rebuild every month

Protection is the star job. The dead, keratin-packed stratum corneum keeps water in. Without it, you would dry out in hours. It also keeps germs, chemicals, and UV out.

Sweat and oil coat the surface in a slightly acidic film called the acid mantle. This film makes life hard for bacteria and fungi. Dendritic immune cells are woven through the epidermis. They grab any invader that slips past. The whole barrier is rebuilt about once a month. So small damage heals with barely a trace.

Temperature, sensation, and a vitamin the skin makes

The skin is also your main thermostat, the part that controls your body heat. When you get too hot, eccrine glands pour out sweat. As the sweat dries, it carries heat away. Blood vessels in the dermis also widen to give off heat at the surface. That is why you turn pink when you are hot. When you are cold, those same vessels squeeze down to trap warmth deep inside.

The dermis is also crowded with sensory receptors. These make the skin the largest sense organ you have. They send your nervous system a steady stream of touch, pressure, temperature, and pain signals.

The skin also makes vitamin D, a job no other organ can start. UV light strikes a molecule like cholesterol in the epidermis. This begins its change into vitamin D₃. Without vitamin D, you cannot absorb the calcium your bones and nerves depend on.

Section 6Burns & skin cancer

Burns: it's all about depth

A burn destroys skin. It can be caused by heat, electricity, chemicals, or radiation. How bad a burn is depends first on how deep it goes. The skin's barrier, water control, and heat control all live in these layers. So the deeper the burn, the more dangerous it is. This is not because a deep burn hurts more. In fact, a deep burn may stop hurting at all.

A serious burn has two big dangers: fluid loss and infection. Once the barrier is gone, water pours out and germs pour in. The body can no longer hold its temperature steady either.

To judge how much skin is burned, doctors and nurses use the rule of nines. It splits the adult body surface into regions worth about 9% each, or a multiple of nine. Each arm is 9%. Each leg is 18%. The front of the trunk is 18%, and so on. A burn is usually called critical in three cases. It covers more than 25% of the body. Or it involves the hands, face, or feet. Or it is third-degree over more than 10% of the body.

Skin cancer: three kinds, one that kills

Skin cancer is the most common cancer of all. Its leading cause is years of UV exposure. There are three main types. Telling them apart matters a lot. Some are just a nuisance, but one can kill. Notice that each one starts in a cell you already met in this chapter.

The dangerous one is melanoma, a cancer of the melanocytes that make pigment. It often starts in or near a mole. It metastasizes, or spreads to other parts of the body, early. So spotting it early is everything.

The ABCDE rule helps you check a mole that looks suspicious. Asymmetry: one half does not match the other. Border: the edges are ragged or blurred. Color: the color is uneven, with several shades. Diameter: it is wider than about 6 mm, the size of a pencil eraser. Evolving: it changes in size, shape, or color over time.

Any mole that shows these signs should be checked by a doctor.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back whenever a word trips you up.

Apocrine sweat gland

A sweat gland in the armpit and groin that empties into a hair follicle. It makes a thick sweat with protein in it, turns on at puberty, and causes body odor once bacteria break the sweat down.

Arrector pili

A tiny smooth muscle attached to a hair follicle. When it tightens, it pulls the hair upright and makes "goosebumps."

Basal cell carcinoma

The most common and least dangerous skin cancer. It starts in cells of the stratum basale and rarely spreads.

Carotene

A yellow-orange pigment from food. It collects in the stratum corneum and hypodermis and adds to skin color.

Dermis

The deep, living layer of skin, made of dense irregular connective tissue. It holds blood vessels, nerves, hair follicles, and glands.

Eccrine (merocrine) sweat gland

The most common sweat gland, found all over the body. It opens right onto the skin, and its watery sweat cools you as it dries.

Epidermis

The top layer of skin. It is a keratinized stratified squamous epithelium (many layers of flat, tough cells) with no blood vessels of its own.

Hair follicle

The tube of tissue around the hair root. The hair grows out of it.

Hypodermis (subcutaneous layer)

The layer of fat and loose tissue under the skin. It is not part of the skin itself. It holds the skin in place, keeps in heat, and stores energy.

Integument

A covering. The integumentary system is the skin plus hair, nails, and glands.

Keratin

A tough, water-resistant protein. It fills skin cells, hair, and nails, and makes them strong and waterproof.

Keratinocyte

The main cell of the epidermis. It is born in the stratum basale, fills with keratin as it is pushed up, dies, and is shed.

Melanin

The brown-to-black (or red-to-yellow) pigment that gives skin most of its color. It soaks up ultraviolet light to protect DNA.

Melanocyte

The pigment-making cell in the stratum basale. It makes melanin and passes it to nearby keratinocytes.

Melanoma

The least common but most dangerous skin cancer. It starts in melanocytes, spreads early, and is checked for with the ABCDE rule.

Rule of nines

A way to estimate what percent of the body's surface is burned. It splits the adult body into regions of about 9% or multiples of nine.

Sebaceous gland

An oil gland that usually empties into a hair follicle. Its sebum keeps skin and hair soft and holds back bacteria.

Sebum

The oil made by sebaceous glands. When a follicle clogs with sebum and dead cells, acne results.

Squamous cell carcinoma

A skin cancer that starts in keratinocytes of the stratum spinosum. It can spread if not treated but is usually curable if caught early.

Stratum basale

The deepest layer of the epidermis. It is one row of dividing stem cells and also holds melanocytes and Merkel cells.

Stratum corneum

The outermost layer of the epidermis. It is 15–30 rows of dead, keratin-filled cells that form the tough, waterproof barrier.

Vitamin D

A vitamin the skin starts to make when ultraviolet light hits a relative of cholesterol in the epidermis. You need it to absorb calcium from food.

Bone Tissue & the Skeletal System

It is easy to picture the skeleton as a dry rack of poles, the part of us that lasts after everything else is gone. That picture is wrong in the most important way. Bone is living tissue. It is laced with blood vessels and nerves. It is rebuilt every day of your life. And it quietly does jobs that have nothing to do with holding you up.

In this chapter you will meet the skeleton as an organ system, a group of organs that work together. You will see how it is built and what its cells do. You will learn how it keeps the calcium in your blood in a very narrow range. You will also see how it heals itself when it breaks. Once you see bone as alive, the rest of the skeleton makes sense.

Section 1What the skeleton is for

A frame that is also a factory

Every bone is an organ. It is made of several tissue types working together. These include bone tissue, cartilage, dense connective tissue, blood, and nerve tissue. And a bone is very much alive. Snap a fresh bone and it bleeds. Leave a broken one alone and it heals.

All your bones, cartilages, and joints together make up the skeletal system. This system does far more than give you a shape. It works as a support beam, a suit of armor, and a set of levers. It is also a mineral bank and a factory for blood cells. Often it does all of these at the same time.

The jobs bones do

It is worth learning the skeleton's main jobs as a short list. Almost everything else in this chapter is one of these jobs seen up close. Bone stores fat and minerals, makes blood, moves you, protects you, and holds you up.

Axial and appendicular: two halves of one set

An adult skeleton has 206 bones. They split into two named groups. The axial skeleton has about 80 bones. It runs down the center line, or axis, of the body. It includes the skull, the backbone (vertebral column), and the rib cage (thoracic cage). Its main jobs are protection and support.

The appendicular skeleton has about 126 bones. It is everything that hangs off the axis. That means the arms and legs. It also includes the two girdles, the shoulder (pectoral) and hip (pelvic) girdles, which attach the limbs to the trunk. Its main job is movement. In the reference figure, the axial bones are one color and the appendicular bones are another. This is a handy way to keep the two straight.

Tap each of the skeleton's jobs in the interactive to see what carries it out.

Section 2Classifying bones by shape

Shape follows the job

Bones come in a few shapes you can learn to spot. As with the rest of the body, the shape is a clue to the job. A bone built to act as a lever looks nothing like a bone built to shield a soft organ. Sorting bones into five shape classes gives you quick words to describe any bone you meet.

The five shapes

Notice that the phalanges of your fingers count as long bones, even though they are tiny. The label is about proportion, not size. A long bone is longer than it is wide and has a shaft. It also works as a lever. The reference figure also shows a sutural (Wormian) bone. This is a small extra bone that can show up in the seams of the skull. Think of it as a special kind of flat bone, not a sixth class.

Tap each shape in the interactive to see the bone and where it lives in the body.

Section 3Inside a long bone

The shaft and the ends

A long bone, like the femur, has a neat layout once you learn its parts. The long middle tube is the diaphysis, or shaft. Its wall is a tube of dense compact bone. Its hollow center is the medullary cavity. In adults it is filled with fatty yellow marrow.

Each rounded end is an epiphysis. It is filled with light spongy (trabecular) bone. The spaces in spongy bone hold red marrow, where blood cells are made. Where the end meets another bone at a joint, it is capped with slick articular cartilage. This cartilage cushions the joint and lets it glide.

Between the shaft and each end is the metaphysis. In a growing child, it holds the epiphyseal (growth) plate. This is a disc of cartilage where the bone gets longer. When growth ends, the plate turns to bone. It leaves a thin epiphyseal line.

The living wrappers

Two thin layers keep bone alive and connected. The periosteum is a tough sheath on the outside of the bone. It covers every surface except the joint caps. It carries blood vessels and nerves. It anchors tendons and ligaments. It also holds bone-making cells for growth and repair.

The inner surfaces are lined by the thin endosteum. It lines the medullary cavity and the spaces in spongy bone. It also holds bone cells. A nutrient artery enters the shaft through a small hole. It feeds the bone from the inside.

Zoom in: the osteon

Under a microscope, compact bone is not a solid block. It is a bundle of tiny pillars that bear weight, called osteons (Haversian systems). Each osteon is a set of rings of bone matrix, like the rings in a tree trunk. These rings are the lamellae. They wrap around a central canal that carries a blood vessel and a nerve.

Small pockets between the rings are called lacunae. Mature bone cells, the osteocytes, are trapped inside them. The cells stay linked to each other and to the central vessel by hair-thin channels called canaliculi. This system of canals and channels works like plumbing. It keeps the buried cells in hard bone fed and alive.

Tap each labeled part in the interactive to find it on the bone and learn its role.

Section 4Bone cells & remodeling

What bone is made of

Bone tissue is a wonder of design. It mixes two opposite qualities. Its organic part is mostly collagen fibers. Collagen gives bone flexibility and tensile strength, the strength to resist being pulled. So bone bends a little under a load instead of snapping.

Its inorganic part is made of mineral salts of calcium and phosphate. These form hard crystals called hydroxyapatite. They give bone its hardness and compressive strength, the strength to resist being squeezed. Take the mineral away, and bone turns rubbery. Take the collagen away, and it turns as brittle as chalk. Healthy bone needs both, in balance.

Four cells, one crew

Four cell types build and care for this matrix. The easiest way to remember them is by their jobs. Osteogenic (osteoprogenitor) cells are like stem cells. They divide to make the builders. Osteoblasts lay down new bone matrix. They build.

An osteoblast can wall itself into the matrix it made. Then it matures into an osteocyte. This is the caretaker cell. It lives in a lacuna and keeps the tissue healthy. The wrecking crew is the osteoclasts. These are large cells with many nuclei. They dissolve old bone and release its minerals back into the blood.

The tug-of-war called remodeling

Bone is never finished. All through life, osteoclasts dissolve old bone. Osteoblasts lay down fresh bone right behind them. This nonstop cycle is called remodeling. It replaces about 10% of the skeleton every year. When the two crews stay balanced, the amount of bone stays steady.

Two things tune the balance. The first is mechanical stress, the load placed on a bone. Bone gets thicker where it is loaded and thinner where it is not. This rule is called Wolff's law. It is why exercise that puts weight on your bones makes them stronger. It is also why bones weaken during bed rest or in space. The second is hormones, especially the two calcium hormones you will meet next.

Sometimes breakdown beats building for years. This often happens after menopause. The result is osteoporosis, bone full of holes that breaks easily.

Watch the crews trade places in the interactive, and meet the four cells one at a time.

Section 5Calcium homeostasis

Why calcium is worth guarding

Blood calcium (Ca²⁺) must stay in a tight range, about 9–10 mg/dL. So much depends on it. Calcium makes muscles contract. It lets nerves fire. It is needed for blood clotting. It also carries messages inside cells.

If calcium drops too low, nerves get jumpy. This causes muscle spasms and cramps, called tetany. If it climbs too high, nerves and muscles get sluggish. This is a classic example of homeostasis, the idea from Chapter 1. The body holds a value near a set point using feedback, much like a thermostat. The skeleton is the store the body draws on to protect that value.

PTH raises calcium

When blood calcium falls, the parathyroid glands respond. These are four small glands on the back of the thyroid. They release parathyroid hormone (PTH). PTH raises calcium in three ways at once.

In bone, it spurs osteoclasts to dissolve matrix. This releases calcium into the blood. In the kidneys, it tells the tubes to take back calcium that would be lost in urine. It also has the kidneys turn vitamin D into its active form, calcitriol. Calcitriol then makes the intestines absorb more calcium from food. Bone, kidneys, and gut all pour calcium into the blood. The level climbs back toward normal.

Calcitonin lowers calcium

When blood calcium runs high, the thyroid gland responds. Its parafollicular (C) cells release calcitonin. Calcitonin does the opposite of PTH. It holds back osteoclasts and favors osteoblasts. So calcium is pulled out of the blood and stored back in bone. The level settles down.

The two hormones work as a push-pull pair. PTH is the gas pedal, and calcitonin is the brake. Together they keep blood calcium very steady.

Knock blood calcium off its set point in the interactive and watch the right hormone respond.

Section 6Fractures & repair

How bones break

A fracture is simply a broken bone. Doctors name the pattern of the break. The pattern hints at what caused it and how to treat it. The first and most important question is whether the skin stayed whole.

A closed (simple) fracture leaves the skin unbroken. An open (compound) fracture breaks through the skin. An open fracture is far more serious. It opens a door for infection. Breaks are also named for the line they follow and how bad they are.

How a bone heals: four stages

A broken bone repairs itself in a set order. The full process can take months. First, torn blood vessels bleed and clot. This forms a fracture hematoma, a blood clot that seals the gap and sets the stage.

Next, cells build a fibrocartilaginous (soft) callus out of collagen and cartilage. It splints the broken ends together. Then osteoblasts replace this soft bridge with a bony (hard) callus of spongy bone. This happens about six weeks after the injury. Last, over months, the bone remodels. Osteoclasts and osteoblasts reshape the callus back toward the bone's first shape. The same stresses that shape every bone guide this step.

Doctors help this along in two steps. If the broken ends have moved apart, the fracture is first reduced. This means the ends are lined back up. It can be done by moving them from the outside (closed reduction) or by surgery (open reduction). Then the bone is immobilized, or held still, in a cast or with metal hardware. This keeps the fragile callus safe while it matures.

Explore each fracture pattern in the interactive, then step through the four stages of repair.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back whenever a word trips you up.

Appendicular skeleton

The bones of the arms and legs, plus the shoulder and hip girdles that attach them to the axis (about 126 bones). Built for movement.

Axial skeleton

The bones along the body's center line: skull, backbone, and rib cage (about 80 bones). Built for support and protection.

Calcitonin

A hormone from the thyroid's parafollicular (C) cells. It lowers blood calcium by holding back osteoclasts.

Calcitriol

The active form of vitamin D. It helps the gut absorb calcium, and PTH helps switch it on.

Callus

The repair bridge that forms across a break. First it is a soft (fibrocartilaginous) callus, then a hard (bony) callus.

Compact bone

Dense, strong bone tissue built of osteons. It forms the outer wall of bones and the shaft of long bones.

Diaphysis

The tube-shaped shaft of a long bone. It is compact bone around the medullary cavity.

Endosteum

The thin layer that lines the inside surfaces of bone: the medullary cavity and the spaces in spongy bone.

Epiphyseal plate

The cartilage growth plate in the metaphysis, where a long bone gets longer. It becomes the epiphyseal line when growth ends.

Epiphysis

The wide end of a long bone. It is spongy bone with red marrow, capped by articular cartilage.

Hematopoiesis

The making of blood cells (red cells, white cells, and platelets) in the red marrow of bone.

Hydroxyapatite

The crystal mineral of calcium and phosphate in bone matrix. It makes bone hard.

Medullary cavity

The hollow center of a long bone's shaft. In adults it stores fatty yellow marrow.

Osteoblast

The bone-building cell that makes new matrix. It becomes an osteocyte once bone surrounds it.

Osteoclast

A large cell with many nuclei. It dissolves bone matrix and releases its minerals into the blood.

Osteocyte

A mature bone cell that lives in a lacuna. It keeps the matrix healthy and senses stress on the bone.

Osteon

The basic building block of compact bone. It is rings of matrix (lamellae) around a central (Haversian) canal.

Parathyroid hormone (PTH)

The hormone from the parathyroid glands that raises blood calcium. It acts on bone, kidneys, and intestine.

Periosteum

The tough outer layer of a bone. It carries vessels and nerves, anchors tendons, and holds bone-making cells.

Remodeling

The lifelong cycle in which osteoclasts break down bone and osteoblasts build it back, in balance.

Sesamoid bone

A small bone set inside a tendon, such as the patella. It protects the tendon and gives it more leverage.

Spongy bone

Light bone tissue full of holes, made of thin struts (trabeculae). It is found in the epiphyses, and its spaces hold red marrow.

The Axial Skeleton

A column of bone runs down the center of your body. It holds you upright, guards your most delicate organs, and gives your muscles something to pull on. This core is the axial skeleton, made of the skull, the spine, and the rib cage, with eighty bones lined up along the long axis of your body. In this chapter, we will take that core apart piece by piece. You will name the bones of the skull and stack the twenty-six bones of the backbone. You will look closely at one vertebra to learn the plan they all share. Then you will build the cage that guards the heart and lungs. Last, you will trace the gentle S-curve that lets your spine carry your weight without bending in. By the end, a chest X-ray or a skull drawing will read like a map you already know.

Section 1Axial vs appendicular

Two skeletons in one

An adult body has 206 bones. Anatomists sort every one of them into two groups. The axial skeleton forms the center line, or axis, of your body. This line runs from the top of the skull, down the spine, and out through the ribs. The axial skeleton has 80 bones. Its job is protection and support. It holds the brain, the spinal cord, the heart, and the lungs, and it gives your trunk a firm core.

The appendicular skeleton is everything that hangs off that center line. These are the appendages, or limbs. It has 126 bones, which make up your arms and legs, plus the two girdles (shoulder and hip) that hold the limbs on. Its job is movement. Here is a quick way to tell them apart. If a bone sits on the midline or wraps around an organ, it is axial. If it helps you reach, walk, or grip, it is appendicular.

What the axial 80 are made of

The eighty axial bones fall into just three groups, and this whole chapter is a tour of them. The skull and the bones that go with it make up twenty-nine. The vertebral column, or backbone, has twenty-six. The thoracic cage, or rib cage, has twenty-five. See how each group matches the organ it guards. The skull wraps the brain, the vertebrae wrap the spinal cord, and the rib cage wraps the heart and lungs. In a real sense, the axial skeleton is your body's suit of armor.

Switch between the two skeletons below. Watch the axial and appendicular bones light up in turn.

Section 2The skull

Eight bones cradle the brain

The skull has 22 bones, and the first split is between the part around the brain and the part that builds the face. The cranium is the braincase, and it is made of eight cranial bones. Four of them are single bones on the midline:

The frontal bone is your forehead. The occipital bone is the back and base of the skull. It has a large hole, the foramen magnum, where the spinal cord leaves the skull. The sphenoid bone is shaped like a bat. It spans the floor of the skull and locks the other bones together, like the keystone of an arch. The ethmoid bone is a thin, fragile bone between the eyes. It forms part of the nose cavity.

The other two come in pairs. The two parietal bones form the roof and upper sides. The two temporal bones sit low on each side and hold the ears. Together these bones form a hard dome whose only job is to keep the brain safe.

Fourteen bones build the face

The 14 facial bones give your face its shape, and they frame the openings for seeing, smelling, and eating. Most come in pairs:

The two maxillae form the upper jaw. They join at the midline. The two zygomatic bones are your cheekbones. The two nasal bones form the bridge of the nose. The two lacrimal bones are tiny bones at the inner corner of each eye. They help drain tears. The two palatine bones form the back of the hard palate, the roof of your mouth. The two inferior nasal conchae are curled shelves inside the nose.

Two facial bones are single. The vomer forms the lower part of the wall between your nostrils (the nasal septum). The mandible is the lower jaw. The mandible is special, because it is the only freely movable bone in the skull. It meets the temporal bone at the jaw joint (the temporomandibular joint), which lets you chew and speak.

Sutures, sinuses, and the infant skull

The cranial bones do not meet at smooth joints. Instead, they lock together along jagged seams that cannot move. These seams are called sutures, and they fit together like the teeth of a zipper. Four are worth naming:

The coronal suture runs between the frontal and parietal bones. The sagittal suture runs front to back between the two parietal bones. The lambdoid suture runs between the parietal bones and the occipital bone. The squamous suture runs along the top of each temporal bone.

In a newborn, these seams have not closed yet. The gaps between them are the fontanelles, or "soft spots." They let the skull bend a little during birth and grow as the brain grows.

Some skull bones are also hollowed out by air-filled spaces. These are the paranasal sinuses, the air pockets around the nose. They make the skull lighter and help your voice ring and carry.

Tap any bone on the side view to find it and name it.

Section 3The vertebral column

Twenty-six bones, five regions

The vertebral column is your backbone, a stack of 26 bones that runs from the base of the skull to the hips. It does three big jobs at once. It supports your head and trunk. It protects the spinal cord that runs down its center. And it bends enough to let you bend and twist.

Each bone in the stack is a vertebra, and together they are called vertebrae. They are grouped into five regions from top to bottom. A classic memory trick links the counts to mealtimes. Eat breakfast at 7, lunch at 12, and dinner at 5:

A child actually starts with 33 separate vertebrae. As you grow, five of them fuse into one sacrum and four more fuse into the coccyx, so the adult total ends up at 26. Notice a pattern, too. The vertebrae get steadily larger as you move down the column. That is because each lower bone carries the weight of everything above it. The lumbar vertebrae hold up the whole upper body, so they are the biggest and bulkiest of all.

Discs, the cushions between

From C2 downward, a pad called an intervertebral disc sits between each pair of vertebrae. It has a tough outer ring of fibers, the anulus fibrosus. That ring wraps around a soft, jelly-like center, the nucleus pulposus. Think of a jelly donut, and you have 23 of them in your spine.

The discs do two jobs. They act as shock absorbers that soften the jolt of every step. They also form joints that move a little, so the spine can bend. Sometimes the soft center pushes out through a weak spot in the ring. This is a herniated ("slipped") disc. It can press on a nearby spinal nerve and send pain shooting down an arm or leg.

Tap each region to pick it out and see its vertebrae.

Section 4A typical vertebra

The body: the weight-bearer

Vertebrae look different from region to region, but they all share one basic plan. The front part of every vertebra is the body, also called the centrum. It is a thick, round block of bone that faces the front (anterior) and does the heavy lifting. The bodies stack on top of each other, with discs between them. Together they form a sturdy pillar that carries the weight of your head and trunk. This is why the bodies get bigger toward the lower back. The lower you go, the more weight there is to hold up.

The arch: the shield

Behind the body is the vertebral arch, a ring of bone that sticks out toward the back. Two short walls rise from the body, and these are the pedicles. Two flat plates form the roof and meet in the middle, and these are the laminae.

The body in front and the arch behind surround a hole. This hole is the vertebral foramen. When all the vertebrae are stacked, these holes line up to form one long tunnel, the vertebral canal. This bony tunnel shelters the spinal cord along its whole length. So the arch works like the spine's version of the skull, and its job is pure protection.

The processes: the handles

Seven bony knobs called processes stick out from the arch, and they work as levers and joints. The single spinous process points straight back. These are the bumps you feel down the middle of your back. The two transverse processes stick out to the sides. Muscles and ligaments grab onto the spinous and transverse processes.

The last four are the paired superior and inferior articular processes. "Superior" means upper, and "inferior" means lower. Each has a smooth, flat surface called a facet. The facets form joints with the vertebra just above and just below, which links the whole column together.

Look straight down on a vertebra. Tap each part to name it.

Section 5The thoracic cage

The sternum: the breastplate

The front of the chest is held in place by the sternum, or breastbone. It is a flat bone shaped like a dagger, with three parts. The wide manubrium sits on top and joins the collarbones. The long body forms the middle. The small xiphoid process is the pointed tip at the bottom, and it starts out as cartilage.

Where the manubrium meets the body, there is a small ridge. This is the sternal angle, also called the angle of Louis. You can feel it through your skin, and it lines up with the second rib. So doctors and nurses use it as a starting point for counting ribs and placing a stethoscope.

Twelve pairs of ribs: true, false, and floating

You have 12 pairs of ribs, which curve from the spine in back around to the front of the chest. Every rib joins a thoracic vertebra in back. But ribs differ in how they reach the sternum in front, or if they reach it at all. That is how we sort them:

The true ribs are pairs 1–7. Each one joins the sternum directly by its own strip of costal cartilage. This is a strip of bendy tissue that links a rib to the breastbone. The false ribs are pairs 8–12. They do not reach the sternum directly. Pairs 8–10 join the costal cartilage of the rib just above them. The last two pairs, 11 and 12, have no front attachment at all. They are called floating ribs, and they end freely in the muscles of the back wall.

A cage that breathes

The thoracic cage is not a solid box. It is a bendable basket that moves with every breath. When you breathe in, muscles lift the ribs up and out. This makes your chest bigger, so the lungs have room to fill with air. When you breathe out, the ribs drop back down. All the while, the cage shields the heart, lungs, and great vessels (the largest blood vessels) inside it. It also gives your breathing and upper-body muscles a frame to pull on, so it is both armor and machine in one.

Tap the sternum, the spine, and each group of ribs to see how the cage fits together.

Section 6Spinal curvatures

Four curves, two born and two learned

Seen from the side, a healthy spine is not straight. It makes a gentle S-shape with four curves. These curves balance your weight over your hips, and they act like a spring that softens every step.

Two of them are primary curves, which you have at birth. They match the curled-up shape of a baby in the womb. They are the thoracic curve (in the chest) and the sacral curve (in the pelvis). Both bend away from the front, so they are hollow in front.

The other two are secondary curves. They form after birth, as a baby learns to hold up its head and then to walk. The cervical curve appears when a baby starts to lift its head. The lumbar curve appears when a toddler stands and walks. Together, the four curves let the spine carry a heavy head and trunk without tipping over.

When curves go wrong

The curves are healthy when they are gentle. But a curve can become too deep or bend the wrong way, and then it gets a medical name.

Kyphosis is a thoracic curve that is too deep. It gives a rounded, hunched upper back, sometimes called "hunchback." It is common in older adults with osteoporosis (weak, thin bones). Lordosis is a lumbar curve that is too deep. The lower back sways inward too much. This is called "swayback." Scoliosis is different from both. It is a sideways (lateral) curve of the spine. Seen from behind, the spine bends into a C or an S shape.

Children are checked for scoliosis at school. Finding it early, while the skeleton is still growing, gives the best chance to fix it.

Compare the normal S-curve with kyphosis, lordosis, and scoliosis side by side.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back when a word trips you up.

Appendicular skeleton

The 126 bones of the limbs and the two girdles (shoulder and hip) that hold them on. They are built for movement.

Atlas (C1)

The first neck vertebra. It holds up the skull and lets your head nod "yes."

Axial skeleton

The 80 bones along the center line of the body: the skull, spine, and rib cage. They protect and support.

Axis (C2)

The second neck vertebra. It has a peg that points up and lets your head turn to say "no."

Coccyx

The small tailbone at the bottom of the spine. It is made of four fused vertebrae.

Costal cartilage

The strip of cartilage that links a rib to the sternum. It joins directly, or through the rib above.

Cranial bones

The eight skull bones that surround the brain. They are the frontal, occipital, sphenoid, and ethmoid, plus the paired parietals and temporals.

Facial bones

The fourteen skull bones that shape the face. They frame the eyes, nose, and mouth.

Floating ribs

Rib pairs 11 and 12. They attach only to the vertebrae in back and have no front connection.

Fontanelle

A soft gap of tough tissue between a baby's skull bones. It lets the skull bend at birth and grow with the brain.

Foramen magnum

The large hole in the occipital bone. Here the spinal cord joins the brain.

Intervertebral disc

A pad of tough cartilage between two vertebral bodies. It absorbs shock and lets the spine move.

Kyphosis

A thoracic curve that bends back too far. It gives a hunched upper back.

Lordosis

A lumbar curve that bends forward too far. It gives an overdone swayback.

Mandible

The lower jaw. It is the only skull bone that moves freely.

Manubrium

The wide top part of the sternum. It forms joints with the collarbones (clavicles) and the first ribs.

Sacrum

A triangle-shaped bone made of five fused vertebrae. It joins the spine to the hip bones.

Scoliosis

An abnormal sideways (lateral) curve of the spine.

Sternal angle

The ridge you can feel where the manubrium meets the body of the sternum. It lines up with the second rib.

Suture

A joint between two cranial bones. The bones lock together and cannot move.

Vertebral foramen

The central hole of a vertebra, formed by the body and arch. The spinal cord runs through it.

Xiphoid process

The small pointed tip at the bottom of the sternum. It is a key landmark for CPR.

The Appendicular Skeleton

Think of the axial skeleton as the central mast of your body. The appendicular skeleton is everything that hangs off that mast and lets you act on the world. It includes the arms that reach and grip and the legs that carry you and spring. It also includes two bony girdles, which are frames that bolt the limbs to the trunk. This part of the skeleton has 126 bones, and almost every one of them is a lever built for movement. In this chapter, we work outward from each girdle to the fingertips and toes. Along the way, you will see one theme again and again. The shoulder is built for freedom, and the hip is built for strength. The whole design is a set of trade-offs between the two. Learn the bones in this order, and the hand and foot will make sense. With 27 and 26 bones each, they stop being a jumble and become a clear map.

Section 1The pectoral girdle

Two bones, one clever compromise

The pectoral girdle is your shoulder girdle. It is the frame that connects each arm to your trunk. It has only two bones on each side. The clavicle (collarbone) is in front, and the scapula (shoulder blade) is in back.

What is surprising is how loosely the whole arm is attached to the rest of the skeleton. The entire arm hangs from the axial skeleton at one small joint. This is the sternoclavicular joint, where the inner end of the clavicle meets the top of the breastbone (the manubrium of the sternum). The scapula is not joined by bone to the ribs or spine at all. It floats on the back of the rib cage, and only muscles hold it in place.

This loose setup is a trade. The shoulder gives up steadiness to gain a huge range of motion, which means how far a joint can move. You can swing your arm around in almost a full circle in every direction. The price is that the shoulder is one of the easiest joints in the body to dislocate, or pop out of place.

The clavicle: a slender strut

The clavicle is an S-shaped bone that works like a brace. It holds the shoulder out and away from the chest, so the arm can swing freely. Its rounded inner end is the sternal (medial) end, and it forms a joint with the sternum. Its flat outer end is the acromial (lateral) end. It meets a part of the scapula called the acromion at the acromioclavicular (AC) joint.

The clavicle is thin, and it sits right under the skin with nothing to pad it. So it is the most frequently fractured bone in the body. It usually breaks in its middle third. This often happens when someone falls onto an outstretched arm or onto the tip of the shoulder.

The scapula: a triangle of landmarks

The scapula is a flat, triangle-shaped bone. It is covered with bumps and ridges where muscles attach. A ridge called the spine runs across its back. On the outer side, the spine ends in the acromion, which is the bony tip of your shoulder that you can feel. A hook called the coracoid process curves forward like a bent finger. It anchors muscles and ligaments.

The most important feature is the glenoid cavity, also called the glenoid fossa. This is a shallow socket on the outer corner of the scapula. The rounded head of the upper arm bone (humerus) fits into it to form the shoulder joint. That shallow socket is the key to the whole design. It lets the arm bone swing widely, but it also lets the head slip out too easily. Picture a ball resting on a saucer instead of sitting in a cup.

Explore both bones and their landmarks in the interactive below.

Section 2Bones of the upper limb

Three long bones between shoulder and wrist

Each upper limb has 30 bones, but three big long bones do most of the work of holding it up. The upper arm is called the brachium, and it has one bone, the humerus. The forearm is called the antebrachium, and it has two bones side by side, the radius and the ulna.

To tell them apart, use the anatomical position. That means standing with your palms facing forward. In this position, the radius is on the thumb (lateral) side. The ulna is on the little-finger (medial) side. Here is a phrase that sticks: the radius rotates around the ulna, and the radius is on the same side as the thumb.

The humerus and the elbow

The humerus is the long bone of the upper arm. Its rounded head fits into the glenoid cavity at the shoulder. Just below the head is the surgical neck. This is a common place for breaks, and it sits very close to an important nerve (the axillary nerve). Farther down the shaft is the deltoid tuberosity, a rough bump where the deltoid muscle attaches.

At the elbow end, two rounded surfaces form the joint. The capitulum meets the radius. The trochlea is shaped like a pulley, and it grips the ulna. On either side of them are two bumps, the medial and lateral epicondyles. The ulnar nerve runs behind the inner (medial) one, in a shallow groove. There it lies just under the skin, with no muscle to cover it. So when you bump that spot, you press right on the nerve. A jolt shoots down your forearm. This is your "funny bone."

Radius and ulna: a rotating pair

The two forearm bones are joined along their length by a sheet of tough tissue, the interosseous membrane. They also meet at small joints near the top and bottom. This setup lets the forearm do a neat trick. It can turn the palm up, which is supination, or turn it down, which is pronation. In pronation, the radius turns and crosses over the ulna.

At the elbow, the ulna is the main partner. Its olecranon forms the bony point of your elbow. Its trochlear notch wraps around the trochlea of the humerus, like a wrench on a bolt. At the wrist, the radius takes over. It is the forearm bone that forms most of the joint with the wrist bones (carpals). Both bones end in a pointed tip called a styloid process.

Tap each of the three bones. Then turn the forearm from supination to pronation.

Section 3The hand & wrist

Twenty-seven bones in three groups

Your hand is the most skillful part of your body. That skill comes from packing 27 bones, and all the joints between them, into a small space. They sort neatly into three groups, from the wrist outward. There are 8 carpals in the wrist, 5 metacarpals in the palm, and 14 phalanges in the fingers. Learn those three numbers (8, 5, 14) in order, and you know the whole hand.

The carpals: eight little stones

The carpals are eight small wrist bones set in two rows of four. Ligaments bind them tightly, so the wrist bends as one unit. Start on the thumb side of the row nearest the forearm, then move to the row nearest the fingers. In that order, they are the scaphoid, lunate, triquetrum, pisiform, then the trapezium, trapezoid, capitate, hamate. A classic memory trick keeps the order: "Sally Left The Party To Take Cathy Home."

The scaphoid needs special attention. It is the carpal bone that breaks most often, usually in a fall onto an outstretched hand. Its blood supply enters from only one end. So a break can cut off blood to part of the bone, and that part can die.

Metacarpals and phalanges

The metacarpals are the five bones of the palm. They are numbered 1 to 5, starting at the thumb. Their rounded ends near the fingers are your knuckles. The thumb's metacarpal meets the trapezium at a saddle joint, shaped like a rider sitting in a saddle. This special joint lets the thumb swing across the palm to touch the other fingers. This is the opposable grip that makes the human hand so useful.

The phalanges are the finger bones. Each finger has three: proximal (nearest the palm), middle, and distal (at the tip). But the thumb, or pollex, has only two. That makes 14 per hand. (One finger bone is called a phalanx.)

Tap each of the three groups below: the wrist, the palm, and the fingers.

Section 4The pelvic girdle

Built for strength, not freedom

The shoulder is built to move freely. The pelvic girdle, or hip girdle, is built for the opposite. Its jobs are strength and weight-bearing. It must pass the whole weight of your upper body down into your legs. It must also take the pounding of every step.

So the pelvic girdle does not float on muscle. Instead, it attaches firmly to the axial skeleton at the sacroiliac joints. These are strong joints that barely move. The girdle itself is the pair of hip bones, also called coxal bones or os coxae. Together with the sacrum and coccyx, they form the bony pelvis, a bowl of bone at the base of your trunk.

Three bones fused into one

Each hip bone starts out as three separate bones. They fuse into one bone during adolescence, as a child grows into an adult. They meet at a deep, cup-shaped socket called the acetabulum. The ilium is the large, fan-shaped upper part. Its rim, the iliac crest, is the "hip" where you rest your hands. The ischium is the lower back part. Its rough ischial tuberosity is the bone you sit on. The pubis is the front part. The two pubic bones meet in front at the pubic symphysis, a joint made of tough cartilage (fibrocartilage).

Notice how different this is from the shoulder. The acetabulum is a deep socket that grips the head of the thigh bone (femur) firmly. So the hip almost never dislocates. It is the mirror image of the shallow glenoid cavity, which moves freely.

Male and female pelvis: shaped by childbirth

The pelvis is the part of the skeleton that differs most clearly between males and females. That is because the female pelvis is shaped to let a baby pass through. Compared with the male pelvis, the female pelvis is wider and shallower. Its opening at the top, the pelvic inlet, is larger and more rounded. The angle under the pubic bones, called the subpubic angle, is also wider. In a female it is greater than 90°.

The male pelvis is narrower and heavier. Its inlet is heart-shaped, and its subpubic angle is less than 90°. These differences are reliable. So anatomists and forensic scientists use the pelvis to tell whether a skeleton is male or female.

Tap the ilium, ischium, and pubis. Then switch between the male and female pelvis.

Section 5Bones of the lower limb

The strongest bones in the body

The lower limb carries your whole body weight and powers every step. So its bones are the largest and strongest you have. Like the arm, each lower limb has 30 bones. Four of them are the big bones that hold it up. The thigh has one bone, the femur. The patella, or kneecap, guards the knee. The lower leg has two bones, the tibia and fibula. They match the radius and ulna of the forearm, but with one key difference, which we will get to soon.

Femur and patella

The femur is the longest, heaviest, and strongest bone in the body. Its ball-shaped head sits deep in the acetabulum. Below the head is the angled neck. This is where a "hip fracture" usually happens in older adults with thin bones. Two bumps, the greater and lesser trochanters, anchor strong hip muscles. A ridge down the back, the linea aspera, gives muscles more places to attach. At the knee, the femur's rounded ends, the condyles, roll on top of the tibia.

The patella is the largest sesamoid bone in the body. A sesamoid bone is a bone that forms inside a tendon. The patella sits in the tendon of the quadriceps, the big muscle on the front of the thigh. It works like a pulley. It gives the thigh muscles better leverage when you straighten your knee. It also shields the knee from a direct hit.

Tibia and fibula: one carries, one steadies

Here is how the leg differs from the forearm. In the leg, the two bones do not share the load. The tibia, or shinbone, is the thick inner (medial) bone. It carries almost all the weight. Its wide, flat top, the tibial plateau, supports the femur. Its lower end forms the medial malleolus, the bump on the inside of your ankle.

The fibula is the thin outer (lateral) bone. It carries almost no weight. Its jobs are to stabilize the ankle, or keep it steady, and to give muscles a place to attach. Its lower end forms the lateral malleolus, the bump on the outside of your ankle. In fact, the fibula does not even reach the knee. Only the tibia meets the femur there.

Tap each of the four bones. Then find the two malleoli at the ankle.

Section 6The foot & ankle

Twenty-six bones, built to bear weight

The foot is a lot like the hand, with three groups of bones. But it is tuned for a very different job. Instead of grabbing, it must bear weight and spring you forward. It has 26 bones. There are 7 tarsals in the ankle and heel, 5 metatarsals in the sole, and 14 phalanges in the toes. It is the same three-group plan as the hand, with slightly different numbers to remember (7, 5, 14).

The tarsals and the weight it takes

The tarsals are seven bones that form the ankle and the back of the foot. Two of them do the hard work. The talus sits on top, and the tibia and fibula grip it. It takes the body's weight from the leg and passes it down. The calcaneus, or heel bone, is the largest tarsal. It forms the heel and takes the weight when your foot hits the ground. The other five tarsals link the ankle to the sole. They are the navicular, the cuboid, and three cuneiforms.

The metatarsals are numbered 1–5, starting at the big toe. They form the sole and the ball of the foot. The phalanges are the toe bones. Each toe has three, but the big toe, or hallux, has only two. This is exactly the same pattern as the hand.

The arches: the foot's suspension

What really sets the foot apart from the hand is its arches. Ligaments and tendons hold the tarsals and metatarsals in three curves. The medial longitudinal arch is the high one along the inside of the foot. The lateral longitudinal arch runs along the outside. The transverse arch runs across the foot.

These arches make the foot a natural spring and shock absorber, like the springs in a car. They flatten a little to soak up the impact of each step. Then they spring back to push you off. When the arches drop, the result is flat feet (pes planus). This can make the feet and legs tired, because the shock absorber is gone.

Tap each of the three groups (ankle, sole, and toes) below, and see the arches.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back when a word trips you up.

Acetabulum

The deep, cup-shaped socket on the hip bone. The head of the femur fits into it to form the hip joint.

Appendicular skeleton

The 126 bones of the limbs and the two girdles (pectoral and pelvic). The girdles attach the limbs to the axial skeleton.

Calcaneus

The heel bone. It is the largest tarsal, and it bears weight when your foot hits the ground.

Carpals

The eight small bones of the wrist, set in two rows of four.

Clavicle

The S-shaped collarbone that holds the shoulder out. It is the bone broken most often in the body.

Femur

The thigh bone. It is the longest, heaviest, and strongest bone in the body.

Fibula

The thin outer (lateral) bone of the lower leg. It carries almost no weight. It steadies the ankle and forms the lateral malleolus.

Glenoid cavity

The shallow socket on the scapula. The head of the humerus fits into it at the shoulder joint.

Hip bone (os coxae)

Also called the coxal bone. Each one is the ilium, ischium, and pubis fused together, and the pair forms the pelvic girdle.

Humerus

The single long bone of the upper arm. It runs from the shoulder to the elbow.

Metacarpals

The five bones of the palm, numbered 1 to 5 from the thumb. Their heads form the knuckles.

Metatarsals

The five bones of the sole of the foot, numbered 1 to 5 from the big toe.

Patella

The kneecap. It is the largest sesamoid bone, and it forms inside the quadriceps tendon to improve leverage.

Pectoral girdle

The shoulder girdle, made of the clavicle and scapula. It attaches each arm to the axial skeleton.

Pelvic girdle

The two hip bones. They attach the legs to the axial skeleton and bear the body's weight.

Phalanges

The bones of the fingers and toes. Each digit has three, except the thumb and big toe, which have two. One bone is a phalanx.

Radius

The outer (thumb-side) bone of the forearm. It turns during pronation and forms most of the wrist joint.

Scapula

The triangle-shaped shoulder blade. Only muscles hold it to the trunk, and it carries the glenoid cavity.

Talus

The tarsal bone that the tibia and fibula grip. It takes the body's weight from the leg.

Tarsals

The seven bones of the ankle and the back of the foot. They include the talus and calcaneus.

Tibia

The inner (medial) shinbone. It is the weight-bearing bone of the lower leg. It forms the knee joint with the femur and forms the medial malleolus.

Ulna

The inner (pinky-side) bone of the forearm. Its olecranon forms the point of the elbow.

Joints

Bones give your body its frame. But a frame that could not bend would be a statue. Any place where two bones meet is a joint, and joints are where movement starts. In this chapter you will learn the two ways to sort every joint in the body. You will meet the six shapes of the free-moving synovial joints, the ones with a pocket of fluid inside. You will open up the fluid-filled case that lets them glide. And you will learn a name for each motion your body can make. Along the way, one big idea ties it all together. There is always a trade-off between how much a joint moves and how stable it stays.

Section 1Two ways to classify a joint

What is a joint, and why do we sort them twice?

A joint (also called an articulation) is any place where two bones come together. It can also be where bone meets cartilage, or where a tooth meets bone. That is a huge, mixed group. The seams of the skull that never move are joints. So is the shoulder, which moves a lot. To make sense of it all, experts describe every joint in two separate ways at once. Get in the habit of giving both.

The first way is structural. It asks what holds the bones together, and is there a fluid-filled space between them? By that test there are just three kinds. Fibrous joints are held by tough, dense tissue, with no space inside. Cartilaginous joints are held by cartilage, a firm, rubbery tissue, again with no space. Synovial joints have a space filled with fluid between the bones.

The second way is functional. It asks a simpler question: how much does it move? A synarthrosis does not move. An amphiarthrosis moves a little. A diarthrosis moves freely.

Movement versus stability: the great trade-off

The two ways are not random. They match up, because what holds a joint together decides how much it can move. Tie two bones with tough fibers and you get a rock-solid joint that barely moves. That is fibrous, and most often a synarthrosis. Pad them with cartilage and you get a little give. That is cartilaginous, and most often an amphiarthrosis.

Now put a slippery, fluid-filled gap between them. They can swing freely. But now only ligaments and muscles hold them in place. That is synovial, and always a diarthrosis.

This is the most important idea in the whole chapter: mobility and stability pull against each other. Mobility means how much a joint can move. Stability means how well it stays in place. Every joint in your body sits somewhere on that scale. Its design is a deal made for the job it does.

Your skull has to guard your brain. So it gives up all movement to be as stable as it can be. Your shoulder has to throw and reach. So it gives up stability to get a huge range of motion. That is exactly why the shoulder is the joint people dislocate most.

Tap through the sorting tree in the interactive. Watch how much each joint really moves.

Section 2The six synovial joints

Same blueprint, six shapes

All synovial joints share the same fluid-filled design. You will see it in the next section. But the shape of the two bone ends that meet decides how the joint can move. There are six shapes, and each allows its own set of motions. A neat way to keep them straight is by their axes of movement. An axis here means a direction in which the bones can swing.

A nonaxial joint slides but does not swing. A uniaxial joint moves in one plane, or one flat path, like a door on a hinge. A biaxial joint moves in two planes. A multiaxial joint moves in all directions and can spin, too. As you go down that list, the joint can move more. And, true to this chapter's theme, it gets less stable.

Reading the shape tells you the move

Once you can picture the shape, you can work out the motion. A hinge is a rounded end sitting in a curved trough. So it can only rock one way. Your elbow cannot swing side to side, just as a door cannot.

A ball-and-socket joint is a round head set in a cup. It can swing in any direction and spin. That is why your shoulder and hip are the two joints that move the most. The saddle joint at the base of your thumb has bones shaped like a rider on a saddle. This joint gives humans a thumb that can truly cross over to touch the fingers. It is the reason you can grip things.

Tap each of the six shapes in the interactive. See how each one moves and where you have one.

Section 3Inside a synovial joint

The essential parts

A synovial joint moves freely because of a small space filled with fluid. This is the joint (synovial) cavity. It sits between the two bone ends, so they never really touch. Every other part of the joint is there to guard, feed, and oil that gap. Learn these parts and you can label any synovial joint in the body.

Each bone end has a smooth cap of articular cartilage. This is glassy hyaline cartilage. It gives an almost friction-free surface and cushions the blow of every step. The whole joint is wrapped in an articular capsule with two layers.

The outer layer is the tough fibrous layer. It joins with the covering of the bones and holds everything together. The thin inner layer is the synovial membrane. It lines the space and makes the fluid inside. That fluid is synovial fluid. It fills the space like oil in a door hinge.

Synovial fluid: oil, food, and cushion

Synovial fluid does three jobs at once. First, it lubricates, or oils, the joint. Its hyaluronic acid makes it slick. So the cartilage surfaces glide with less friction than ice sliding on ice.

Second, it feeds the cartilage. Articular cartilage has no blood supply of its own. So it soaks up oxygen and food from the fluid, and it sends its waste back out into it.

Third, it absorbs shock. When you press on a joint, fluid squeezes out of the pressed cartilage. When you let go, the fluid seeps back in. This self-oiling action is called weeping lubrication. It is one reason gentle movement is good for sore joints.

Bursae: the joint's shock pads

Many joints have small, flat sacs of synovial fluid around them. These are called bursae. They sit wherever a tendon, ligament, or patch of skin would rub against bone. They work like greased cushions. If you overuse one, it can get swollen and painful. Kneeling all day or throwing overhead again and again can do this. The problem is called bursitis.

Tap each labeled part in the interactive. See how the capsule, space, cartilage, and fluid fit together.

Section 4The movements of the body

Motions come in opposite pairs

Each movement a synovial joint makes has an exact name. The best part is that the names come in opposing pairs. Learn one and you almost know its partner. All of them are described from the anatomical position. This is the same pose you met in Chapter 1: standing tall, palms facing forward.

The most common pair is flexion and extension. Flexion makes the angle at a joint smaller. Bending your elbow or curling forward at the waist are examples. Extension makes the angle bigger and straightens the joint back out. Push a joint past straight and you get hyperextension. Tilting your head back to look at the ceiling is one example.

The next pair moves limbs toward or away from the midline, the center line of the body. Abduction moves a limb away from the midline. Adduction brings it back toward the midline. Now hold your arm out and trace a full cone in the air. That is circumduction. It links flexion, abduction, extension, and adduction into one circle. Last, rotation turns a bone around its own long axis. Shaking your head “no” rotates your skull on your spine.

The special movements

A few joints make motions that need their own names. At the forearm, supination turns the palm up, as if you were carrying soup. Pronation turns it down. At the ankle, lifting your toes toward your shin is dorsiflexion. Pointing them down is plantar flexion. Turning the sole inward is inversion, and turning it outward is eversion.

The jaw and shoulders can protract (move forward) and retract (move back). They can also elevate (lift) and depress (lower). And the thumb can do opposition. It crosses the palm to meet the fingertips. This is the movement that makes the human hand such a good tool.

Tap each movement in the interactive. Watch a body do it, and see its opposite.

Section 5Fibrous & cartilaginous joints

Joints without a cavity

Not every joint is built for motion. Two of the structural classes have no synovial space. These are fibrous and cartilaginous joints. They give up movement to get the stability the body needs in certain spots. They hold bones together with tissue, not with a gap of fluid. Most of them do not move at all, or move only a little.

Fibrous joints: bound by tough fibers

In a fibrous joint, tough, dense tissue stitches the bones right together. There are three kinds.

A suture is the jagged, zipper-like seam between the flat bones of the skull. A thin layer of fibrous tissue holds it. It is basically immovable (a synarthrosis). That is just what a case that guards the brain should be.

A syndesmosis joins two bones that lie side by side. They are linked by a wide sheet or cord of fibrous tissue. One example is the interosseous membrane between the radius and ulna in your forearm. This joint allows a little give (an amphiarthrosis).

A gomphosis is a peg-in-a-hole joint. It holds each tooth in its bony socket. The periodontal ligament holds it in place. It is also immovable.

Cartilaginous joints: bound by cartilage

In a cartilaginous joint, cartilage holds the bones together. A synchondrosis uses hyaline cartilage, a smooth, glassy kind of cartilage. The classic example is the epiphyseal (growth) plate in a long bone that is still growing. Another is the joint between the first rib and the breastbone (sternum). Both are immovable.

A symphysis uses fibrocartilage, a tough, springy kind of cartilage. It allows a little movement (an amphiarthrosis). There are two you must know. One is the pubic symphysis at the front of the pelvis. The other is the intervertebral discs between the bones of the spine. Each disc lets the spine bend a little. Added together, all that bending gives you a flexible back.

Tap each fibrous and cartilaginous type in the interactive. See how each one gives up movement to gain stability.

Section 6Four joints that do the work

Shoulder and hip: two ball-and-sockets, two philosophies

The shoulder and the hip are both ball-and-socket joints. Yet they solve the mobility–stability problem in opposite ways.

The shoulder (glenohumeral) joint is built for reach. The ball of the humerus rests against a shallow socket, like a ball on a saucer. This gives it the widest range of any joint in the body. The price is weakness. The socket barely holds the ball. So the muscles of the rotator cuff do most of the work of holding it in place. It is the joint people dislocate, or pop out of place, most often.

The hip joint makes the opposite deal. The head of the femur sits deep in a true cup, called the acetabulum. Thick ligaments ring it. It gives up some range to become one of the most stable joints in the body. It has to be, because it carries your whole body weight with every step.

The knee: a hinge that does more than hinge

The knee is the largest and most complex joint in the body. It works mainly as a hinge, bending and straightening the leg. But it also allows a little rotation when it is bent. That is where trouble starts.

Two half-moon pads of fibrocartilage sit on top of the tibia. These are the menisci. They spread out the load and cushion the joint. Deep inside, two ligaments cross each other in an X. These are the anterior and posterior cruciate ligaments (ACL and PCL). They keep the femur and tibia from sliding past each other. A hard twist on a planted foot can tear the ACL and a meniscus together. That is why knee injuries end so many sports seasons.

The elbow: stability by design

The elbow shows what a joint looks like when the body picks stability. It is a hinge. The humerus and ulna lock together so snugly that the joint moves cleanly in one plane. It rarely dislocates. Strong collateral ligaments on each side make it even stronger.

A separate pivot joint at the radius lets the forearm rotate. This gives supination and pronation without loosening the hinge. The elbow is a quiet lesson in this chapter's theme. A joint that gives up range gains a toughness the shoulder can only wish for.

Tap each of the four joints in the interactive. Compare how each one balances mobility against stability.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back when a word trips you up.

Abduction

Movement of a limb away from the midline of the body. Raising your arm out to the side is an example.

Adduction

Movement of a limb toward the midline, back to the body.

Amphiarthrosis

A joint that moves a little. Examples are the pubic symphysis and an intervertebral disc.

Articular capsule

The two-layer sleeve that wraps a synovial joint. It has a tough fibrous outer layer and a synovial membrane inside that makes fluid.

Articular cartilage

The smooth hyaline cartilage that caps each bone end in a synovial joint. It cuts friction and absorbs shock.

Bursa

A small sac of fluid that cushions a spot where a tendon, ligament, or skin rubs over bone. When it swells, that is bursitis.

Circumduction

Moving a limb in a cone shape. It joins flexion, abduction, extension, and adduction in order.

Diarthrosis

A joint that moves freely. All synovial joints are diarthroses.

Fibrous joint

A joint where tough, dense tissue binds the bones right together, with no space inside. Types are the suture, syndesmosis, and gomphosis.

Flexion

A movement that makes the angle at a joint smaller, such as bending the elbow.

Gomphosis

The immovable peg-in-a-hole fibrous joint that holds a tooth in its socket. The periodontal ligament holds it in place.

Ligament

A band of tough, dense tissue that joins bone to bone and makes a joint stronger.

Meniscus

A half-moon pad of fibrocartilage between bone ends, as in the knee. It spreads the load and helps the bones fit.

Suture

An immovable fibrous joint. Sutures form the zipper-like seams between the flat bones of the skull.

Symphysis

A cartilaginous joint that moves a little, joined by fibrocartilage. Examples are the pubic symphysis and intervertebral discs.

Synarthrosis

A joint that does not move, such as a skull suture.

Synchondrosis

A cartilaginous joint joined by hyaline cartilage, such as the growth (epiphyseal) plate.

Syndesmosis

A fibrous joint where a sheet or cord of tissue links two bones. It allows a little movement, as between the radius and ulna.

Synovial fluid

The slippery fluid that fills a joint space. It is rich in hyaluronic acid. It oils the joint, feeds the cartilage, and cushions.

Synovial joint

A joint that moves freely. It has a fluid-filled space between bones capped with cartilage, all wrapped in a capsule.

Muscle Tissue

Muscle is the tissue that turns a choice into a movement. Every heartbeat, every blink, and every step is muscle at work. So is every push of food through your gut. Each one is a muscle cell doing the one thing muscle does best: shortening on command.

In this chapter we take one skeletal muscle apart, layer by layer. We keep going until we reach the tiny engine that actually pulls: the sarcomere. Then we watch that engine run. You will see how a nerve signal crosses into the muscle and how calcium flips the switch. You will see how threads of protein slide past each other. And you will see how the body turns the force up from a gentle flicker to an all-out heave. By the end, muscle action will not seem like magic. It will be a neat chain of events you can follow from thought to motion.

Section 1How a muscle is built

Three kinds of muscle tissue

Before we open up one muscle, it helps to know that the body makes three kinds of muscle. Each is tuned to a different job.

Skeletal muscle is attached to bones and moves the body. It is under voluntary control, which means you choose when it works. Under the microscope its cells are long and striped (striated), and each has many nuclei.

Cardiac muscle is found only in the wall of the heart. It is striated too, but it is involuntary. That means it works without you thinking about it. Its cells are branched and most often have one nucleus. They are joined end to end by intercalated discs. These links let the whole heart squeeze as one.

Smooth muscle lines the walls of hollow organs, such as blood vessels, the gut, the bladder, and the airways. There it squeezes slowly and never seems to tire. It is involuntary and has no stripes. Its cells taper at both ends and have one nucleus in the middle.

This chapter focuses on skeletal muscle, because it shows every idea most clearly. But keep the other two in mind. Most of the machinery for squeezing is shared.

Bundles inside bundles

A whole skeletal muscle, such as your biceps, is not one solid block. It is a set of bundles inside bundles. Each bundle is wrapped in its own sheet of connective tissue. Let's go from the outside in.

The whole muscle is wrapped in the epimysium. Inside that, fibers are grouped into bundles called fascicles. Each fascicle is wrapped in perimysium. Inside each fascicle, every single muscle cell (a muscle fiber) is wrapped in a thin endomysium.

These three sheets are all joined to each other. At the ends of the muscle, they merge and reach past it. They form a tendon (a cord) or an aponeurosis (a flat sheet) that ties the muscle to bone. This setup is more than neat packing. The connective tissue carries blood vessels and nerves to each fiber. It also passes the pull of every cell to the tendon, so the whole muscle moves as one.

Inside a single fiber

Zoom into one muscle fiber and you find a cell built for pulling. Its cell membrane is called the sarcolemma. Its cytoplasm, the jelly inside, is the sarcoplasm. The word part sarco- means “flesh.” The sarcolemma dives deep into the cell as tunnels called transverse (T) tubules. They carry the electrical signal into the core of the fiber.

Wrapped around the working parts is the sarcoplasmic reticulum (SR). It is a special kind of smooth endoplasmic reticulum. It stores and releases the calcium that sets off a contraction. Most of the cell is filled with hundreds of long threads called myofibrils. Each myofibril is a chain of the units that shorten, the sarcomeres. These are made of myofilaments of two proteins, actin and myosin.

A muscle fiber forms when many cells fuse together as the body develops. So it holds many nuclei, pushed to the edge just under the sarcolemma. Nearby sit spare satellite cells that help repair damage.

Follow all of this in the interactive. Start at the whole muscle and zoom in, one wrapping at a time, down to the sarcomere.

Section 2The sarcomere

Stripes with a purpose

Skeletal and cardiac muscle have stripes. These stripes are not just for show. They are the outward sign of very orderly machinery inside. In every myofibril, the proteins that pull are lined up in the same repeating segments. Each segment is a sarcomere. It is the functional unit of contraction, the smallest part that can do the job. One sarcomere runs from one Z-disc to the next Z-disc.

When millions of these little units all shorten at once, lined up, the whole muscle shortens. Each unit has dark and light parts. Stacked side by side across the fiber, they look like stripes to our eyes. These are the striations.

The bands, zones, and lines

A sarcomere is built from two kinds of filament, or thread. The thick filaments are made of myosin. Each has a hinged head that sticks out. The thin filaments are made of actin. Where the two overlap, the sarcomere looks dark. Where only one kind is found, it looks lighter. It is worth learning this map of bands. In the next section you will watch these parts change size as the muscle contracts.

Thick, thin, and the switch proteins

The thin filament is more than plain actin. A rope-like protein called tropomyosin lies along it. In a resting muscle, it covers the spots where myosin would grab on. These spots are the myosin-binding sites. A three-part protein group called troponin holds tropomyosin in place. One part of it, troponin C, senses calcium.

This is the switch. When calcium binds to troponin, the whole group shifts. Tropomyosin slides aside, and the binding sites open up.

Two more proteins keep the sarcomere in order. Titin is a huge, springy protein. It runs from the Z-disc to the M-line. It keeps the thick filament centered and lets muscle spring back when stretched. Nebulin runs along the thin filament and sets its length. Once again, the build is made exactly for the job.

Tap each part in the interactive to see how the filaments make the stripes.

Section 3The sliding filament theory

Sliding, not shrinking

Here is the most important idea in this chapter. It surprises almost everyone the first time: the filaments do not get shorter. When a muscle contracts, the thick and thin filaments slide past each other. They overlap more and drag the Z-discs closer together. Each filament keeps its exact length. It is the sarcomere that shortens.

That is why the A band never changes in the reference figure. The A band is the length of the thick filament. Meanwhile the I band and H zone shrink, and the Z-discs move inward. Now add up that tiny slide across millions of sarcomeres lined up end to end. The whole muscle clearly shortens and pulls on its tendon.

The cross-bridge cycle

The myosin heads power the sliding. They act like tiny oars. Each head reaches out and grabs actin, forming a cross-bridge. Then it swivels. This is the power stroke. It pulls the thin filament a few nanometers toward the center. Then the head lets go, gets cocked again, and grabs again farther along. The cycle repeats many times each second. It runs on ATP, the cell's energy molecule:

Notice that ATP has two jobs. It is needed to release the head (step 3). It is also needed to re-cock it (step 4). This leads to something you may have heard of. After death, the body stops making ATP. The myosin heads bind actin and can no longer let go. So the body stiffens. This is called rigor mortis.

Calcium throws the switch: excitation–contraction coupling

Cross-bridges can form only if the binding sites are uncovered. That is calcium's job. The link between an electrical signal and a pull is called excitation–contraction coupling. Here is how it works.

A muscle action potential, an electrical signal, races along the sarcolemma. It dives down the T-tubules into the fiber. That change in voltage tells the sarcoplasmic reticulum to release its stored Ca²⁺ into the sarcoplasm. Calcium binds to troponin. Troponin pulls tropomyosin off the actin. The binding sites open, and cross-bridges begin to form. The filaments start to slide.

Relaxing is just the reverse. When the signal stops, a pump called SERCA moves Ca²⁺ back into the SR. Tropomyosin covers the sites again. Cross-bridges can no longer form, and the muscle goes slack. No calcium, no contraction.

In the interactive, watch the Z-lines pull together while the filaments keep their length.

Section 4The neuromuscular junction

From nerve to muscle

A skeletal muscle fiber will not contract on its own. It waits for a command from a nerve. That command is delivered at a special synapse, or meeting point, called the neuromuscular junction (NMJ). A motor neuron reaches out from the spinal cord. The end of its axon rests just above a folded patch of sarcolemma. This patch is the motor end plate.

The two never touch. A tiny gap, the synaptic cleft, separates them. To cross that gap, the neuron uses a chemical messenger. This is the neurotransmitter acetylcholine (ACh).

The handoff, step by step

Passing the signal from nerve to muscle is one of the most reliable events in the body. A single nerve impulse almost always sets off a muscle impulse. Follow the chain:

See how this fits with the last section. Step 6 is the very same muscle action potential that starts excitation–contraction coupling. The NMJ is the doorway. The sliding filaments are the room it opens into.

Switching the signal off, and where it breaks

A signal that never ended would leave the muscle stuck in contraction. So the cleft holds an enzyme called acetylcholinesterase (AChE). It chops ACh apart within milliseconds. This neat on/off system is also where many drugs and diseases act. That makes the NMJ one of the most important spots in medicine.

In myasthenia gravis, the immune system destroys ACh receptors. So muscles tire quickly and grow weak. Curare, an arrow poison, blocks the receptors and causes paralysis. Organophosphate bug sprays and nerve agents block AChE. Then ACh piles up and muscles seize. Botulinum toxin (Botox) blocks the release of ACh, so muscle relaxes. This is useful for treating spasms and wrinkles.

Step through the chemical handoff yourself in the interactive.

Section 5Motor units & muscle tension

The motor unit

One motor neuron rarely controls just one muscle fiber. It branches and connects to a group of fibers. The neuron plus all the fibers it controls is a motor unit. When that neuron fires, every fiber in its unit contracts at once. They act as an all-or-nothing team.

The size of the unit sets how finely a muscle can be controlled. Muscles that need a gentle touch, like those that move the eye or the fingers, have small motor units. Each neuron links to only a few fibers. Muscles built for power, like the quadriceps or the calf, have large motor units. Each neuron links to hundreds of fibers. Small units mean fine control. Large units mean strong, rough force.

Twitch, summation, tetanus

One short stimulus gives a single quick contraction and relaxation. This is a twitch. It has three phases. First is a latent period, a hidden delay while calcium is released. Next is a contraction phase, then a relaxation phase. A twitch is a flick, not a useful pull.

To get smooth, strong movement, the nervous system fires the motor neuron again before the fiber fully relaxes. So the second contraction stacks on top of the first. This stacking is wave (temporal) summation. Fire even faster, and the twitches blur into a partly smooth pull. This is unfused (incomplete) tetanus. Faster still, and you get a steady, perfectly smooth, all-out pull. This is fused (complete) tetanus. Almost all normal movements use tetanus, not single twitches.

Two dials for force: recruitment and rate

The body controls how hard a muscle pulls with two dials at once. The first dial is recruitment, which means turning on more motor units. It follows the size principle. Small, weak units that resist tiring are turned on first. Larger, stronger ones join only when more force is needed. So movements start smooth and easy to control.

The second dial is rate coding. This means firing each active unit faster, to climb from summation up to tetanus. With both dials, the same muscle can lift a feather or a heavy weight. Even at rest, a few units are always active, taking turns. They keep muscles firm and ready. This steady, low tension is muscle tone.

Pick a stimulus pattern in the interactive. Watch tension climb from twitch to tetanus.

Section 6The three fiber types

Speed versus stamina

Not every skeletal muscle fiber is the same. There are three types. They trade speed against endurance, or staying power. No one design can be both as fast as possible and as tireless as possible.

Slow oxidative (Type I) fibers contract slowly and make only modest force. But they can keep going for hours. They are the endurance experts. Fast glycolytic (Type IIx) fibers contract quickly and with great power. But they tire within minutes. They are the experts at sprinting and lifting. In between are fast oxidative (Type IIa) fibers. They are fast and fairly slow to tire, a useful middle ground.

Aerobic versus anaerobic engines

The difference comes down to how each fiber makes its ATP. Oxidative fibers use aerobic respiration, which needs oxygen. They are packed with mitochondria, the cell's power plants. They have lots of capillaries, the tiniest blood vessels. They are also loaded with myoglobin, a pigment that holds oxygen and makes them look red. This red, oxygen-fed system is slow but can keep going.

Glycolytic fibers rely on anaerobic glycolysis, which does not need oxygen. They have few mitochondria and little myoglobin, so they look pale. They store lots of glycogen, a sugar fuel, for quick bursts. They give explosive power. But they fill up with lactate and tire fast.

Everybody is a mixture, plus a word on the other two muscles

Almost every real muscle has all three fiber types. The mix is set partly by your genes and partly by how you train. Endurance training moves fibers toward the oxidative end, where they resist tiring. Strength and sprint training push toward bigger, more powerful fibers. That is why a marathon runner's calves and a sprinter's thighs are built for different feats.

Now recall the two muscle types we set aside at the start. Cardiac muscle acts like tissue that is always tuned for endurance. It sets its own rhythm, and intercalated discs join its cells, so the heart never truly rests. Smooth muscle has no sarcomeres at all. It can hold tension for a long time using very little energy. This “latch” state is perfect for keeping a blood vessel or a sphincter squeezed all day.

Tap each fiber type in the interactive to compare speed, color, and stamina.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back when a word trips you up.

A band

The dark part of a sarcomere that spans the full length of the thick filaments. It does not change length during contraction.

Acetylcholine (ACh)

The neurotransmitter a motor neuron releases at the neuromuscular junction. It sets off a muscle action potential.

Actin

The protein of the thin filament. It has the myosin-binding sites where cross-bridges attach.

Cross-bridge

The link made when a myosin head binds an open site on actin. Its cycle of binding, power stroke, and release makes the filaments slide.

Endomysium

The thin sheet of connective tissue that wraps each single muscle fiber.

Epimysium

The sheet of connective tissue that wraps a whole skeletal muscle.

Excitation–contraction coupling

The chain of events that links a muscle action potential to contraction. The signal travels down the T-tubules and makes the sarcoplasmic reticulum release Ca²⁺.

Fascicle

A bundle of muscle fibers wrapped in perimysium.

Motor unit

One motor neuron plus all the muscle fibers it controls. They contract as one.

Myofibril

A long thread inside a muscle fiber. It is made of a chain of sarcomeres.

Myosin

The protein of the thick filament. Its hinged heads form cross-bridges and do the power stroke.

Neuromuscular junction

The synapse between a motor neuron and a skeletal muscle fiber. Here acetylcholine passes on the signal.

Recruitment

Raising muscle force by turning on more motor units. The smallest units, which tire the least, go first (the size principle).

Sarcolemma

The cell membrane of a muscle fiber. Its tunnels into the cell are the T-tubules.

Sarcomere

The functional unit of striated muscle, the part that shortens. It runs from one Z-disc to the next.

Sarcoplasmic reticulum (SR)

The special smooth ER of a muscle fiber. It stores and releases the Ca²⁺ that sets off contraction.

Sliding filament theory

The idea that muscle shortens because thick and thin filaments slide past each other. The filaments themselves do not get shorter.

Tetanus

A lasting contraction. It happens when stimuli come faster than the fiber can fully relax between them. It is unfused (incomplete) when some relaxing still shows as slight wobbles. It is fused (complete), smooth and all-out, when stimuli come at a very high rate.

Tropomyosin

The thin-filament protein that covers the myosin-binding sites at rest. It slides aside when calcium binds troponin.

Troponin

The thin-filament protein group that binds Ca²⁺. It moves tropomyosin to uncover the binding sites.

Twitch

A single short contraction and relaxation of a muscle fiber after one stimulus. It has latent, contraction, and relaxation phases.

The Muscular System

Every move you choose to make is a muscle pulling on a bone or on your skin. A smile, a step, a breath and a signature all work this way. In the last chapters you looked inside a single muscle fiber. Now we zoom out to the whole body. You have more than 600 skeletal muscles, and each one can only pull. So how do they team up to make smooth, controlled moves? And why do their names sound like Latin riddles? You will learn to crack those riddles, so you can know a muscle you never memorized. This chapter tours the body's muscles, one region at a time. We go from the tiny muscles that shape your face to the big ones that launch you off the ground.

Section 1How muscles work together

A muscle can only pull

Everything in this section rests on one fact: a muscle can only pull, never push. When a muscle contracts, its fibers get shorter. This draws its two ends toward each other. That is the only trick it has. It cannot stretch itself out to shove a bone away.

So how does your forearm bend up and then straighten back down? Muscles almost never work alone. They are set up in opposing teams on each side of a joint. To reverse a move, the body relaxes one team and contracts the other.

To describe the pull clearly, we name a muscle's two ends. The origin is where the muscle attaches to the bone that stays fairly still. It is usually the heavier bone, closer to the body's center (proximal). The insertion is where it attaches to the bone that moves. That bone is usually farther out (distal).

Between the two ends is the thick middle part that contracts. This is the belly. When the muscle gets shorter, the insertion is pulled toward the origin. Muscles attach to bone by tough cords called tendons. Some attach by wide, flat sheets called aponeuroses.

Four roles in every movement

Muscles work in teams, so every move is like a small crew at work. Each crew member has a job. Learn these four roles, and you can study any movement in the body:

The same muscle can switch roles. It depends on the move. When you bend your elbow, the biceps is the agonist and the triceps is the antagonist. When you straighten it, they swap jobs. So no muscle is just a “flexor” in general. A muscle is a flexor or an extensor only for one joint in one move.

Bones are levers, joints are fulcrums

Muscles do not move bones by brute force alone. They work through levers. A lever is a stiff bar that turns on a fixed point. Your body is full of them. The bone is the bar. The joint is the turning point, called the fulcrum. The muscle's pull is the effort. The weight being moved is the load. How these three points are lined up decides what a joint is good at. It favors either power or speed and range.

Most joints in the body are third-class levers. In this setup, the effort is between the fulcrum and the load. Your elbow is a good example. The biceps attaches just past the joint, while the load sits far out at your hand. This setup is weak for lifting heavy things. But it gives you something more useful. A small, quick shortening of the muscle makes your hand sweep a long way, fast. Your body traded raw force for speed and reach. That is why you can throw a ball hard but strain to curl a heavy dumbbell.

Bend and straighten the elbow in the interactive. Watch the biceps and triceps trade the agonist and antagonist roles.

Section 2How muscles are named

Names are descriptions in disguise

The first time you see a name like extensor carpi radialis longus, it can feel like a wall of Latin to memorize. It is not. Scientists named muscles the way sailors named knots. They described them. Almost every muscle name is a short description. It tells you where the muscle is, what it looks like, how big it is, which way its fibers run, how many heads it has, or what it does. Once you know the words, a new name becomes a set of directions. It tells you where to find the muscle and what it does.

The six clues

A name can carry six kinds of clues. Many muscles pack several clues into one name:

Watch the clues stack up. Extensor digitorum longus means “the long muscle that extends the digits.” It gives the action, then the body part, then the size. Rectus femoris is “the straight muscle of the femur.” Even a monster like flexor carpi radialis is simple. It is “the muscle that flexes the wrist (carpus) on the radius side.” You do not have to memorize it as one blob. You read it like a sentence.

Tap each of the six clues in the interactive. See how a name reveals a muscle.

Section 3Muscles of the head and neck

The muscles that make a face

The muscles of facial expression are the muscles that shape your face. They are unlike almost any others in the body. Most of them do not move a bone at all. Instead, they insert into the skin. When they contract, they tug the skin of your face into shapes. That is how you smile, frown, raise an eyebrow, or wink. One nerve runs this whole group. It is the facial nerve (cranial nerve VII).

Here are the key players. The orbicularis oris is a ring around the mouth. It purses your lips to whistle or kiss. The orbicularis oculi is a ring around the eye. It closes the eye to blink and squint.

The buccinator is in the cheek. It flattens the cheek to blow out a candle. It also holds food against your teeth while you chew. The zygomaticus pulls the corner of the mouth up to smile. The occipitofrontalis raises the eyebrows and wrinkles the forehead.

The muscles that chew

The second job up here is mastication, which means chewing. The muscles of mastication are not like the face muscles. They do move a bone. They close the mandible (lower jaw) and grind it at the jaw joint. That joint is called the temporomandibular joint. This group answers to a different nerve, the trigeminal nerve (cranial nerve V).

The masseter runs from the cheekbone to the angle of the jaw. For its size, it is one of the strongest muscles in the body. You can feel it bulge when you clench your teeth. The fan-shaped temporalis spreads over the side of the skull. It helps close the jaw and pull it back. The deep pterygoid muscles move the jaw from side to side to grind food.

Down in the neck, two large muscles move the head itself. There is a sternocleidomastoid muscle on each side of the neck. When both work together, they bend the neck forward (flex it). When one works alone, it turns the head to the opposite side. The wide trapezius tips the head back (extends it). It also steadies the shoulder girdle behind it.

Tap each muscle in the interactive to light it up. See whether it makes a face or chews.

Section 4Muscles of the trunk

The muscles that breathe

The trunk muscles do the body's quiet but vital work. None of it is more vital than breathing. The diaphragm is the main muscle of breathing (respiration). It is a wide dome of muscle that seals off the bottom of the chest. When it contracts, it flattens. This makes the chest space (the thoracic cavity) bigger. The pressure inside drops, and air is pulled into the lungs. When the diaphragm relaxes, the dome springs back up and pushes air out.

Other muscles help. The external intercostals are muscles between the ribs. They lift the rib cage when you breathe in. The internal intercostals pull the ribs down when you breathe out hard. You use them when you cough or blow out a candle.

The abdominal wall and the back

Four flat muscles wrap the front and sides of your belly. They are layered like plywood. Their fibers run in different directions, which makes the wall strong. The rectus abdominis is the “six-pack.” It runs straight up the middle. It flexes (bends) the spine, the vertebral column. That is the move of a sit-up.

Three more muscles cover the sides. They are the external oblique, the internal oblique, and the deepest one, the transversus abdominis. The fibers of the transversus wrap sideways around you like a corset. Together, this wall squeezes the belly. It bends and twists the trunk. It protects the soft organs behind it. It also raises the pressure you need to cough, give birth, and have a bowel movement.

Along the back runs the erector spinae. It is a set of three columns of muscle that run the length of the spine. It does the opposite of the belly muscles. It extends the spine, which means it straightens you up after you bend over. It also holds you upright against gravity all day long.

Tap each trunk muscle in the interactive. See whether it breathes, bends, or protects.

Section 5Muscles of the upper limb

Anchoring and moving the shoulder

Your upper limb is built for reaching and for skilled hand work. It starts at the shoulder. The shoulder moves more than any other joint, so it is also the least stable. Several large muscles anchor the shoulder blade and swing the arm. The trapezius and serratus anterior hold and turn the scapula (shoulder blade). This makes it a moving base for the arm.

The fan-shaped pectoralis major on the chest pulls the arm forward and across the body. The wide latissimus dorsi on the back pulls the arm down and back. It is the muscle of a swimmer's stroke. On top of the shoulder sits the deltoid. This triangle-shaped muscle abducts the arm, which means it lifts the arm out to the side.

Deep around the joint itself is the rotator cuff. It is a set of four small muscles: supraspinatus, infraspinatus, teres minor, and subscapularis. Together they are called the “SITS” group. They do not make big moves. Their job is to hold the ball at the top of the humerus snugly in its shallow socket. They do this while the bigger muscles move the arm.

The arm and forearm

The arm runs from the shoulder to the elbow. It holds the classic agonist and antagonist pair. On the front, the biceps brachii flexes (bends) the elbow. It also turns the palm up, which is called supination. The brachialis, under the biceps, helps it. On the back, the triceps brachii extends the elbow to straighten the arm. It is the only major muscle there.

Past the elbow, the forearm has two opposing groups. On the palm side, flexors curl the wrist and fingers. On the back side, extensors straighten them. Their long tendons run down into the hand. That is why much of the power for your grip comes from muscle bellies up in the forearm.

Tap each muscle in the interactive. Find the movers of the shoulder, arm, and forearm, and their pairs.

Section 6Muscles of the lower limb

The hip and the buttock

Your lower limb carries the whole weight of your body. It drives every step. So it holds the biggest, strongest muscles you have. At the hip is the gluteus maximus. It makes up most of the buttock. It is the largest single muscle in the body. It extends the hip with great power. It drives you up from a squat, up the stairs, and into a run.

Under it and to the side sit the gluteus medius and gluteus minimus. They abduct the thigh, moving it out to the side. Even more important, they steady the pelvis each time you stand on one leg to take a step. On the front of the hip, the deep iliopsoas is the main hip flexor. It lifts the thigh toward the chest.

The thigh and the leg

The thigh has opposing groups. On the front is the quadriceps femoris. It is four muscles: the rectus femoris and the three vastus muscles. They share one tendon over the kneecap. They extend the knee to straighten the leg for a kick or a step. On the back are the three hamstrings: the biceps femoris, semitendinosus, and semimembranosus. They do the reverse. They flex the knee and help extend the hip. On the inner thigh, the adductors squeeze the legs together.

Below the knee, the calf gives you your push-off. The gastrocnemius has two heads. It and the deeper soleus join into the Achilles (calcaneal) tendon. Together they plantarflex the ankle. That means they point the toes and lift the heel. This launches you into every step and jump. On the shin, the tibialis anterior works against them. It dorsiflexes the ankle, pulling the toes up. This keeps your toes off the ground as your foot swings forward.

Tap each muscle in the interactive. Trace the movers of the hip, knee, and ankle.

ReferenceWords worth knowing

Here is a quick guide to the terms in this chapter. Skim it now, or come back whenever a word trips you up.

Abduction

Moving a limb away from the midline of the body. The deltoid abducts the arm.

Adduction

Moving a limb toward the midline of the body. The thigh adductors squeeze the legs together.

Agonist (prime mover)

The main muscle that makes a certain move.

Antagonist

The muscle that works against an agonist. It relaxes and gets longer while the agonist contracts.

Aponeurosis

A wide, flat sheet of connective tissue. It attaches a muscle to bone or to another muscle.

Belly

The thick middle part of a muscle that contracts. It lies between the origin and insertion.

Diaphragm

The dome-shaped main muscle of breathing. It separates the chest (thoracic) and belly (abdominal) cavities. The phrenic nerve runs it.

Erector spinae

A group of deep back muscles. They extend the spine and hold you upright.

Fixator

A muscle that holds the origin, or a nearby joint, steady. This lets the prime mover work well.

Insertion

Where a muscle attaches to the bone that moves. It is usually the bone farther out (distal).

Intercostal muscles

The muscles between the ribs. The externals lift the rib cage to breathe in. The internals pull it down to breathe out hard.

Lever

A stiff bar (a bone) that turns on a fulcrum (a joint) to move a load. Most levers in the body are third-class.

Masseter

A strong chewing muscle that runs from the cheekbone to the angle of the jaw. It closes the mandible.

Mastication

Chewing. Muscles that move the mandible grind the food. The trigeminal nerve (CN V) runs them.

Muscles of facial expression

Muscles that insert into the skin of the face to make expressions. The facial nerve (CN VII) runs them.

Origin

Where a muscle attaches to the bone that stays fairly still. It is usually the bone closer in (proximal).

Quadriceps femoris

The four-headed muscle group on the front of the thigh. It extends the knee.

Rotator cuff

Four muscles that hold the shoulder joint steady. They are the supraspinatus, infraspinatus, teres minor, and subscapularis (“SITS”).

Sternocleidomastoid

A neck muscle that links the sternum, clavicle, and mastoid process. It bends the neck and turns the head.

Synergist

A muscle that helps an agonist or holds a joint steady during a move.

Tendon

A tough, cord-like band of dense connective tissue. It attaches a muscle to a bone.

The Nervous System & Nervous Tissue

Every thought you have had was carried by a wave of electricity. So was every move you have made and every feeling you have felt. That wave races along a living wire. This chapter tells the story of that wire, the neuron, and the tissue it lives in. First we map how the whole nervous system is set up. Then we meet the loyal helper cells that keep neurons alive. Then we get to the heart of it. You will see how a neuron builds up an electrical charge and fires an action potential. You will see how that signal flies down an axon wrapped in myelin. And you will see how it hands the message to the next cell across a tiny gap. Learn these ideas well. The rest of the nervous system, from the brain and spinal cord to reflexes and the senses, is built on this one theme.

Section 1How the nervous system is organized

One control center, wires everywhere

The nervous system is the body's fast message network. Scientists split it up in two ways. One way is by where the parts are. The other is by which way the signals flow.

By location, there are just two parts. The central nervous system (CNS) is the brain and spinal cord. It is the headquarters, where information is handled and choices are made. The peripheral nervous system (PNS) is everything else. It is the nerves and ganglia (clusters of nerve cell bodies) that fan out from the CNS to every corner of the body. Think of the CNS as a computer. The PNS is the set of cables that run to every sensor and every motor.

Three jobs: sense, decide, act

A nerve pathway may look complex. But it is always doing one of three things. First comes sensory input. Receptors sense a change, inside or outside the body, and send that news toward the CNS. Second is integration. The CNS sorts through the incoming signals and decides what to do, if anything. Third is motor output. A command travels away from the CNS to an effector, which is a muscle or a gland. The effector carries out the response.

Touch a hot stove, and all three happen in a heartbeat. Skin receptors sense heat (input). The spinal cord decides "pull back" (integration). Arm muscles yank the hand away (output).

The words for these directions are worth learning. You will see them on every diagram. Neurons that carry signals toward the CNS are afferent, or sensory. Neurons that carry commands away from the CNS are efferent, or motor.

The efferent side splits again by where its signals go. The somatic nervous system carries commands you control to skeletal muscle. These are the moves you choose. The autonomic nervous system (ANS) runs the automatic chores inside you. It sends signals to cardiac muscle, smooth muscle, and glands without you having to think about it. The ANS itself has two arms that usually work against each other. The sympathetic division is the "fight-or-flight" gas pedal. The parasympathetic division is the "rest-and-digest" brake.

Tap through each division in the interactive below. Try central and peripheral, sensory and motor, and somatic and autonomic.

Section 2Neurons and their support crew

The neuron: a cell built to signal

Before we meet the helper cells, meet the star. A neuron is a nerve cell. It is an excitable cell, which means it can fire an electrical signal. Its shape suits one purpose. It takes in a signal at one end and passes it on from the other. Three parts do the work.

The dendrites are branches, like antennae, that receive incoming signals. The cell body, or soma, holds the nucleus. It keeps the cell alive and running. It is also where incoming signals are added up. The axon is one long fiber that carries the outgoing signal away. It is often wrapped in a fatty myelin sheath. It ends in axon terminals that hand the message to the next cell.

Neurons are also amitotic. This means most of them cannot divide to replace themselves. That is exactly why damage to the brain and spinal cord is so hard to repair.

The support crew: six kinds of glia

For every neuron, there are about as many neuroglia, or more. These are also called glial cells. They are the crew that feeds, insulates, cleans, and protects neurons. Glia do not fire action potentials. But without them, neurons would starve, short-circuit, and drown in waste. There are six types. Four live in the CNS, and two live in the PNS.

Two cells make myelin, so keep them straight

People often mix up the two cells that make myelin. So let's pin them down now. In the CNS, an oligodendrocyte reaches out several arms. It wraps many different axons at once. In the PNS, a Schwann cell wraps itself around a single segment of one axon, like a jelly roll.

Both make the same fatty insulation. But the difference matters in medicine. Multiple sclerosis attacks CNS myelin. Guillain–Barré syndrome attacks PNS myelin. They are totally separate diseases, and they play out in different ways.

Tap each of the six glial cells. See where it sits and what it does.

Section 3The resting membrane potential

A charged battery, waiting

A neuron at rest is not doing nothing. It is armed and ready. There is a tiny voltage across its membrane. It is called the resting membrane potential, and it is about −70 mV. This means the inside of the cell is 70 millivolts more negative than the outside.

A membrane that keeps charges apart like this is called polarized. It is like a charged battery, ready to be used. Every signal a neuron sends is a quick, controlled release of that stored charge. So the first question is simple. How does the cell build up the charge in the first place?

Why the inside is negative

The answer is unequal ion distribution. Ions are charged particles. The cell keeps different amounts of each ion inside and outside. Sodium (Na⁺) is piled up outside the cell. Potassium (K⁺) is packed inside. The inside is also stuffed with large proteins and phosphates that carry negative charge. They are too big to leave.

Now add the membrane's channels. At rest, the membrane lets potassium leak through much more easily than sodium. It has many more open leak channels for K⁺. So potassium trickles out, from where there is more of it to where there is less. It carries positive charge with it. This leaves the inside more negative. The potassium leak, along with the trapped negative particles inside, is the main reason the resting voltage lands near −70 mV.

The pump that never sleeps

Over time, leaks would run the gradients down. Sodium would trickle in. Potassium would drift out. The battery would go flat. To stop this, the membrane runs the sodium–potassium pump (Na⁺/K⁺ ATPase) all day and night. It burns ATP for fuel. It pumps 3 Na⁺ out for every 2 K⁺ in. This keeps rebuilding both gradients.

The pump moves three positive charges out for every two it brings in. So it also removes a little positive charge overall. This makes the inside slightly more negative. The pump is so vital that it uses a large share of the body's energy at rest. That is a big reason the brain is such a hungry organ.

Explore the ion gradients, the leak channels, and the tireless pump in the interactive below.

Section 4The action potential

All or nothing

An action potential is the neuron's signal. It is a quick, dramatic flip of the voltage across the membrane. This flip shoots down the axon. But it only happens if the membrane is pushed up from its resting −70 mV to a key level. That level is the threshold, at about −55 mV.

Below threshold, nothing fires. Reach threshold, and a full-sized spike goes off every time. The spike is the same height whether the push was barely enough or huge. This is the famous all-or-nothing rule. A neuron cannot fire part of an action potential, just as a gun cannot fire part of a bullet. So how does a neuron signal a strong stimulus? Not with bigger spikes. It fires more often.

The upstroke: sodium floods in

The whole event is run by voltage-gated channels. These are ion gates that open when the voltage changes. At threshold, voltage-gated Na⁺ channels snap open. Sodium was piled up outside. It is pulled in by two forces: its concentration gradient and the negative inside. So it floods in.

Each positive charge that enters makes the inside less negative. That opens even more sodium gates. This fast, self-boosting rush is called depolarization. It drives the voltage from −55 all the way up to about +30 mV. For an instant, the inside of the cell is actually positive.

The downstroke and the reset

The spike ends as fast as it began. At the peak, the Na⁺ channels inactivate, which means they shut on their own. Now the slower voltage-gated K⁺ channels finish opening. Potassium rushes out. It carries positive charge away, and the voltage drops back down. This is repolarization.

The potassium gates are slow to close. So a little too much K⁺ leaves. The membrane briefly dips below rest, to about −80 mV. This dip is called hyperpolarization, or the undershoot. Then the leak channels and the Na⁺/K⁺ pump bring things back to a tidy −70 mV.

During the recovery there is a refractory period. In the first part (absolute), the axon cannot fire at all. In the later part (relative), it needs an extra-strong stimulus to fire.

Hit "Stimulate" in the interactive below. Follow the voltage through every phase.

Section 5Saltatory conduction & speed

Crawling versus leaping

Once an action potential starts, it has to travel the whole length of the axon. There are two ways to do that. Some axons are bare, or unmyelinated. On these, the spike rebuilds itself at every patch of membrane in a row. It marches along step by step. This is continuous conduction. It works, but it is slow.

Other axons are myelinated. The fatty sheath insulates long stretches, so current cannot leak out there. The action potential can only rebuild itself at the bare gaps between the myelin segments. So the signal seems to jump from gap to gap. It skips the insulated parts in between. This leaping is saltatory conduction. The name comes from the Latin word saltare, which means "to leap." It is much faster.

Nodes of Ranvier

The bare gaps have a name worth knowing: the nodes of Ranvier. These small bare rings are packed with voltage-gated sodium channels. So the action potential is rebuilt fresh and at full strength at each node. Between the nodes is a covered stretch called the internode. It carries the current quickly and passively, with no channels to slow it down. So the impulse hops from node to node. It rebuilds, glides, rebuilds, and glides. This covers far more ground in the same time than a steady crawl.

What sets conduction speed

Two features decide how fast a nerve fiber carries a signal. The first is myelination. A myelinated axon is much faster than an unmyelinated one of the same size. The second is axon diameter, or how thick the axon is. Thick axons are faster because current flows through them more easily. In the same way, a wide pipe moves water faster than a narrow one.

The body uses both of these on purpose. Large fibers with thick myelin carry reflexes and the sense of where your limbs are. They can carry signals at over 100 meters per second. Thin fibers with no myelin carry dull, aching pain. They crawl along at barely one meter per second. Sharp pain that you can pinpoint travels on thinner myelinated fibers. These are faster than the dull kind, but far short of that top speed.

Race a myelinated signal against an unmyelinated one in the interactive below. Watch it leap the nodes.

Section 6The synapse: passing the message on

Neurons don't quite touch

An action potential reaches the end of an axon. Now it has a problem. The next cell is not connected to it. There is a tiny gap in between, called the synaptic cleft. The whole meeting point is the synapse. The neuron sending the message is presynaptic ("before the synapse"). The cell getting it is postsynaptic ("after the synapse").

The electrical signal cannot jump the gap by itself. So most synapses turn it into a chemical messenger, called a neurotransmitter. The chemical drifts across the gap. On the far side, it creates the signal again. The message goes from electrical, to chemical, to electrical again.

Six steps across the gap

Chemical transmission always follows the same six steps. (1) The action potential arrives at the presynaptic axon terminal. (2) Its voltage opens voltage-gated calcium (Ca²⁺) channels, and calcium flows into the terminal. (3) Calcium makes tiny sacs called synaptic vesicles merge with the membrane and dump what they hold.

(4) The neurotransmitter is released (by exocytosis) and drifts across the cleft. (5) It binds receptors on the postsynaptic membrane. This opens ion channels, which create a small graded potential. (6) The neurotransmitter is quickly cleared away. Enzymes may break it down. It may be taken back into the sending cell (reuptake). Or it may drift away. This keeps the signal crisp and short.

Excitatory or inhibitory?

What the postsynaptic cell does next depends on which neurotransmitter and receptor are involved. An excitatory signal depolarizes the postsynaptic membrane. It nudges the cell toward threshold. This is an excitatory postsynaptic potential (EPSP). An inhibitory signal hyperpolarizes the membrane. It pushes the cell away from threshold. This is an inhibitory postsynaptic potential (IPSP).

One neuron gets thousands of these signals at once. It sums, or adds up, all of them. It fires only if the "go" signals beat the "stop" signals by enough to reach threshold. In other words, the synapse is where the nervous system actually makes decisions.

Step through the electrical-to-chemical-to-electrical handoff in the interactive below.

ReferenceWords worth knowing

Here is a quick guide to the terms in this chapter. Skim it now, or come back whenever a word trips you up.

Action potential

The neuron's electrical signal. It is a quick, all-or-nothing flip of the membrane voltage that travels down an axon.

Afferent (sensory) neuron

A neuron that carries signals toward the central nervous system. Afferent arrives.

All-or-nothing

The rule that a neuron fires either a full-sized action potential or none at all. The strength of a stimulus is coded by how often the neuron fires.

Astrocyte

The most common glial cell in the CNS. It ties neurons to blood vessels, controls the chemicals around them, and helps form the blood–brain barrier.

Autonomic nervous system

The automatic (involuntary) motor division. It controls heart muscle, smooth muscle, and glands. It splits into sympathetic and parasympathetic arms.

Axon

The one long fiber of a neuron. It carries the action potential away from the cell body.

Depolarization

A shift of the membrane voltage toward positive (less negative). It happens mainly when Na⁺ enters the cell.

Efferent (motor) neuron

A neuron that carries commands away from the central nervous system to an effector. Efferent exits.

Myelin sheath

Fatty insulation wrapped around an axon that speeds up signals. Oligodendrocytes make it in the CNS, and Schwann cells make it in the PNS.

Neuroglia

The helper cells of nervous tissue. They feed, insulate, clean, and protect neurons. There are six types across the CNS and PNS.

Neuron

The nerve cell that sends signals. It has dendrites (receive), a cell body (adds up signals), and an axon (sends the signal on).

Neurotransmitter

A chemical messenger released across a synapse. It passes a signal to the next cell.

Node of Ranvier

A bare gap between myelin segments, packed with Na⁺ channels. The action potential is rebuilt here during saltatory conduction.

Refractory period

A short time after a spike when that patch of membrane cannot fire again (absolute) or needs a stronger push (relative). It makes signals travel one way.

Repolarization

The return of the membrane voltage toward its negative resting value. It happens mainly when K⁺ leaves the cell.

Resting membrane potential

The voltage across a resting neuron's membrane, about −70 mV with the inside negative. Ion gradients and the K⁺ leak set it.

Saltatory conduction

Fast signaling along a myelinated axon. The signal leaps from node to node.

Schwann cell

The PNS glial cell that wraps one segment of an axon in myelin.

Sodium–potassium pump

A membrane pump powered by ATP. It moves 3 Na⁺ out and 2 K⁺ in, which keeps up the ion gradients.

Synapse

The meeting point where one neuron passes a message to another cell, across the synaptic cleft.

Threshold

The membrane voltage (about −55 mV) that must be reached to set off an action potential.

Anatomy of the Nervous System

Inside your skull sits the most complex object we know of. It is about three pounds of soft tissue that lets you read this line, remember when you were small, and pull your hand off a hot stove before you even feel the burn. This chapter is a guided tour of that brain. We will walk over the wrinkled top of the cerebrum. We will drop down through its inner regions to the brainstem, which keeps you alive, and look at the cerebrospinal fluid that cushions it all. We will peel back the three meninges, the wraps that guard it. We will follow the spinal cord down its bony tunnel. Last, we will count off the twelve cranial nerves that leave the brain directly. By the end, you will be able to picture a whole brain and name what you see.