Exam 3 Study Guide

These four chapters are about movement and the signal that starts it. First you learn how bones meet, and the one trade-off that shapes every joint. Then you go inside a muscle, down to the tiny engine that pulls. Then you learn how more than six hundred muscles team up, and how their names give them away. Last you learn the electrical story of the neuron, with its exact voltages. This set rewards two habits: learn the sequences in order, and learn the numbers exactly. The cram sheet at the end holds both.

Chapter 9Joints

What to know

A joint is any place where two bones come together. It can also be where bone meets cartilage or where a tooth meets bone. Every joint gets described two ways at once, so get in the habit of giving both.

The first way is by structure. It asks what holds the bones and whether there is a fluid-filled space. Fibrous joints are held by tough dense tissue with no space. Cartilaginous joints are held by cartilage, again with no space. Synovial joints have a fluid-filled space between the bones.

The second way is by function. It asks how much the joint moves. A synarthrosis does not move. An amphiarthrosis moves a little. A diarthrosis moves freely.

The two ways match, because the material sets the motion. That leads to the one big idea of the chapter. Mobility and stability pull against each other. Every joint sits somewhere on that scale, and its build is a deal made for its job. The skull gives up all movement to guard the brain. The shoulder gives up stability to reach and throw, which is why it dislocates most often.

All synovial joints share one design, but the shape of the bone ends decides the motion. There are six shapes. Sort them by axes, which means the directions the bones can swing. Nonaxial slides without swinging. Uniaxial swings in one plane. Biaxial swings in two. Multiaxial swings every way and spins too.

The key feature of a synovial joint is the fluid-filled joint cavity, which keeps the bone ends from touching. Everything else guards, feeds and oils that gap. Synovial fluid does three jobs at once. It oils the joint. It feeds the articular cartilage, which has no blood supply of its own. And it absorbs shock, by squeezing out of the pressed cartilage and seeping back in when the load lifts.

Every movement has an exact name, and the names come in opposite pairs. All of them are read from the anatomical position, standing tall with palms forward.

Worth memorizing

The self-oiling action of synovial fluid has a name. Pressing a joint squeezes fluid out of the cartilage, and letting go lets it seep back in. That is weeping lubrication, and it is one reason gentle movement helps a sore joint. An overused bursa swells, and that is bursitis. Worn-away articular cartilage gives osteoarthritis.

Push a joint past straight and you get hyperextension. Tilting the head back to look at the ceiling is one example.

A growth plate is a short-term joint. Later it fuses into solid bone, and that fused joint is called a synostosis.

The knee is the largest and most complex joint in the body. Two half-moon pads of fibrocartilage, the menisci, sit on top of the tibia and spread the load. Deep inside, the anterior and posterior cruciate ligaments cross in an X and keep the femur and tibia from sliding past each other. A hard twist on a planted foot can tear a cruciate ligament and a meniscus together.

How to reason through it

Chapter 10Muscle Tissue

What to know

Muscle does one thing well. It shortens on command. There are three kinds, each tuned to a different job, and most of the machinery for shortening is shared.

A whole skeletal muscle is bundles inside bundles, and each bundle gets its own wrapper. The three wrappers join each other and reach past the muscle to form a tendon, or a flat sheet called an aponeurosis. That setup carries blood vessels and nerves to every fiber and passes the pull of every cell to the tendon.

Inside one fiber, the cell membrane is the sarcolemma and the cytoplasm is the sarcoplasm. The sarcolemma dives inward as tunnels called T-tubules, which carry the electrical signal to the core. The sarcoplasmic reticulum wraps the working parts and stores the calcium. Most of the cell is filled with myofibrils, and each myofibril is a chain of sarcomeres. A fiber forms when many cells fuse, so it holds many nuclei pushed to the edge.

The sarcomere is the functional unit of contraction. It runs from one Z-disc to the next. Thick filaments are myosin, with hinged heads. Thin filaments are actin. Where they overlap it looks dark, and where only one kind sits it looks lighter. Lined up across the fiber, those dark and light parts are the stripes you can see.

Here is the most important idea in the chapter. The filaments do not get shorter. They slide past each other, overlap more, and drag the Z-discs closer. Each filament keeps its exact length. That is why the A band never changes while the I band and the H zone shrink.

The myosin heads do the sliding, acting like tiny oars. Each head grabs actin to form a cross-bridge, then swivels in a power stroke, then lets go and gets cocked again. The cycle repeats many times a second and runs on ATP.

Cross-bridges can only form if the binding sites are open, and that is calcium's job. The link between the electrical signal and the pull is called excitation and contraction coupling. A skeletal muscle fiber never fires on its own. It waits for a motor neuron, and the handoff happens at the neuromuscular junction.

One motor neuron plus all the fibers it controls is a motor unit, and the whole unit fires as one. Force is set by two dials at the same time. Recruitment turns on more units, smallest first. Rate coding fires each active unit faster.

Three fiber types trade speed against staying power, because no design can be both as fast and as tireless as possible. Almost every real muscle has all three, in a mix set partly by genes and partly by training.

Worth memorizing

Learn the wrappers as a zoom from outside in, and note that every one of them joins the tendon at the end.

1. Epimysium — wraps the whole muscle.
2. Perimysium — wraps each bundle of fibers, called a fascicle.
3. Endomysium — wraps each single muscle fiber.

The thin filament carries two guard proteins. Tropomyosin lies along it and covers the myosin-binding sites at rest. Troponin holds tropomyosin in place, and one part of troponin senses calcium. When calcium binds troponin, the group shifts, tropomyosin slides aside, and the sites open. Two more proteins keep order. Titin is a huge springy protein running from the Z-disc to the M-line, and it keeps the thick filament centered. Nebulin runs along the thin filament and sets its length.

ATP has two jobs here, and that explains rigor mortis. After death the body stops making ATP, so the heads bind actin and cannot let go, and the body stiffens.

Excitation and contraction coupling runs in one fixed order. A muscle action potential races along the sarcolemma. It dives down the T-tubules. The sarcoplasmic reticulum releases stored calcium into the sarcoplasm. Calcium binds troponin. Troponin pulls tropomyosin off the actin. The sites open and cross-bridges form. Relaxing is the reverse: the signal stops, a pump called SERCA moves the calcium back into the store, tropomyosin covers the sites again, and the muscle goes slack.

An enzyme in the cleft, acetylcholinesterase, chops acetylcholine apart within milliseconds, so the signal switches off cleanly. Many drugs and diseases act at this one spot. In myasthenia gravis the immune system destroys the receptors, so muscles tire and weaken. Curare blocks the receptors and causes paralysis. Bug sprays and nerve agents block the enzyme, so acetylcholine piles up and muscles seize. Botulinum toxin blocks the release of acetylcholine, so muscle relaxes.

A twitch has three phases: a latent period while calcium is released, then a contraction phase, then a relaxation phase. Almost all real movement uses tetanus, not single twitches. Small motor units give fine control, as in the muscles that move an eye or a finger. Large units give rough power, as in the quadriceps or the calf. Recruitment follows the size principle, turning on the small, tire-resistant units first. Even at rest a few units are always active, taking turns, and that steady low tension is muscle tone.

Cardiac muscle acts like tissue always tuned for endurance, because the heart never truly rests. Smooth muscle has no sarcomeres at all, and it can hold tension a long time on very little energy.

How to reason through it

Chapter 11The Muscular System

What to know

You have more than 600 skeletal muscles, and every one of them can only pull. A contracting muscle draws its two ends toward each other, and that is the only trick it has. So muscles 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.

Each muscle has two named ends. The origin attaches to the bone that stays fairly still, usually the heavier bone closer in. The insertion attaches to the bone that moves, usually the one farther out. The thick middle that contracts is the belly. When the muscle shortens, the insertion is pulled toward the origin.

Every movement uses a small crew with four roles. The same muscle swaps roles from one move to the next, so no muscle is a flexor in general. It is a flexor only for one joint in one move.

Muscles work through levers. The bone is the bar, the joint is the turning point, and the muscle's pull is the effort. Most joints in the body are third-class levers, with the effort between the turning point and the load. That setup is weak for lifting heavy things, but a small quick shortening sweeps the hand a long way fast. The body traded raw force for speed and reach.

Muscle names are descriptions, not random labels. Almost every name tells you the location, the shape, the size, the fiber direction, the number of heads, or the action. Read a name from left to right like a sentence and you can work out a muscle you never memorized.

In the head, two groups do very different jobs. The muscles of facial expression mostly insert into skin, not bone, and tug it into shapes. The muscles of mastication move the lower jaw to chew. Each group answers to a different cranial nerve.

In the trunk, the diaphragm is the main breathing muscle. When it contracts the dome flattens, the chest space gets bigger, the pressure inside drops, and air is pulled in. Four flat muscles wrap the belly in layers like plywood, with fibers running different ways, which makes the wall strong. Along the back, the erector spinae does the opposite of the belly muscles and holds you upright all day.

In the limbs, learn the opposing pairs first. The front and back of the arm. The front and back of the thigh. The calf against the shin.

Worth memorizing

Watch the clues stack up. "Extensor digitorum longus" is the long muscle that extends the digits, which gives the action, then the body part, then the size. "Rectus femoris" is the straight muscle of the femur. "Flexor carpi radialis" is the muscle that flexes the wrist on the radius side.

Know a few members by what they do. The orbicularis oris purses the lips. The orbicularis oculi closes the eye. The buccinator flattens the cheek and holds food against the teeth. The zygomaticus pulls the mouth corner up to smile. The occipitofrontalis raises the eyebrows. For chewing, the masseter is one of the strongest muscles in the body for its size, the fan-shaped temporalis helps close and pull back the jaw, and the deep pterygoids grind side to side. A sternocleidomastoid on each side bends the neck forward when both work, and turns the head to the opposite side when one works alone.

The external intercostals lift the rib cage to breathe in. The internal intercostals pull the ribs down to breathe out hard, as when you cough or blow out a candle. The rectus abdominis runs straight up the middle and bends the spine, which is the move of a sit-up. The transversus abdominis is the deepest layer, and its fibers wrap sideways like a corset.

The rotator cuff is four small muscles: supraspinatus, infraspinatus, teres minor and subscapularis, which gives the word SITS. They make no big moves. They hold the ball of the humerus snugly in its shallow socket while the bigger muscles swing the arm. The supraspinatus tears most often. The trapezius and serratus anterior hold and turn the scapula, which makes it a moving base for the arm.

The quadriceps is four muscles: the rectus femoris plus three vastus muscles, sharing one tendon over the kneecap. The hamstrings are three: the biceps femoris, semitendinosus and semimembranosus. The gastrocnemius has two heads and joins the deeper soleus in the Achilles tendon.

How to reason through it

Chapter 12The Nervous System & Nervous Tissue

What to know

The nervous system is split two ways. By location there are two parts. The central nervous system is the brain and spinal cord, and it is where choices are made. The peripheral nervous system is the nerves and ganglia that fan out to every corner of the body.

By job, every pathway is doing one of three things. Sensory input brings news of a change toward the center. Integration sorts the signals and decides what to do. Motor output sends a command out to an effector, which is a muscle or a gland. Touch a hot stove and all three happen in a heartbeat.

Two direction words show up on every diagram. Afferent neurons carry signals toward the center, so afferent arrives. Efferent neurons carry commands away, so efferent exits. The efferent side splits again. The somatic nervous system carries commands you choose to skeletal muscle. The autonomic nervous system runs the automatic chores, with a sympathetic gas pedal and a parasympathetic brake.

A neuron is an excitable cell, which means it can fire an electrical signal. Dendrites receive, the cell body adds up the input and keeps the cell alive, and the axon carries the outgoing signal away. Most neurons are amitotic, which means they cannot divide to replace themselves, and that is exactly why damage to the brain and cord is so hard to repair.

Six kinds of neuroglia support them. Four live in the center and two live in the periphery. Glia never fire action potentials, but without them neurons would starve and drown in waste.

A resting neuron is not doing nothing. It is armed. There is a voltage across the membrane of about −70 mV, with the inside more negative than the outside. A membrane holding charges apart like that is called polarized, and every signal is a quick controlled release of that stored charge.

The charge comes from unequal ion distribution. Sodium is piled up outside. Potassium is packed inside. Large proteins and phosphates with negative charge are trapped inside because they are too big to leave. At rest the membrane lets potassium leak out far more easily than sodium comes in, and that leak is the main thing setting the resting voltage. The sodium and potassium pump then runs day and night, moving 3 sodium out for every 2 potassium in, which keeps rebuilding both gradients and also makes the inside a little more negative.

An action potential is the signal. It only fires if the membrane is pushed from rest up to threshold, about −55 mV. Below threshold nothing happens. At threshold a full-sized spike goes off every time, the same height whether the push was barely enough or huge. That is the all-or-nothing rule. A neuron signals a strong stimulus not with a bigger spike but by firing more often.

Once started, the spike has to travel the whole axon, and there are two ways to do that. Bare axons rebuild the spike at every patch in a row, which works but is slow. Myelinated axons can only rebuild it at the bare gaps, so the signal seems to leap from gap to gap.

At the end of the axon the signal meets a gap it cannot jump. So most synapses turn it into a chemical messenger that drifts across and rebuilds the signal on the far side. The message goes electrical, then chemical, then electrical again. One neuron gets thousands of these at once and sums them, and it fires only if the go signals beat the stop signals by enough to reach threshold. That is where the nervous system actually makes decisions.

Worth memorizing

Keep the two myelin makers straight, because the diseases differ. An oligodendrocyte reaches out several arms and wraps many axons at once. A Schwann cell wraps itself around a single segment of one axon, like a jelly roll. Multiple sclerosis attacks central myelin. Guillain-Barre syndrome attacks peripheral myelin.

During the recovery there is a refractory period. In the first part the axon cannot fire at all. In the later part it needs an extra-strong push to fire. That period keeps the signal moving one way and limits how fast a neuron can fire.

Large fibers with thick myelin can carry signals at over 100 meters per second. Thin fibers with no myelin crawl along at barely one meter per second. Sharp pain you can pinpoint travels on thinner myelinated fibers, faster than the dull kind but far short of that top speed.

Chemical transmission across a synapse always follows the same six steps. The action potential arrives at the sending terminal. Its voltage opens calcium channels and calcium flows in. Calcium makes small sacs merge with the membrane. The neurotransmitter is released and drifts across the cleft. It binds receptors on the far membrane and opens ion channels, which makes a small graded potential. Then it is cleared away, by an enzyme, by being taken back into the sending cell, or by drifting off.

An excitatory signal depolarizes the far membrane and nudges the cell toward threshold, and that is an excitatory postsynaptic potential. An inhibitory signal hyperpolarizes it and pushes the cell away from threshold, and that is an inhibitory postsynaptic potential.

How to reason through it

One pageCram sheet