Psychology · Chapter 3

Biopsychology

Every thought you have ever had is a physical event. So is every feeling, every memory, and every choice. Each one is a pattern of electricity and chemistry moving through living cells. Biopsychology is the part of psychology that takes this idea seriously. It asks how the nervous system, and the glands that tune it, give rise to mind and behavior. This chapter starts with one working unit, the neuron. Then it follows the signal outward. You will see how one cell fires and how it passes a message to the next cell across a synapse. You will see how those cells are wired into the main lines of the nervous system and how the brain groups them into regions with jobs. Last, you will meet a slower chemical message service, the endocrine system, which sets the body's background mood. Once you know these pieces, the rest of psychology stops feeling like magic. It starts to feel like biology.

By the end of this chapter you can

Label the main parts of a neuron and say what each part does.
Describe the resting potential and walk through the steps of the action potential.
Explain how a signal crosses the synapse. Name the main neurotransmitters and their jobs.
Tell drugs that act as agonists apart from drugs that act as antagonists.
Map the parts of the nervous system: central vs peripheral, somatic vs autonomic, and sympathetic vs parasympathetic.
Find the hindbrain, midbrain, and forebrain, and give the jobs of the four cortical lobes.
Explain how the endocrine system sends signals with hormones, and how that differs from nerve signals.
Link real cases and researchers to the brain and body systems they helped explain. These include Phineas Gage, Broca, Wernicke, Sperry, and Selye.

Section 1The neuron

Drawing of a neuron with its dendrites, cell body, axon, and myelin sheath labeled — **Reference.** The neuron: dendrites take in signals, the cell body adds them up, and the myelin-wrapped axon carries the signal out to the terminals. OpenStax Anatomy & Physiology, CC BY 4.0 (via Wikimedia Commons)

The parts of a neuron

The neuron is the basic cell of the nervous system that handles information. All of psychology runs on neurons. That includes seeing, remembering, feeling, and moving. Your brain has about 86 billion of them, all wired together. A neuron does three jobs in order. It receives a signal. It decides whether to pass the signal on. Then it sends it.

The receiving end is a spray of branches called dendrites. They carry incoming signals toward the cell body, also called the soma. The cell body holds the nucleus. It also adds up everything that arrives. If the total is big enough, the signal leaves along one long fiber, the axon. The axon ends in small swollen tips called terminal buttons. There, the signal is handed off to the next cell.

Many axons are wrapped in a fatty white coating called the myelin sheath. It works like the plastic coating on a wire. Support cells lay it down. Myelin is not one smooth tube. It comes in segments with bare gaps between them. These gaps are the nodes of Ranvier, and they are the secret to how fast a myelinated axon can be.

Neurons do not work alone. They have a crew of helpers called glial cells, or glia. The brain has roughly 85 billion glia. That is about one glial cell for every neuron in the brain as a whole. In the cerebral cortex, though, glia outnumber neurons several times over. Glia build myelin, feed neurons, clean up waste, and hold everything in place. For a long time, people treated glia as simple packing material. An old textbook claim said there were ten glia for every neuron, and you may still hear it. Careful cell counts have retired both ideas. We now know glia help shape how signals travel.

Part	Job
Dendrites	Take in signals from other neurons and carry them toward the cell body
Cell body (soma)	Holds the nucleus; integrates (adds up) all incoming signals to decide whether to fire
Axon	One long fiber that carries the outgoing signal away from the cell body
Myelin sheath	Fatty coating that speeds the signal; its gaps are the nodes of Ranvier
Terminal buttons	Axon tips that release neurotransmitter onto the next cell
Glial cells	Helper cells that make myelin, feed neurons, and protect them

How a neuron fires: the action potential

At rest, a neuron is like a tiny charged battery. It keeps pumping charged particles, called ions, across its outer membrane. A pump called the sodium-potassium pump does much of this work. The result is that the inside of the cell sits at about −70 millivolts compared with the outside. This state is the resting potential. The cell is polarized, which means the inside and outside carry different charges. It is primed and waiting.

Signals from other neurons nudge that voltage up or down. Some push the cell toward firing. Others push it away. If enough push-toward signals add up, the voltage climbs to a tipping point near −55 mV. That tipping point is the threshold. Once the cell crosses it, the neuron fires.

Firing is the action potential. It is a quick electrical spike that races down the axon. It always runs in the same order. First, gates in the membrane fly open. Positively charged sodium (Na⁺) ions rush in. For a moment the inside turns positive. This step is depolarization. A split second later, the sodium gates shut and potassium (K⁺) ions flow out. That drags the voltage back down. This step is repolarization. The voltage often dips a little below rest before it settles.

During the spike and just after it, there is a short refractory period. In this window the cell cannot fire again, or it needs an extra strong push to do so. This keeps the signal moving one way down the axon. Then the cell settles back to rest, and the pump keeps the ion balance steady over time. The neuron is ready to go again.

One rule is key. The action potential is all-or-none. A neuron fires a full spike, or it does not fire at all. Think of a gun trigger. You cannot fire half a shot. So how does the brain know a strong touch from a light one? It does not use bigger spikes. Instead, a stronger stimulus makes neurons fire faster, and it makes more neurons fire.

In a myelinated axon, the spike does not crawl along the fiber. It leaps from one node of Ranvier to the next. This shortcut is called saltatory conduction. The name comes from the Latin word saltare, which means "to jump." This is why myelin matters so much. It can make a signal many times faster. It lets a signal travel from your spinal cord to your toe in a small part of a second.

Damaged myelin is a real disease. In multiple sclerosis, the immune system strips myelin off axons in the brain and spinal cord. With the coating gone, saltatory conduction fails. Signals slow down, scatter, or die out. That causes the numbness, weakness, and vision loss of the disease. The lesson is plain: the health of a neuron's wiring is the health of the behavior it supports.

Interactive Set the strength of a stimulus and see whether the neuron reaches threshold, then step through the five phases of an action potential on a live voltage graph. Open full ↑

Key points

A neuron takes in signals at its dendrites, adds them up in the cell body, and sends the result down its axon to the terminals.
The resting potential (about −70 mV) is a primed, polarized state. Crossing threshold (about −55 mV) makes the neuron fire.
The action potential is an all-or-none spike. Na⁺ flows in (depolarize), then K⁺ flows out (repolarize).
Myelin allows fast saltatory conduction. When myelin is damaged, signals and behavior suffer.

Section 2Neurotransmitters

A chemical synapse with small sacs releasing neurotransmitter across a gap to receptors on the next neuron — **Reference.** The chemical synapse: neurotransmitter leaves the vesicles, crosses the gap, and binds to receptors on the next neuron. OpenStax Anatomy & Physiology, CC BY 4.0 (via Wikimedia Commons)

Crossing the synapse

At most junctions, neurons do not touch. A very small gap sits between the end of one neuron and the dendrite of the next. This gap is the synaptic cleft. The whole junction is called the synapse. (A few synapses in the body are electrical and pass current straight through. Most, and the ones this chapter is about, are chemical.)

The electrical spike cannot jump the gap. So the message is turned into a chemical one. When the spike reaches the terminal buttons, tiny sacs called vesicles empty their contents into the cleft. What spills out are molecules of neurotransmitter. These chemical messengers drift across the gap. They lock onto receptors on the receiving neuron. Each receptor is shaped to fit a certain neurotransmitter, the way a lock fits one key.

The effect can be excitatory, which nudges the next neuron toward firing. Or it can be inhibitory, which nudges it away. Either way, the signal has to be shut off fast. If not, the receptor would keep going. The body turns off the signal in two ways. Enzymes in the cleft break the neurotransmitter apart. Or the sending neuron sucks the leftover molecules back up to use again. This second way is called reuptake. Reuptake will matter a lot in a moment, because many drugs for mental health conditions act on exactly this step.

The major neurotransmitters

Scientists have found dozens of neurotransmitters. A handful do most of the work that matters for psychology. Each is loosely tied to certain jobs. Each is also tied to certain disorders when its system runs too high or too low.

Neurotransmitter	Linked with	When it is out of balance
Acetylcholine (ACh)	Muscle movement, learning, memory	Loss is linked to Alzheimer's disease
Dopamine	Reward, motivation, movement, pleasure	Loss of dopamine cells causes Parkinson's; too much activity in some pathways is linked to schizophrenia
Serotonin	Mood, sleep, appetite	Linked to depression, though not as a simple shortage
Norepinephrine	Alertness, arousal, the stress response	Tied to anxiety and mood
GABA	The brain's main inhibitory messenger; calming	Low activity linked to anxiety and seizures
Glutamate	The brain's main excitatory messenger; learning	Too much can overexcite and damage neurons
Endorphins	Pain relief and pleasure (the body's own opioids)	Copied by morphine and heroin

A word of care about serotonin. You may hear that depression is "low serotonin." Research does not back that simple story. Large reviews have not found a clear serotonin shortage in most people with depression. Serotonin is part of the picture. So are stress, sleep, other brain chemicals, life events, and how brain networks change over time.

How drugs act: agonists and antagonists

Almost every drug that changes the mind does it by meddling with this chemical hand-off. The effects fall into two camps. An agonist is anything that increases a neurotransmitter's action. It might copy the neurotransmitter and bind its receptor directly. It might cause more of it to be released. Or it might block reuptake, so the neurotransmitter stays in the cleft longer. An antagonist does the opposite. It blocks or reduces a neurotransmitter's action. Often it plugs the receptor, so the real neurotransmitter cannot dock.

This split is the heart of psychopharmacology, the study of drugs that affect the mind. SSRIs are the most common antidepressants. They are agonists that block the reuptake of serotonin, which leaves more of it in the synapse. Many antipsychotic drugs are dopamine antagonists. They calm an overactive dopamine system. Caffeine, nicotine, opioids, and antihistamines all get their effects by pulling one of these levers.

An agonist turns the volume up. An antagonist turns it down. Botox (a toxin) and curare (an arrow poison) both shut down acetylcholine at the muscle. But they work on opposite sides of the synapse. Botox works on the sending side. It stops the terminal from releasing acetylcholine at all, so the muscle relaxes and the wrinkle smooths out. Curare works on the receiving side. It sits in the receptor on the muscle, so released acetylcholine cannot dock. The result is paralysis. Both lower acetylcholine's action, by two different routes. The lesson: to predict what a drug does, ask which way it pushes the synapse and where it acts.

Interactive Release a neurotransmitter across the synaptic cleft. Then switch on an agonist, an antagonist, or a reuptake blocker and see how each one changes the signal in the next neuron. Open full ↑

Key points

Signals cross the synapse as chemicals. Vesicles release neurotransmitter, which binds to receptors on the next neuron.
The signal is cleared by enzymatic breakdown (enzymes break it apart) or by reuptake back into the sending neuron.
Key neurotransmitters include dopamine, serotonin, acetylcholine, GABA, glutamate, and endorphins. Each is tied to certain jobs and disorders.
Agonists increase a neurotransmitter's action. Antagonists block it. Most psychiatric drugs work one of these two ways.

Section 3The nervous system

Outline of the body showing the central nervous system and the nerves of the peripheral nervous system — **Reference.** The nervous system splits into the central part (brain and spinal cord) and the peripheral part (all the other nerves). OpenStax Anatomy & Physiology, CC BY 4.0 (via Wikimedia Commons)

Central and peripheral

All those neurons are organized into two main parts. The central nervous system (CNS) is the brain and the spinal cord. It is the command center, where information is combined and decisions are made. Everything else is the peripheral nervous system (PNS). This is the huge web of nerves that runs out to the muscles, skin, and organs. Its job is to carry messages back and forth between the CNS and the rest of the body.

Sensory neurons carry messages in toward the CNS. Motor neurons carry messages out to muscles and glands. The spinal cord is not just a cable, either. It can run simple reflexes on its own. That is why your hand jerks off a hot stove before your brain even notices the pain.

Somatic and autonomic

The peripheral system splits again. This time the split is about whether you control the action. The somatic nervous system handles what you do on purpose. It carries sense information in. It also sends commands out to the skeletal muscles you control, as when you lift a cup, type, or walk. The autonomic nervous system runs the background jobs you never think about. These include heartbeat, digestion, breathing rate, pupil size, and sweating. The words "autonomic" and "automatic" sound alike for a reason. This system runs the body on autopilot.

Sympathetic and parasympathetic

The autonomic system has two branches that work against each other, like a gas pedal and a brake. The sympathetic nervous system is the gas pedal. It gets the body ready for action. This is the famous fight-or-flight response. When you face a threat, it speeds up your heart and widens your pupils. It sends sugar into your blood and shifts resources to your muscles, all in an instant.

The parasympathetic nervous system is the brake. It is sometimes called rest-and-digest. Once the danger passes, it slows the heart and restarts digestion. It brings the body back to a calm baseline. Both branches are almost always working at some level. Health depends on the balance between them. Trouble starts when the sympathetic gas pedal is held down too long, as happens in long-term stress.

Organ	Sympathetic (“fight-or-flight”)	Parasympathetic (“rest-and-digest”)
Heart	Speeds up	Slows down
Pupils	Dilate (widen)	Constrict (narrow)
Digestion	Slowed	Stimulated
Lungs	Airways widen	Airways narrow
Adrenal glands	Release stress hormones	No direct role
Overall effect	Spend energy for action	Save energy, restore calm

The pounding heart before a test is your sympathetic system doing its job. The racing pulse, dry mouth, and shaky hands of stage fright are the fight-or-flight response. Here it is firing at a threat that is not physical. It evolved to help you outrun a predator. It cannot tell a lion from an audience. Knowing that the reaction is automatic, and not a flaw in your character, is often the first step in managing it.

Interactive Explore the branching tree of the nervous system, then test yourself by sorting everyday moments into the branch that handles them. Open full ↑

Key points

The CNS (brain plus spinal cord) combines information. The PNS carries signals to and from the body.
The somatic system controls the skeletal muscles you move on purpose. The autonomic system runs organs automatically.
The sympathetic branch powers fight-or-flight. The parasympathetic branch brings back rest-and-digest.
Health is the balance between the two. Keeping the sympathetic branch switched on for weeks is the biology of chronic stress.

Section 4The brain

Side view of the brain with the frontal, parietal, temporal, and occipital lobes shaded in different colors — **Reference.** The four lobes of the cerebral cortex. Each one is specialized for different jobs. OpenStax Anatomy & Physiology, CC BY 4.0 (via Wikimedia Commons)

Hindbrain, midbrain, forebrain

One useful way to read the brain is from the bottom up, with the oldest parts first. The hindbrain sits where the spinal cord enters the skull. It runs the machinery of staying alive. It includes three parts. The medulla controls heartbeat and breathing. The pons is a relay station that helps with sleep and arousal. The cerebellum, which means "little brain," coordinates balance, posture, and smooth, practiced movement.

The small midbrain sits just above the hindbrain. It helps you turn toward sudden sights and sounds. It also helps control movement and alertness. Together, the midbrain, pons, and medulla make up the brainstem, the brain's survival core. The cerebellum is part of the hindbrain but is not part of the brainstem. It attaches behind it.

The forebrain is the newest and largest region. It is where most of psychology lives. Deep inside it are several parts we will meet again and again. The thalamus is a relay station for the senses. It routes incoming signals to the right places. The hypothalamus controls hunger, thirst, body temperature, and the endocrine system. The limbic system includes the amygdala, which handles emotion, especially fear. It also includes the hippocampus, which helps form new memories. Draped over all of this is the wrinkled outer layer that makes us most human: the cerebral cortex.

The four cortical lobes

The cerebral cortex is the thin, folded sheet of neurons that covers the brain. It is split into two halves, called hemispheres. Each half is divided into four lobes, and each lobe has typical jobs.

The frontal lobe sits at the front. It handles planning, judgment, personality, and voluntary movement. Its back strip is the motor cortex, which sends out movement commands. It also holds Broca's area, which is needed to produce speech.
The parietal lobe sits just behind it. It handles touch, temperature, and body position. It does this along a strip called the somatosensory cortex.
The temporal lobe sits at the side, near your ears. It handles hearing and understanding language. It is home to Wernicke's area, and it helps with memory.
The occipital lobe sits at the very back. It is almost all about vision.

Lobe	Location	Main jobs
Frontal	Front	Planning, judgment, personality, voluntary movement (motor cortex), speech production (Broca's area)
Parietal	Top, toward the back	Touch, temperature, pain, body position (somatosensory cortex)
Temporal	Sides	Hearing, understanding language (Wernicke's area), memory
Occipital	Back	Vision

Two hemispheres, one brain

The cortex shows lateralization. That means the two hemispheres are not exact copies. For most people, language is centered in the left hemisphere. The right side leans toward spatial tasks, like reading a map or judging a face. Each hemisphere also controls the opposite side of the body. The left motor cortex moves your right hand.

A thick bundle of fibers called the corpus callosum links the two halves. In the 1960s, some people with severe seizures had this bundle cut to stop the seizures from spreading. Roger Sperry and Michael Gazzaniga studied these split-brain patients. If a picture was flashed only to the left hemisphere, the person could name it. If it was flashed only to the right hemisphere, the person often could not say what it was. But the left hand, which the right hemisphere controls, could still pick it out by touch. Sperry shared a Nobel Prize in 1981 for this work.

These findings are real. But a popular myth grew out of them. It says people are "left-brained" (logical) or "right-brained" (creative). Brain scans do not support this. A large study of over a thousand people found no sign that some people use one side more than the other overall. In a healthy brain, the two halves work as a team on almost every task.

None of this wiring is fully fixed, either. The brain shows great plasticity. That means it can rewire itself. After an injury, or with practice, healthy regions can take over lost jobs. This is strongest in young people, but it lasts across life.

Phineas Gage lost part of his frontal lobe and, for a time, part of himself. In 1848, a blast drove an iron rod through the front of the railroad foreman's brain. He survived. He could still walk, talk, and reason. Yet the steady, responsible man became impulsive and rude. Friends said he was “no longer Gage.” The case became early evidence that the frontal lobe supports personality and self-control. Historians add an important footnote. Many later retellings made his changes sound worse and more lasting than the records show. In his final years he held a steady job driving a stagecoach in Chile, which suggests he regained much of his social skill. So the case also shows the brain's power to recover.

Interactive Tap the four cortical lobes and the deep structures beneath them to see what each region does, then solve short brain-injury cases. Open full ↑

Interactive Run a split-brain experiment: flash a picture to one visual field, choose how the person answers, and see which hemisphere can respond. Open full ↑

Key points

The hindbrain (medulla, pons, cerebellum) and the midbrain form the brain's survival core. The brainstem is that core minus the cerebellum.
The forebrain holds the thalamus, hypothalamus, limbic system (amygdala, hippocampus), and cortex.
The four lobes are frontal (planning, movement, speech), parietal (touch), temporal (hearing, language), and occipital (vision).
Broca's area, Wernicke's area, split-brain studies, and Phineas Gage each tie a job to certain brain tissue. "Left-brained" and "right-brained" personalities are a myth, and the brain stays plastic.

Section 5The endocrine system

Outline of the body showing the major glands that release hormones — **Reference.** The endocrine system: glands that release hormones into the blood to send signals to organs far away. OpenStax Anatomy & Physiology, CC BY 4.0 (via Wikimedia Commons)

Glands and hormones

The nervous system is not the body's only message service. The endocrine system is a network of glands. These glands release chemical messengers called hormones straight into the bloodstream. The blood then carries them to organs far away. A neuron speaks privately to the one cell across its synapse. A gland broadcasts to the whole body at once. Any cell with the right receptor will answer.

The "master gland" is the pituitary. It is tucked under the brain, and it takes orders from the hypothalamus. This is the place where the nervous system and the endocrine system shake hands. The pituitary, in turn, directs the other glands.

Gland	Key hormone(s)	Main effect
Pituitary	Growth hormone; hormones that signal other glands	The “master gland” directs other glands and controls growth
Thyroid	Thyroxine	Sets the body's energy use (metabolic rate)
Adrenal glands	Epinephrine (adrenaline), cortisol	Drive the stress response; arousal and energy
Pancreas	Insulin, glucagon	Keep blood sugar in balance
Gonads (ovaries and testes)	Estrogen, testosterone	Reproduction and sex characteristics
Pineal	Melatonin	Helps time sleep and the daily body clock

Hormones versus nerve signals

Both systems use chemical messengers. Sometimes they even use close cousins. Epinephrine (adrenaline) travels in the blood as a hormone. Its close relative, norepinephrine, works as a neurotransmitter in the brain. But the two systems run on very different time scales, and that difference is the point.

Nerve signals are fast, precise, and brief. An action potential fires in about a thousandth of a second. It aims at one cell, and it is over almost as soon as it starts. Hormone signals are slow, spread out, and long-lasting. A hormone may take seconds to minutes to reach its target. It washes over the whole body, and its effects can last for hours. Think of how long it takes to calm down after a fright, long after the danger is gone. That lingering quality is why hormones shape mood and long-running states, rather than split-second reactions.

Feature	Nerve signals	Hormone signals
Messenger	Neurotransmitter	Hormone
Route	Across a synapse to one cell	Through the bloodstream to the whole body
Speed	Milliseconds	Seconds to minutes
How long it lasts	Brief	Long-lasting
Precision	Targeted, private	Broadcast, spread out

Hans Selye showed that chronic stress wears the body down through hormones. Selye studied how animals responded to long-lasting stress. He described the general adaptation syndrome, which has three stages: alarm, resistance, and exhaustion. He put the adrenal glands at its center. The chain works like this. The hypothalamus signals the pituitary. The pituitary signals the adrenal glands. The adrenal glands pour out cortisol. A short burst protects you. But weeks of it weaken the immune system, disturb sleep, and wear down health. This is the endocrine reason that long-term stress can make people sick.

Interactive Tap each gland on the body to see the hormone it releases and what it does, then run the stress chain and watch cortisol rise and fall. Open full ↑

Key points

The endocrine system is a network of glands that release hormones into the bloodstream.
The hypothalamus and the pituitary gland link the nervous and endocrine systems and direct the other glands.
Compared with nerve signals, hormones are slower, more spread out, and longer-lasting. They are the chemistry of mood and stress.
Selye's work ties chronic stress and cortisol to real harm to health.

The chapter in one breath

The neuron takes in signals at its dendrites, adds them up in its cell body, and sends them down its axon. Myelin speeds the signal.
Crossing threshold sets off an all-or-none action potential. Na⁺ flows in to depolarize, and K⁺ flows out to repolarize.
Signals cross the synapse as neurotransmitters. They bind to receptors and are then cleared by reuptake or by enzymes.
Drugs act as agonists (boost a neurotransmitter) or antagonists (block it). SSRIs and antipsychotics work this way.
The nervous system splits into central vs peripheral, somatic vs autonomic, and sympathetic (fight-or-flight) vs parasympathetic (rest-and-digest).
The brain runs from the hindbrain and midbrain survival core up to the forebrain. Its cortex has four lobes: frontal, parietal, temporal, and occipital. The two hemispheres differ in some jobs but work as a team.
The endocrine system signals with hormones through the blood. Hormones are slower, more spread out, and longer-lasting than nerve signals, and they drive the stress response.

ReferenceWords worth knowing

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

Action potential: The quick, all-or-none electrical spike a neuron fires and sends down its axon.
Agonist: A drug or substance that increases a neurotransmitter's action. It may copy it, boost its release, or block its reuptake.
Antagonist: A drug or substance that blocks or reduces a neurotransmitter's action, often by sitting in its receptor.
Autonomic nervous system: The part of the peripheral system that runs automatic jobs like heartbeat and digestion. It has a sympathetic branch and a parasympathetic branch.
Axon: The one long fiber that carries a neuron's outgoing signal away from the cell body.
Central nervous system (CNS): The brain and spinal cord, where information is combined and decisions are made.
Cerebral cortex: The wrinkled outer layer of the brain. It has two hemispheres and four lobes and handles higher mental jobs.
Corpus callosum: The thick bundle of fibers that connects the left and right hemispheres.
Dendrite: A branching fiber that takes in signals and carries them toward the cell body.
Endocrine system: The network of glands that release hormones into the bloodstream to signal organs far away.
Glial cell: A helper cell of the nervous system that makes myelin, feeds neurons, and protects them.
Hormone: A chemical messenger that a gland releases into the bloodstream.
Lateralization: The way some jobs lean more on one hemisphere than the other, such as language on the left for most people.
Myelin sheath: The fatty coating around many axons that speeds the signal. Its gaps are the nodes of Ranvier.
Neuron: The basic cell of the nervous system that handles information.
Neurotransmitter: A chemical released across a synapse to carry a signal to the next neuron.
Parasympathetic nervous system: The autonomic branch that calms the body and restores it: “rest-and-digest.”
Peripheral nervous system (PNS): All the nerves outside the brain and spinal cord. They connect the CNS to the body.
Plasticity: The brain's ability to rewire itself after injury or with practice.
Resting potential: The neuron's charged, polarized resting state, about −70 mV inside compared with outside.
Reuptake: The sending neuron taking leftover neurotransmitter back in.
Somatic nervous system: The part of the peripheral system that carries sense information and controls the skeletal muscles you move on purpose.
Sympathetic nervous system: The autonomic branch that gets the body ready for action: “fight-or-flight.”
Synapse: The junction between two neurons, including the tiny gap (synaptic cleft) the signal crosses.
Threshold: The level of charge (about −55 mV) a neuron must reach to fire an action potential.

Check yourself

1. Follow an incoming signal through the three working parts of a neuron. Name each part and its job.

The dendrites take in the signal. The cell body (soma) adds up all the inputs to decide whether to fire. The axon carries the outgoing signal to the terminal buttons. Myelin on the axon speeds that outgoing signal along.

2. What is the resting potential, and what has to happen for a neuron to fire?

The resting potential is the neuron's charged, polarized state, about −70 mV inside. Incoming push-toward-firing signals must raise the voltage past threshold, about −55 mV. Only then does an action potential fire.

3. What does “all-or-none” mean? How does the brain then signal how strong a stimulus is?

All-or-none means a neuron fires a full action potential or none at all. There is no half spike. Since the spike size never changes, the brain signals a stronger stimulus by firing more neurons and by firing them more often.

4. Why does damage to myelin, as in multiple sclerosis, harm how the body works?

Myelin allows fast saltatory conduction, where the action potential jumps between nodes of Ranvier. Strip the myelin away and signals slow, scatter, or fail. That causes numbness, weakness, and vision problems.

5. A signal reaches the end of an axon. Describe how it crosses to the next neuron and how it is then switched off.

Vesicles release neurotransmitter into the synaptic cleft. The molecules cross and bind to receptors on the next neuron. The signal is cleared by enzymes that break the neurotransmitter apart, or by reuptake back into the sending neuron.

6. Match each to its main role: dopamine, serotonin, GABA, endorphins.

Dopamine: reward, motivation, and movement. Serotonin: mood, sleep, and appetite. GABA: the main inhibitory (calming) messenger. Endorphins: natural pain relief and pleasure. Serotonin is linked to depression, though not as a simple shortage. Loss of dopamine cells causes Parkinson's.

7. Define agonist and antagonist. Which one is an SSRI antidepressant, and why?

An agonist increases a neurotransmitter's action. An antagonist blocks or reduces it. An SSRI is a serotonin agonist. It blocks reuptake, so serotonin stays in the synapse longer and its effect grows.

8. Tell the central and peripheral nervous systems apart. Then tell the somatic and autonomic parts apart.

The CNS is the brain and spinal cord, where information is combined and decisions are made. The PNS is all the nerves that connect the CNS to the body. Inside the PNS, the somatic system controls the skeletal muscles you move on purpose. The autonomic system runs the organs automatically.

9. A car swerves toward you and your heart pounds. Name the system in charge and three things it does. Then name what brings you back to calm.

The sympathetic nervous system drives fight-or-flight. It speeds the heart, widens the pupils, and slows digestion. It also releases sugar and stress hormones. The parasympathetic system (“rest-and-digest”) then slows the heart and restores the baseline.

10. Name the three main parts of the brain from bottom to top. Give one structure or job in each.

Hindbrain (the medulla for heartbeat and breathing, or the cerebellum for balance). Midbrain (turning toward sights and sounds, and alertness). Forebrain (thalamus, hypothalamus, limbic system, and cortex). The brainstem is the midbrain, pons, and medulla. The cerebellum is left out.

11. Name the four cortical lobes and the main job of each.

Frontal: planning, judgment, movement, and speech production (Broca's area). Parietal: touch and body position. Temporal: hearing and understanding language (Wernicke's area). Occipital: vision. The case of Phineas Gage tied the frontal lobe to personality and self-control.

12. Give two ways hormone signals differ from nerve signals. Why are hormones better suited to mood and stress?

Hormones travel through the blood to the whole body, and they act slowly and for a long time. Nerve signals are fast, brief, and aimed at one cell. Because hormone effects linger, they set long-running states like mood. Weeks of high cortisol, as Selye showed, wear the body down.

13. In a split-brain patient, a picture of a key is flashed only to the right hemisphere. What can the person do, and what can they not do?

The person usually cannot say "key," because speech is centered in the left hemisphere for most people. But the left hand, which the right hemisphere controls, can pick out the key by touch. With the corpus callosum cut, the two halves cannot share what they saw.

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