Week 3 · Chapter

Chapter 3 — Biological Bases of Behavior

Introduction to Psychology Free sample edition

The module's primary reading · ~30–40 minutes · Objective 3 (describe the biological bases of behavior: neurons and neural transmission, the major neurotransmitters, and the structures and functions of the nervous system and brain) · Read it first this week; the lecture, the tutorial, and the quiz all build on it.

In plain language

Everything you do, from blinking to deciding what to eat, runs on messages sent by your body's nerve cells. This week follows one of those messages from start to finish. You will meet the neuron (a nerve cell that carries messages) and see how it fires. You will see how a message jumps from one cell to the next with a neurotransmitter (a chemical messenger). Then you will zoom out to the whole nervous system and take a tour of the brain, one structure at a time.

Why you already care about this

You reach for a pan on the stove. The handle is hot. Your hand jerks back before you have time to think "ouch." A second later your heart is pounding, your fingertip stings, and you are already turning toward the sink for cold water.

That whole scene took about as long as a sneeze. Yet it involved a long chain of cells, a trip through your spinal cord, an alarm in your chest, and a new memory that will make you reach for a potholder next time. This chapter asks a simple question about that moment: where did that behavior live? The answer takes us from a single cell to the whole brain.

You will also keep using the habit you built last week. When a headline says a brain chemical "causes" a mood, you will know which question to ask.

By the end of this chapter you will be able to:

trace one message through the parts of a neuron;
explain why a stronger stimulus changes how often a neuron fires but not how big each impulse is;
describe how a synapse passes a message on and how reuptake clears it;
match six neurotransmitters to what each is associated with;
sort the nervous system into its divisions;
match brain structures and lobes to their jobs;
explain what the case of H.M. shows and why the "10 percent" claim is a myth.

Carry this sentence with you: a stronger push changes the rate — not the size.

Where we are

Already covered: Week 1 defined psychology as the science of behavior and mental processes and introduced the biological perspective. Week 2 built the research toolkit, from case studies to experiments, and the rule that correlation is not causation.
This week: the biological perspective up close: the neuron, the neural impulse, the synapse and its chemical messengers, the divisions of the nervous system, and the major structures of the brain.
How they connect: Week 1 told you the biological perspective looks at the brain and body; this week shows you what it actually looks at. Week 2's design questions keep this week's biology honest.
Where we're going next: Week 4, Sensation & Perception, follows the messages your senses send into this same nervous system and asks how the brain turns them into a world you experience.

Two ideas from Week 2 matter most here. First, a case study is a descriptive method that examines one person or a small group in great depth; it can suggest ideas to test but cannot show cause or be assumed to fit everyone. Second, a correlation tells you that two measured things move together, not that one causes the other. You will need both before this chapter ends.

1. Meet the neuron: the cell that carries the message

Think of a long line of people passing a note across a crowded stadium. Each person takes the note from the person behind, decides whether it is worth passing on, and hands it forward. Your nervous system works in a similar way, except the people are cells.

A neuron is a nerve cell: the basic unit of the nervous system, which receives, combines, and sends messages to other cells. Every neuron has the same basic parts, and each part has one job.

Dendrites are the branching fibers that receive incoming messages and carry them toward the cell body.
The soma is the cell body. It adds up the incoming signals and "decides" whether the neuron will fire.
The axon is the long fiber that carries the neuron's impulse away from the soma.
The myelin sheath is a fatty layer wrapped around many axons. It insulates the axon and makes the impulse travel faster.
The terminal buttons are the knob-like endings of the axon's branches. They release chemical messengers to the next cell.

A quick way to hold the order: the dendrites receive, the soma combines, the axon carries, the myelin speeds, and the terminal buttons release.

Worked example — the hot-stove signal trace, part 1. Follow the message from your fingertip.

Nerve endings at the dendrites of a sensory neuron in your fingertip respond to the heat of the pan handle.
The soma adds up the incoming signals from those dendrites.
The combined signal is strong enough, so the neuron fires, and the impulse travels down the axon.
The myelin sheath around the axon speeds the impulse along.
The impulse reaches the terminal buttons, which get ready to pass the message on.

In words: the message enters at the dendrites, gets added up in the soma, and leaves along the axon, faster because of the myelin.

Misconception check. Students often think a message can flow either way through a neuron, like water in a pipe. It cannot. Inside one neuron the message runs one way: in at the dendrites, out at the terminal buttons. If you mix up the two ends, every later step of the trace goes wrong.

2. Follow the impulse: all-or-none firing

Picture the starter's pistol at a track meet. The official either pulls the trigger or does not. There is no half-bang for a nervous runner and no extra-loud bang for a fast one. A neuron fires in the same way.

Before anything happens, a neuron sits at its resting potential: the electrical charge across its membrane when it is not firing, about −70 mV, with the inside negative compared with the outside. The unit, mV, is a millivolt: one thousandth of a volt. The minus sign tells you the inside of the cell is negative.

Incoming signals nudge that charge. If they push it far enough, the neuron reaches its threshold: the level of stimulation it must reach before it fires. Input below threshold produces no impulse at all.

At threshold the neuron fires an action potential: a brief electrical impulse that travels down the axon when a neuron fires. It is all-or-none, so it either happens at full strength or does not happen. Right after each action potential comes a refractory period: a brief recovery time during which the neuron cannot fire again.

Worked example — the hot-stove signal trace, part 2. Compare a warm handle with a very hot one.

At rest, before you touch the pan, the sensory neuron in your fingertip sits at about −70 mV.
A warm handle pushes the neuron past threshold, so it fires action potentials.
A much hotter handle also pushes it past threshold. Each action potential is the same size as before.
What changes is the rate: the hotter handle makes the neuron fire more impulses each second, with a refractory period after each one.

In words: the brain learns "this is very hot" from how often the neuron fires, not from bigger impulses.

Misconception check. Students often think a stronger stimulus makes a bigger action potential. It is tempting, because a harder push feels like it should make a bigger signal. But the action potential is all-or-none. A stronger stimulus changes the rate of firing, not the size of each impulse. Remember the carry line: a stronger push changes the rate — not the size.

Pause and reflect — choose one. Pick either prompt; they count the same.

(a) Describe a device or event in your own life that is all-or-none, like the starter's pistol, and explain how it matches an action potential.
(b) Describe a time a sensation got stronger, such as a sound getting louder, and explain in your own words how your neurons could signal "stronger" without making bigger impulses.

3. Cross the synapse: chemical messengers and reuptake

Here is a surprise: neurons do not touch. Between the terminal button of one neuron and the next cell is a tiny gap. The message has to cross that gap as chemicals, a bit like a note tossed across a narrow stream rather than handed over.

A synapse is the junction between the sending neuron's terminal button and the receiving cell, including the tiny gap the message must cross chemically. A neurotransmitter is a chemical messenger released by a neuron's terminal buttons that crosses the synapse and affects whether the receiving cell fires. When the job is done, reuptake cleans up: the sending neuron reabsorbs leftover neurotransmitter molecules, which clears the synapse for the next message.

Worked example — the hot-stove signal trace, part 3. Cross the gap.

The impulse reaches the terminal buttons of the sensory neuron.
The terminal buttons release neurotransmitter molecules into the synapse.
The molecules cross the gap and affect the next neuron, which may fire in turn.
Reuptake pulls the leftover molecules back into the sending neuron, so the gap is clear for the next message.

In words: electricity carries the message inside a neuron, and chemistry carries it between neurons.

Different neurotransmitters are linked to different jobs. The table below lists the six this course uses and what each one is associated with.

Six major neurotransmitters and the functions each is associated with
Neurotransmitter	Associated with
Acetylcholine	muscle action, attention, and memory
Dopamine	reward, motivation, learning, and movement
Serotonin	mood, sleep, and appetite
GABA	calming; the main inhibitory transmitter, which makes neurons less likely to fire
Glutamate	the main excitatory transmitter, which makes neurons more likely to fire; involved in learning and memory
Endorphins	pain relief and pleasure

Notice the wording in the right-hand column. Each chemical is associated with several functions. None of them is the single cause of a feeling or an action. Your hand jerking away from the pan, for example, depends on acetylcholine, the neurotransmitter associated with muscle action, at the junction where a motor neuron meets your arm muscle.

Misconception check. Students often swap dopamine and serotonin. A quick sort helps: dopamine goes with reward, motivation, and movement, the "go get it" chemical; serotonin goes with mood, sleep, and appetite. If you catch yourself linking serotonin to movement, check the table again.

4. Read a brain headline the Week 2 way

Imagine scrolling past an ad for a supplement. "Low mood? Low serotonin causes sadness. Our capsule boosts serotonin, so it makes you happy." It sounds scientific. It even names a real chemical. But last week gave you the questions that take this kind of claim apart.

The rule this course uses for biology is hedged language. Write "is associated with," "is involved in," or "helps regulate" for any chemical or structure. Never write that one chemical causes a mood.

Why the caution? Much of the evidence linking a chemical to a mood is correlational: researchers measured the chemical and measured the mood and found that they move together. Week 2 showed you what that can and cannot support. A third variable, such as long-term stress, poor sleep, or illness, might affect both. And the arrow might run the other way: a low mood might change brain chemistry rather than the reverse.

Worked example — rewriting a supplement ad. Fix the claim one step at a time.

Original claim: "Low serotonin causes sadness."
Name the problem: the word "causes" claims more than correlational evidence allows.
Hedged rewrite: "Serotonin is associated with mood, sleep, and appetite."
Name a third variable a careful reader would raise: long-term stress could affect both serotonin and mood.
Name what would be needed to claim a cause: an experiment that randomly assigns people to conditions and measures mood as the dependent variable.

In words: the honest version says serotonin is linked to mood, names something else that could explain the link, and admits that a cause needs a stronger design.

Misconception check. Students often hear that "serotonin causes depression." The evidence supports "associated with," not "causes." Depression is a health condition with biological, psychological, and social influences acting together, which is the biopsychosocial model from Week 1. One chemical is not the whole story.

Honesty note. The ad's capsule may or may not do anything. This chapter cannot tell you, and neither can the ad's wording. Only a well-designed study could, which is exactly the point.

5. Zoom out: the divisions of the nervous system

You are biking to class. A car door swings open in front of you. You swerve, your heart slams, your breath goes quick and shallow. Ten minutes later, sitting in a lecture hall, your heart rate drifts back down and you notice you are hungry. Two different systems ran those two moments.

The nervous system sorts into a few divisions, each split in two.

The central nervous system is the brain and the spinal cord, often shortened to CNS.
The peripheral nervous system is every nerve outside the brain and spinal cord, which links the CNS to the senses, muscles, and organs; it is often shortened to PNS.
The PNS splits in two. The somatic nervous system carries messages from the senses and controls voluntary movement of the skeletal muscles. The autonomic nervous system runs the internal organs and glands, such as the heart and digestion, mostly without conscious control.
The autonomic system splits again. The sympathetic nervous system mobilizes the body for action, speeding the heart and breathing: the fight-or-flight response. The parasympathetic nervous system calms the body and conserves energy after arousal: the rest-and-digest response.

Worked example — the hot-stove signal trace, part 4. Sort each moment into its division.

The message travels from your fingertip into the spinal cord. The spinal cord is part of the central nervous system (CNS).
In the spinal cord, the message passes to motor neurons of the somatic nervous system. They signal your arm muscles with acetylcholine, and your hand jerks away.
Your heart speeds up. That is the sympathetic division of the autonomic nervous system (fight-or-flight).
A few minutes later your heart slows again. That is the parasympathetic division (rest-and-digest).

In words: your spinal cord and somatic nerves moved your hand, and the two halves of your autonomic system first sped up and then calmed your body.

Misconception check. Students often swap sympathetic and parasympathetic, because "sympathetic" sounds gentle. Here is a hook that works: the sympathetic system is for stress, getting you ready to fight or flee; the parasympathetic system is for peace, calming you down to rest and digest.

Pause and reflect — choose one. Pick either prompt; they count the same.

(a) Describe a moment of sudden alarm you have lived through, such as a near-miss in traffic, and label which division acted first and which one acted afterward.
(b) Describe a calm daily routine, such as a slow meal, and explain which division is most active during it and what it is doing for your body.

6. Tour the brain: structures and lobes

Picture a basketball player at the free-throw line. The arena is loud. Someone in the stands shouts her name, and she hears it through the noise. She bends her knees, sets her elbow, and releases the ball in one smooth motion she has practiced a thousand times. Many brain structures share that moment, and each has its own job.

Start low in the brain and work up.

The medulla, at the base of the brainstem, helps control breathing and heart rate. It keeps the player breathing without her thinking about it.
The cerebellum, at the back of the brain below the lobes, coordinates movement and balance and supports learning motor skills. Her smooth, practiced shot leans on it.
The thalamus is the brain's relay station. It receives incoming sensory messages, except smell, and routes them to the cortex.
The hypothalamus, a small structure below the thalamus, helps regulate hunger, thirst, body temperature, and the release of hormones. It is involved in her thirst after a long game.
The amygdala, deep in each temporal lobe, is central to fear and strengthens memories that carry emotional weight.
The hippocampus, also deep in each temporal lobe, is needed to form new declarative memories, the facts and events a person can consciously recall.

The outer layer of each half of the brain is divided into four lobes.

The frontal lobe, at the front, is involved in planning, decision-making, judgment, and the control of voluntary movement.
The parietal lobe, at the top and middle, is involved in processing touch and the body's position in space.
The temporal lobe, above the ears, is involved in hearing, understanding language, and memory. Hearing her name across the arena relies on it.
The occipital lobe, at the back, processes visual information. Seeing the rim relies on it.

Worked example — the hot-stove signal trace, part 5. Finish the trace in the brain.

The thalamus relays the message upward.
The parietal lobe registers touch and where on your body it happened.
The frontal lobe plans your next move: run cold water.
The amygdala flags the fear.
The cerebellum coordinates the quick movement.
The hippocampus helps store the episode as a new memory: "use a potholder."

In words: one sharp moment in the kitchen called on at least six parts of the brain, each doing its own job.

Misconception check. Students often think each structure does exactly one thing, like a single switch. The list above names each structure's best-known jobs, but the brain works as a network. Structures are "involved in" functions together; no one structure is the whole cause of a behavior.

7. Ask "how do we know?": the case of H.M.

How could anyone learn what the hippocampus does? For much of the brain, some of the earliest evidence came from people whose brains were damaged by illness, injury, or surgery.

A man known in research as H.M. had brain surgery to control severe seizures. Surgeons removed much of the medial temporal lobe on both sides, including much of both hippocampi. Scoville and Milner (1957) reported what followed. H.M. could no longer form new declarative memories. He kept most of his memories from before the operation, and he could still learn new motor skills.

Worked example — reading the H.M. case with Week 2's questions. Ask four questions of the case.

What design is it? A case study: one person examined in great depth.
What does it suggest? That the hippocampus is needed to form new declarative memories, while older memories and motor learning rely on other systems.
What can it not claim? On its own, one case cannot show that the result fits everyone, and his surgery removed more than the hippocampus alone.
What makes it convincing? Its findings pointed researchers toward ideas that later studies could test.

In words: H.M.'s case linked a brain structure to a kind of memory, and a careful reader treats it as a strong clue rather than the final word. You will meet H.M. again in Week 7, when memory gets its own week.

Misconception check. Students often assume that a vivid, famous case proves a general rule. It does not. A case study can suggest ideas to test but cannot show cause or be assumed to fit everyone. H.M. mattered because later research kept checking what his case suggested.

Honesty note. This chapter tells the H.M. story in only a few sentences. The full history is longer, and a single case always leaves questions that only further studies can answer.

8. Rewire for life: neuroplasticity and the 10 percent myth

Think of someone who picks up the guitar in their forties. At first, every chord feels clumsy. Months later, their fingers find the frets without looking. Something in their brain changed.

Neuroplasticity is the brain's ability to change its connections and organization in response to experience, practice, and injury, throughout life. Practice strengthens some connections, and after an injury other areas can sometimes take over part of a lost job. Your brain at the end of this course will not be wired exactly as it was at the start.

Now a popular claim: "We use only 10 percent of our brains." Brain-training apps sometimes promise to "unlock the other 90 percent." The claim is a myth. Every structure in this chapter has known jobs. Damage to almost any part of the brain affects some ability, which would not happen if most of the brain sat unused. Over a normal day, you use all of it.

Worked example — checking a brain-training ad. Test the claim against this chapter.

The ad says most of your brain is idle until you buy the app.
Check it against the tour in section 6: the medulla, cerebellum, thalamus, hypothalamus, amygdala, hippocampus, and all four lobes have known jobs.
Check it against section 7: losing one structure, as H.M. did, removed a major ability, so that structure was in use.
Conclusion: the "10 percent" premise is false. Any real benefit of the app would have to be shown by a study, not by the myth.

In words: practice can change your brain, but not because most of it was asleep.

Misconception check. Students sometimes hear neuroplasticity and conclude that the 10 percent myth must be true, since the brain can "grow into" unused space. The two ideas point the opposite way. Neuroplasticity is about changing connections in a brain that is already fully in use.

Key terms

neuron — A nerve cell: the basic unit of the nervous system, which receives, combines, and sends messages to other cells. (first used: 1. Meet the neuron: the cell that carries the message)
dendrites — The branching fibers of a neuron that receive incoming messages from other cells and carry them toward the cell body. (first used: 1. Meet the neuron: the cell that carries the message)
soma — The cell body of a neuron, which holds the nucleus and adds up the incoming signals to decide whether the neuron will fire. (first used: 1. Meet the neuron: the cell that carries the message)
axon — The long fiber that carries a neuron's electrical impulse away from the cell body toward the terminal buttons. (first used: 1. Meet the neuron: the cell that carries the message)
myelin sheath — A fatty layer wrapped around many axons that insulates them and makes the impulse travel faster. (first used: 1. Meet the neuron: the cell that carries the message)
terminal buttons — The knob-like endings of an axon's branches, which release neurotransmitter molecules into the synapse when an impulse arrives. (first used: 1. Meet the neuron: the cell that carries the message)
resting potential — The electrical charge across a neuron's membrane when the neuron is not firing, about 70 millivolts below zero, with the inside negative compared with the outside. (first used: 2. Follow the impulse: all-or-none firing)
threshold — The level of stimulation a neuron must reach before it fires; input below that level produces no impulse. (first used: 2. Follow the impulse: all-or-none firing)
action potential — A brief electrical impulse that travels down the axon when a neuron fires; it is all-or-none, so it either happens at full strength or does not happen. (first used: 2. Follow the impulse: all-or-none firing)
refractory period — A brief recovery time right after an action potential during which the neuron cannot fire again. (first used: 2. Follow the impulse: all-or-none firing)
synapse — The junction between the sending neuron's terminal button and the receiving cell, including the tiny gap the message must cross chemically. (first used: 3. Cross the synapse: chemical messengers and reuptake)
neurotransmitter — A chemical messenger released by a neuron's terminal buttons that crosses the synapse and affects whether the receiving cell fires. (first used: 3. Cross the synapse: chemical messengers and reuptake)
reuptake — The sending neuron's reabsorption of leftover neurotransmitter molecules, which clears the synapse for the next message. (first used: 3. Cross the synapse: chemical messengers and reuptake)
acetylcholine — A neurotransmitter associated with muscle contraction, attention, and memory. (first used: 3. Cross the synapse: chemical messengers and reuptake)
central nervous system — The brain and the spinal cord, often shortened to CNS. (first used: 5. Zoom out: the divisions of the nervous system)
peripheral nervous system — Every nerve outside the brain and spinal cord, which links the central nervous system to the senses, muscles, and organs; often shortened to PNS. (first used: 5. Zoom out: the divisions of the nervous system)
somatic nervous system — The branch of the peripheral nervous system that carries messages from the senses and controls voluntary movement of the skeletal muscles. (first used: 5. Zoom out: the divisions of the nervous system)
autonomic nervous system — The branch of the peripheral nervous system that runs the internal organs and glands, such as the heart and digestion, mostly without conscious control. (first used: 5. Zoom out: the divisions of the nervous system)
sympathetic nervous system — The division of the autonomic nervous system that mobilizes the body for action, speeding the heart and breathing in the fight-or-flight response. (first used: 5. Zoom out: the divisions of the nervous system)
parasympathetic nervous system — The division of the autonomic nervous system that calms the body and conserves energy after arousal, the rest-and-digest response. (first used: 5. Zoom out: the divisions of the nervous system)
medulla — The structure at the base of the brainstem that helps control breathing and heart rate. (first used: 6. Tour the brain: structures and lobes)
cerebellum — The structure at the back of the brain, below the lobes, that coordinates movement and balance and supports learning motor skills. (first used: 6. Tour the brain: structures and lobes)
thalamus — The brain's relay station, which receives incoming sensory messages, except smell, and routes them to the cortex. (first used: 6. Tour the brain: structures and lobes)
hypothalamus — A small structure below the thalamus that helps regulate hunger, thirst, body temperature, and the release of hormones. (first used: 6. Tour the brain: structures and lobes)
amygdala — A structure deep in each temporal lobe that is central to fear and that strengthens memories carrying emotional weight. (first used: 6. Tour the brain: structures and lobes)
hippocampus — A structure deep in each temporal lobe that is needed to form new declarative memories, the facts and events a person can consciously recall. (first used: 6. Tour the brain: structures and lobes)
frontal lobe — The front section of each brain half, involved in planning, decision-making, judgment, and the control of voluntary movement. (first used: 6. Tour the brain: structures and lobes)
parietal lobe — The top-middle section of each brain half, involved in processing touch and the body's position in space. (first used: 6. Tour the brain: structures and lobes)
temporal lobe — The section of each brain half above the ears, involved in hearing, understanding language, and memory. (first used: 6. Tour the brain: structures and lobes)
occipital lobe — The rear section of each brain half, which processes visual information. (first used: 6. Tour the brain: structures and lobes)
neuroplasticity — The brain's ability to change its connections and organization in response to experience, practice, and injury, throughout life. (first used: 8. Rewire for life: neuroplasticity and the 10 percent myth)

Summary

A neuron's dendrites receive, the soma combines, the axon carries, the myelin sheath speeds, and the terminal buttons release.
A resting neuron sits at about 70 millivolts below zero, inside negative.
The action potential is all-or-none; a stronger stimulus raises the firing rate, not the impulse size.
Neurotransmitters cross the synapse, and reuptake clears it.
Each neurotransmitter is associated with functions; none is the single cause of a mood.
The CNS is the brain and spinal cord; the PNS is everything else.
Sympathetic means fight-or-flight; parasympathetic means rest-and-digest.
Each brain structure and lobe has known jobs, and the brain works as a network.
H.M.'s case study linked the hippocampus to forming new declarative memories.
Neuroplasticity is real; the 10 percent claim is a myth.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

At the beach, you step barefoot on a sharp shell and lift your foot. Trace the message through the five parts of the sensory neuron in your sole, in order.
A friend says, "A gentle tap on your arm sends a small action potential, and a hard slap sends a big one." What is wrong with that sentence, and how would you fix it?
A fitness app's ad says, "Our streak badges raise your dopamine, so they cause motivation." Rewrite the claim with hedged language, name one third variable, and say how the causal arrow could run the other way.
During a job interview your mouth goes dry and your heart pounds. Over lunch an hour later you feel calm and hungry. Name the division active in each moment and the larger branch both belong to.

Answers (worked reasoning — read after attempting)

Dendrites, soma, axon, myelin sheath, terminal buttons. The dendrites receive the signal from the sole of your foot, and the soma adds up the incoming signals. The axon carries the impulse away, the myelin sheath speeds it along, and the terminal buttons release neurotransmitter to the next cell. The order matters because inside one neuron the message runs one way.
The action potential is all-or-none, so a gentle tap and a hard slap make impulses of the same size once threshold is reached. The fix: "A hard slap makes my neurons fire more impulses each second than a gentle tap does." The difference is in the rate, not the size. This item is built to catch the "bigger stimulus, bigger impulse" misconception.
A hedged rewrite: "Dopamine is associated with reward and motivation, and people who use the badges may feel more motivated." A third variable: a new fitness goal could make a person both use the app more and feel more motivated. The arrow could also run the other way: people who were already motivated may be the ones who keep long streaks, so motivation could drive the badges rather than the badges driving motivation. That is the directionality problem. Only an experiment that randomly assigns people to use badges or not could support a cause.
In the interview, the sympathetic nervous system is active: fight-or-flight dries your mouth and speeds your heart. At lunch, the parasympathetic nervous system is active: rest-and-digest slows your heart and supports digestion. Both are divisions of the autonomic nervous system, which runs the organs and glands mostly without conscious control.

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, and the quiz — and next week, Sensation & Perception follows how your senses send their signals into this same nervous system.

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