Lecture: Biological Bases of Behavior
Lecture script · about 15–20 minutes spoken · Week 3
Opening
You reach for a pan on the stove, and the handle is hot. Your hand jerks back before you've had time to think a single word. A second later your heart is pounding and your fingertip stings, and you're already turning toward the sink. I want to ask a simple-sounding question about that moment: where did that behavior live? By the end of this lecture, you'll be able to follow that one message from your fingertip to your muscles and up into your brain, station by station.
In Week 1, you met psychology as the science of behavior and mental processes, and you met the biological perspective, the angle that explains behavior through the brain and body. In Week 2, you built a research toolkit. You learned what a case study can and can't tell us, and you learned that a correlation is not a cause. This week, we take the biological perspective up close, and we keep that Week 2 toolkit in our pocket, because brain claims are some of the easiest claims to overstate. Next week, in Week 4, we'll follow the messages your senses send into this same nervous system.
Here's the plan. We'll meet the cell that carries the message, watch it fire, and watch the message cross to the next cell. Then we'll read a brain headline carefully, zoom out to the nervous system, tour the brain, and meet a famous case.
1. The neuron: the cell that carries the message
Picture a long line of people passing a note across a crowded stadium. Each person takes the note from behind, decides whether it's worth passing on, and hands it forward. Your nervous system works in a similar way, except the people are cells.
That cell is a neuron: 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. The dendrites are branching fibers that receive incoming messages and carry them toward the cell body. The soma is the cell body, and it adds up the incoming signals to decide whether the neuron will fire. The axon is the long fiber that carries the impulse away from the cell body. Many axons are wrapped in a myelin sheath, a fatty layer that insulates them and makes the impulse travel faster. At the far end are the terminal buttons, the knob-like endings that release chemical messengers to the next cell.
Here's a short way to hold that order. The dendrites receive, the soma combines, the axon carries, the myelin speeds, and the terminal buttons release.
Now put it back on the stove. Nerve endings at the dendrites of a sensory neuron in your fingertip respond to the heat. The soma adds up those signals. The impulse races down the axon, sped along by the myelin, to the terminal buttons. Inside one neuron, the message runs one way only, from dendrites to terminal buttons.
Pause and try this. A message has reached the soma of your fingertip neuron. Which part carries it next, and which part speeds it along?
Say your answer out loud before I go on.
Here's the answer. The axon carries it next, and the myelin sheath speeds it along. The soma has done its adding up, so the impulse leaves along the axon, and the myelin wrapped around that axon is what makes the trip fast.
2. The impulse: all or nothing
Think about the starter's pistol at a track meet. The official either pulls the trigger or doesn't. There's no half-bang for a nervous runner and no extra-loud bang for a fast one. A neuron fires in that same spirit.
Before anything happens, a neuron sits at its resting potential: the electrical charge across its membrane when it isn't firing. It's about minus 70 millivolts, which means the inside of the cell is negative compared with the outside. 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. Below threshold, nothing happens at all.
At threshold, the neuron fires an action potential: a brief electrical impulse that travels down the axon. It's all-or-none. It happens at full strength, or it doesn't happen. Right after each one comes a refractory period, a brief recovery time when the neuron can't fire again.
So how does your brain tell a warm handle from a scorching one? Not by size. A hotter handle makes the neuron fire more impulses each second, with a refractory period after each one. Every impulse is the same size. Here's a sentence I'd like you to carry all through this course: a stronger push changes the rate, not the size.
Pause and try this. A friend says a hard slap on the arm sends a bigger action potential than a gentle tap. What would you tell them?
Think about the starter's pistol, then decide.
Here's the answer. Once threshold is reached, both make impulses of the same size, because an action potential is all-or-none. What the hard slap changes is the rate: the neurons fire more impulses each second. That's how a stronger touch reaches your brain as "stronger."
3. The synapse: a chemical handoff
Here's something that surprises most people. Neurons don't touch. Between the terminal button of one neuron and the next cell, there's a tiny gap. The message has to cross that gap as chemicals, a bit like tossing a note across a narrow stream instead of handing it over.
That junction is the synapse: the meeting point between the sending neuron's terminal button and the receiving cell, including the tiny gap the message must cross. The chemical that crosses is a neurotransmitter, a chemical messenger released by the terminal buttons that affects whether the receiving cell fires. When the job's done, there's cleanup. In reuptake, the sending neuron reabsorbs the leftover neurotransmitter molecules, which clears the synapse for the next message.
Different neurotransmitters are linked to different jobs, and the wording matters. Acetylcholine is associated with muscle action, attention, and memory, and it's the one at work when a motor neuron signals your arm to pull away from the pan. Dopamine is associated with reward, motivation, learning, and movement. Serotonin is associated with mood, sleep, and appetite. GABA is the main inhibitory transmitter, the calming one that makes neurons less likely to fire. Glutamate is the main excitatory one, which makes neurons more likely to fire and is involved in learning and memory. And endorphins are associated with pain relief and pleasure.
People often mix up dopamine and serotonin. Dopamine goes with reward and movement; serotonin goes with mood and sleep.
Pause and try this. After the neurotransmitter crosses the gap and does its job, what clears the synapse so the next message can get through?
Give it a moment before I answer.
Here's the answer. Reuptake clears it. The sending neuron pulls the leftover molecules back in, so the gap is clean and ready. Electricity carries the message inside a neuron, and chemistry carries it between neurons.
4. Reading a brain headline the careful way
Imagine an ad for a supplement. It says low serotonin causes sadness, so its capsule, which boosts serotonin, will make you happy. It sounds scientific. It even names a real chemical. But Week 2 gave you the questions that take this claim apart.
This course writes biology with hedged language. We say a chemical "is associated with," "is involved in," or "helps regulate" a function. We never say one chemical causes a mood. Why so careful? A lot of the evidence linking a chemical to a mood is correlational. Researchers measured the chemical, measured the mood, and found that the two move together. And you know what a correlation can't do. A third variable, like ongoing stress, poor sleep, or illness, might affect both. Or the arrow might run backward, so that a low mood changes brain chemistry rather than the reverse.
You've probably heard the line that serotonin causes depression. The evidence supports "associated with," not "causes." Depression is a health condition with biological, psychological, and social influences acting together. That's the biopsychosocial model you met in Week 1. One chemical isn't the whole story.
So what would it take to show a cause? You'd need an experiment. People would be randomly assigned to conditions, and mood would be measured as the dependent variable. Without that kind of design, the honest sentence stays at "associated with."
Pause and try this. Rewrite the ad's claim, "low serotonin causes sadness," as one honest sentence.
Try your own wording first.
Here's the answer. Something like "serotonin is associated with mood, sleep, and appetite" works. It keeps the real link and drops the word "causes," because correlational evidence can't carry a cause, and a third variable such as stress could explain the link.
5. The nervous system: two big divisions, then two more
Picture yourself biking to class when a car door swings open in front of you. You swerve, your heart slams, your breathing goes quick. A little later, sitting in a lecture hall, your heart slows down and you notice you're hungry. Two different systems ran those two moments.
The nervous system sorts into divisions, and each one splits in two. The central nervous system, or CNS, is the brain and the spinal cord. The peripheral nervous system, or PNS, is every nerve outside the brain and spinal cord, linking the CNS to the senses, muscles, and organs.
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, like your heart and your digestion, mostly without conscious control.
Then the autonomic system splits again. The sympathetic nervous system mobilizes the body for action. It speeds the heart and breathing, and we call that fight-or-flight. The parasympathetic nervous system calms the body and conserves energy afterward, and we call that rest-and-digest. Here's a hook. Sympathetic starts with S, for stress. Parasympathetic starts with P, for peace.
Back to the stove. The message from your fingertip enters the spinal cord, which is part of the CNS. There it passes to motor neurons of the somatic system, which signal your arm muscles with acetylcholine, and your hand jerks away. Your heart speeds up, which is the sympathetic division. A few minutes later it slows again, which is the parasympathetic division.
Pause and try this. In the bike story, which division swerved your heart into high gear, and which one calmed it down in the lecture hall?
Use the S-and-P hook.
Here's the answer. The sympathetic division sped you up for fight-or-flight, and the parasympathetic division calmed you down to rest and digest. Both belong to the autonomic nervous system, which is why you didn't have to decide either one.
6. A tour of the brain
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 and releases the ball in one smooth motion she's practiced many times. Lots of brain structures share that moment, and each one has its own job.
Let's start low and work up. The medulla, at the base of the brainstem, helps control breathing and heart rate. The cerebellum, at the back of the brain below the lobes, coordinates movement and balance and supports learning motor skills, so her practiced shot leans on it. The thalamus is the brain's relay station: it receives incoming sensory messages, except smell, and routes them onward. The hypothalamus, below the thalamus, helps regulate hunger, thirst, body temperature, and hormones. Deep inside, the amygdala is central to fear, and the hippocampus is needed to form new declarative memories, the facts and events you can consciously recall.
The outer layer of each half of the brain has four lobes. The frontal lobe, at the front, is involved in planning, decisions, and voluntary movement. The parietal lobe, at the top, is involved in touch and the body's position in space. The temporal lobe, above the ears, is involved in hearing, language, and memory, so hearing her name relies on it. The occipital lobe, at the back, processes vision.
Let's finish the stove story. The thalamus relays the message upward. The parietal lobe registers the touch and where it happened. The frontal lobe plans the next move: cold water. The amygdala flags the fear. The cerebellum coordinates the quick movement. And the hippocampus helps store a new memory: use a potholder next time.
Pause and try this. Which structure helps you form the new memory "use a potholder," and which lobe registers where on your body the burn happened?
Picture the stove, then answer.
Here's the answer. The hippocampus helps form the new memory, and the parietal lobe registers touch and where it happened on your body. Remember, though, that the brain works as a network; each structure is involved in a job, not the sole cause of it.
7. How do we know? The case of H.M.
How could anyone learn what the hippocampus does? A lot of the earliest evidence came from people whose brains were changed 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 reported what followed in 1957. H.M. could no longer form new declarative memories. Yet he kept most of his memories from before the operation, and he could still learn new motor skills.
Now put on your Week 2 hat. What design is this? A case study, which examines one person or a small group in great depth. It can suggest ideas to test, but it can't show cause or be assumed to fit everyone. H.M.'s surgery also removed more than the hippocampus alone. So his case is a powerful clue, and its real value came from the later studies that tested what it suggested. You'll meet H.M. again in Week 7, when memory gets its own week.
Pause and try this. Why can't H.M.'s case, on its own, prove a rule about every human brain?
Think about what a case study can and can't do.
Here's the answer. It's one person, so it can't be assumed to fit everyone, and a single case can't show cause the way an experiment can. It points researchers toward an idea that other studies then have to test.
8. A brain that keeps changing, and a myth that won't quit
Think about someone who picks up the guitar as an adult. At first every chord feels clumsy. Months later, their fingers find the frets without looking. Something in their brain changed.
That's neuroplasticity: the brain's ability to change its connections and organization in response to experience, practice, and injury, throughout life. Practice strengthens some connections. After an injury, other areas can sometimes take over part of a lost job.
Now a claim you've probably heard: we use only 10 percent of our brains. Some brain-training apps even promise to unlock the other 90 percent. It's a myth. Every structure on our tour has known jobs. And damage to almost any part of the brain affects some ability, which wouldn't happen if most of it sat unused. Over a normal day, you use all of it. Neuroplasticity doesn't rescue the myth, either. It's about rewiring a brain that's already fully in use.
Pause and try this. Use H.M.'s case to argue against the 10 percent myth in one sentence.
Connect sections 7 and 8.
Here's the answer. Losing one structure took away H.M.'s ability to form new declarative memories, so that structure was in use. If most of the brain sat idle, damage to most places would change nothing, and that isn't what researchers find.
Closing
Let me pull it together. A neuron's dendrites receive, the soma combines, the axon carries, the myelin speeds, and the terminal buttons release. A neuron rests at about minus 70 millivolts and fires all-or-none, so a stronger push changes the rate, not the size. Neurotransmitters cross the synapse, and reuptake clears it. Each one is associated with functions, and none is the single cause of a mood. The sympathetic division revs you up, and the parasympathetic division calms you down. Each brain structure and lobe has its jobs, working as a network. H.M.'s case linked the hippocampus to forming new memories, and a case study is a clue, not a proof. Your brain keeps rewiring for life, and you use all of it.
Next in this module come the slide deck, the Readings & Resources page, the lecture tutorial, the practice set, and the quiz. Next week, in Week 4, Sensation & Perception follows how your senses send their signals into the nervous system you met here.
Built with The Course Maker · samples.thecoursemaker.com