The Brain & Nervous System

🧠 Biological Bases and Brain

Part 1 of 7 — Neurons and Neural Firing

🔑 Key Concept: Every thought, feeling and movement starts as electrical and chemical signaling between cells. A neuron either fires completely or not at all, so the brain codes "how strong" by how often neurons fire and how many fire, not by making each signal bigger.

Two kinds of cells

The CED names two common cell types in the nervous system:

Glial cells far outnumber neurons. They are not just "glue": a nervous system without healthy glia cannot send signals quickly or reliably.

Parts of a neuron

Follow a message in one direction, from the receiving end to the sending end:

Memory aid: Dendrites → Cell body → Axon → Terminals ("Dogs Can Always Tell").

Concept Check — Parts of the Neuron 🎯

Three types of neurons and the reflex arc

Worked example: touching a hot pan

1. Receptors in the skin detect intense heat.
2. A sensory neuron carries the signal into the spinal cord.
3. An interneuron in the spinal cord passes the message straight to a motor neuron.
4. A motor neuron signals the arm muscles, and the hand jerks back.
5. Meanwhile, other neurons carry the message up to the brain, and only then do you feel pain.

That loop (sensory neuron → interneuron → motor neuron) is the reflex arc. Because the spinal cord handles the response before the brain processes it, the hand moves before you are aware of the pain. The reflex arc shows the central nervous system (spinal cord) and the peripheral nervous system (the sensory and motor nerves) working together.

🔑 AP Connection: If a question asks why someone reacts "before they realize what happened," think reflex arc. If it asks what lets the person feel the pain, the answer involves the brain.

How a neuron fires

A neuron's message travels down its axon as an action potential, a brief electrical impulse. The CED terms you need, in order:

The all-or-none principle

A neuron either fires at full strength or does not fire at all. Pushing harder past threshold does not make a bigger action potential, the same way squeezing a trigger harder does not make a bullet go faster.

Worked example: how the brain codes intensity

Because every action potential is the same size, the nervous system signals intensity with frequency (how often a neuron fires) and number (how many neurons fire).

Recall Practice — Neural Firing ✍️

When neural transmission is disrupted

The CED names two disorders that show what happens when this orderly process breaks down.

How to tell them apart: MS is a problem with the cable (insulation on the axon), while myasthenia gravis is a problem at the connection between nerve and muscle (the receiving side of the synapse).

Misconceptions to drop

🔑 AP Connection: The AAQ asks you to state an operational definition. In a neuroscience study, "nerve conduction speed" might be operationalized as meters per second measured between two electrodes, and "muscle fatigue" as the number of grip squeezes before strength falls by half.

Applied Scenarios — Firing and Disorders 🔬

🧠 Biological Bases and Brain

Part 2 of 7 — Neurotransmitters and Psychoactive Drugs

🔑 Key Concept: Part 1 followed a message down one neuron. This part follows it across the synapse to the next one. Neurotransmitters carry the message, and almost every psychoactive drug works by changing what happens at that gap.

Crossing the synapse

1. An action potential reaches the terminal buttons.
2. Tiny sacs release neurotransmitter molecules into the synaptic gap.
3. The molecules bind to receptor sites on the receiving neuron, fitting like a key in a lock. Only the matching neurotransmitter fits a given receptor.
4. The binding sends an excitatory or inhibitory message.
5. Leftover neurotransmitter is cleared, mostly by reuptake: the sending neuron reabsorbs it so it can be reused.

Whether the receiving neuron fires depends on the sum of all excitatory and inhibitory messages arriving at once. Whether it passes threshold is still all-or-none.

The eight neurotransmitters you need

The CED limits AP Psychology to these eight. A neurotransmitter's job can depend on where in the nervous system it acts, so learn each one's main links.

Two pairs worth memorizing

• Glutamate vs. GABA: the brain's main accelerator vs. main brake.
• Substance P vs. endorphins: the pain messenger vs. the pain reliever.

Concept Check — Neurotransmitters 🎯

How drugs act at the synapse

Psychoactive drugs change perception, mood, thinking or behavior. The CED groups their synaptic actions into three kinds:

Picture the receptor as a lock. An agonist is a copied key that opens it. An antagonist is a key that fits but will not turn, and it jams the lock so the real key cannot get in. A reuptake inhibitor keeps the real keys from being collected, so they keep trying the lock.

Worked example: an opioid overdose and naloxone

1. A person takes a strong opioid. It acts as an agonist at opioid (endorphin) receptors, relieving pain and producing euphoria.
2. At high doses, the same receptors in the brain stem slow breathing dangerously.
3. A responder gives naloxone, an antagonist that binds the opioid receptors more strongly than the opioid and does not activate them.
4. The opioid is pushed off the receptors, and breathing returns within minutes.

The two drugs act on the same receptor with opposite results. That pairing is the clearest way to remember agonist vs. antagonist.

Classify the Drug Action 🔎

Drug classes and what repeated use does

Tolerance, addiction and withdrawal

Withdrawal symptoms are often the opposite of the drug's effects. Caffeine makes people alert, and withdrawal makes them tired. Opioids relieve pain and calm, and withdrawal brings aches and agitation. This happens because the brain has adjusted to the drug's constant presence, and removing the drug leaves that adjustment unopposed.

Misconceptions to drop

Applied Scenarios — Drugs and Behavior 🔬

🧠 Biological Bases and Brain

Part 3 of 7 — The Nervous System and the Endocrine System

🔑 Key Concept: Neurons are wired into systems. The nervous system sends fast, targeted electrical-chemical messages. The endocrine system sends slower chemical messages through the blood. Most behavior, from a racing heart to falling asleep, uses both.

The map of the nervous system

Nervous system
├── Central nervous system (CNS): brain + spinal cord
└── Peripheral nervous system (PNS): every nerve outside the CNS
    ├── Somatic nervous system: VOLUNTARY control of skeletal muscles
    └── Autonomic nervous system: INVOLUNTARY control of organs and glands
        ├── Sympathetic division: AROUSING ("fight or flight")
        └── Parasympathetic division: CALMING ("rest and digest")

Quick test: Could you choose to do it? If yes, it is somatic. If it happens on its own, it is autonomic.

Sympathetic vs. parasympathetic

The two autonomic divisions work like a seesaw: when one is more active, the other eases off.

Memory aid: Sympathetic = Stress; Parasympathetic = Peace.

Worked example: a fire drill during a test

1. Alarm sounds. The sympathetic division takes over: heart pounds, breathing quickens, pupils widen and digestion pauses. The body is ready to move.
2. Students walk outside. Their legs and feet move by somatic commands, which are voluntary.
3. "All clear" is announced. Over the next several minutes the parasympathetic division slows the heart and restarts digestion.
4. Some students still feel jittery after returning to their seats. That lingering arousal comes from hormones already in the blood, which leads to the endocrine system.

🔑 AP Connection: A polygraph ("lie detector") measures sympathetic arousal (heart rate, breathing, sweating), not lying itself. An anxious but honest person can look "deceptive." This is a classic validity problem.

Concept Check — Divisions of the Nervous System 🎯

The endocrine system

The endocrine system sends chemical messengers called hormones through the bloodstream. Hormones act on target tissues much as neurotransmitters act on receptors, but the delivery is very different.

The hypothalamus and the pituitary gland

The hypothalamus, a small brain structure, links the two systems. It directs the pituitary gland at the base of the brain. The pituitary is often called the "master gland" because its hormones influence other glands throughout the body. The pituitary is the only gland you need to know by name for AP Psychology.

The five hormones on the AP exam

The CED limits AP Psychology to these five:

Memory aid: ghrelin = "growling" stomach (hunger); leptin = "leave the table" (full).

Recall Practice — Hormones ✍️

Putting the systems together

Worked example: a near-miss on the highway

The nervous system explains the instant reaction. The endocrine system explains why the feeling lingers.

Misconceptions to drop

🔑 AP Connection: In an EBQ about stress, school start times or eating, you can earn explanation points by naming the specific system or hormone. "Ghrelin rises when meals are skipped, increasing hunger" is stronger than "the body gets hungry."

Applied Scenarios — Nervous and Endocrine Systems 🔬

🧠 Biological Bases and Brain

Part 4 of 7 — Brain Structures and Their Functions

🔑 Key Concept: Brain structures are roughly layered from basic survival at the bottom (breathing, heartbeat) to complex thought at the top and front (planning, judgment). On the exam you will usually work backward: from a symptom or behavior to the structure most likely involved.

Lower-level structures

Researchers James Olds and Peter Milner found in the 1950s that rats would press a lever again and again to receive mild electrical stimulation in certain deep brain areas, evidence for a reward system that motivates behavior.

🔑 AP Connection: Alcohol impairs the cerebellum early, which is why roadside sobriety tests check balance and coordinated movement.

Concept Check — Lower-Level Structures 🎯

The limbic system

The CED groups five structures under the limbic system. They sit around the top of the brain stem, beneath the cortex, and are tied to emotion, memory, drives and hormones.

Memory aids:

• Hippocampus = "hippo on campus learning new facts" (memory).
• Amygdala = "Afraid" (fear and threat).
• Hypothalamus = the body's thermostat for drives: it keeps hunger, thirst and temperature in balance ("hypo" = below the thalamus).

The cerebral cortex and its four lobes

The cerebral cortex is the thin, wrinkled outer layer of the two cerebral hemispheres. Its folds pack more surface area, and so more neurons, into the skull.

Association areas are cortex regions not devoted to a single sense or movement. They integrate information for thinking, remembering and speaking, and they make up much of the frontal, parietal and temporal lobes.

The motor and somatosensory strips

The motor cortex (rear of the frontal lobe) and the somatosensory cortex (front of the parietal lobe) sit side by side. In the 1930s–1950s, neurosurgeon Wilder Penfield stimulated these strips in awake patients during surgery. Stimulating the motor strip made body parts move; stimulating the somatosensory strip made patients report being touched. Body parts that need fine control or high sensitivity, such as the hands, lips and tongue, have the largest areas of cortex.

Match the Case to the Structure 🔎

Case files: reasoning from damage to function

Much of what we know about brain structures came from case studies of people with brain damage. The logic: if a function disappears when an area is damaged, that area is likely necessary for the function.

Case 1: Phineas Gage (1848)

A railroad construction foreman survived an iron rod that passed through his frontal lobe. Afterward, according to his doctor's reports, he became impulsive, profane and unable to stick to plans, though his memory and movement were largely intact.

• Reasoning: damage to the prefrontal cortex disrupted executive functioning and impulse control.
• Limitation: one person, with an injury no one could control or measure precisely, and later accounts exaggerated parts of his story. A case study suggests hypotheses but cannot establish general cause and effect on its own.

Case 2: a practice patient with a stroke

A 70-year-old has a stroke on the left side of her motor cortex. Her right arm and leg are weak.

• Reasoning: the motor cortex controls skeletal movement, and each hemisphere controls the opposite side of the body (more on this in Part 5).

Case 3: a practice patient with a tumor

A tumor presses on a patient's somatosensory cortex in the area that serves the face. He reports numbness and tingling in his cheek and lips, while his ability to move his face is normal.

• Reasoning: touch is processed in the somatosensory cortex; movement depends on the neighboring motor cortex, which is unaffected.

Misconceptions to drop

Applied Scenarios — Brain Structures 🔬

🧠 Biological Bases and Brain

Part 5 of 7 — Hemispheres, Plasticity and Brain Research Methods

🔑 Key Concept: The two hemispheres look alike but specialize. Normally the corpus callosum shares information between them so instantly that we never notice. Cutting it, as in split-brain surgery, reveals what each side can do alone.

Contralateral organization

Each hemisphere mainly controls and senses the opposite side of the body:

• The left hemisphere moves and feels the right hand, arm and leg.
• The right hemisphere moves and feels the left side.
• Vision works by visual field, not by eye. Whatever you see to the left of your focus point (with either eye) goes to the right hemisphere, and the right visual field goes to the left hemisphere.

Hemispheric specialization

Specialization is a matter of degree. Both hemispheres work together on almost every task, and there is no evidence that people are "left-brained" or "right-brained" types.

Split-brain research

For some people with severe epilepsy, surgeons have cut the corpus callosum so seizures cannot spread from one hemisphere to the other. Roger Sperry and Michael Gazzaniga studied these split-brain patients by flashing information to just one visual field.

Worked example: one trial, step by step

Setup: The patient stares at a dot in the center of a screen. A word flashes for a fraction of a second, too fast for the eyes to move.

Conclusion: both hemispheres perceive, but in most people only the left can put what it saw into words. The right hemisphere can still recognize and act on information. With an intact corpus callosum, the two sides would share the word instantly.

🔑 AP Connection: For split-brain questions, trace three links in order: visual field → opposite hemisphere → can that hemisphere speak or control that hand?

Concept Check — Split Brain and Language 🎯

Language areas and aphasia

Aphasia is an impairment of language caused by brain damage. The two classic areas are both usually in the left hemisphere:

Memory aid: Broca = Broken speech output; Wernicke = Word salad.

Brain plasticity

Brain plasticity (neuroplasticity) is the brain's ability to rewire itself: to modify or create new connections throughout development and, often, to let one area take over the function of a damaged area.

Plasticity is greatest in childhood but continues throughout life.

Recall Practice — Hemispheres and Plasticity ✍️

How we study the brain

Reading brain-research data: a practice study

Practice study (hypothetical): 24 adult volunteers viewed photos of angry and neutral faces in an fMRI scanner. Researchers measured amygdala activity as the percent change in blood-oxygen signal.

Reading it like an AAQ:

• Difference in means: 0.42 − 0.18 = 0.24 percentage points. On average, angry faces produced more amygdala activity than neutral faces.
• Spread: the standard deviations are small compared with the difference, so the two conditions overlap little.
• What it supports: the amygdala is associated with processing threat.
• What it does not show: that the amygdala causes fear. fMRI shows activity that accompanies a task; to test necessity you need lesion or damage evidence.
• Generalizability: 24 adult volunteers is a small, self-selected sample; results may not extend to children or to other populations.

Applied Scenarios — Methods and Plasticity 🔬

🧠 Biological Bases and Brain

Part 6 of 7 — Sleep, Dreams and Sleep Disorders

🔑 Key Concept: Sleep is not "the brain switching off." It is an active, organized state of consciousness that cycles through distinct stages, each with its own EEG pattern, and it does real work for memory and for restoring the body.

Consciousness

Consciousness is our awareness of ourselves and our environment: thoughts, feelings, behavior and events inside and outside us. It comes in varying levels. Wakefulness and sleep are two states of consciousness, and even asleep we are partly aware: a parent may sleep through traffic noise but wake at a baby's cry.

The circadian rhythm

The sleep/wake cycle is a circadian rhythm, a biological cycle of about 24 hours. Light is its main timekeeper: morning light helps reset the clock each day, and darkness lets melatonin rise in the evening, bringing sleepiness. Body temperature and alertness also rise and fall on this daily cycle.

🔑 AP Connection: Teenagers' circadian rhythms shift later during puberty, so they naturally feel sleepy later at night. This is a common setting for EBQ prompts about schedules and performance.

The stages of sleep

Researchers identify sleep stages by their EEG patterns. A night has two kinds of sleep: NREM (non-rapid eye movement), in Stages 1–3, and REM (rapid eye movement).

Why REM is "paradoxical"

During REM sleep, the EEG looks almost like an awake brain, yet the body's muscles are at their most relaxed. Brain busy, body still: that contradiction is why REM is called paradoxical sleep.

The nightly cycle

A full cycle (Stage 1 → 2 → 3 → 2 → REM) takes about 90 minutes, and most adults pass through four to six cycles a night. The cycles change as the night goes on:

So NREM-3 decreases across the night while REM increases. Cutting sleep short in the morning mostly cuts REM.

REM rebound

When people are deprived of REM sleep (for example, woken whenever REM begins), they enter REM sooner and spend more time in it on the next undisturbed night. This REM rebound suggests REM serves a need the brain works to make up.

Concept Check — Sleep Stages 🎯

Why do we sleep?

The CED highlights two current theories:

The two are not rivals; sleep likely does both. Losing sleep impairs attention, reaction time, mood, judgment and immune function, and drowsy driving causes many crashes.

Why do we dream?

The five sleep disorders

The CED limits AP Psychology to these five:

Treating sleep disorders and keeping a regular sleep schedule improve daytime performance and overall well-being.

Identify the Sleep Disorder 🔎

Sleep by the Numbers ✍️

Applied Scenarios — Sleep and Dreams 🔬

🧠 Biological Bases and Brain

Part 7 of 7 — AP Practice and Synthesis

🔑 Key Concept: The free-response section has two 7-point questions. The Article Analysis Question (AAQ) gives you one study summary and asks six parts (A–F) about its method, variables, statistics, ethics, generalizability and support for a hypothesis; you get 25 minutes, including a 10-minute reading period. The Evidence-Based Question (EBQ) gives you three source summaries; you make a claim and support it with evidence from two different sources, each explained with a different psychological concept, in 45 minutes including a 15-minute reading period.

AAQ practice study (hypothetical)

Introduction. Researchers tested the hypothesis that restricting sleep impairs sustained attention.

Participants. Sixty undergraduate volunteers (ages 18–22, 34 women and 26 men) from one university responded to campus flyers. People with a diagnosed sleep disorder or who worked night shifts were excluded. All gave informed consent and were told they could leave the study at any time without penalty.

Method. A computer randomly assigned participants to one of two groups. For four nights in a sleep lab, the 8-hour group was allowed 8 hours in bed and the 5-hour group was allowed 5 hours in bed. Each morning, participants completed a 10-minute reaction-time task: they pressed a button as soon as a dot appeared on a screen at unpredictable intervals. Attention was measured as mean reaction time in milliseconds on the fourth morning. Afterward, participants were debriefed and paid $100.

Results.

The difference was statistically significant (p = .002).

Discussion. The researchers concluded that four nights of restricted sleep slowed reactions, suggesting reduced sustained attention.

Walking through the six AAQ tasks

Common AAQ mistakes

AAQ Elements — Practice Study 🔎

AAQ Reasoning — Practice Study 🎯

EBQ practice: caffeine, sleep and school performance

Prompt: Using the sources, develop and justify an argument about whether daily caffeine use helps or hurts high school students' academic performance.

Source 1 (practice experiment). Eighty college students were randomly assigned to drink a beverage with 200 mg of caffeine or an identical placebo 30 minutes before a 20-minute attention test. The caffeine group made fewer errors (mean 4.1 vs. 6.3).

Source 2 (practice correlational survey). In a survey of 1,200 high school students, those who drank caffeine after 4 p.m. reported sleeping 48 fewer minutes per school night on average. Evening caffeine and sleep duration were negatively correlated (r = −.31).

Source 3 (practice longitudinal study). Researchers followed 300 teens for a school year. Daily heavy caffeine users reported headaches, fatigue and low mood on weekend mornings before their first drink, and average nightly sleep was positively correlated with grade point average (r = .28).

A model response

A. Claim: Daily caffeine use is more likely to hurt than help high school students' academic performance, because it shortens sleep and creates dependence.

B(i). Evidence: Students who drank caffeine after 4 p.m. slept 48 fewer minutes per school night (Source 2). B(ii). Explanation: Caffeine is a stimulant that increases neural activity, so evening use delays sleep onset while school start times stay fixed. Less sleep means less time for memory consolidation, so material studied that day is stored less well, which hurts performance on tests.

C(i). Evidence: According to Source 3, daily heavy users reported headaches, fatigue and low mood before their first drink on weekends. C(ii). Explanation: These symptoms are withdrawal, a sign that repeated use has produced tolerance and dependence. Students then need caffeine just to feel normal, so any benefit like the one in Source 1 shrinks, while fatigue during withdrawal works against concentration in class.

Notice the structure: each explanation uses a different psychological concept (stimulants and memory consolidation; tolerance and withdrawal), each cites a different source, and the evidence is specific (numbers, groups), not a vague summary. A strong writer may also acknowledge Source 1 and explain why a short-term lab boost does not outweigh the nightly sleep loss.

EBQ Reasoning — Caffeine Sources 🎯

Synthesis: from one neuron to a night's sleep

One chain that connects everything

A student stays up late on a phone. Bright light delays melatonin and shifts the circadian rhythm. Cut short in the morning, the night loses mostly REM sleep, which shows up as REM rebound on the weekend. Tired the next day, the student drinks coffee, a stimulant, and over weeks develops tolerance. In class, a slower prefrontal cortex (executive functioning) and less memory consolidation make learning harder. Every piece of that chain is on the AP exam.

Integrated Review 🔬