AP Psychology Unit 1: Biological Bases of Behavior Review - Neurons, Brain & Sensation - StarSpark

Unit 1 is where AP Psychology gets biological. You're learning the physical foundations of everything we do, think, and feel. Your brain, nervous system, and the chemicals coursing through your body determine how you perceive the world, form memories, sleep, and react to stress. This unit covers about 15-25% of the AP exam, and understanding these concepts helps you make sense of every unit that follows. Let's break it down.

Topic 1.1: Interaction of Heredity and Environment

Your behavior comes from two sources: what you inherited (nature) and what you experience (nurture). The AP exam loves testing whether you understand this interplay.

This topic explores the classic nature versus nurture debate, but modern psychology has moved beyond it. Today we ask: which traits are influenced by genetics, which by environment, and how do they interact? You'll also need to understand key research methods and perspectives that shaped this discussion.

Key concepts to know:

• Twin studies and adoption studies measure how much behavior is influenced by genetics. Identical twins share 100% of DNA; fraternal twins share 50%. If identical twins are more similar on a trait than fraternal twins (even when raised apart), that suggests genetic influence.
• Evolutionary psychology explains behavior through natural selection. Our ancestors who had helpful traits (like fear of snakes) survived and reproduced more, passing those traits down. This explains why we have innate fears, social instincts, and certain cognitive biases.
• Gene-environment interaction is the key modern concept. A genetic predisposition for depression only develops into actual depression if triggered by environmental stress. Genes load the gun; environment pulls the trigger.
• Heritability is the proportion of variation in a trait within a population caused by genetic differences. A high heritability doesn't mean a trait is genetic in origin or unchangeable.
• Eugenics movements tried to "improve" populations by controlling reproduction. This is a historical cautionary tale about misusing genetics research. The AP exam may ask you to identify eugenics as pseudoscience.

Topic 1.2: Overview of the Nervous System

The nervous system is organized into layers, and you need to know the names and functions of each.

At the top level, the nervous system divides into the central nervous system (CNS) and peripheral nervous system (PNS). Your brain and spinal cord are the CNS. Everything else is the PNS. The PNS further divides into somatic (skeletal muscles you control consciously) and autonomic (heart, digestion, and other unconscious functions).

The autonomic nervous system has two branches that often work against each other. The sympathetic nervous system is "fight or flight" – it speeds your heart rate, dilates your pupils, and shuts down digestion when you face stress. The parasympathetic nervous system is "rest and digest" – it slows your heart, stimulates digestion, and conserves energy. These usually work in balance.

Key concepts to know:

• Central nervous system (CNS) consists of your brain and spinal cord. It's where all the processing happens.
• Peripheral nervous system (PNS) includes all neurons outside the brain and spinal cord. It carries signals to and from the CNS.
• Somatic nervous system controls voluntary skeletal muscles. You use it to move your fingers right now.
• Autonomic nervous system controls involuntary functions like heart rate, breathing, and digestion. You don't consciously direct these.
• Sympathetic nervous system activates during stress or excitement. Your body mobilizes resources: pupils dilate, heart rate increases, digestion slows.
• Parasympathetic nervous system activates during calm states. It's your parasympathetic brake: heart rate slows, digestion resumes, pupils constrict.

Topic 1.3: The Neuron and Neural Firing

Now we zoom in to individual neurons. This is the heart of Unit 1 and a major AP exam focus.

A neuron has a cell body (soma) with a nucleus, dendrites that receive signals from other neurons, and an axon that sends signals to other neurons. Some axons are covered in myelin sheath, a fatty insulation that speeds up signal transmission.

When a neuron fires, it sends an electrical signal down the axon called an action potential. This happens when the neuron's charge becomes less negative (depolarization). If the signal reaches threshold (about -55 millivolts), the neuron fires an all-or-nothing response. It doesn't fire harder or faster depending on stimulus strength; it either fires or doesn't. Stronger stimuli simply cause more neurons to fire, not individual neurons to fire more intensely.

At the synapse (the gap between neurons), the sending neuron releases neurotransmitters, chemicals that cross the gap and affect the receiving neuron. Different neurotransmitters have different effects.

Key concepts to know:

• Resting membrane potential is about -70 millivolts. The inside of the neuron is more negative than the outside.
• Depolarization means the neuron becomes less negative. If it reaches threshold, the neuron fires.
• Action potential is the electrical impulse that travels down the axon. It's an all-or-nothing event.
• Synapse is the junction between neurons where neurotransmitters are released.
• Reuptake is when the sending neuron reabsorbs neurotransmitters from the synapse, ending the signal. Drugs that block reuptake (like SSRIs for depression) keep neurotransmitters in the synapse longer.
• Agonists mimic the effect of a neurotransmitter; antagonists block it.

You need to know eight major neurotransmitters and their general functions:

1. Dopamine – motivation, reward, movement, pleasure
2. Serotonin – mood, sleep, appetite
3. Norepinephrine – alertness, arousal
4. Glutamate – the main excitatory neurotransmitter; involved in learning and memory
5. GABA (gamma-aminobutyric acid) – the main inhibitory neurotransmitter; reduces neural activity, promotes calm
6. Endorphins – natural pain relief and pleasure (released during exercise, laughing)
7. Substance P – transmits pain signals
8. Acetylcholine – movement, learning, memory

You also need five hormones:

1. Adrenaline (epinephrine) – released during stress, increases heart rate and blood pressure
2. Leptin – suppresses appetite, signals satiety
3. Ghrelin – stimulates appetite
4. Melatonin – regulates sleep-wake cycles
5. Oxytocin – involved in bonding, trust, social connection

Topic 1.4: The Brain

The brain is the most complex organ you'll study in AP Psychology. You need to know the major structures and their functions.

Start with the brain stem at the base of the brain. It controls automatic functions like breathing and heart rate. The reticular activating system (RAS) is a network within the brain stem that controls your level of alertness and arousal.

The cerebellum sits behind the brain stem. It coordinates movement, balance, and motor learning. People with cerebellar damage move clumsily.

The limbic system includes structures involved in emotion and memory. The amygdala processes emotions (especially fear), the hippocampus forms memories, and the hypothalamus controls homeostasis and hormone release.

The cerebral cortex is the outermost layer of the brain, divided into four lobes. The frontal lobe handles decision-making, planning, impulse control (prefrontal cortex), and movement (motor cortex). The parietal lobe processes touch and spatial awareness. The temporal lobe handles hearing and memory. The occipital lobe processes vision.

Two language areas are critical: Broca's area (in the frontal lobe) produces speech, and Wernicke's area (in the temporal lobe) comprehends speech.

The corpus callosum connects the left and right hemispheres, letting them communicate. Split-brain patients (whose corpus callosum is severed) show interesting behavior: the left hemisphere (usually dominant for language) might speak about one thing while the right hemisphere acts on a different intention.

Key concepts to know:

• Brain stem – controls basic life functions
• Reticular activating system – controls arousal and sleep-wake cycles
• Cerebellum – coordinates movement and balance
• Limbic system – emotion and memory (amygdala, hippocampus, hypothalamus)
• Cerebral cortex – conscious thought, sensation, movement
• Frontal lobe – decision-making, planning, impulse control, speech production (Broca's area)
• Parietal lobe – touch and spatial processing
• Temporal lobe – hearing and memory (Wernicke's area is here)
• Occipital lobe – vision
• Hemispheric specialization – left brain typically handles language and logic; right brain handles spatial and emotional processing
• Brain plasticity – the brain's ability to reorganize after injury, especially in younger people
• Neuroimaging – fMRI shows brain activity, PET scans show brain metabolism, EEG records electrical activity

Topic 1.5: Sleep

Sleep isn't just downtime. It's essential for memory consolidation, emotional regulation, and physical restoration. You spend about a third of your life sleeping, so it matters for the AP exam.

Your sleep follows a circadian rhythm, roughly a 24-hour cycle controlled mainly by the brain's suprachiasmatic nucleus responding to light. When light decreases, your brain releases melatonin, triggering sleepiness.

Sleep has several stages. When you first fall asleep, you enter light sleep (stages 1 and 2). Your brain waves slow, your heart rate drops, and your body temperature declines. Then you enter deep sleep (stage 3), where your brain produces slow-wave sleep (delta waves). Deep sleep is restorative; growth hormone is released, and memory consolidation happens.

After about 90 minutes, you cycle into REM sleep (rapid eye movement). Your brain becomes active, your eyes move rapidly, and you dream vividly. REM sleep is crucial for emotional processing and memory. You cycle through these stages multiple times, spending more time in REM later in the night.

Dreams happen most vividly in REM but can occur in any stage. The activation-synthesis hypothesis suggests dreams result from the brain trying to make sense of random neural activity during sleep. Consolidation theory suggests dreams help consolidate memories and process information from the day.

Sleep disorders are common. Insomnia (chronic difficulty falling or staying asleep) affects about 30% of adults. Narcolepsy causes sudden loss of muscle tone and uncontrollable sleep attacks. Sleep apnea involves brief pauses in breathing during sleep. Somnambulism (sleepwalking) and REM sleep behavior disorder involve movement during sleep.

Key concepts to know:

• Circadian rhythm – roughly 24-hour biological cycle that regulates sleep-wake timing
• Light as a circadian cue – light exposure triggers alertness; darkness triggers melatonin
• Sleep stages – light sleep (stages 1-2), deep sleep (stage 3), and REM sleep
• REM sleep – rapid eye movement, vivid dreams, high brain activity
• Slow-wave sleep – delta waves, physically restorative
• Sleep cycle – repeats every 90 minutes; more REM later in the night
• Activation-synthesis hypothesis – dreams result from the brain interpreting random neural activity
• Insomnia – difficulty falling or staying asleep
• Narcolepsy – sudden sleep attacks and muscle tone loss
• Sleep apnea – breathing pauses during sleep
• Somnambulism – sleepwalking, typically in stage 3 sleep

Topic 1.6: Sensation

Sensation is how your sensory organs detect physical energy. The AP exam tests specific sensory processes and thresholds.

An absolute threshold is the minimum stimulus intensity needed to detect a sensation. A just noticeable difference (JND) is the smallest change in stimulation you can detect. Weber's law states that JND is proportional to the magnitude of the stimulus. If you're holding 1 pound, you might notice a 0.1-pound increase (10% change). If you're holding 10 pounds, you'd need a 1-pound increase to notice it.

Sensory adaptation is when repeated stimulation causes a sensory receptor to become less responsive. You stop noticing your clothes on your skin or the hum of a refrigerator because these stimuli are constant and unchanging.

Vision involves light entering your eye through the cornea and lens, which focus it onto the retina. The retina contains rod cells (sensitive to light; see in black and white) and cone cells (see color; need bright light). These photoreceptors convert light into neural signals. The optic nerve carries these signals to the brain's visual cortex in the occipital lobe.

Hearing involves sound waves traveling into your ear canal and vibrating your eardrum. The middle ear bones (hammer, anvil, stirrup) amplify these vibrations, which then move the fluid in the inner ear's cochlea. Hair cells in the cochlea convert these vibrations into neural signals, which travel via the auditory nerve.

Smell and taste work through chemical receptors. Odorant molecules bind to olfactory receptors in your nose; taste molecules (sweet, salty, sour, bitter, umami) bind to taste receptors on your tongue.

Touch, pain, and temperature sensations come from receptors in your skin. Touch receptors respond to pressure; temperature receptors respond to hot and cold. Pain is modulated by gate-control theory: non-painful stimulation (like rubbing an injury) can close the "gate," reducing pain perception.

The vestibular sense detects your head's position and movement in space (inner ear). Kinesthetic sense detects your body's position and movement (joint and muscle receptors).

Key concepts to know:

• Absolute threshold – minimum stimulus intensity to detect
• Just noticeable difference (JND) – smallest detectable change
• Weber's law – JND is proportional to stimulus magnitude
• Sensory adaptation – reduced responsiveness to constant stimulation
• Photoreceptors – rods (light sensitive, black and white) and cones (color, bright light)
• Retina – contains photoreceptors and sends signals via optic nerve
• Cochlea – coiled structure in inner ear; hair cells convert vibrations to neural signals
• Olfactory receptors – detect smell
• Taste receptors – detect sweet, salty, sour, bitter, umami
• Gate-control theory – non-painful stimulation reduces pain perception
• Vestibular sense – detects head position and movement
• Kinesthetic sense – detects body position and movement

Study Tips for Unit 1

If you're looking for a more interactive way to lock in these concepts, StarSpark's flashcard sets for Unit 1 break down neurotransmitters, brain structures, and sleep stages into focused study sessions. You can quiz yourself on anything from neuron anatomy to the limbic system. Check out StarSpark's AP Psychology study tools to see flashcards and quizzes built for this unit.

Summary: What Actually Matters for the Exam

You've covered all the topics in Unit 1. Before you move on, test yourself with the scenario-based questions below. If you can answer them confidently, you're in great shape for this section of the exam.

Explore the Full AP Psychology Study Guide

Check out the full AP Psychology study plan to see how Unit 1 connects to the rest of the course.

Other Unit Reviews:

• Unit 2: Cognition
• Unit 3: Development and Learning
• Unit 4: Social Psychology and Personality
• Unit 5: Mental and Physical Health

For official AP Psychology resources, visit apcentral.collegeboard.org.

This review is aligned with the AP Psychology Course and Exam Description. AP is a registered trademark of the College Board, which was not involved in the production of this guide.