Biological Bases of Behavior

What this unit covers

The topics below follow the published Psychology course framework for Unit 1. This unit is worth 15–25% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Neurons & Neural Firing14 min · 3 objectivesLabel the parts of a neuron and describe how a signal travels through one · Explain the action potential, the all-or-none principle, and the role of the synapse · Connect specific neurotransmitters to their behavioral effects
• The Brain & Tools for Studying It14 min · 3 objectivesLocate major brain structures and state the function of each · Explain the roles of the two cerebral hemispheres and the cortex lobes · Compare the tools neuroscientists use to study brain structure and activity
• Genetics, Evolution & the Nature–Nurture Debate13 min · 3 objectivesExplain how twin and adoption studies untangle genetic from environmental influence · Define heritability and interpret it correctly · Apply an evolutionary perspective to explain behavioral tendencies
• Sensation & Sensory Systems13 min · 3 objectivesDistinguish sensation from perception and define transduction · Explain absolute and difference thresholds and sensory adaptation · Describe how the eye and ear transduce physical energy into neural signals

Every term in Unit 1

All 28 terms we publish for Biological Bases of Behavior, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Neuron structure

Dendrites receive signals, the soma integrates them, the axon carries the impulse, and terminal buttons release neurotransmitter into the synapse.

Action potential

An all-or-none electrical impulse. Sodium rushes in to depolarize the membrane, then potassium exits to repolarize it — strength of stimulus changes frequency, never amplitude.

All-or-none principle

A neuron either fires fully or not at all once threshold is reached. Intensity is coded by how often it fires and how many neurons fire, not by a bigger spike.

Refractory period

A brief interval after firing during which the neuron cannot fire again, setting an upper limit on firing rate and forcing the impulse to travel one way.

Myelin sheath

Fatty insulation around an axon that speeds transmission by letting the impulse jump between nodes of Ranvier. Its breakdown causes multiple sclerosis.

Reuptake

Reabsorption of neurotransmitter by the sending neuron, clearing the synapse. SSRIs work by blocking serotonin reuptake, leaving more in the cleft.

Excitatory vs inhibitory signals

Excitatory neurotransmitters make the receiving neuron more likely to reach threshold; inhibitory ones make it less likely. The neuron sums both.

Dopamine

Involved in reward, motivation and motor control. Oversupply is linked to schizophrenia symptoms; loss of dopamine neurons produces Parkinson's.

Serotonin

Affects mood, hunger and sleep. Low levels are associated with depression, which is the rationale for SSRIs.

Acetylcholine

Triggers muscle contraction and is involved in learning and memory. Deteriorating ACh neurons are a feature of Alzheimer's.

GABA and glutamate

GABA is the main inhibitory neurotransmitter and glutamate the main excitatory one. Too little GABA is linked to seizures, too much glutamate to overstimulation.

Endorphins

Natural opiates that reduce pain and produce pleasure. Persistent opioid drug use suppresses natural production, which is part of why withdrawal hurts.

Sympathetic vs parasympathetic

Sympathetic mobilises for fight-or-flight — heart rate up, digestion down. Parasympathetic restores the resting state. Together they are the autonomic nervous system.

Endocrine system

Glands releasing hormones into the bloodstream. Slower than neural signaling but longer-lasting; the pituitary is the master gland.

Adrenal glands

Release adrenaline and cortisol during stress, raising heart rate, blood sugar and alertness. Prolonged cortisol damages the hippocampus.

Hippocampus

Forms new explicit memories. Damage produces anterograde amnesia — the inability to lay down new conscious memories while old ones remain.

Amygdala

Processes fear and aggression and attaches emotional weight to memory, which is why frightening events are remembered vividly.

Hypothalamus

Regulates hunger, thirst, body temperature and sexual behavior, and links the nervous system to the endocrine system through the pituitary.

Cerebellum

Coordinates balance, fine movement and procedural memory. Damage disrupts smooth movement without paralyzing.

Frontal lobe

Handles planning, judgement, personality and voluntary movement. The Phineas Gage case is the classic evidence for its role in personality.

Broca's vs Wernicke's area

Broca's (frontal) produces speech — damage yields halting, effortful speech. Wernicke's (temporal) comprehends it — damage yields fluent but meaningless speech.

Neuroplasticity

The brain's ability to reorganize by forming new connections, greatest in childhood but present throughout life and central to recovery after injury.

Split-brain research

Severing the corpus callosum shows the hemispheres process independently; Sperry and Gazzaniga demonstrated the left hemisphere's dominance for language.

Circadian rhythm

Roughly 24-hour biological cycle regulated by the suprachiasmatic nucleus responding to light. Disrupting it produces jet lag and shift-work fatigue.

Sleep stages and REM

NREM stages deepen into slow-wave sleep, then REM brings vivid dreaming with near-paralyzed muscles. REM proportion rises across the night.

Sensory adaptation

Reduced sensitivity to an unchanging stimulus, which is why you stop noticing a smell. It frees attention for change, which carries more information.

Absolute vs difference threshold

Absolute threshold is the minimum stimulus detectable half the time; difference threshold (JND) is the smallest detectable change.

Weber's law

The JND is a constant proportion of the original stimulus, not a constant amount — so noticing a change depends on how strong the baseline is.

What examiners penalize here

• For neurotransmitter items, memorize one disorder or behavior per transmitter: dopamine–movement/reward, serotonin–mood, ACh–muscles/memory, GABA–anxiety (inhibitory), glutamate–excitatory, endorphins–pain. A single anchor per transmitter is usually enough to earn the point.
• When a question distinguishes brain-imaging tools, sort by structure vs. function: **CT/MRI = structure** (a photograph), **fMRI/PET = function** (a movie of activity), **EEG = electrical activity over time**. Naming the structure/function split usually earns the point on FRQs.
• On nature–nurture FRQs, name the *method* that isolates the influence: **twin studies** (identical vs. fraternal) and **adoption studies** (biological vs. adoptive relatives). Then note that the modern answer is always **interaction**, not one or the other.
• For color-vision items, pair the two theories: **trichromatic (three cones)** explains detection at the receptor level, while **opponent-process** explains **afterimages** and color pairs. Questions about staring at a color then seeing its complement are always opponent-process.

Practice Psychology

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 5 units of AP Psychology

1. Unit 1 · Biological Bases of Behavior
2. Unit 2 · Cognition
3. Unit 3 · Development & Learning
4. Unit 4 · Social Psychology & Personality
5. Unit 5 · Mental & Physical Health