AP Psych Unit 1 — Biological Bases of Behavior Complete Study Guide (2026)
AP Psychology Unit 1 covers the biological foundations of behavior — how the brain, nervous system, and neurotransmitters produce thought, emotion, and action. It is one of the most content-heavy units and accounts for approximately 8–10% of the AP exam. This guide covers every major topic with the level of detail AP readers expect.
What Is in AP Psych Unit 1?
Unit 1 (Biological Bases of Behavior) covers:
- The neuron and neural communication
- Neurotransmitters and their effects
- The nervous system (central and peripheral)
- The brain: structures and functions
- The endocrine system and hormones
- Genetics and behavior
- Research methods in biological psychology (brain scanning, lesion studies)
The Neuron — Structure and Function
A neuron is the basic unit of the nervous system. Understanding how neurons work is foundational to everything else in Unit 1.
Parts of a Neuron
| Part | Function |
|---|---|
| Dendrites | Receive signals from other neurons |
| Cell body (soma) | Integrates incoming signals; contains nucleus |
| Axon | Carries electrical signal away from cell body |
| Myelin sheath | Insulates axon; speeds up signal transmission |
| Nodes of Ranvier | Gaps in myelin where signal jumps (saltatory conduction) |
| Terminal buttons | Release neurotransmitters into synapse |
| Synapse | Gap between terminal button and next neuron |
How Neurons Fire — The Action Potential
At rest, the inside of a neuron is negatively charged (resting potential: −70 mV). When stimulation exceeds the threshold, the neuron fires an action potential — an all-or-nothing electrical signal that travels down the axon.
Key principle: The all-or-nothing law means a neuron either fires completely or not at all. Stimulus intensity is communicated by firing frequency, not signal strength.
After firing, the neuron enters a refractory period — a brief window where it cannot fire again.
Synaptic Transmission
- Action potential reaches terminal buttons
- Neurotransmitters released into synapse
- Neurotransmitters bind to receptors on next neuron
- Remaining neurotransmitters reabsorbed via reuptake or broken down by enzymes
Neurotransmitters — The Big 7
| Neurotransmitter | Function | Deficit/Excess |
|---|---|---|
| Acetylcholine (ACh) | Muscle movement, memory, arousal | Deficit → Alzheimer's disease |
| Dopamine | Reward, movement, motivation | Deficit → Parkinson's; excess → schizophrenia |
| Serotonin | Mood, sleep, appetite | Deficit → depression |
| Norepinephrine | Alertness, arousal, fight-or-flight | Deficit → depression |
| GABA | Main inhibitory neurotransmitter; reduces anxiety | Deficit → anxiety disorders |
| Glutamate | Main excitatory neurotransmitter; learning/memory | Excess → seizures |
| Endorphins | Pain relief, pleasure; released during exercise | Basis for "runner's high" |
Agonists enhance a neurotransmitter's effect (e.g., heroin mimics endorphins). Antagonists block a neurotransmitter's effect (e.g., some antipsychotics block dopamine receptors).
The Nervous System
Central Nervous System (CNS)
- Brain — command center
- Spinal cord — relays signals between brain and body; controls reflexes
Peripheral Nervous System (PNS)
- Somatic nervous system — controls voluntary muscle movement
- Autonomic nervous system — controls involuntary functions (heart rate, digestion)
- Sympathetic — activates fight-or-flight response (increases heart rate, dilates pupils)
- Parasympathetic — restores rest-and-digest state (slows heart rate, aids digestion)
AP exam tip: Sympathetic = arousal/stress. Parasympathetic = calm/recovery. Remember: "Parasympathetic = Peace."
The Brain — Structures and Functions
The Brain Stem (Oldest Structures)
| Structure | Function |
|---|---|
| Medulla | Heartbeat, breathing, reflexes (swallowing, vomiting) |
| Pons | Sleep, arousal, coordinates movement signals |
| Reticular formation | Filters sensory input; controls arousal and sleep/wake |
| Cerebellum | Coordinates movement, balance, procedural memory |
| Thalamus | Sensory relay station (except smell) |
The Limbic System (Emotion and Memory)
| Structure | Function |
|---|---|
| Hypothalamus | Hunger, thirst, sex drive, body temperature; controls pituitary gland |
| Amygdala | Fear, aggression, emotional memories |
| Hippocampus | Formation of new explicit (declarative) memories |
Classic case: Patient H.M. had his hippocampus removed and could no longer form new long-term memories — demonstrating the hippocampus's role in memory consolidation.
The Cerebral Cortex (Thinking and Perception)
The cortex is divided into four lobes:
| Lobe | Location | Function |
|---|---|---|
| Frontal | Front | Higher reasoning, planning, impulse control, voluntary movement (motor cortex), Broca's area (speech production) |
| Parietal | Top/rear | Touch, spatial processing, somatosensory cortex |
| Temporal | Sides | Hearing, language comprehension (Wernicke's area), face recognition |
| Occipital | Back | Vision, visual processing |
Two Key Speech Areas
- Broca's area (left frontal lobe): Speech production. Damage → Broca's aphasia (understands speech but can't produce it fluently)
- Wernicke's area (left temporal lobe): Speech comprehension. Damage → Wernicke's aphasia (produces fluent but meaningless speech)
Brain Lateralization
The two cerebral hemispheres handle different functions:
- Left hemisphere: Language, logic, math, writing
- Right hemisphere: Spatial tasks, facial recognition, music, creativity
The hemispheres communicate via the corpus callosum. Split-brain patients (corpus callosum severed) demonstrate that the hemispheres can function independently.
The Endocrine System
The endocrine system communicates via hormones — slower than neural signals but longer-lasting.
| Gland | Hormone(s) | Function |
|---|---|---|
| Pituitary | Growth hormone, many others | "Master gland" — controls other glands |
| Adrenal glands | Epinephrine (adrenaline), cortisol | Stress response; fight-or-flight |
| Thyroid | Thyroxine | Metabolism, energy |
| Pancreas | Insulin, glucagon | Blood sugar regulation |
| Testes/Ovaries | Testosterone, estrogen | Sex characteristics, sexual behavior |
Hypothalamus → Pituitary → Adrenal glands is the key stress axis tested on the AP exam (HPA axis).
Brain Research Methods
| Method | What It Shows | Limitation |
|---|---|---|
| EEG (electroencephalogram) | Electrical activity; good for timing | Poor spatial resolution |
| CT scan | Brain structure (X-rays) | Radiation; no function |
| MRI | Detailed brain structure | Expensive; no real-time function |
| fMRI | Brain activity during tasks (blood flow) | Slow; expensive |
| PET scan | Metabolic activity (glucose use) | Radiation; low resolution |
| Lesion studies | Function of damaged areas | Can't control damage precisely |
AP Psych Unit 1 Practice Questions
1. A patient can understand speech normally but cannot produce fluent sentences. Which brain area is most likely damaged?
Broca's area (left frontal lobe)
2. Which neurotransmitter is most directly associated with the "runner's high" experienced after prolonged exercise?
Endorphins
3. After an action potential, the neuron enters a period during which it cannot fire again. This is called the:
Refractory period
4. The sympathetic nervous system would be most active during which situation?
Being chased — fight-or-flight activation (increased heart rate, pupil dilation, adrenaline release)
5. A researcher removes the amygdala of a rat. What behavior change is most likely?
Reduced fear response — the rat no longer avoids stimuli associated with pain or threat