What you'll learn
Everything we think, feel and do is rooted in biology — in the brain, the nervous system, and the chemicals that carry signals between cells. This is the focus of the Biological Bases of Behavior unit of AP Psychology. For AP Psychology you need to understand the neuron and how it communicates, neurotransmitters, the nervous and endocrine systems, the structures of the brain, and how the brain is studied. This guide covers neurons and neural firing, neurotransmitters, the nervous and endocrine systems, brain structures, and research methods. By the end you should be able to explain how the biology of the body produces behavior.
Key terms and definitions
Neuron — A nerve cell that transmits information through the nervous system.
Action potential — The electrical impulse that travels along a neuron.
Synapse — The gap between two neurons where chemical signals pass.
Neurotransmitter — A chemical that carries a signal across the synapse.
Central nervous system (CNS) — The brain and spinal cord.
Peripheral nervous system (PNS) — The nerves outside the brain and spinal cord.
Endocrine system — The system of glands that release hormones into the blood.
Neuroplasticity — The brain's ability to change and reorganise itself.
Core concepts
The neuron and neural firing
The neuron is the basic unit of the nervous system. A neuron receives signals through its dendrites, passes an electrical impulse (the action potential) along its axon, and sends signals from the axon terminals. Neural firing follows the all-or-none principle: a neuron either fires a full action potential or does not fire at all, once the stimulation reaches a threshold. Many axons are covered in a myelin sheath, which speeds up the impulse.
Communication across the synapse
Neurons do not touch; they are separated by a tiny gap called the synapse. When an action potential reaches the end of a neuron, it triggers the release of neurotransmitters, which cross the synapse and bind to receptors on the next neuron, passing the signal on. Afterwards, leftover neurotransmitter is reabsorbed in a process called reuptake. This chemical communication is how neurons form the vast networks that produce thought and behavior.
Neurotransmitters and their effects
Different neurotransmitters have different effects on mood, behavior and body functions. Examples include:
- Dopamine — involved in reward, motivation and movement.
- Serotonin — affects mood, sleep and appetite.
- Acetylcholine — involved in muscle movement and memory.
- GABA — the main inhibitory neurotransmitter, calming activity.
- Endorphins — reduce pain and produce feelings of pleasure.
Imbalances in neurotransmitters are linked to disorders (for example, low serotonin with depression), and many drugs work by affecting neurotransmitter systems.
The nervous system
The nervous system has two main divisions:
- The central nervous system (CNS) — the brain and spinal cord, which process information and coordinate responses.
- The peripheral nervous system (PNS) — the nerves connecting the CNS to the rest of the body. The PNS includes the somatic system (voluntary movement) and the autonomic system (automatic functions), which is further split into the sympathetic (arousing, "fight or flight") and parasympathetic (calming, "rest and digest") divisions.
The endocrine system
The endocrine system is the body's slower chemical communication system. Glands release hormones into the bloodstream, which affect organs and behavior. For example, the adrenal glands release adrenaline during stress, and the pituitary gland (the "master gland") controls other glands. Unlike the fast, brief signals of neurons, hormones act more slowly but have longer-lasting effects.
Structures of the brain
Key brain structures include:
- The brainstem and medulla — control vital functions like heartbeat and breathing.
- The cerebellum — coordinates movement and balance.
- The limbic system — including the amygdala (emotion, especially fear), the hippocampus (memory), and the hypothalamus (drives such as hunger and body regulation).
- The cerebral cortex — the outer layer, responsible for higher thinking, divided into lobes (frontal, parietal, temporal, occipital) with different functions.
The brain also shows neuroplasticity — the ability to change and form new connections, especially important in learning and recovery from injury.
Studying the brain
Psychologists study the brain in several ways: by observing the effects of damage to particular areas (case studies of lesions), and by using brain imaging techniques such as the EEG (recording electrical activity) and fMRI (showing which areas are active during tasks). These methods have revealed which brain regions are involved in different functions.
Genetics, evolution and behavior
The biological approach also considers how genes and evolution shape behavior. Behavioral traits can be influenced by heredity, and psychologists study this using twin studies (comparing identical and fraternal twins) and adoption studies to estimate how much of a trait is due to nature versus nurture. The evolutionary perspective argues that some behaviors exist because they helped our ancestors survive and reproduce, so they were naturally selected. For example, fear responses to certain dangers may have had survival value. These ideas connect the biology of individuals to the broader questions of why certain behaviors are common, and they are part of the biological bases of behavior alongside the nervous and endocrine systems.
The effects of drugs on neurotransmission
Because neurons communicate with neurotransmitters, many drugs work by affecting this process, and this is commonly tested. Agonists are drugs that increase or mimic the action of a neurotransmitter, while antagonists block or reduce its action. Some drugs affect reuptake — for example, blocking the reuptake of serotonin leaves more of it in the synapse, which is how some antidepressants work. Other substances affect neurotransmitter release or receptor binding. Understanding that drugs act at the synapse — enhancing, blocking or prolonging neurotransmitter effects — links the biology of neural communication to real effects on mood and behavior, and explains how medications and recreational drugs change the way people think and feel.
Worked examples
Example 1: The all-or-none principle
Explain what the all-or-none principle means. It means a neuron either fires a full action potential or does not fire at all, once stimulation reaches the threshold. The strength of a stimulus does not change the size of the action potential, only whether the neuron fires and how often.
Example 2: How neurons communicate
Describe how a signal passes from one neuron to the next. When the action potential reaches the axon terminal, it triggers the release of neurotransmitters into the synapse. These cross the gap and bind to receptors on the next neuron, passing on the signal. Leftover neurotransmitter is then reabsorbed by reuptake.
Example 3: Sympathetic nervous system
A person is frightened and their heart races. Which part of the nervous system is responsible? The sympathetic division of the autonomic nervous system, which produces the "fight or flight" response, increasing heart rate and preparing the body for action.
Example 4: Linking a structure to a function
Which brain structure is most associated with forming new memories? The hippocampus, part of the limbic system, is most associated with forming new memories. Damage to it impairs the ability to form new long-term memories.
Common mistakes and how to avoid them
A common error is thinking a stronger stimulus makes a bigger action potential. Because of the all-or-none principle, the action potential is the same size each time; a stronger stimulus makes the neuron fire more often, not more strongly.
Students often confuse the nervous and endocrine systems. The nervous system uses fast electrical and chemical signals; the endocrine system uses hormones in the blood, which are slower but longer-lasting.
Another mistake is mixing up brain structures. Learn the key ones: amygdala (fear/emotion), hippocampus (memory), hypothalamus (drives and regulation), cerebellum (movement/balance). Matching structure to function is frequently tested.
When describing the synapse, do not say the impulse "jumps" the gap. A chemical (neurotransmitter) is released and crosses the synapse to the next neuron.
Finally, remember the sympathetic and parasympathetic divisions are opposites: sympathetic arouses ("fight or flight"), parasympathetic calms ("rest and digest"). Do not confuse them.
Exam technique for "Biological Bases of Behavior"
Be ready to describe neural firing (all-or-none, threshold, action potential) and synaptic transmission (neurotransmitters, reuptake) clearly, using correct terms.
Know the divisions of the nervous system and the sympathetic/parasympathetic distinction, and be able to link neurotransmitters and brain structures to their functions — these matching relationships are common multiple-choice items.
For free-response questions, apply concepts to scenarios (for example, explaining a stress response biologically) rather than just defining terms. Use precise terminology — neuron, synapse, neurotransmitter, autonomic, endocrine — and connect biology to behavior throughout.
Quick revision summary
- The neuron fires an action potential on the all-or-none principle; the myelin sheath speeds it up.
- Neurons communicate across the synapse using neurotransmitters (e.g. dopamine, serotonin, GABA), which are then reabsorbed by reuptake.
- The CNS is the brain and spinal cord; the PNS includes the somatic and autonomic systems (sympathetic = "fight or flight", parasympathetic = "rest and digest").
- The endocrine system uses hormones in the blood — slower but longer-lasting than neural signals.
- Key brain structures: amygdala (emotion/fear), hippocampus (memory), hypothalamus (drives), cerebellum (movement), cortex (higher thinking); the brain shows neuroplasticity.
- The brain is studied through damage/lesions and imaging (EEG, fMRI).