Unit 1: Biological Bases of Behavior

Foundations: Neurons, Glia, and the Body–Brain Connection

Biological psychology starts from a simple but powerful idea: everything you think, feel, and do is supported by physical processes in your body, especially your nervous system. That does not mean your experiences are “nothing but chemicals.” It means your experiences have a biological implementation. If you want to understand why caffeine makes you alert, why damage to one brain region can change personality, or why chronic stress can affect memory, you need the basics of how the nervous system is built.

The nervous system’s “parts list”: neurons and glia

Your nervous system is made of specialized cells, and the two big categories to know are neurons and glial cells (glia).

A neuron is the basic unit of structure and function of the nervous system. Neurons transmit information using electrical signals within the cell and chemical signals between cells. Patterns of neural activity are how the brain represents information: sensations, decisions, memories, emotions, and actions all correspond to coordinated activity across networks of neurons.

Glial cells are support cells that help neurons function. They guide the growth of developing neurons, provide nutrients, help clean up wastes, and form an insulating myelin sheath around many axons to speed conduction. A common misconception is that only neurons matter for behavior; in reality, glia influence how efficiently neural circuits work and how the brain responds to injury.

Basic neuron structure (and why each part matters)

Most neurons share a similar layout. Knowing these parts helps you predict what happens when something goes wrong.

• Cell body (also called cyton or soma): contains cytoplasm and the nucleus, which directs synthesis of substances such as neurotransmitters. The cell body integrates incoming signals and is often described as the neuron’s “decision area.”
• Dendrites: branching tubular processes capable of receiving information from other neurons.
• Axon: emerges from the cell body as a single conducting fiber (typically longer than a dendrite) that carries impulses away from the soma.
• Myelin sheath: fatty insulating layer (formed by glial cells) that speeds up neural conduction.
• Axon terminals: the axon branches and ends in tips called terminal buttons, axon terminals, or synaptic knobs, which release chemical messengers.
• Synaptic vesicles: structures inside terminal buttons that store neurotransmitters.

Why this matters: Many biological explanations of behavior reduce to “communication got faster/slower” or “the signal didn’t reach the right place.” For example, if myelin is damaged, signals slow down, which can impair coordinated movement and information processing.

Types of neurons: sensory, motor, and interneurons (plus effectors)

Neural pathways are often described using three functional neuron types:

• Sensory (afferent) neurons carry information from sensory receptors (skin, eyes, ears, etc.) to the central nervous system.
• Motor (efferent) neurons carry commands from the central nervous system to muscles and glands.
• Interneurons connect neurons within the brain and spinal cord and handle much of the integration and interpretation that the CNS performs.

Effectors are the muscle and gland cells that actually carry out the response: muscles contract and glands secrete.

A common exam trap is mixing up afferent and efferent. A helpful memory aid is: Afferent arrives (to the CNS); Efferent exits (from the CNS).

Neurogenesis and neural networks

Neurogenesis, the growth of new neurons, takes place throughout life. At the same time, most mental functions depend less on a single “special neuron” and more on neural networks: interconnected groups of neurons that work together. A single neuron can participate in many networks, and complex mental processes emerge from coordinated patterns across many areas. This network perspective helps explain why brain damage often causes complicated effects: injury may disrupt a network, not just one isolated function.

Exam Focus

• Typical question patterns
  • Identify neuron parts in a diagram and explain what each part does (especially dendrites, axon, myelin, terminals).
  • Distinguish sensory vs. motor vs. interneurons in a reflex or pathway description; identify the effector.
  • Predict an outcome if myelin is damaged or if dendrites are reduced.
• Common mistakes
  • Treating glia as irrelevant “filler” rather than active support cells that influence neural functioning.
  • Confusing dendrites (input) with axons (output).
  • Assuming one neuron equals one behavior instead of thinking in circuits and networks.

Neural Communication: How Neurons Generate Signals and Talk to Each Other

To connect biology to behavior, you need a clear picture of how neurons send information. Neural signaling is electrochemical: electricity within the neuron (movement of charged particles) and chemistry between neurons (neurotransmitters).

Resting potential: polarization, ions, and selective permeability

When a neuron is “at rest,” it is not inactive; it is ready. The neuron is more negative inside the cell membrane relative to outside. This resting potential (also described as polarization) comes from the membrane’s selective permeability and from different concentrations of electrically charged particles called ions inside and outside the cell.

A frequent misunderstanding is to think the neuron is “full of electricity” that gets released. Instead, the neuron’s signal comes from controlled changes in membrane permeability and ion movement.

Action potentials: threshold, all-or-none, and recovery

An action potential is a brief electrical impulse that travels down the axon.

Key ideas:

1. Threshold: The neuron fires only if incoming signals push it to a minimum level of activation.
2. All-or-none principle: If threshold is reached, the neuron fires fully. If stimulation is not strong enough, the neuron doesn’t fire. Action potentials do not vary in size; stronger stimulation is typically coded by firing rate (how often the neuron fires).
3. Depolarization and repolarization: When sufficiently stimulated, a net flow of sodium ions into the cell causes the rapid change in membrane potential (the action potential). The wave of depolarization and repolarization moves down the axon to the terminal buttons.
4. Refractory period: After firing, the neuron needs a short recovery time before it can fire again. This limits firing rate and helps impulses move in one direction.

Myelin, nodes of Ranvier, and saltatory conduction

The myelin sheath speeds up action potentials. Spaces between segments of myelin are called nodes of Ranvier. In myelinated axons, depolarizations can “jump” from node to node, increasing conduction speed. This is saltatory conduction.

In practice, faster communication supports quicker reflexes, smoother coordinated movement, and more efficient processing. Developmental changes in myelination are often tied to changes in cognitive and motor abilities.

Synapses: neurotransmitters, receptors, and the excitatory/inhibitory balance

Neurons do not usually touch. The tiny gap between a sending neuron’s terminal and a receiving neuron’s dendrite is the synaptic gap (or synaptic cleft). The junction is the synapse.

Across the synapse, communication is chemical:

1. The action potential reaches the axon terminals.
2. The sending neuron releases neurotransmitters from synaptic vesicles into the synaptic gap.
3. Neurotransmitters bind to receptor sites on the receiving neuron.
4. This binding changes the likelihood that the receiving neuron will fire.

Neurotransmitter effects can be:

• Excitatory: increases the likelihood the receiving neuron will fire.
• Inhibitory: reduces or prevents neural impulses.

A helpful analogy is a car: excitatory input is like the gas pedal; inhibitory input is like the brake. Inhibition is essential for control, focus, and coordinated movement; without it, neural activity can become unstable.

Clearing neurotransmitters: reuptake and breakdown

After neurotransmitters send a message, they must be removed from the synapse.

• Reuptake: the sending neuron reabsorbs neurotransmitters.
• Enzymatic breakdown: enzymes in the synapse break neurotransmitters down.

Many medications and drugs work by altering reuptake or breakdown, changing how long neurotransmitters stay active.

Major neurotransmitters to know (and what they’re associated with)

AP Psychology emphasizes a set of “big name” neurotransmitters. Effects depend on receptor type and brain region, so avoid the idea that one neurotransmitter equals one emotion.

• Acetylcholine (ACh): involved in muscle action, learning, and memory; problems in ACh systems are associated with memory-related disorders.
• Dopamine: involved in movement, learning, attention, and reward; it also stimulates the hypothalamus to synthesize hormones and affects alertness and movement. Too much or too little activity in dopamine pathways has been linked to different disorders.
• Serotonin: affects mood, sleep, and appetite; also associated with sexual activity, concentration/attention, moods, and emotions.
• Norepinephrine (noradrenaline): involved in alertness and arousal; also associated with attentiveness, sleeping, dreaming, and learning.
• GABA (gamma-aminobutyric acid): major inhibitory neurotransmitter that inhibits firing of neurons.
• Glutamate: major excitatory neurotransmitter involved in information processing throughout the cortex and especially memory formation in the hippocampus.
• Endorphins / opioid peptides: natural painkillers linked to pain control and pleasure.

Drugs at the synapse: agonists and antagonists

A lot of exam questions describe a drug’s effect and ask what it is doing at the synapse.

• An agonist increases a neurotransmitter’s action. It may mimic a neurotransmitter and bind to its receptor site to produce the effect of the neurotransmitter; it can also increase availability by blocking reuptake or otherwise boosting signaling.
• An antagonist decreases a neurotransmitter’s action, often by blocking a receptor site and inhibiting the effect of the neurotransmitter or agonist.

How to reason through drug questions:

1. Identify the neurotransmitter system involved.
2. Decide whether the described effect increases or decreases signaling.
3. Map that to agonist vs. antagonist.

Examples (conceptual)

• Reuptake and mood-related medications: If a medication blocks the reuptake of serotonin, serotonin stays in the synapse longer, increasing its effects on the receiving neuron. That is an agonist-like effect on serotonin transmission.
• Blocking receptors: If a substance binds to dopamine receptors without activating them, it prevents dopamine from binding effectively and reduces dopamine’s effects (antagonist).

Exam Focus

• Typical question patterns
  • Explain the sequence of neural transmission from action potential to neurotransmitter release to receptor binding.
  • Use threshold/all-or-none to explain why intensity is often coded by firing rate.
  • Determine whether a drug is an agonist or antagonist based on its effect (reuptake blocked, receptors blocked, neurotransmitter mimicked).
  • Identify which neurotransmitter is most associated with a described function (movement, mood, arousal, learning, pain control).
• Common mistakes
  • Saying action potentials vary in size (they’re all-or-none).
  • Treating neurotransmitters as inherently excitatory or inhibitory in all contexts (effects depend on receptor type and location).
  • Confusing reuptake (reabsorption by sending neuron) with receptor binding (on receiving neuron).

Organization of the Nervous System: From Reflexes to Regulation

Once you understand how a single neuron communicates, the next step is seeing how neurons are organized into larger systems.

Central vs. peripheral nervous systems

• Central nervous system (CNS): the brain and spinal cord.
• Peripheral nervous system (PNS): the nerves that connect the CNS to the rest of the body.

The PNS lies outside the midline portion of the nervous system, carrying sensory information to and motor information away from the CNS via spinal and cranial nerves.

A simple way to remember it: the CNS is the “command center,” while the PNS is the “wiring.”

Somatic vs. autonomic nervous systems (within the PNS)

The PNS is subdivided into:

• Somatic nervous system: carries sensory information to the CNS and has motor neurons that stimulate skeletal (voluntary) muscles.
• Autonomic nervous system (ANS): has motor neurons that stimulate smooth (involuntary) and heart muscle; it controls involuntary functions like heart rate, digestion, and gland activity.

Sympathetic vs. parasympathetic: arousal and recovery

The autonomic nervous system is subdivided into antagonistic branches that work together to keep the body in balance.

• Sympathetic nervous system (“fight-or-flight”): mobilizes the body’s resources in stressful situations. Sympathetic stimulation can include dilation of pupils, release of glucose from the liver, dilation of bronchi, inhibition of digestive functions, acceleration of heart rate and breathing rate, secretion of adrenaline from the adrenal glands, and inhibition of tear gland secretion.
• Parasympathetic nervous system (“rest-and-digest”): calms the body following sympathetic activation by restoring digestive processes (salivation, peristalsis, enzyme secretion), returning pupils to normal size, stimulating tear glands, and restoring normal bladder contractions.

Psychologically, many emotions include autonomic changes. Anxiety often includes sympathetic activation (racing heart), while relaxation involves parasympathetic rebound. A common misconception is that sympathetic and parasympathetic are simple on/off switches; in reality, they adjust continuously and can both be active in complex ways.

The spinal cord and reflexes: reflex arcs and effectors

The spinal cord is more than a cable connecting brain and body. It can coordinate fast, automatic responses called reflexes.

The spinal cord is protected by membranes called meninges and by the spinal column of bony vertebrae. It starts at the base of your back and extends upward to the base of your skull where it joins the brain.

A reflex often involves impulse conduction over a few (perhaps three) neurons; this path is a reflex arc:

1. Sensory (afferent) neuron transmits impulses from sensory receptors to the spinal cord or brain.
2. Interneurons (within brain/spinal cord) intervene between sensory and motor neurons.
3. Motor (efferent) neuron transmits impulses to effectors (muscle cells that contract or gland cells that secrete).

Example: stepping on something sharp
You withdraw your foot quickly before you’ve fully processed what happened. The immediate withdrawal is largely spinal (reflex arc), while the conscious recognition (“That hurt!”) and planning (“I should look down”) involve the brain.

Exam Focus

• Typical question patterns
  • Classify a function as CNS vs. PNS, and then as somatic vs. autonomic.
  • Compare sympathetic and parasympathetic effects in a scenario (test anxiety, relaxation, exercise) using concrete body changes (pupils, digestion, heart rate).
  • Trace a reflex arc and correctly label afferent, interneuron, efferent, and effector.
• Common mistakes
  • Mixing up sympathetic (arousal) and parasympathetic (calming).
  • Treating the autonomic system as only about “stress” instead of ongoing regulation.
  • Describing reflexes as “decisions” rather than automatic circuits.

Brain Structure and Function: How Different Regions Support Behavior

AP Psychology expects you to connect brain regions with the behaviors they support and to reason from damage or activation to predicted behavioral changes.

Localization, networks, and plasticity

Some brain functions are localized (certain areas are especially important for certain tasks), but the brain also relies on distributed processing across networks.

Plasticity means that although specific regions are associated with specific functions, if one region is damaged the brain can sometimes reorganize to take over its function. Plasticity can involve strengthening or weakening synaptic connections, forming new connections, and (especially in younger brains) reassigning functions after injury. Plasticity is powerful but not limitless.

The triune brain model (an evolutionary overview)

One influential (though simplified) evolutionary model is the triune brain idea: the human brain has three major, overlapping divisions, with more recent systems nearest the front and top.

• Reptilian brain: maintains homeostasis and instinctive behaviors; roughly corresponds to the brainstem, which includes the medulla, pons, and cerebellum.
• Old mammalian brain: roughly corresponds to the limbic system, including the septum, hippocampus, amygdala, cingulate cortex, hypothalamus, and thalamus; important in emotional behavior, some aspects of memory, and (in this model’s broad framing) vision.
• New mammalian brain (neocortex): synonymous with the cerebral cortex, about 80% of brain volume; associated with higher functions such as judgment, decision making, abstract thought, foresight, hindsight and insight, language, and “computing,” as well as sensation and perception.

The cerebral cortex: folds and surface area

The surface of the cortex has peaks called gyri and valleys called sulci. These folds (called convolutions) increase cortical surface area. Deeper valleys are fissures.

Brainstem and cerebellum: survival and coordination

• Medulla oblongata: regulates heart rhythm, blood flow, breathing rate, digestion, and vomiting. Damage can be life-threatening.
• Pons: involved in sleep and arousal; includes a portion of the reticular activating system/ reticular formation critical for arousal and wakefulness; sends information to and from the medulla, cerebellum, and cerebral cortex.
• Reticular formation: network involved in arousal and alertness.
• Cerebellum: controls posture, equilibrium, and movement; helps coordinate voluntary movements, balance, and posture, and contributes to learning that involves timing and fine motor adjustments.

Basal ganglia

The basal ganglia help regulate initiation of movements, balance, eye movements, and posture, and they function in processing implicit memories.

Thalamus: sensory relay

The thalamus relays visual, auditory, taste, and somatosensory information to and from appropriate areas of the cerebral cortex (often described as routing sensory information except smell). It also helps regulate attention by influencing what information gets prioritized.

Limbic system: emotion, motivation, and memory

Key limbic structures commonly tested include:

• Amygdala: processes emotion, particularly fear and threat detection; helps tag experiences as emotionally significant.
• Hippocampus: enables formation of new long-term (explicit) memories.
• Hypothalamus: controls feeding behavior, drinking behavior, body temperature, sexual behavior, and threshold for rage behavior; activates sympathetic and parasympathetic systems; and controls the endocrine system through regulation of pituitary hormone secretion.

The cerebral cortex: lobes, primary areas, and association areas

The cortex is the wrinkled outer layer associated with complex processing.

Lobes of the cortex

• Frontal lobe: decision-making, planning, judgment, impulse control, and voluntary movement.
• Parietal lobe: touch and body position (somatosensation) and spatial processing.
• Occipital lobe: visual processing.
• Temporal lobe: auditory processing; involved in language and memory.

Motor and somatosensory cortices

• Motor cortex (frontal lobe): controls voluntary movement; body mapping is uneven (hands/face take more space).
• Somatosensory cortex (parietal lobe): processes touch and body sensations; also mapped unevenly.

Association areas
Association areas do not have specific sensory or motor functions; they integrate information and support higher mental functions such as thinking, planning, remembering, and communicating. Many AP-style questions describe someone who can see/hear but can’t interpret, plan, or connect information; association areas are a key concept for those scenarios.

Language areas and classic aphasias (plus the historical evidence)

For most right-handed people (and many left-handed people), key language areas are in the left hemisphere.

• Broca’s area (left frontal lobe): speech production. Damage can cause expressive aphasia, difficulty producing fluent speech.
• Wernicke’s area (left temporal lobe): language comprehension. Damage can cause receptive aphasia, difficulty comprehending written and spoken language.

How we learned this (classic cases):
In 1861, Paul Broca performed an autopsy on a patient nicknamed Tan, who had lost the capacity to speak even though his mouth and vocal cords weren’t damaged and he could still understand language. Broca found deterioration in part of the left frontal lobe, a pattern also seen in similar cases. Carl Wernicke similarly identified a left temporal lobe area involved in understanding language.

Hemispheric specialization and split-brain research

The two hemispheres are connected by the corpus callosum, which allows communication between hemispheres.

Hemispheric specialization (lateralization) refers to the tendency for certain functions to be more dominant in one hemisphere. A broad, test-relevant pattern is that language is often left-lateralized, while some spatial and facial processing tasks are often right-lateralized.

Split-brain research (notably by Roger Sperry and Michael Gazzaniga) studied patients whose corpus callosum was cut (historically, sometimes to reduce severe epilepsy). Because the hemispheres cannot share information effectively, researchers could test what each hemisphere can do.

A classic pattern:

• Information presented to the right visual field goes to the left hemisphere (often language-dominant), so the person can usually name what they saw.
• Information presented to the left visual field goes to the right hemisphere, so the person may be able to point to what they saw with the left hand but may struggle to name it.

A misconception to avoid: split-brain patients do not typically have “two completely separate minds” in everyday life; many functions remain coordinated.

Damage-based reasoning examples

• If someone has trouble forming new long-term memories of events after a brain injury but can still remember older events and can still learn some skills with practice, think about the hippocampus and the idea that different memory systems exist.
• If a person shows reduced fear responses to threats after damage to a structure involved in threat processing, the amygdala becomes a strong candidate.

Exam Focus

• Typical question patterns
  • Match structures to functions: medulla, pons/reticular formation, cerebellum, basal ganglia, thalamus, hypothalamus, amygdala, hippocampus, cortical lobes, motor/somatosensory cortex, association areas.
  • Predict behavioral changes from damage (aphasias; memory formation issues; impaired coordination; changes in arousal).
  • Interpret split-brain scenarios involving visual fields and naming vs. pointing.
• Common mistakes
  • Treating the limbic system as a single structure rather than a group with different roles.
  • Confusing Broca’s (production) with Wernicke’s (comprehension).
  • Overstating “left brain vs. right brain” myths (both hemispheres contribute to most tasks).

How Psychologists Study the Brain: Tools, Tradeoffs, and Evidence

Knowing brain parts is only half the story. You also need to know the methods used to link brain and behavior and the limitations of each.

Lesions and ablation

A lesion is damage to brain tissue. Lesion studies examine what abilities change after damage to a particular area. Lesions can occur naturally (injury/stroke) or be induced in controlled ways in animal research.

More precise lesions (sometimes described as precise destruction of brain tissue) allow systematic study of loss of function resulting from surgical removal (ablation), cutting neural connections, or destruction by chemical applications.

Key limitation: Lesions are rarely neat; damage can affect multiple areas, and the brain can reorganize.

EEG (and evoked potentials)

An EEG (electroencephalogram) is an amplified tracing of brain activity produced when electrodes positioned over the scalp transmit signals about the brain’s electrical activity (“brain waves”) to an electroencephalograph machine.

• Strength: excellent temporal resolution (timing), useful for sleep and seizure activity.
• Limitation: poor spatial resolution (hard to pinpoint deep sources).

When the recorded change in voltage results from a response to a specific stimulus presented to the subject, the tracings are called evoked potentials.

Structural imaging: CT/CAT and MRI

• CT/CAT (computerized axial tomography): uses X-rays taken from various angles to create computerized two-dimensional “slices” that can be arranged to show the extent of a lesion.
• MRI (magnetic resonance imaging): uses a magnetic field and pulses of radio waves; faint radio frequency signals depend on tissue density, producing detailed soft-tissue images.

Structural scans show what the brain looks like, not what it is doing in the moment.

Functional imaging: PET and fMRI

• PET (positron emission tomography): produces color computer graphics that depend on the amount of metabolic activity in an imaged region (often via a radioactive tracer related to glucose use).
• fMRI (functional MRI): shows the brain at work at higher resolution than PET. Changes in oxygen in the blood of an active brain area alter its magnetic qualities; the scanner records these changes (often described as the BOLD signal).

Functional imaging infers neural activity indirectly, and activation does not automatically prove that a region caused a behavior.

MEG / MSI

A magnetic source image (MSI), produced by magnetoencephalography (MEG), is similar to an EEG but detects the slight magnetic fields caused by electric potentials in the brain.

Converging evidence and interpretation pitfalls

Strong conclusions often come from converging evidence: multiple methods pointing to the same relationship (for example, lesions showing necessity plus fMRI showing task-related activation).

Common pitfalls:

1. Correlation vs. causation in imaging: fMRI/PET show associations.
2. Reverse inference: activation in an area does not prove a specific mental state.
3. Overprecision: colorful scans can look exact, but interpretation depends on analysis choices and comparison conditions.

Exam Focus

• Typical question patterns
  • Choose the best method for a goal: EEG for timing/sleep; MRI/CT for structure; fMRI/PET for function; lesion for whether an area is necessary.
  • Interpret what a scan can and cannot conclude.
  • Explain why combining methods strengthens a claim.
• Common mistakes
  • Claiming fMRI “reads thoughts” rather than inferring activity from blood oxygenation.
  • Assuming EEG can precisely locate deep-brain activity.
  • Treating activation as proof an area is exclusively responsible for a behavior.

The Endocrine System: Hormones as Slower, Longer-Lasting Messengers

Neural communication is fast and precise, but the body also communicates using hormones, which tend to act more slowly and for longer periods.

Hormones vs. neurotransmitters

A hormone is a chemical messenger secreted by endocrine glands into the bloodstream. Hormones travel to target organs/cells and bind to specific receptors.

Compared with neurotransmitters:

• Neurotransmitters act across synapses and often have rapid, localized effects.
• Hormones travel through the bloodstream and often have broader, longer-lasting effects.

A useful analogy: neurotransmitters are like a targeted text message; hormones are like a radio broadcast that only cells with the right “receiver” (receptor) can respond to.

Major endocrine glands and their roles

The endocrine system consists of glands that secrete hormones into the blood. Key glands include the pineal gland, hypothalamus, and pituitary in the brain; the thyroid and parathyroids in the neck; adrenal glands atop the kidneys; the pancreas near the stomach; and the gonads (testes or ovaries).

• Pineal gland: produces melatonin, which helps regulate circadian rhythms and is associated with seasonal affective disorder.
• Hypothalamus: acts as an endocrine gland and produces hormones that stimulate (releasing factors) or inhibit pituitary hormone secretion; it links nervous system activity to endocrine control.
• Pituitary gland: often called the “master gland,” but it is directed by the hypothalamus. It produces stimulating hormones that promote secretion by other glands, including:
  • TSH (thyroid-stimulating hormone)
  • ACTH (adrenocorticotropic hormone), which stimulates the adrenal glands
  • FSH (follicle-stimulating hormone), which stimulates egg or sperm production
  • ADH (antidiuretic hormone), which helps retain water
  • HGH (human growth hormone)
• Thyroid gland: produces thyroxine, which stimulates and maintains metabolic activities.
• Parathyroids: produce parathyroid hormone, helping maintain calcium ion levels in the blood necessary for normal neuron functioning.
• Adrenal glands: produce stress-related hormones that mobilize energy and support arousal (including adrenaline/epinephrine in common descriptions of the stress response).
• Pancreas: secretes insulin and glucagon, regulating blood sugar that fuels behavioral processes.
• Ovaries and testes: produce hormones necessary for reproduction and development of secondary sex characteristics.

Stress as a brain–body loop

Stress shows the nervous and endocrine systems working together:

• The brain evaluates a situation (cognitive appraisal).
• The hypothalamus coordinates bodily responses.
• Endocrine hormones support sustained arousal and energy use.

This helps explain why chronic stress can affect sleep, immune function, and mood: the alarm system is useful short-term but costly when constantly activated.

Example: stage fright
Before a performance, sympathetic activation can increase heart rate quickly, while stress hormones can sustain heightened alertness longer. You can feel “revved up” even after the immediate threat is gone because hormonal effects persist.

Exam Focus

• Typical question patterns
  • Distinguish neurotransmitter signaling from hormonal signaling (synapse vs. bloodstream; fast vs. slower/longer-lasting).
  • Identify the hypothalamus–pituitary relationship and how it regulates other glands.
  • Apply endocrine concepts to stress and energy scenarios (adrenals; blood glucose; sustained arousal).
• Common mistakes
  • Calling the pituitary the master gland without noting hypothalamic control.
  • Treating hormones as acting instantly like neurotransmitters.
  • Forgetting that hormones require receptor sites; effects depend on target cells.

Genetics, Evolution, and Behavior: Nature and Nurture Working Together

Biological bases of behavior include genetic influences and evolutionary processes. AP Psychology emphasizes that genes and environment work together rather than competing.

Nature–nurture and what behavioral geneticists study

The nature–nurture controversy asks how much heredity and environment each influence behavior. Behavioral geneticists study how genes and environment contribute to individual differences in mental ability, emotional stability, temperament, personality, and interests.

Genes, chromosomes, genotype, and phenotype

A gene is a DNA segment on a chromosome that contributes to traits by influencing protein production and biological regulation. Genes influence behavior indirectly by shaping neural development, neurotransmitter systems, hormones, and sensitivity to environmental inputs.

• Chromosomes carry genetic information to new cells during reproduction.
• Normal human body cells have 46 chromosomes; eggs and sperm have 23.
• Genotype: an individual’s genetic makeup for a trait.
• Phenotype: the observable expression of genes.
• Dominant vs. recessive: if paired genes differ, the expressed gene is called dominant and the hidden gene is recessive.

Twin studies, adoption studies, and heritability

Because we can’t ethically manipulate human genes, psychologists use natural experiments.

• Identical (monozygotic) twins: develop from the same fertilized egg (zygote) and share (essentially) all genes.
• Fraternal (dizygotic) twins: develop from two different fertilized eggs and share about half their genes, like typical siblings.

If identical twins are more similar than fraternal twins on a trait, that suggests genetic influence.

Adoption studies compare adopted individuals to biological relatives (shared genes, different environment) and adoptive relatives (different genes, shared environment).

Heritability is the proportion of variation among individuals in a population that is due to genetic causes. Key clarifications:

• It describes variation in a group, not “percent genetic” for a person.
• High heritability does not mean a trait is unchangeable.
• Heritability can change when environments change.

A key limitation in both twin and adoption work is that environments aren’t random (for example, identical twins may be treated more similarly, and adoption placement can be systematic).

Gene–environment interaction and epigenetics

A gene–environment interaction means the effect of genes depends on the environment, and the effect of environment depends on genes (for example, a stress-sensitive predisposition expressed differently depending on support and coping resources).

Epigenetics refers to changes in gene expression that occur without changing the DNA sequence. Environmental factors (stress, nutrition, toxins) can influence whether genes are “turned on” or “turned down,” helping explain how early experiences can have long-term effects.

Evolutionary psychology and natural selection

Natural selection is the process by which traits that increase survival and reproduction become more common over generations. Evolutionary psychologists study how natural selection may have favored behaviors that helped ancestors survive and spread their genes, and they often examine universal behaviors shared across cultures.

Evolutionary explanations describe tendencies shaped over many generations, not rigid rules for individuals. Strong explanations connect traits to plausible adaptive value without claiming certainty.

Example: fear and preparedness
Humans may be biologically prepared to acquire certain fears more readily than others, consistent with the idea that learning to avoid historically dangerous stimuli could have improved survival.

Example: taste aversion
People can develop strong aversions to foods that made them sick after a single pairing, even with a delay between eating and illness. This pattern makes sense if avoiding contaminated food improved survival.

Genetic and chromosomal conditions and disorders (key examples)

• Turner syndrome: individuals have only one X sex chromosome (XO).
• Klinefelter’s syndrome: arises from an XXY zygote; males with Klinefelter’s are often described as tending to be more passive.
• Down syndrome: results from three copies of chromosome 21.
• Tay-Sachs syndrome: progressive loss of nervous function leading to death in infancy.
• Albinism: failure to synthesize or store pigment; also involves abnormal nerve pathways to the brain, which can include quivering eyes and reduced depth perception using both eyes.
• Phenylketonuria (PKU): causes severe, irreversible brain damage unless the baby is fed a special diet low in phenylalanine within 30 days of birth; the infant lacks an enzyme needed to process phenylalanine, which can build up and poison nervous system cells.
• Huntington’s disease: an example of a dominant-gene defect involving degeneration of the nervous system.
• Familial Alzheimer’s disease: one form has been attributed to a gene on chromosome 21, but not all Alzheimer’s cases are associated with that gene.

Exam Focus

• Typical question patterns
  • Interpret twin/adoption study patterns to infer genetic and environmental influence.
  • Explain what heritability does and does not mean.
  • Apply gene–environment interaction or epigenetics to scenarios involving stress vulnerability, resilience, or development.
  • Connect evolutionary explanations to likely adaptive problems (prepared fears; taste aversion) using careful wording.
• Common mistakes
  • Treating heritability as “percent genetic” for a person rather than a population statistic.
  • Saying “genes cause behavior” without mentioning biological pathways and environmental context.
  • Treating evolutionary psychology as a collection of just-so stories rather than hypothesis-driven explanations.

Levels of Consciousness

Consciousness is often discussed in terms of different levels of awareness and processing.

Core levels and processes

• Preconscious: outside of awareness but contains feelings and memories you can easily bring into conscious awareness.
• Nonconscious: processes completely inaccessible to conscious awareness, such as blood flow, kidney filtration, hormone secretion, and lower-level sensory processing (detecting edges, estimating size/distance, pattern recognition).
• Unconscious (subconscious): includes often unacceptable feelings, wishes, and thoughts not directly available to conscious awareness.
• Dual processing: processing information on conscious and unconscious levels at the same time.
• Unconsciousness: characterized by loss of responsiveness to the environment due to disease, trauma, or anesthesia.

Exam Focus

• Typical question patterns
  • Classify an example as preconscious vs. nonconscious vs. unconscious.
  • Identify dual processing in everyday cognition (simultaneous conscious and automatic processing).
• Common mistakes
  • Treating “nonconscious” and “unconscious” as identical categories.
  • Assuming unconscious processing is always emotional or always “Freudian,” rather than also including basic sensory computations.

Sleep and Dreams

Sleep is a complex combination of states of consciousness, each with its own level of awareness, responsiveness, and physiological arousal.

Circadian rhythm and hypothalamic regulation

The hypothalamus systematically regulates changes in body temperature, blood pressure, pulse, blood sugar levels, hormonal levels, and activity levels over about a day.

A circadian rhythm is a natural internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours. It’s also known as your body’s clock and mainly responds to light and darkness in the environment.

Measuring sleep with EEG

Electroencephalograms (EEGs) can be recorded with electrodes on the surface of the skull to track brain-wave patterns across sleep stages.

Sleep stages

• Hypnagogic state: as you feel relaxed, fail to respond to outside stimuli, and begin the first stage of sleep.
• NREM-1: EEG shows theta waves, which are higher in amplitude and lower in frequency than alpha waves.
• NREM-2: EEG shows high-frequency bursts called sleep spindles and K complexes.
• NREM-3: EEG shows very high amplitude and very low-frequency delta waves.
• REM sleep: occurs about 90 minutes after falling asleep and is associated with rapid eye movements.

Nightmares are frightening dreams that occur during REM sleep.

Lucid dreaming (being aware of and directing one’s dreams) has been used to help people make recurrent nightmares less frightening.

Dream theories

• Freud’s theory: dreams can be analyzed to uncover unconscious desires and fears. The remembered storyline is manifest content; the underlying meaning is latent content.
• Activation-synthesis theory (McCarley & Hobson): dreams reflect the brain’s attempt to make sense of neural activity. The pons generates bursts of action potentials to the forebrain (activation), and the cortex synthesizes that activity into a narrative.

Sleep disorders

• Insomnia: inability to fall asleep and/or stay asleep.
• Narcolepsy: an awake person suddenly and uncontrollably falls asleep, often directly into REM.
• Sleep apnea: temporary cessations of breathing that repeatedly awaken the sufferer.
• Night terrors: most frequently childhood disruptions from the deepest part of NREM-3 (formerly stage 4), marked by a bloodcurdling scream and intense fear.
• Sleepwalking (somnambulism): most frequently childhood disruption during deep NREM-3, involving trips out of bed or complex activities.

Exam Focus

• Typical question patterns
  • Identify stages using EEG patterns (theta, spindles/K complexes, delta) and relate REM to dreaming.
  • Apply circadian rhythm concepts and hypothalamus regulation to sleep-wake timing.
  • Distinguish nightmares (REM) from night terrors (deep NREM-3).
• Common mistakes
  • Confusing NREM-3 (delta) with REM because both can involve unusual experiences.
  • Treating all dreaming as exclusively Freudian rather than knowing multiple theories.

Hypnosis and Meditation

Hypnosis

Hypnosis is an altered state of consciousness characterized by deep relaxation and heightened suggestibility. Under hypnosis, subjects can change aspects of perceived reality and let those changes influence behavior. Hypnotized individuals may report sensations like floating or sinking; perceive sights, sounds, smells, tastes, or touch that are not present; lose sense of touch or pain; feel as if they are passing back in time; act as if they are out of their own control; and respond strongly to suggestions by others.

According to the dissociation theory, hypnotized individuals experience two or more streams of consciousness cut off from each other.

Meditation

Meditation is a set of techniques used to focus concentration away from thoughts and feelings to create calmness, tranquility, and inner peace. Meditation is popular in Asia; for example, Zen Buddhists meditate. EEGs of meditators often show alpha waves characteristic of relaxed wakefulness.

Exam Focus

• Typical question patterns
  • Identify hypnosis as relaxation plus suggestibility and connect it to dissociation theory.
  • Recognize alpha waves as a relaxed wakefulness pattern (often linked to meditation).
• Common mistakes
  • Assuming hypnosis is sleep (it is not the same as sleep stages).
  • Overclaiming that hypnosis guarantees accurate memory retrieval rather than understanding it as heightened suggestibility.

Drugs and Behavior

Many questions about biological bases of behavior also involve how drugs alter the nervous system.

Psychoactive drugs and dependence concepts

Psychoactive drugs are chemicals that can pass through the blood-brain barrier into the brain and alter perception, thinking, behavior, and mood. Effects range from mild relaxation or increased alertness to vivid hallucinations.

• Psychological dependence: intense desire to achieve the drugged state despite adverse effects.
• Tolerance: decreasing responsivity to a drug (needing more to get the same effect).
• Physiological dependence (addiction): changes in brain chemistry from drug use make continued use necessary to prevent withdrawal.
• Withdrawal symptoms: intense craving and effects opposite to those the drug usually induces.

Major categories of psychoactive drugs

• Depressants: reduce activity of the CNS and induce relaxation. Examples include sedatives (such as barbiturates), tranquilizers, and alcohol.
• Narcotics (opioids): analgesics (pain reducers) that work by depressing the CNS; they can also depress the respiratory system.
• Stimulants: activate motivational centers and reduce activity in inhibitory centers of the CNS by increasing activity of serotonin, dopamine, and norepinephrine neurotransmitter systems.
• Hallucinogens (psychedelics): a diverse group that alters moods, distorts perceptions, and evokes sensory images in the absence of sensory input.

Exam Focus

• Typical question patterns
  • Classify a drug scenario as depressant vs. stimulant vs. hallucinogen vs. narcotic.
  • Use tolerance, dependence, and withdrawal vocabulary correctly in application-based questions.
  • Connect drug effects back to synapses (often using agonist vs. antagonist logic).
• Common mistakes
  • Confusing psychological dependence with physiological dependence.
  • Assuming withdrawal always looks the same across drug classes rather than often producing effects opposite the drug’s usual effects.
  • Treating “stimulant” as meaning “always positive” rather than as a category defined by CNS effects and neurotransmitter activity.