Biological Basis of Behavior

Behavior ultimately arises from the nervous and endocrine systems. This chapter bridges biology and psychology, covering neuron structure and signaling, the major neurotransmitters, the organization of the nervous system, the functions of brain regions, and hormonal control. The AAMC outline expects you to link specific structures and chemicals to their behavioral roles.

Key exam topics. Neuron parts and the action potential (resting potential, depolarization, all-or-none); neurotransmitters and their functions; CNS vs PNS, somatic vs autonomic, sympathetic vs parasympathetic; forebrain/midbrain/hindbrain divisions and lobes; the limbic system; and the endocrine glands and their hormones.

Neurons and Signaling

A neuron has dendrites (receive input), a cell body (soma), an axon (carries the signal), a myelin sheath (insulates, speeds conduction via nodes of Ranvier) and axon terminals. Glial cells (oligodendrocytes in CNS, Schwann cells in PNS) form myelin.

The action potential

Resting potential ~ −70 mV, maintained by the Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in).
Depolarization — a stimulus reaching threshold opens voltage-gated Na⁺ channels; Na⁺ rushes in.
Repolarization — K⁺ channels open, K⁺ exits.
Hyperpolarization then return to rest; the refractory period ensures one-way propagation.

The action potential is all-or-none; stronger stimuli increase firing frequency, not amplitude. At the synapse, the signal triggers neurotransmitter release into the synaptic cleft.

Neurotransmitters

Key neurotransmitters and roles
Neurotransmitter	Function / clinical link
Acetylcholine	Muscle contraction, memory; deficits in Alzheimer’s
Dopamine	Reward, movement; excess linked to schizophrenia, deficit to Parkinson’s
Serotonin	Mood, sleep, appetite; low levels linked to depression
Norepinephrine	Arousal, alertness, fight-or-flight
GABA	Main inhibitory transmitter; anxiety when low
Glutamate	Main excitatory transmitter; learning
Endorphins	Natural pain relief, pleasure

Organization of the Nervous System

The central nervous system (CNS) = brain + spinal cord. The peripheral nervous system (PNS) divides into:

Somatic — voluntary control of skeletal muscle; sensory and motor pathways.
Autonomic — involuntary control, split into:
- Sympathetic — "fight or flight" (raises heart rate, dilates pupils, inhibits digestion).
- Parasympathetic — "rest and digest" (lowers heart rate, stimulates digestion; via the vagus nerve).

Brain Regions

Hindbrain: Medulla (breathing, heart rate), pons (relay, sleep), cerebellum (balance, coordinated movement).
Midbrain: Reflexes, arousal, visual/auditory relay (reticular formation).
Forebrain: Thalamus (sensory relay, except smell), hypothalamus (homeostasis, hunger, thirst, the "four F’s"), limbic system (amygdala — fear/emotion; hippocampus — memory), and the cerebral cortex.

Cortical lobes: frontal (executive function, motor cortex, Broca’s area), parietal (somatosensory cortex, spatial), temporal (hearing, Wernicke’s area, memory), occipital (vision).

The Endocrine System

The endocrine system uses hormones (slower, longer-lasting than neural signals). The hypothalamus controls the pituitary ("master gland"). Key glands: adrenal medulla (epinephrine/norepinephrine — fight-or-flight), adrenal cortex (cortisol — stress), thyroid (metabolism), pancreas (insulin/glucagon), and gonads (sex hormones).

Common trap. The action potential is all-or-none — stimulus strength changes firing frequency, not amplitude. Sympathetic = arousal (fight-or-flight), parasympathetic = rest-and-digest. Dopamine excess → schizophrenia; dopamine deficit → Parkinson’s.

Worked MCQs

Five MCQs covering the high-yield testing patterns for the biological basis of behavior.

Q1. The influx of which ion is directly responsible for the depolarization phase of the action potential?

Potassium (K⁺)
Sodium (Na⁺)
Chloride (Cl⁻)
Calcium (Ca²⁺)

Depolarization occurs when voltage-gated Na⁺ channels open and sodium rushes into the cell, raising the membrane potential toward positive. K⁺ efflux causes the later repolarization.

Q2. A degeneration of dopamine-producing neurons is most associated with:

Alzheimer’s disease
Parkinson’s disease
Depression
Generalized anxiety disorder

Parkinson’s disease results from the loss of dopaminergic neurons in the substantia nigra, causing tremor and rigidity. Alzheimer’s is more associated with acetylcholine deficits.

Q3. Which division of the nervous system prepares the body for "fight or flight"?

Parasympathetic
Sympathetic
Somatic
Central

The sympathetic division of the autonomic nervous system increases heart rate, dilates pupils and mobilizes energy for fight-or-flight. The parasympathetic division promotes rest-and-digest.

Q4. Damage to the hippocampus would most likely impair:

Balance and coordination
Vision
Formation of new long-term memories
Breathing rate

The hippocampus is essential for consolidating new explicit long-term memories. Balance is the cerebellum, vision the occipital lobe, and breathing the medulla.

Q5. The "master gland" that regulates other endocrine glands and is controlled by the hypothalamus is the:

Thyroid
Adrenal gland
Pituitary gland
Pancreas

The pituitary gland is the master gland, secreting hormones that control other glands. It is itself directed by the hypothalamus, linking the nervous and endocrine systems.

Quick Recap

Neuron: dendrites → soma → axon (myelinated) → terminals; synapse uses neurotransmitters.
Action potential: resting (−70 mV) → Na⁺ in (depolarize) → K⁺ out (repolarize); all-or-none.
Dopamine (reward/Parkinson’s/schizophrenia), serotonin (mood), GABA (inhibitory), glutamate (excitatory).
CNS (brain + cord) vs PNS (somatic + autonomic; sympathetic vs parasympathetic).
Hindbrain (medulla, cerebellum), midbrain, forebrain (thalamus, hypothalamus, limbic, cortex).
Lobes: frontal (executive), parietal (touch), temporal (hearing), occipital (vision).
Pituitary = master gland; adrenal, thyroid, pancreas, gonads secrete hormones.

Test yourself. Take a timed practice test or browse topic-wise MCQs to lock these concepts in.