📚Study Guide: Biological Bases of Behavior
Unit 2: Biological Bases of Behavior
The biological perspective in psychology asserts that all psychological phenomena—thoughts, emotions, behaviors—have physical bases in the brain, nervous system, endocrine system, and genetics. This unit provides a comprehensive overview of the structures and functions of the nervous system, the electrochemical processes underlying neural communication, the organization of the brain, and the complex interplay between genes and environment in shaping behavior. Students will learn to identify major brain regions and their associated functions, understand how neurotransmitters influence mood and behavior, and appreciate the evolutionary logic that shaped human psychology. The unit also examines the endocrine system's slower but wide-reaching hormonal influences and introduces behavioral genetics, including heritability estimates and twin studies. In an era of advancing neuroscience, brain imaging, and genetic editing, understanding the biological foundations of behavior is essential for evaluating claims about human nature, mental illness, and the limits of biological reductionism. A sophisticated psychological perspective recognizes that biology sets the stage, but environment writes the script—and that the two are inextricably intertwined.
KEY CONCEPTS
- Neuron Structure and Function: Neurons are the basic functional units of the nervous system. A typical neuron consists of dendrites (receive messages), a cell body (soma, processes information), an axon (transmits signals), and terminal branches (pass signals to other neurons). Signals travel electrochemically via action potentials and synaptic transmission.
- Action Potential: A brief electrical charge that travels down an axon. It operates on the all-or-none principle: once the threshold is reached, the neuron fires completely. Intensity of stimulation is coded by frequency of firing, not amplitude.
- Neurotransmitters: Chemical messengers that cross synapses to transmit signals between neurons. Key neurotransmitters include dopamine (reward, movement), serotonin (mood, sleep), acetylcholine (muscle action, memory), GABA (inhibition), glutamate (excitation), norepinephrine (alertness, stress), and endorphins (pain relief, pleasure).
- The Central and Peripheral Nervous Systems: The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) connects the CNS to the body and includes the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of glands and internal organs).
- The Autonomic Nervous System: Divided into the sympathetic division (arouses the body for action—fight or flight) and the parasympathetic division (calms the body to conserve energy—rest and digest). These two divisions typically operate in antagonistic fashion.
- Brain Plasticity: The brain's ability to change and reorganize itself by forming new neural connections throughout life. Plasticity is greatest in childhood but continues into adulthood, underlying learning, recovery from injury, and adaptation to experience.
- Gene-Environment Interaction: Behavior emerges from the interplay of genetic predispositions and environmental influences. Heritability estimates indicate the proportion of phenotypic variation in a population attributable to genetic variation, but they do not apply to individuals and change with environmental conditions.
VOCABULARY
- Dendrite: The bushy, branching extensions of a neuron that receive messages and conduct impulses toward the cell body.
- Axon: The long, thin fiber that transmits signals away from the cell body toward other neurons or muscles.
- Myelin Sheath: A fatty tissue layer that insulates axons and speeds neural transmission. Myelin is produced by glial cells (oligodendrocytes in the CNS, Schwann cells in the PNS). Degeneration of myelin characterizes multiple sclerosis.
- Synapse: The junction between the axon tip of the sending neuron and the dendrite or cell body of the receiving neuron. The tiny gap at this junction is called the synaptic cleft.
- Reuptake: A neurotransmitter's reabsorption by the sending neuron, terminating the synaptic message. Many antidepressants (SSRIs) work by blocking serotonin reuptake.
- Agonist: A molecule that increases a neurotransmitter's action by mimicking it or blocking reuptake.
- Antagonist: A molecule that blocks or decreases a neurotransmitter's action, often by occupying receptor sites without activating them.
- Heritability: The proportion of variation among individuals in a trait that can be attributed to genetic factors. Heritability is a population statistic, not a measure of how "genetic" a trait is in any individual.
MODELS, THEORIES, AND FRAMEWORKS
- The Hindbrain, Midbrain, and Forebrain: The brain develops from bottom (oldest) to top (newest). The hindbrain includes the medulla (heartbeat, breathing), pons (sleep, arousal), and cerebellum (coordination, timing). The midbrain includes structures involved in reward and sensory reflexes. The forebrain includes the thalamus (sensory relay), hypothalamus (hunger, temperature, emotion), hippocampus (memory), amygdala (emotion, especially fear), and cerebral cortex (higher cognition).
- Lobes of the Cerebral Cortex: The frontal lobe (motor function, planning, personality, Broca's area for speech production), parietal lobe (sensory processing, spatial reasoning), temporal lobe (auditory processing, Wernicke's area for language comprehension), and occipital lobe (visual processing). Each lobe has specialized regions but functions through integrated networks.
- Split-Brain Research (Sperry): Studies of patients whose corpus callosum was severed to treat epilepsy revealed that the left hemisphere typically specializes in language and logical processing, while the right hemisphere specializes in spatial and visual tasks. However, later research showed this lateralization is relative, not absolute, and both hemispheres collaborate in most tasks.
- Evolutionary Psychology: The application of principles of evolution, including natural selection, to explain psychological processes. Evolutionary psychologists argue that many human behaviors and cognitive mechanisms are adaptations that solved recurrent problems in ancestral environments. Critics caution against "just-so stories" that lack empirical testability.
COMMON MISTAKES ON AP EXAMS
- Confusing the functions of the amygdala and hippocampus: The amygdala is primarily involved in emotion, particularly fear and threat detection. The hippocampus is primarily involved in memory formation and spatial navigation. While they interact during emotional memory encoding, their core functions are distinct.
- Stating that people are "left-brained" or "right-brained": This popular myth vastly oversimplifies hemispheric specialization. While language is typically left-lateralized and spatial processing somewhat right-lateralized, both hemispheres work together in virtually all complex tasks. The brain is not neatly divided into creative and logical halves.
- Interpreting heritability as applying to individuals: A heritability of 0.60 for intelligence means that 60% of the variation in intelligence in that population is associated with genetic variation. It does NOT mean that 60% of any individual's intelligence is genetically determined.
- Confusing agonists and antagonists: Agonists activate or enhance receptor activity (they are "pro" the neurotransmitter). Antagonists block or reduce receptor activity (they are "anti" the neurotransmitter). A helpful mnemonic: agonists are protagonists—they promote action.
AP EXAM STRATEGIES
- Draw and label the brain from memory: FRQs frequently ask you to identify brain regions and their functions. Practice sketching the brain with the four lobes, key subcortical structures (thalamus, hypothalamus, hippocampus, amygdala), brainstem, and cerebellum.
- Match neurotransmitters to functions and disorders: Be ready to state that dopamine is involved in Parkinson's disease and schizophrenia; serotonin in depression and OCD; acetylcholine in Alzheimer's disease; and GABA in anxiety disorders.
- Apply the fight-or-flight vs. rest-and-digest distinction: When presented with a stress scenario, identify which autonomic division is activated, what physiological changes occur (increased heart rate, dilated pupils, inhibited digestion), and how the parasympathetic system restores homeostasis.
- Evaluate nature vs. nurture as interaction, not opposition: High-scoring responses emphasize gene-environment interaction, epigenetics, and reaction ranges rather than asserting that behavior is purely biological or purely environmental.
REAL-WORLD APPLICATIONS
- Pharmacological Treatments for Depression: SSRIs (selective serotonin reuptake inhibitors) increase synaptic serotonin levels by blocking reuptake. While effective for many patients, the serotonin hypothesis of depression has been challenged, and treatment increasingly combines medication with psychotherapy and lifestyle interventions.
- Brain Imaging in Legal Contexts: fMRI and PET scans are increasingly introduced as evidence in criminal trials to argue for reduced culpability based on brain abnormalities. This raises profound ethical questions about free will, responsibility, and the limits of neuroscientific explanation.
- Neuroplasticity and Rehabilitation: Stroke victims and individuals with traumatic brain injuries can recover function through targeted rehabilitation that harnesses brain plasticity. Constraint-induced movement therapy, for example, forces use of impaired limbs to stimulate cortical reorganization.