02 Biological Bases of Behavior

Foundations of Biological Behavior

Biological bases of behavior concern how neurons, brain structures, body systems, genes, development, experience, and the environment contribute to behavior and mental processes. Biology does not determine behavior in isolation. Instead, behavior emerges from ongoing interactions among these levels.

A useful organizing principle is to move from communication within cells, to coordination across neural systems, to regulation by hormones, and finally to genetic and environmental influences. For example, a perceived threat can activate sensory systems, brain networks, the autonomic nervous system, and the endocrine system. Past learning and current context influence how the threat is interpreted, while biological differences can affect sensitivity to stress.

Takeaway: Biological psychology explains behavior through interacting systems rather than through a single brain region, chemical, or gene.

Neurons and Glial Cells

A neuron is a specialized cell that receives, processes, and transmits information. Its main parts work in sequence:

• Dendrites receive signals from other neurons or sensory receptors.

• The cell body, or soma, contains the nucleus and maintains the cell.

• The axon carries signals away from the cell body.

• Axon terminals release chemical messengers to other neurons, muscles, or glands.

Neurons are often classified by function:

1. Sensory neurons carry information from sensory receptors toward the central nervous system.

2. Interneurons process and connect information within the brain and spinal cord.

3. Motor neurons carry instructions to muscles and glands.

Neural signals generally travel from dendrites and the cell body, along the axon, to the axon terminals. Myelin is a fatty substance that insulates many axons and increases conduction speed. Gaps in the myelin sheath, called nodes, help the electrical signal move rapidly from one node to the next.

Glial cells, or glia, support neurons by nourishing them, maintaining the chemical environment, producing myelin, removing cellular debris, protecting neural tissue, and helping organize neural development. Neural communication therefore depends on neurons and glial cells working together.

Takeaway: Neurons transmit information, while glial cells support and organize the conditions that make neural communication possible.

Neural Communication

A neuron maintains an electrical difference between the inside and outside of its cell membrane. When stimulation reaches a sufficient threshold, an action potential occurs. This rapid electrical change travels along the axon.

Action potentials are commonly described as all-or-none events. A neuron either produces one after reaching threshold or does not. Stronger stimulation is generally represented by a greater frequency of action potentials rather than by a larger individual action potential.

The junction between two neurons is a synapse, and the small space between them is the synaptic cleft. When an action potential reaches the presynaptic axon terminal, neurotransmitters are released into the cleft. These chemical messengers bind to receptors on the postsynaptic cell.

Neurotransmitter effects can be:

• Excitatory, making the receiving neuron more likely to fire.

• Inhibitory, making the receiving neuron less likely to fire.

The effect depends on the receptor and neural circuit, not simply on the identity of the neurotransmitter. After release, neurotransmitters may undergo reuptake, be broken down by enzymes, or diffuse away from the synapse.

Important examples include:

• Glutamate: a major excitatory neurotransmitter involved in learning, memory, and neural plasticity.

• GABA: a major inhibitory neurotransmitter that helps regulate neural activity.

• Dopamine: involved in movement, reward learning, motivation, attention, and reinforcement.

• Serotonin: involved in mood regulation, sleep, appetite, and other functions.

• Acetylcholine: important for muscle activation, attention, learning, and memory.

• Norepinephrine: involved in alertness, attention, arousal, and stress responses.

• Endorphins: neuropeptides that can reduce pain and contribute to pleasure or well-being.

An agonist mimics or increases the effect of a neurotransmitter. An antagonist blocks or reduces that effect. Drugs can influence behavior by changing neurotransmitter release, receptor binding, reuptake, or breakdown. Because neurotransmitter systems are distributed throughout the brain and body, changing one system can produce several behavioral and physiological effects.

Takeaway: Neural communication combines electrical signals within neurons with chemical signaling between cells, and the result depends on receptors and networks.

The Nervous System

The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

The central nervous system (CNS) consists of the brain and spinal cord. It receives sensory information, integrates it, and coordinates responses. The spinal cord can also organize some rapid reflexes without requiring conscious processing by the brain.

The peripheral nervous system (PNS) includes neural tissue outside the brain and spinal cord. It connects the CNS with sensory receptors, muscles, organs, and glands. Its major functional divisions include:

• The somatic nervous system, which carries sensory information and controls voluntary skeletal-muscle movement.

• The autonomic nervous system (ANS), which regulates internal organs and glands.

The autonomic nervous system (ANS) includes several divisions:

• The sympathetic nervous system mobilizes the body during challenge or emergency. It can increase heart rate, widen airways, and redirect blood flow toward muscles.

• The parasympathetic nervous system supports rest, digestion, energy conservation, and recovery.

• The enteric nervous system helps regulate the digestive tract.

The sympathetic and parasympathetic divisions usually work together as part of coordinated regulation rather than functioning as simple opposites.

Takeaway: The nervous system links sensory information, brain processing, voluntary action, and internal regulation.

Brain Structures and Networks

Brain structures have specialized contributions, but behavior is produced by interacting neural networks rather than isolated regions.

• The brainstem connects the brain with the spinal cord and helps regulate breathing, heart rate, sleep-wake regulation, and arousal.

• The cerebellum contributes to balance, posture, coordination, timing, refinement of skilled movements, and some learning and cognitive processing.

• The thalamus acts as a major relay and processing center for sensory information traveling to the cerebral cortex. Most sensory systems, except smell, send information through the thalamus before reaching the cortex.

• The hypothalamus helps regulate homeostasis and links the nervous system to the endocrine system.

• The amygdala contributes to emotional learning, threat detection, and evaluation of emotionally significant events.

• The hippocampus is important for forming and organizing many explicit memories and for spatial memory.

• The cingulate cortex participates in attention, emotion, motivation, and monitoring behavior.

• The basal ganglia are involved in movement initiation, habit learning, action selection, and reward-related processes.

The cerebral cortex is the outer layer of the cerebrum. It supports complex perception, voluntary movement, language, reasoning, decision-making, and conscious awareness. Its lobes make different major contributions:

• The frontal lobe supports planning, decision-making, impulse control, voluntary movement, and aspects of language.

• The parietal lobe processes touch and other body sensations, spatial information, and integrated sensory information.

• The temporal lobe contributes to hearing, language comprehension, memory, and aspects of object recognition.

• The occipital lobe is central to visual processing.

The motor cortex controls voluntary movement, and the somatosensory cortex processes touch and related body sensations. The two cerebral hemispheres communicate through the corpus callosum. Although some functions show hemispheric specialization, normal behavior generally depends on communication between both hemispheres.

Takeaway: Brain regions make specialized contributions, but thoughts, emotions, and actions depend on communication across networks.

The Endocrine System and Stress

The endocrine system consists of glands that release hormones into the bloodstream. Hormones generally act more slowly and for longer periods than many neural signals, although the distinction is not absolute. Some chemical messengers, such as epinephrine, can function as both neurotransmitters and hormones.

Important endocrine glands include the pituitary, thyroid, adrenal glands, pancreas, ovaries, and testes. The hypothalamus monitors the body and directs the pituitary gland, which influences several other endocrine glands.

Hormone concentrations are often regulated by negative feedback. When hormone levels rise sufficiently, signals to the releasing glands decrease. This feedback helps maintain homeostasis.

The hypothalamic-pituitary-adrenal (HPA) axis illustrates the connection between biology and behavior:

1. The hypothalamus detects or interprets a stressor and releases a signaling hormone.

2. The pituitary gland releases a hormone that acts on the adrenal glands.

3. The adrenal cortex releases cortisol, which helps regulate energy use and supports the stress response.

4. Negative feedback helps reduce HPA-axis activity when the threat has passed.

The sympathetic nervous system and adrenal medulla also contribute to rapid stress responses by releasing catecholamines such as epinephrine and norepinephrine. Short-term stress can support attention and action, whereas prolonged or severe stress may disrupt sleep, mood, memory, immune function, and cardiovascular health.

Takeaway: Neural and hormonal systems work together, combining rapid communication with slower, longer-lasting regulation.

Genes, Environment, and Behavior

Genes are segments of DNA that contribute to the production and regulation of biological molecules. A person's observable characteristics, or phenotype, result from interactions between genetic factors and environmental influences.

Most psychological characteristics are complex traits. They are influenced by many genes, each usually contributing a small effect, as well as by development, learning, relationships, culture, health, and life events. It is therefore misleading to describe a complex behavior as being caused by a single gene.

Behavioral genetics investigates genetic and environmental contributions to individual differences in behavior. Family, twin, adoption, and molecular-genetic designs can help estimate these contributions.

Heritability describes the proportion of variation in a trait within a particular population and environment that is statistically associated with genetic differences. It does not indicate how genetic a trait is for one individual, and it does not imply that a trait is fixed or unchangeable. A highly heritable trait can still be influenced by environmental change.

Researchers distinguish between:

• Shared environmental influences, which tend to make family members more alike.

• Nonshared environmental influences, which contribute to differences among people raised in the same family.

A gene-environment interaction occurs when the effect of a genetic difference depends on the environment, or when the effect of an environment differs according to genetic characteristics. A gene-environment correlation occurs when genetic differences are related to the environments people experience, such as the activities, relationships, or settings a person seeks or receives.

Epigenetics refers to changes in gene regulation that affect when and how strongly genes are expressed without changing the DNA sequence itself. Nutrition, stress, toxins, sleep, and other experiences can influence biological regulation. Epigenetic mechanisms are one possible pathway through which environments affect brain development and behavior, but they should not be interpreted as simple or deterministic.

Takeaway: Genes influence behavioral possibilities in interaction with development and experience; they do not provide simple, fixed explanations for complex behavior.

Integrating the Systems

The interaction of biological systems can be followed through a sudden crash heard at night:

1. Sensory neurons transmit information about the sound to the central nervous system.

2. The thalamus and cortical areas help process the sensory input.

3. The amygdala may rapidly evaluate the sound as potentially threatening.

4. The hypothalamus activates autonomic and endocrine stress systems.

5. The sympathetic nervous system increases arousal, heart rate, and readiness for action.

6. The adrenal glands release hormones that help provide energy.

7. The prefrontal cortex evaluates the situation more deliberately and may determine that the sound was harmless.

8. Parasympathetic activity and hormonal feedback help the body return toward baseline.

This sequence is not a purely automatic reaction. Past learning, current context, social information, and individual biological differences influence how the sound is interpreted and how strongly the body responds.

The example illustrates a multilevel approach:

• Genes influence the development and regulation of cells and neural systems.

• Neurons and neurotransmitters transmit information rapidly across circuits.

• Brain structures and networks integrate sensory information and support thought, emotion, memory, and action.

• The nervous system coordinates rapid responses throughout the body.

• The endocrine system produces slower, longer-lasting chemical effects.

• Experience and environment modify neural connections, gene expression, hormone responses, and behavior.

Final takeaway: Biological bases of behavior are best understood as dynamic interactions among genes, brain systems, body systems, experience, and context.