Biological Basis of Behaviour

Overview

Behaviour begins with transduction: physical energy from the world is converted into neural signals by specialised receptors. Those signals travel through neurons, are integrated in the spinal cord and brain, and return as movement, glandular secretion or further neural activity. The nervous system is organised hierarchically and in parallel: reflexes can be handled at spinal level, while perception, planning and language require cortical networks that integrate information across regions. The endocrine system works alongside the nervous system, slower but longer-lasting, coordinating growth, metabolism, stress and reproduction through hormones carried in blood. Muscles execute behaviour, glands modulate it, and genes set the developmental programme within which learning operates. Understanding any one behaviour, such as fear, therefore requires several levels at once: the stimulus, the receptor, the circuit, the transmitter, the hormone, the muscle response and the learned history that gives the stimulus its meaning.

Sensory systems

Vision begins when light passes through the cornea and lens and forms an image on the retina, where photoreceptors transduce light into neural activity. Rods serve dim-light and peripheral vision with high sensitivity but poor colour and detail; cones serve bright-light, central vision with colour and fine detail, concentrated in the fovea. Signals pass through bipolar and ganglion cells, whose axons form the optic nerve, with partial crossing at the optic chiasm so that each hemisphere processes the opposite visual field. The lateral geniculate nucleus relays information to the visual cortex, where features such as orientation, movement and colour are processed in specialised areas before being integrated into perception. Colour vision is explained jointly by trichromatic theory, three cone types, and opponent-process theory, opponent pairs that explain afterimages and colour contrasts.

Hearing begins when sound waves vibrate the eardrum and ossicles, transmitting vibration to the oval window of the cochlea, where hair cells on the basilar membrane transduce movement into neural signals. Place theory explains high-frequency hearing by location of maximum vibration along the membrane, while frequency theory explains low-frequency hearing by firing rate, with volley theory bridging the middle range. Auditory signals travel to the auditory cortex, where location is computed from timing and intensity differences between ears. Smell uses olfactory receptors whose axons project directly to olfactory bulb and limbic structures, which helps explain the strong emotional and memory links of odour. Taste uses taste buds sensitive to sweet, salty, sour, bitter and savoury qualities, integrated with smell to produce flavour. Touch, temperature, pain and proprioception use skin and body receptors, with pain carried by fast and slow fibres that explain sharp immediate pain and dull aching pain, and modulated by descending control and psychological factors such as attention and expectation.

Neurons and neurotransmitters

The neuron is the basic signalling unit of the nervous system. Dendrites receive input, the cell body integrates it, the axon transmits the signal, and terminal buttons release transmitter at synapses. Many axons are insulated by myelin, formed by glial cells, which speeds conduction through saltatory jumping between nodes. At rest the neuron maintains a negative resting potential through ion distribution and pumps. When stimulation depolarises the membrane to threshold, voltage-gated channels open and an action potential fires, travelling along the axon in an all-or-none fashion: stimulus strength is coded by firing rate and number of neurons recruited, not by the size of a single action potential. At the synapse, the action potential triggers transmitter release into the synaptic cleft, where transmitter binds to receptors on the postsynaptic cell, producing excitation or inhibition, before being removed by reuptake or enzymatic breakdown. Summation of excitatory and inhibitory inputs decides whether the postsynaptic neuron fires.

Neurotransmitters are chemical messengers with distinct, though overlapping, roles. Acetylcholine supports muscle contraction, attention and memory, and its disruption is implicated in memory disorders. Dopamine supports reward, motivation and motor control, and its pathways are central to reinforcement, movement disorders and psychosis theories. Serotonin modulates mood, sleep, appetite and impulse control. Norepinephrine supports arousal, vigilance and stress response. GABA is the main inhibitory transmitter, calming neural activity, while glutamate is the main excitatory transmitter, essential for learning and synaptic plasticity. Endorphins modulate pain and pleasure. No transmitter does one thing alone: behaviour emerges from patterns of release, receptor type, circuit location and interaction with other transmitters, which is why drugs that alter one transmitter have wide effects and why simple chemical slogans about happiness or aggression mislead.

Central and peripheral nervous systems

The central nervous system comprises the brain and spinal cord. The spinal cord carries ascending sensory and descending motor pathways and mediates reflexes through local circuits. The brainstem, including medulla, pons and midbrain, controls vital functions such as breathing, heart rate and arousal, and relays information upward. The cerebellum coordinates movement, balance and timing, and contributes to procedural learning. The thalamus relays most sensory information to cortex, while the hypothalamus regulates hunger, thirst, temperature, sleep and endocrine activity through the pituitary, linking neural and hormonal control. The limbic system, including amygdala and hippocampus, supports emotion and memory: the amygdala is central to fear learning and emotional salience, while the hippocampus supports formation of new explicit memories. The cerebral cortex, with frontal, parietal, temporal and occipital lobes, supports planning, language, spatial processing and vision respectively, though functions are distributed across networks and the two hemispheres specialise, language typically left-dominant, with the right hemisphere contributing spatial and emotional processing.

The peripheral nervous system connects centre to body. The somatic division serves voluntary movement and sensory input from skin and muscles. The autonomic division controls involuntary organs and glands through sympathetic and parasympathetic branches: sympathetic activation prepares for action, raising heart rate and mobilising energy, while parasympathetic activation restores and conserves, slowing heart rate and supporting digestion. Most organs receive dual innervation, allowing fine control by balance rather than by either branch alone. Neuroplasticity is the capacity of the nervous system to change its structure and function with experience: synaptic strengthening and weakening, growth of connections, reorganisation after injury, and adult neurogenesis in limited regions. Plasticity explains learning, rehabilitation after damage, and the lasting effects of enriched or deprived environments, but it also means that repeated stress, drug use and trauma can reshape circuits in harmful directions.

Methods of physiological psychology

Physiological psychology uses methods that intervene in, record from, or image the nervous system. Lesion studies infer function from the deficit after damage to a structure, but damage is rarely clean, compensation occurs, and human lesions are uncontrolled. Electrical stimulation can activate a region and reveal its contribution, while recording from single cells or populations shows how neurons respond to stimuli. Electroencephalography records summed electrical activity from the scalp with excellent time resolution but poor spatial precision, making it ideal for sleep stages and rapid cognitive events. Brain imaging adds spatial detail: structural imaging shows anatomy, functional magnetic resonance imaging tracks blood-flow changes linked to activity with good spatial but slower temporal resolution, positron emission tomography tracks metabolic activity using tracers, and magnetoencephalography combines good time and spatial information at high cost. Each method trades time against space, invasiveness against precision, and correlation against causation.

Other methods measure the body as a window on psychology. Autonomic measures such as heart rate, skin conductance and pupil dilation index arousal and emotion, though they are non-specific and influenced by many factors. Hormonal assays from blood or saliva track stress, reproductive and metabolic states. Genetic methods compare twins and adopted individuals, study family patterns, and examine molecular associations, but they estimate influence, not destiny. Optogenetics and related animal techniques allow precise control of identified neurons, advancing causal understanding in animals while raising translation questions for humans. The methodological rule is triangulation: a claim about a brain basis is strongest when lesion, recording, imaging and behavioural evidence converge, and weakest when it rests on a single colourful brain image.

Muscular and glandular system

Muscles execute behaviour. Skeletal muscles, under voluntary control, move the skeleton through contraction of fibres organised into motor units, each motor neuron serving a set of fibres. Smooth muscles line organs and blood vessels under involuntary control, while cardiac muscle powers the heart with its own rhythmic properties. Muscle contraction depends on the sliding of actin and myosin filaments triggered by calcium release after neural stimulation at the neuromuscular junction, where acetylcholine is the transmitter. Graded force comes from recruiting more motor units and increasing firing rates. Fatigue, training and disease alter this machinery, so motor performance reflects neural command, muscle condition and motivation together.

The glandular or endocrine system secretes hormones into blood for slower, widespread action. The pituitary, under hypothalamic control, is the master gland releasing hormones that regulate growth, thyroid, adrenal and reproductive glands. The thyroid regulates metabolism, the adrenals release cortisol for sustained stress response and adrenaline for rapid mobilisation, the pancreas regulates blood sugar through insulin and glucagon, and the gonads release sex hormones that organise development and influence behaviour. Feedback loops maintain balance: rising hormone levels typically inhibit further release, preventing runaway secretion. Endocrine disorders illustrate psychology from the gland side: thyroid imbalance alters energy, mood and cognition, and chronic cortisol elevation affects memory, immunity and mood. Behaviour therefore sits at the intersection of fast neural signals and slow hormonal tides.

Biological basis of motivation and emotion

Motivation has clear biological roots without being reducible to them. Hunger is regulated by hypothalamic centres that integrate stomach signals, blood glucose, leptin from fat tissue signalling satiety, and ghrelin signalling hunger, together with learned preferences, palatability and social context. Thirst responds to cellular dehydration and blood volume through hormonal signals that drive drinking. Sexual motivation involves hypothalamic circuits, sex hormones and learned experience, with human sexuality shaped far more by culture, relationship and meaning than by hormones alone. Sleep and wakefulness are regulated by circadian rhythms from the suprachiasmatic nucleus and by sleep-promoting processes that build with wakefulness, producing cycles of rapid-eye-movement and non-rapid-eye-movement sleep with distinct brain activity and functions for memory and restoration.

Emotion is organised by interacting systems: the amygdala flags threat and salience, the hypothalamus and brainstem drive autonomic and endocrine responses, the cortex appraises meaning and regulates expression, and feedback from the body itself shapes felt experience. The sympathetic branch mobilises the body for action, while cortisol sustains the response over time. Different emotions share much physiology, so bodily arousal alone does not identify a specific emotion; appraisal, context and expression complete the pattern. Stress illustrates the integration: acute stress mobilises resources adaptively, while chronic stress through sustained cortisol and sympathetic load wears down immunity, memory and mood. Biological accounts of motivation and emotion therefore explain the machinery of drive and feeling, while psychology explains why the same machinery produces different behaviour in different people and situations.

Genetics and behaviour

Genetics contributes to behaviour through genes that build proteins, shape brain development and set sensitivities, but behaviour is not written directly in genes. Heritability is a population statistic: it states the proportion of variation in a trait within a particular population and environment that is associated with genetic variation. It is not a percentage of an individual's trait caused by genes, and it changes when environments change. Twin studies compare identical twins, who share nearly all genes, with fraternal twins, who share about half, while adoption studies separate genetic from rearing influences. These designs consistently show moderate genetic influence on many traits, including aspects of intelligence, temperament and vulnerability to disorder, alongside substantial environmental influence. Molecular genetics seeks specific gene variants, but most psychological traits are polygenic, influenced by many variants of small effect interacting with experience.

Three interaction concepts prevent genetic determinism. Gene-environment correlation means genes influence the environments people seek and evoke: an active child evokes different responses from adults than a withdrawn child. Gene-environment interaction means the effect of a gene depends on environment, so a vulnerability may appear only under stress or adversity. Epigenetics means experience can alter gene expression without changing DNA sequence, through chemical marks that switch genes on or off, providing a mechanism by which early care, nutrition and stress leave lasting biological traces. Evolutionary psychology adds a distal level, asking which psychological mechanisms would have aided survival and reproduction in ancestral environments, but its hypotheses must be tested against cross-cultural and developmental evidence rather than accepted as stories. The balanced conclusion is that genes shape probabilities and sensitivities, environments shape expression and learning, and development is their continuous interaction.

Major neurotransmitters

Brain divisions and functions