Explain how neurons, the nervous system and brain structures underpin human behaviour.

What this dot point is asking

The biological approach assumes that all thought, emotion and behaviour have a physical basis in the nervous system. To answer this dot point you build from the smallest unit (the neuron) up to whole brain structures.

The neuron and the synapse

The neuron is the basic cell of the nervous system. A typical neuron has dendrites (which receive signals), a cell body (soma), and an axon (which carries the signal away). Many axons are wrapped in a myelin sheath that speeds transmission.

Communication within a neuron is electrical: when stimulation reaches a threshold, the neuron "fires" an action potential, an all-or-none electrical impulse that travels down the axon. Communication between neurons is chemical. At the synapse (the tiny gap between neurons), the action potential triggers the release of neurotransmitters from the presynaptic neuron. These chemicals cross the gap and bind to receptors on the next neuron.

Key neurotransmitters to know: dopamine (reward, movement; depleted in Parkinson's disease), serotonin (mood, sleep; targeted by SSRIs for depression), and GABA (the main inhibitory transmitter, calming neural activity).

Organisation of the nervous system

The nervous system divides into two parts:

• Central nervous system (CNS): the brain and spinal cord.
• Peripheral nervous system (PNS): all nerves outside the CNS.

The PNS splits into the somatic nervous system (voluntary skeletal muscle and sensory input) and the autonomic nervous system (involuntary control of organs). The autonomic system in turn has the sympathetic branch (the "fight-or-flight" arousal response, increasing heart rate and releasing adrenaline) and the parasympathetic branch (the "rest-and-digest" response that returns the body to baseline).

Brain structures and localisation of function

Localisation of function is the idea that specific brain areas carry out specific tasks. Useful structures to name:

• Cerebral cortex, divided into four lobes: frontal (planning, decision-making, personality, motor control), parietal (touch and spatial processing), temporal (hearing, memory, language comprehension), and occipital (vision).
• Cerebellum: coordination, balance and fine motor control.
• Hippocampus: forming new long-term memories.
• Amygdala: emotional responses, especially fear and aggression.
• Hypothalamus: homeostasis, hunger, thirst and hormone regulation.

Hemispheric specialisation

The cortex has two hemispheres joined by the corpus callosum. Sperry and Gazzaniga's split-brain studies (1960s) tested patients whose corpus callosum had been cut to treat epilepsy. When an object was shown only to the right visual field (left hemisphere), patients could name it; when shown only to the left visual field (right hemisphere), they could not name it but could select it by touch with the left hand. This demonstrated that language is typically lateralised to the left hemisphere and that the two hemispheres can process information independently.

Language areas are also localised: Broca's area (frontal lobe) governs speech production, and Wernicke's area (temporal lobe) governs language comprehension. Damage to each produces a different type of aphasia.

Putting it together

A strong answer moves up the levels: a neuron fires and releases a neurotransmitter, networks of neurons form structures, structures specialise in functions, and these functions combine to produce behaviour. Naming evidence (Gage for the frontal lobe, Sperry and Gazzaniga for lateralisation, the lock-and-key model for chemical transmission) shows the marker that you can connect biology to observable behaviour rather than just listing parts.