Explain the structure and function of the nervous system, the role of neurotransmitters, and the biological stress response

What this dot point is asking

SCSA Unit 3 begins with the biological machinery that underlies every thought, feeling and action. You need to describe the organisation of the nervous system, explain how a single neuron transmits and receives signals, name the major neurotransmitters and their effects, and apply Selye's model of the stress response. This material is examined in both the school assessment program and the external written examination.

Organisation of the nervous system

The nervous system has two main divisions.

• The central nervous system (CNS) is the brain and spinal cord. It processes information and coordinates responses.
• The peripheral nervous system (PNS) is all the nerves outside the CNS, carrying messages to and from the body.

The PNS itself splits into the somatic nervous system (voluntary control of skeletal muscle and sensory input) and the autonomic nervous system (involuntary control of glands and smooth muscle). The autonomic system has two complementary branches:

• The sympathetic nervous system activates the body for action (the fight-or-flight response): heart rate rises, pupils dilate, digestion slows.
• The parasympathetic nervous system restores the body to a calm resting state (rest-and-digest), conserving energy.

The neuron and synaptic transmission

The neuron is the basic functional cell of the nervous system. Key parts:

• Dendrites receive incoming signals from other neurons.
• The soma (cell body) integrates incoming signals.
• The axon carries the electrical impulse (action potential) away from the soma.
• The myelin sheath insulates the axon and speeds transmission.
• Axon terminals release chemical messengers.

Communication within a neuron is electrical (the action potential travels down the axon). Communication between neurons is chemical. When the action potential reaches the axon terminal, vesicles release neurotransmitters into the synaptic gap. These molecules cross the gap and bind to receptors on the next neuron's dendrites, either exciting or inhibiting it. Leftover neurotransmitter is broken down or reabsorbed (reuptake).

Neurotransmitters

Neurotransmitters are the chemicals that carry messages across the synapse. Major examples examinable at WACE:

• Dopamine: reward, motivation and movement. Low dopamine is linked to Parkinson's disease; dysregulation is linked to schizophrenia.
• Serotonin: mood, sleep and appetite. Low serotonin activity is associated with depression.
• GABA (gamma-aminobutyric acid): the main inhibitory neurotransmitter, reducing neural excitability and anxiety.
• Glutamate: the main excitatory neurotransmitter, involved in learning and memory.
• Adrenaline (epinephrine) and noradrenaline: released during the stress response to mobilise the body.

The biological stress response

A stressor is any event that challenges the body's equilibrium. The acute response is driven by the sympathetic nervous system and the adrenal glands, which flood the body with adrenaline and the stress hormone cortisol, producing increased heart rate, faster breathing and heightened alertness.

For prolonged stress, Hans Selye proposed the General Adaptation Syndrome (GAS), a three-stage model:

1. Alarm reaction. The body detects the stressor; sympathetic arousal and adrenaline release produce a temporary drop then a sharp rise in resistance (shock followed by countershock).
2. Resistance. The body adapts and tries to cope, keeping arousal elevated. Cortisol remains high. The person may appear to function normally, but resources are being depleted.
3. Exhaustion. If the stressor persists, the body's resources are drained, resistance collapses, and the person becomes vulnerable to illness, fatigue and burnout.