Describe the structure and function of neurons, the transmission of nerve impulses, synaptic transmission and the reflex arc in homeostatic control

What this dot point is asking

WACE wants you to explain how the nervous system achieves fast, precise control, in contrast to the slow, widespread endocrine system. The nervous system has two divisions: the central nervous system (the brain and spinal cord) and the peripheral nervous system (all the nerves outside it). The peripheral system splits into the somatic (voluntary) and autonomic (involuntary) divisions, and the autonomic into the sympathetic (fight or flight) and parasympathetic (rest and digest) branches that often act antagonistically.

Neuron structure and the three neuron types

A neuron is the basic cell of the nervous system. Dendrites receive signals and carry them toward the cell body, which contains the nucleus. The axon carries the impulse away from the cell body to the axon terminals. Many axons are wrapped in a myelin sheath made of fatty cells, with gaps called nodes of Ranvier between the segments.

You need three functional types. Sensory (afferent) neurons carry impulses from receptors to the central nervous system. Interneurons (connector neurons) lie within the central nervous system and link sensory to motor neurons. Motor (efferent) neurons carry impulses from the central nervous system to effectors (muscles and glands).

How an impulse travels along a neuron

A nerve impulse is an electrical signal called an action potential. At rest the inside of the neuron is negative relative to the outside (the resting potential), maintained by the sodium-potassium pump. When a stimulus is strong enough to reach threshold, sodium ions rush in and reverse the charge: this is depolarisation. The region then repolarises as potassium ions leave, and the action potential moves along the axon as a wave.

Synaptic transmission

Neurons do not touch. A synapse is the tiny gap between one neuron and the next. The impulse cannot jump the gap electrically, so it is carried chemically.

When the action potential reaches the axon terminal of the presynaptic neuron, it triggers calcium entry, which makes vesicles release a neurotransmitter into the synaptic gap. The neurotransmitter diffuses across the gap and binds to specific receptors on the postsynaptic neuron, which can start a new action potential there. The neurotransmitter is then broken down or reabsorbed so the signal does not continue indefinitely.

The reflex arc

A reflex is a rapid, automatic, involuntary response that protects the body, for example pulling your hand off a hot object before you feel pain. The reflex arc is the pathway: a receptor detects the stimulus, a sensory neuron carries the impulse to the spinal cord, an interneuron in the spinal cord relays it, a motor neuron carries the impulse to the effector, and the effector (a muscle) produces the response. Because the signal acts at the spinal cord without waiting for the brain, the response is very fast; the brain is informed afterwards.

How this maps to the exam

Common questions ask you to label a neuron, sequence an action potential, explain synaptic transmission in order, or trace a reflex arc. The highest-value comparison is nervous versus endocrine control: fast, targeted and brief versus slow, widespread and sustained. Use the reflex arc to show why the nervous system suits emergency homeostatic responses.