The nervous system: WACE Year 12 Human Biology Unit 3
“Describe the structure and function of neurons, the transmission of nerve impulses, synaptic transmission and the reflex arc in homeostatic control”
The nervous system carries fast electrical impulses along neurons and passes signals chemically across synapses; sensory neurons detect stimuli, interneurons in the central nervous system process them, and motor neurons drive effectors, giving rapid, targeted, short-lived homeostatic responses such as the reflex arc.
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.
The myelin sheath insulates the axon so the action potential cannot form under it. Instead the impulse jumps from one node of Ranvier to the next, which is called saltatory conduction. This makes myelinated neurons conduct far faster than unmyelinated ones, which is why loss of myelin (as in multiple sclerosis) slows or blocks signals.
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.
From presynaptic axon to postsynaptic dendrite
An action potential arrives at the axon terminal of the presynaptic neuron. This causes calcium ions to enter the terminal. The calcium makes synaptic vesicles fuse with the membrane and release neurotransmitter into the synaptic gap by exocytosis. The neurotransmitter molecules diffuse across the gap and bind to specific receptors on the membrane of the postsynaptic neuron. This binding opens ion channels, depolarises the postsynaptic membrane, and if threshold is reached a new action potential is generated. The neurotransmitter is then removed (broken down by an enzyme or reabsorbed), which resets the synapse. The one-way release of neurotransmitter is why synapses make signals travel in a single direction.
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.
A frequent error is describing the electrical impulse crossing the synaptic gap. The gap is bridged chemically by a neurotransmitter, not electrically. State that the action potential triggers neurotransmitter release, the neurotransmitter diffuses across the gap, and binding to receptors on the next neuron may start a fresh action potential. Also remember neurotransmitter release happens only at the presynaptic side, which is what makes transmission one-directional.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksDescribe how a nerve impulse is transmitted across a chemical synapse from one neuron to the next.Show worked answer →
A 6 mark response needs the ordered sequence of events at the synapse.
- Arrival of impulse
- An action potential arrives at the synaptic knob of the presynaptic neuron.
- Calcium entry
- This causes voltage-gated calcium channels to open and calcium ions enter the knob.
- Neurotransmitter release
- Calcium triggers vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft by exocytosis.
- Diffusion and binding
- The neurotransmitter diffuses across the cleft and binds to specific receptors on the postsynaptic membrane.
- Postsynaptic response
- This opens ion channels, and if threshold is reached a new action potential is generated in the postsynaptic neuron.
- Removal
- The neurotransmitter is then broken down or reabsorbed so the synapse can transmit again.
Markers reward the calcium-triggered vesicle release, diffusion across the cleft, receptor binding and a new impulse.
WACE 2024-style5 marksDescribe the components of a reflex arc and explain why a reflex action is rapid and protective.Show worked answer →
A 5 mark answer needs the pathway in order plus why it is fast.
Pathway. Stimulus to receptor, then a sensory neuron carries the impulse to the spinal cord, where an interneuron (relay neuron) connects to a motor neuron, which carries the impulse to an effector (muscle or gland) that produces the response.
Why rapid and protective. The reflex arc is processed in the spinal cord without first passing to the conscious brain, so there are few synapses and the response is very fast. This speed allows the body to withdraw from harm, such as a hot surface, before damage occurs.
Markers reward the receptor to sensory to relay to motor to effector pathway and the spinal-cord, few-synapse reason for speed.