The nerve impulse and action potential: WACE Year 12 Human Biology Unit 3
“Explain the resting membrane potential, the generation and propagation of the action potential, the all-or-none principle, the refractory period and saltatory conduction”
A neuron at rest is polarised, with the inside negative relative to the outside, maintained by the sodium-potassium pump. A strong enough stimulus reaches threshold and triggers an action potential: sodium ions rush in (depolarisation), then potassium ions leave (repolarisation), and this self-propagating wave travels along the axon. The signal is all-or-none, a refractory period forces it in one direction, and myelin makes it jump between nodes (saltatory conduction) for speed.
What this dot point is asking
WACE wants the detailed electrical story of a single impulse, separate from the overall structure of the neuron. This is the level of detail that earns the higher marks: naming the ions, the directions they move, and the stages in order. Get the resting state right first, because every later stage is described relative to it.
The resting membrane potential
When a neuron is not transmitting, it is polarised. The inside of the axon is about 70 millivolts negative compared with the outside, called the resting potential. This is set up and maintained by the sodium-potassium pump, which uses energy (ATP) to pump three sodium ions out for every two potassium ions it pumps in. Combined with the membrane being more permeable to potassium leaking back out, this leaves the inside negative. The neuron is now charged and ready, like a battery waiting to discharge.
The resting potential is the electrical charge difference across the membrane of a neuron that is not conducting an impulse, with the inside negative relative to the outside (about minus 70 millivolts). It is maintained by the sodium-potassium pump and is the starting state from which an action potential is generated.
Generating the action potential
A stimulus changes the membrane permeability. If the stimulus is strong enough to push the membrane to a threshold value, voltage-gated sodium channels open and sodium ions rush into the axon. This makes the inside briefly positive, an event called depolarisation, and this reversal of charge is the action potential. Almost immediately the sodium channels close and potassium channels open, so potassium ions move out and the inside becomes negative again: repolarisation. The sodium-potassium pump then restores the original ion distribution, returning the membrane to its resting potential.
Propagation along the axon
An action potential at one point depolarises the neighbouring region, pushing it to threshold so it fires in turn. In this way the action potential moves along the axon as a self-propagating wave, regenerating at full strength at each point so it does not fade with distance.
A neuron either fires a full action potential or it does not fire at all; there is no partial impulse. A stimulus below threshold produces no action potential, while any stimulus at or above threshold produces an action potential of the same size. A stronger stimulus is signalled not by a bigger impulse but by a higher frequency of impulses.
The refractory period
Just after an action potential, the region of membrane cannot be re-stimulated for a short time, called the refractory period, while the ion channels reset. This has two important effects: it ensures the impulse travels in only one direction, because the region behind cannot fire again immediately, and it sets an upper limit on how frequently a neuron can fire.
Saltatory conduction and myelin
Many axons are wrapped in a fatty myelin sheath with gaps called nodes of Ranvier. The myelin insulates the membrane so the action potential can only form at the nodes. The impulse therefore jumps from node to node, which is called saltatory conduction. This is much faster than the continuous conduction in an unmyelinated axon and uses less energy. Loss of myelin, as in multiple sclerosis, slows or blocks impulses.
Naming the stages in order
A marker wants the events in order with the ions named. Resting state: the sodium-potassium pump keeps the inside negative (about minus 70 millivolts), so the membrane is polarised. Stimulus reaches threshold: voltage-gated sodium channels open and sodium ions diffuse into the axon. Depolarisation: the inflow of sodium makes the inside positive, which is the action potential. Repolarisation: sodium channels close, potassium channels open, and potassium ions move out, restoring the negative inside. Recovery: the sodium-potassium pump restores the ion distribution and the membrane returns to the resting potential, with a refractory period before the region can fire again. This signal regenerates at each point along the axon, so it does not weaken with distance.
How this maps to the exam
Expect questions that ask you to label a graph of the action potential (resting, depolarisation, repolarisation), explain the role of the sodium-potassium pump, state the all-or-none principle, or explain why myelinated neurons conduct faster. Use the ions and directions precisely: sodium in for depolarisation, potassium out for repolarisation.
The all-or-none principle means the size of the action potential does not change with stimulus strength. A stronger stimulus increases the frequency of impulses, not their size. Another common error is reversing the ions: depolarisation is sodium moving in, repolarisation is potassium moving out. Also do not say the impulse travels because charge flows down the axon like a wire; it is regenerated at each point as neighbouring regions reach threshold.
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-style7 marksDescribe the resting membrane potential of a neuron, and explain the ionic movements that produce depolarisation and repolarisation during an action potential.Show worked answer →
A 7 mark response needs the resting state plus the two ion movements of the action potential.
- Resting membrane potential
- At rest the inside of the axon is negative relative to the outside (about mV). The sodium-potassium pump moves three sodium ions out for every two potassium ions in, and the membrane is more permeable to potassium leaking out, so the outside is more positive.
- Depolarisation
- When a stimulus reaches threshold, voltage-gated sodium channels open and sodium ions rush into the axon down their gradient. The inside becomes positive, reaching about mV.
- Repolarisation
- Sodium channels close and voltage-gated potassium channels open, so potassium ions move out of the axon. This restores the negative inside, returning the potential toward resting. The sodium-potassium pump then restores the original ion distribution.
Markers reward the resting potential value and direction, sodium in for depolarisation, and potassium out for repolarisation.
WACE 2023-style4 marksExplain the all-or-none principle and describe how saltatory conduction increases the speed of impulse transmission in a myelinated neuron.Show worked answer →
A 4 mark answer needs the all-or-none idea plus the saltatory mechanism.
All-or-none principle. Once a stimulus reaches threshold, a full action potential of the same size is generated; a stronger stimulus does not make a larger impulse. Below threshold, no action potential occurs. Stimulus strength is instead coded by the frequency of impulses.
Saltatory conduction. In a myelinated axon, the myelin sheath insulates the membrane except at the gaps called nodes of Ranvier. The action potential jumps from node to node rather than moving continuously along the whole membrane, so transmission is much faster.
Markers reward the threshold-triggered fixed-size response and the impulse jumping between nodes of Ranvier.