Explain the resting membrane potential, the generation and propagation of the action potential, the all-or-none principle, the refractory period and saltatory conduction

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.

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.

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.

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.