Local currents caused by Na⁺ influx stimulates voltage-gated Na⁺ channels to open further along the axon, initiating depolarisation there.

This spreads the action potential along the axon membrane.

Na⁺ channels close and K⁺ channels open, allowing repolarisation to occur.

In myelinated axons, saltatory conduction occurs, which involves the action potential jumping between nodes of Ranvier.

This is faster than continuous propagation in unmyelinated axons.

Saltatory conduction increases the speed of propagation as the myelin sheath prevents ion flow, so depolarisation only occurs at the nodes of Ranvier. The impulse jumps node to node, which is faster than propagating continuously.

Saltatory conduction is also more energy efficient, as it requires less ATP for the sodium-potassium pump to repolarise the membrane at fewer positions along the axon membrane. 

Once triggered, an action potential self-propagates through local currents along the axon without any decrease in size.

Wider axons allow faster propagation speed of a nerve impulse due to reduced resistance to the flow of ions in the axon cytoplasm.

A higher temperature increases the rate of ion diffusion, increasing the propagation speed of a nerve impulse.

At very high temperatures, proteins like the sodium-potassium pump denature, greatly reducing the propagation speed of a nerve impulse.

The refractory period is the time after an action potential when no new action potential can be generated due to closed Na⁺ channels.

1. Ensuring action potentials don't overlap
2. Limiting the frequency at which impulses are transmitted
3. Guaranteeing that impulses travel in only one direction