15.1.6—Membrane potential changes

An action potential at one axon region changes the local electrical charge across the membrane. Local Na⁺ movement depolarises the adjacent membrane, bringing its voltage-gated Na⁺ channels towards opening and regenerating the impulse there.

• The action potential therefore passes from one membrane section to the next rather than travelling as one unchanged object along the whole axon.
• Immediately behind the active region, the refractory membrane is temporarily unresponsive, so a new action potential is generated ahead of the original event.
• This local regeneration plus the refractory boundary gives the impulse one-way propagation.

Myelin electrically insulates much of the axon, while uninsulated nodes of Ranvier remain available for membrane exchange. The impulse is therefore regenerated at exposed membrane regions; the detailed speed advantage of saltatory conduction is treated separately.

Local current is not a flow of electrons like an ordinary wire, and the refractory region does not create a second impulse behind the original one. This card explains propagation along adjacent axon regions, not synaptic transmission, muscle activation, or quantitative conduction speed.