The outer surface of the axon membrane has a positive charge, compared to the inside. When a nervous impulse passes along the axon, voltage-gated sodium ion channels
open, causing sodium ions to enter by diffusion, down an electrical/chemical gradient
. This causes depolarisation
of the membrane at that point
, so that here the inside temporarily has a positive charge - typically about +45 mV.
This means that an electrical circuit exists between this (-ve) region and the neighbouring (+ve) section ("adjoining region") of the axon membrane. This sets up a local current, increasing the permeability of the adjoining region as a result of voltage-gated sodium ion channels opening, allowing sodium ions to enter at that point. In fact the voltage change stimulates more sodium ion channels to open - a positive feedback
effect. At the peak of the action potential, sodium ion channels close. There is a section within the channel called the inactivation gate which plugs the channel, preventing flow of sodium ions. At a later stage when the resting potential is restored, 'deinactivation' occurs and the neurone can transmit another impulse.
Next, voltage-gated potassium ion channels open and potassium ions diffuse out of the axon, down an electrical/chemical gradient. This has the effect of reversing the axon surface potential back towards its resting (negative) value: repolarisation
In fact this reduces the membrane potential below the normal resting potential - hyperpolarisation
- shown as "undershoot". The potassium ion channels now close. Another action potential cannot pass until the normal resting potential is restored - the 'refractory period
'. This means that nervous impulses are distinct and discrete pulses, which move in one direction, with distinct gaps (in time) between each .
Before and after action potential
This process occurs progressively along the length of the axon, so that the action potential gradually moves along it.
This is sometimes called the propagation
of a nervous impulse, because it involves the repeated creation of an identical event, travelling in a set direction.
After an impulse, the sodium/potassium ion pump can once again pump sodium ions out, and potassium ions in.