Conduction of the action potential

Action potentials can occur in excitable cells. An action potential is a local event leading to depolarisation, or reversal in polarity, where the inside of the cell becomes positively charged relative to the outside (in neurones this is typically a change from -70mV to +40mV).

Conduction in unmyelinated nerve fibres

The outside of the resting cell membrane is typically positively charged with respect to the inside. The negative charge of the action potential is drawn to the positive charge of adjacent resting cells. This induces a local current or

electronic depolarisation

Electronic depolarisation causes voltage gated Na⁺ channels to open followed by voltage gated K⁺ channels i.e. an action potential occurs in the adjacent cell. This process continues as the action potential propagates along the nerve segment until it reaches the end.

This type of conduction, seen in unmyelinated nerve fibres, where depolarisation spreads to the immediately adjacent cell is called continuous conduction.



Note that the refractory period of already activated cells prevents the impulse flowing backwards.

Conduction in myelinated nerve fibres

The principle of conduction in myelinated nerve fibres is similar to that of continuous conduction which is described above. Myelinated fibres are insulated by myelin. The areas covered by myelin cannot be depolarised. The small areas that lack myelin are called nodes of Ranvier. Depolarisation therefore jumps from one node to another in a process called saltatory conduction.



Saltatory conduction is rapid and conserves energy as much smaller areas of axon use energy to power repolarization via active pumping of Na⁺ and K⁺.