Action potential occurs when a nerve cell is stimulated. The mechanisms responsible for action potential products depend on two main features

• transmembrane ionic concentration gradients
• voltage dependent changes in the membrane permeability to sodium and potassium ions.


It has several important properties:
 

• All or none law: subthreshold stimuli do not elicit any active voltage changes while any stimulus which exceeds threshold will lead to a full action potential. Increasing the stimulus further has no effect on the shape and size of the action potential
• During an action potential it is either impossible or more difficult than normal to stimulate a further action potential, the cell is said to be refractory
• Action potentials are self-propagating ie once an action potential has been generated by an external stimulus at one site it will automatically be conducted along the whole of the excitable membrane.