Action Potentials

An action potential is initiated through a depolarization of the nerve fiber. Sodium permeable channels open allowing sodium to enter the membrane causing the surface of the membrane to become less negative. As the depolarization reaches and crosses the threshold, the action potential is triggered. Multiple action potentials can be generated with continuous depolarization. A nerve can be depolarized by stimulating it with an electrical current. The rate of action potential generation depends on the magnitude of the continuous depolarizing current. The firing frequency of an action potential reflects the magnitude of the depolarizing current. Although the firing frequency increases with the amount of depolarizing current, there is a limit to the rate at which a neuron can generate action potentials, the absolute refractory period. Once an action potential is initiated, it is impossible to initiate until the membrane potential goes sufficiently negative to allow the sodium channels to be reactivated. The relative refractory period follows the absolute refractory period and makes it difficult to initiate another action potential immediately after, but can be initiated with a larger amount of current to bring the membrane potential past the threshold.

Once generated, an action potential transfers information from one point in the nervous system to another. An action potential at one end of an axon only propagates in one direction because the membrane just behind it is still in refractory period. However, the action potential can be generated by depolarization at either end of the axon and therefore propagate in either direction, but will never turn back on itself. An action potential continues to propagate down an axon because the result of an action potential generates a new action potential.