A stereotyped change in electrical potential across the MEMBRANE of a neuron or muscle cell as a result of stimulation. The occurrence of an action potential is dependent on the cell being depolarized (see DEPOLARIZATION) past the point called the THRESHOLD. Once the threshold is reached, a positive feedback loop initiates a sequence of depolarization and repolarization that is independent of the initial cause. This allows an action potential to maintain the same amplitude as it travels (or propagates) along the cell membrane. In the case of neurons, this property allows information to be encoded in terms of the rate of firing of action potentials. This information can then be carried without change from one part of the nervous system to another via the neuron’s axon. An action potential is also called a spike. The simultaneous occurrence of many action potentials in a peripheral nerve or muscle is recorded as a COMPOUND ACTION POTENTIAL.
An action potential is dependent on the presence of SODIUM CHANNELS that are sensitive to membrane voltage. Depolarization of the membrane opens these channels rapidly (sodium activation) and closes them slowly (sodium inactivation). The initial opening allows positive sodium ions to flow into the cell, which depolarizes the membrane more; this depolarization in turn opens more sodium channels. At the point where the positive feedback effect of the inflowing sodium ions cannot be stopped (the threshold) the action potential is initiated. The membrane voltage becomes positive inside the cell as the membrane potential approaches the equilibrium potential for sodium. This large depolarization is terminated by progressive sodium inactivation as well as by the opening of voltage-dependent POTASSIUM CHANNELS which permits potassium ions to flow out of the cell. The slower time course of potassium activation often produces a HYPERPOLARIZATION at the end of the action potential. Voltage-dependent sodium channels cannot be reopened until sodium inactivation is complete. This results in an ABSOLUTE REFRACTORY PERIOD which limits the frequency of action potentials. The absolute refractory period is followed by a relative refractory period, during which a second action potential can be initiated if a stronger stimulus is applied. In other words, the threshold is raised during the relative refractory period.
Propagation of the action potential along the axon results from the fact that the depolarization at one point on the membrane produces passive spread of current in surrounding membrane. Therefore this adjacent membrane will also be depolarized and it too will produce an action potential when it reaches threshold. The movement of an action potential along the cell membrane is therefore analogous to the burning of a fuse. The rate of action potential propagation, or conduction velocity, is dependent on two factors: the internal axonal resistance and the membrane resistance. Conduction velocity is indirectly proportional to the internal resistance, which is primarily determined by the diameter of the axon. In a manner analogous to water flow in a pipe, the smaller the diameter of the axon the greater will be the resistance and consequently the slower the conduction velocity. Membrane resistance affects the velocity in the opposite manner: the greater the membrane resistance the greater the velocity. This is because the greater membrane resistance prevents the current generated by the action potential from leaking out through the membrane. Greater current therefore travels along the axon and is better able to depolarize the next segment of the axon. The membrane resistance is primarily determined by the presence or absence of MYELIN. The NODES OF RANVIER are the regions between myelinated segments and are characterized by a high density of voltage-sensitive sodium channels. The high resistance underlying the myelin and the high density of sodium channels at the nodes of Ranvier allows the action potential to ‘jump’ from one node to the next. This type of action potential conduction is therefore called SALTATORY and is much faster than can be obtained by increasing the axonal diameter.
Diseases that affect conduction velocity can impair sensory and/or motor function because information cannot travel from one end of neuron to the other. This can be due to damage to the neurons themselves or to loss of myelin (DEMYELINATION). Peripheral neuropathies, associated with diabetes and many other diseases, can cause pain or loss of sensation in various parts of the body . The more peripheral parts of the body (hands and feet) are most affected by these diseases because the axons to and from the hands and feet travel the longest distances and are therefore the most easily affected. Other types of peripheral demyelination may result from viral infection (for instance, Guillain-Barr disease). The major demyelinating disease of the central nervous system is MULTIPLE SCLEROSIS.
