Definition:
An all-or-none electrical event in a neuron or muscle fiber, in which the polarity of the membrane potential is rapidly reversed and then reestablished.

Description:
When a threshold stimulus is applied to the membrane of a neuron or muscle fiber, a sequence of events changes the resting membrane potential to an action potential, or impulse. The events are:

1. Adequate stimulation of membrane.
2. Increased membrane permeability to sodium at site of stimulation (opens gates).
3. Sodium ions move inward; transmembrane potential reaches zero (depolarization).
4. Sodium ions continue to move inward and the inside of the membrane becomes positively charged (reverse polarization) relative to the outside.
5. Reverse polarization acts as a stimulus to the adjacent regions.
6. Decreased permeability to sodium and increased permeability to potassium.
7. Potassium ions move out, making the outside positive (repolarization).
8. Pumps transport sodium back out of the cell, and potassium back into the cell.

Properties:: Nerve and muscle cells obey the all-or-none law, meaning that a threshold stimulus evokes an action potential, and that a subthreshold stimulus evokes no response. During the interval from the onset of an action potential until repolarization is about 1/3 completed, a second stimulus cannot elicit another response because the gates are already open and the membrane is in an absolute refractory period. Following the absolute refractory period is an interval during which the neuron will not respond to a normal threshold stimulus, but a very strong stimulus can depolarize the membrane and produce a second action potential; this is the relative refractory period.

Clinical Information:
Recording of action potentials. Recordings of the electrical activity of nerves and muscles provide information about the physiological condition of these tissues. An electrocardiogram (EKG) records cardiac muscle activity; an electroencephalogram (EEG) records brain activity; and an electromyogram (EMG) records skeletal muscle activity.