Action potential: generation and propagation

In the normal resting state, the plasma membrane of nerve and muscle cells generates a transmembrane electrical potential difference – the intracellular surface of the membrane being approximately 70–80 millivolts (mV) negative to the extracellular surface. This is a result of markedly different concentrations of ions inside and outside the cell, together with different membrane permeabilities to different ions that permits K+ to flow down their concentration gradient from inside to outside the cell. Nerve and muscle cells are ‘excitable’ because they can react to external stimuli by generating an extremely rapid change in transmembrane electrical potential difference known as the action potential. This comprises an initial explosive increase in membrane Na+ permeability that allows these ions to flood down their concentration gradient into the cell, thereby depolarizing the membrane such that the potential difference is transiently reversed to a positive value. However, in nerve and skeletal muscle this lasts for only a millisecond, at which time the membrane potential is just as rapidly restored to its resting negative value (repolarization). These events are controlled by the brief opening and closing of voltage-activated sodium and potassium channels in the membrane. The key features of the action potential are that it is: (1) an all-or-none event, rather than a graded response; (2) it is self-propagating, such that the wave of depolarization travels rapidly along the plasma membrane; and (3) it is transient, such that membrane excitability is quickly restored. These features of the action potential allow rapid transfer of information along nerve axons in the nervous system.