The basic long-distance signal of the nervous system is a self-propagating depolarization called the action potential. The action potential arises because of a sequence of voltage-dependent changes in the ionic permeability of the neuron membrane. This voltage-dependent behavior of the membrane is due to gated sodium and potassium channels. The conducting state of the sodium channels is controlled by m gates, which are closed at the usual resting Em and open rapidly upon depolarization, and by h gates, which are open at the usual resting Em and close slowly upon depolarization. The voltage-sensitive potassium channels are controlled by a single type of gate, called the n gate, which is closed at the resting Em and opens slowly upon depolarization. In response to depolarization, pNa increases dramatically as m gates open, and Em is driven up near ENa. With a delay, h gates close, restoring pNa to a low level, and n gates open, increasing pK. As a result, pNa/pK falls below its normal resting value, and Em is driven back to near EK. The resulting repolarization restores the membrane to its resting state.

The behavior of the voltage-dependent sodium and potassium channels can explain (1) why depolarization is the stimulus for generation ofan action potential; (2) why action potentials are all-or-none events; (3) how action potentials propagate along nerve fibers; (4) why the membrane potential becomes positive at the peak of the action potential; (5) why the membrane potential is transiently more negative than usual at the end of an action potential; and (6) the existence of a refractory period after a neuron fires an action potential.

Action potentials of some neurons have components contributed by voltage-dependent calcium channels, which open upon depolarization like voltage-dependent sodium channels but specifically allow influx of calcium ions. The influx of calcium ions through these channels can increase the intracellular concentration of calcium. Calcium-activated potassium channels open when internal calcium is elevated, contributing to the repolarization of the action potential and producing a prolonged period of elevated potassium permeability during which the membrane potential is more negative than the usual resting membrane potential.

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