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The nerve impulse is a dramatic change in membrane potential, and therefore also known as action potential or "spike" (Figure 2-9). The membrane potential at certain position in the cell depends on the local charge density which in turn is determined by the local concentration of ions. Thus, the membrane potential and the local concentration of ions are directly related. The influx of positively charged sodium ions at somewhere in a cell will raise the membrane potential, resulting in depolarization at that position. The influx of negatively charged chloride ions will lead to hyperpolarization, and the outflux of positively charged potassium ions will cause hyperpolarization.
Alterations in membrane potential can affect the voltage-gated ion channels. An ion channel has two main states: "open" and "closed." In the closed state, all ions cannot pass through the channel; in the open state, the selected ions may pass through. The direction of ionic flux depends on membrane potential and ion concentrations. Under normal conditions, sodium ions will flow inward when the sodium channel is open, resulting in depolarization; potassium ions will flow outward when the potassium channel is open, reducing the membrane potential (Figure 2-10).
When a neuron is at rest, the membrane potential at any position is approximately -70 mV. As the membrane potential rises to a threshold value, nerve impulses could be generated. For most neurons, the threshold is about 15 mV higher than the resting potential. A neuron "fires" means that nerve impulses have been generated in the neuron.
The generation of nerve impulses is mainly due to the voltage-gated sodium and potassium channels whose open probability increases with increasing depolarization. As the membrane potential is depolarized to threshold, the open probability of sodium channels has been significantly increased, so that a large amount of sodium ions can flow inward, resulting in further depolarization, and even more sodium influx. In the mean time, opening of potassium channels will reduce membrane potential. However, the potassium channels open more slowly than sodium channels. Only around the peak of nerve impulses, can the potassium channels begin to exert its influence.
In addition to the opening of potassium channels, "sodium inactivation" also plays an important role in the repolarization phase of nerve impulses. The sodium channel will enter an inactivated state after it is open for less than 1 millisecond. In the inactivated state, not only the channel is closed, it cannot be opened by depolarization either. The channel may gradually recover from the inactivated state to the resting state when the membrane potential returns to resting value or less. Before fully recovered (it takes a few milliseconds), the sodium channels become more difficult to be opened by depolarization, and thus it is harder to generate another nerve impulse. This recovery period is known as the "refractory period".