Generation of Nerve Impulse

A resting (non-conducting) neuron maintains a difference in electrical charge across its plasma membrane, called the resting membrane potential, with the inside of the membrane about 70 millivolts more negative than the outside (roughly -70 mV). This potential is not accidental; it is actively maintained. The Na+-K+ pump, a membrane protein that uses the energy of ATP, continuously moves three sodium ions (Na+) out of the cell for every two potassium ions (K+) it brings in, against their concentration gradients. Because of this pump, and because the resting membrane is far more permeable to K+ (which leaks out down its gradient through 'leak' channels) than to Na+, the outside of the axon ends up with a much higher concentration of Na+ and the inside a much higher concentration of K+, and the net effect is an inside-negative resting potential.

When a stimulus of sufficient strength reaches a point on the axon, voltage-gated Na+ channels at that point open. Because Na+ is far more concentrated outside the cell, it rushes in, and the inside of the membrane rapidly becomes less negative and then positive relative to the outside — this swing is called depolarization, and it typically overshoots to about +30 to +40 mV at the peak of the impulse. Almost as soon as they have opened, the Na+ channels begin to inactivate, stopping further Na+ entry, while voltage-gated K+ channels — which open more slowly — now open fully, allowing K+ to rush out of the cell down its concentration gradient. This outflow of positive charge restores the inside-negative condition, a phase called repolarization. K+ channels typically stay open a little longer than needed to reach the resting value, so the membrane potential briefly dips below the normal resting level before settling back — this brief dip is called hyperpolarization (the after-potential). During this recovery, and for a short period afterwards known as the refractory period, the membrane cannot immediately fire a second impulse; the Na+-K+ pump then work …