21 Propagation of the Action Potential
The action potential is initiated at the beginning of the axon, at what is called the initial segment. There is a high density of voltage-gated Na+ channels so that rapid depolarization can take place here. Going down the length of the axon, the action potential is propagated because more voltage-gated Na+ channels are opened as the depolarization spreads. This spreading occurs because Na+ enters through the channel and moves along the inside of the cell membrane. As the Na+ moves, or flows, a short distance along the cell membrane, its positive charge depolarizes a little more of the cell membrane. As that depolarization spreads, new voltage-gated Na+ channels open and more ions rush into the cell, spreading the depolarization a little farther.
Because voltage-gated Na+ channels are inactivated at the peak of the depolarization, they cannot be opened again for a brief time—the absolute refractory period. Because of this, depolarization spreading back toward previously opened channels has no effect. The action potential must propagate toward the axon terminals; as a result, the polarity of the neuron is maintained, as mentioned above.
Propagation, as described above, applies to unmyelinated axons. When myelination is present, the action potential propagates differently. Sodium ions that enter the cell at the initial segment start to spread along the length of the axon segment, but there are no voltage-gated Na+ channels until the first node of Ranvier. Because there is not constant opening of these channels along the axon segment, the depolarization spreads at an optimal speed. The distance between nodes is the optimal distance to keep the membrane still depolarized above threshold at the next node. As Na+ spreads along the inside of the membrane of the axon segment, the charge starts to dissipate. If the node were any farther down the axon, that depolarization would have fallen off too much for voltage-gated Na+ channels to be activated at the next node of Ranvier. If the nodes were any closer together, the speed of propagation would be slower.
Propagation along an unmyelinated axon is referred to as continuous conduction; along the length of a myelinated axon, it is saltatory conduction. Continuous conduction is slow because there are always voltage-gated Na+ channels opening, and more and more Na+ is rushing into the cell. Saltatory conduction is faster because the action potential basically jumps from one node to the next (saltare = “to leap”), and the new influx of Na+ renews the depolarized membrane. Along with the myelination of the axon, the diameter of the axon can influence the speed of conduction. Much as water runs faster in a wide river than in a narrow creek, Na+-based depolarization spreads faster down a wide axon than down a narrow one. This concept is known as resistance and is generally true for electrical wires or plumbing, just as it is true for axons, although the specific conditions are different at the scales of electrons or ions versus water in a river.