PROPAGATION OF THE ACTION POTENTIAL
When an AP is initiated at a specific site of the axonal membrane (usually the initial segment), the inward flow of Na+ alters the extracellular ion environment, causing a local flow of charge from adjacent regions of the axon. This induces a depolarized state in the adjacent node of Ranvier (myelinated axon) or patch of axonal membrane (unmyelinated axon), bringing that region to threshold and resulting in the reinitiation of the action potential. The presence of myelination along axonal segments results in the reinitiation of the action potential at the next node, thus hastening the velocity of conduction of the AP. The resultant appearance of the AP skipping from node to node down the axon is called saltatory conduction.
An action potential is an explosive reversal of the neuronal membrane potential that takes place because of an increase in Na+ conductance induced by depolarization, usually due to the cumulative effects of graded potentials from incoming neurotransmitters; this explosive reversal is followed later by an increase in K+ conductance, restoring the membrane back toward the resting potential. This process normally takes place at the initial segment of an axon. The conduction of an AP down a myelinated axon, saltatory conduction, requires the reinitiation of the AP at each bare patch of axonal membrane, a node of Ranvier. The reinitiation of the AP occurs because of a voltage change at the next node brought about by passive current flow from the AP at its present site. If several nodes distal to the site of AP propagation are blocked with a local anesthetic, preventing Na+ conductance, then the AP will die, or cease, because the closest fully functional, nonblocked node is too far from the point of AP propagation to reach threshold by means of passive current flow. This mechanism of block- ing reinitiation of the action potential at nodes of Ranvier underlies the use of the -caine derivatives, as in novocaine and xylocaine, for local anesthesia during surgical and dental procedures.