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.
CLINICAL POINT
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.
