ACTION POTENTIALS
Action potentials (APs) are all-or-nothing,
nondecremental, electrical potentials that allow an electrical signal to travel
for very long distances (a meter or more) and trigger neurotransmitter release
through electrochemical coupling (excitationsecretion coupling).
APs are
usually initiated at the initial segment of axons when temporal and spatial
summation of EPSPs cause sufficient excitation (depolarization) to open Na+
channels, allowing the membrane to reach threshold. Thresh-old is the point at
which Na+ influx through these Na+ channels cannot be countered by efflux of K+.
When threshold is reached, an action potential is fired. As the axon rapidly
depolarizes during the rising phase of the AP, the membrane increases its K+
conductance, which then allows efflux of K+ to counter the rapid
depolarization and bring the membrane potential back toward its resting level.
Once the action potential has been initiated, it rapidly propagates down the
axon by reinitiating itself at each node of Ranvier (myelinated axon) or
adjacent patch of membrane (unmyelinated axon) by locally bringing that next zone of axon membrane to threshold.
