FoundationalConceptsNeuroscience

end of the axon without stopping. No new input of energy is required
to power the propagation of the action potential along the axon’s
length. The energy has already been stored in the form of the mem-
brane voltage and the concentration differences of sodium and potas-
sium, the result of the continuous work of Na/K pumps charging up
the “batteries” of the neuron.
To recap: an action potential consists of the sequential opening and
closing of voltage-gated sodium and potassium channels, resulting
in a rapid depolarization of the membrane, followed by a rapid return
to resting membrane potential. Axonal action potentials initiate in
the region where the axon emerges from the soma of the cell. This
region is called the axon hillock or axon initial segment (Fig. 5.8). Here
voltage-gated sodium and potassium channels occur in high density.
As signals received from other cells (see Chapter 6) result in changes
in membrane potential that diffuse through neuron, the first place
where voltage-gated channels are encountered in high density is the
axon hillock. The channels are triggered to open, an action potential
results, and propagation continues without stopping along the entire
length of the axon.
The action potential propagates in only one direction along the
axon, from the hillock to the terminus. It does not propagate from
the hillock into the soma, because there are few voltage-gated Na
channels in that direction. And as it moves along the axon, it does not
go backward because of a phenomenon called the refractory period:
after the voltage-gated sodium and potassium channels are triggered
to open and close, they require several milliseconds to return toa
state that can be triggered again to open. (Again, there is an analogy
with a flushing toilet: after the toilet is flushed, it cannot be flushed
again until the system refills.) When Na comes flowing in, although
it drifts in both directions in the interior of the axon from its point of