FoundationalConceptsNeuroscience

polarize the immediately adjacent region of the membrane, causing
the voltage-gated sodium channels there to open and more Na to
come pouring in and drift around to depolarize the next immediately
adjacent region of the axon, and so on. Thus, each small region of the
axon initiates a depolarization of the immediately adjacent region,
and the action potential propagates along the axon in a more or less
continuous way.
In a myelinated axon, when the sodium ions come pouring in
through opened voltage-gated sodium channels, the immediately
adjacent region of the axon membrane is not available to produce an
action potential because it is covered by myelin and ions cannot flow
through channels and change the membrane voltage. The next place
the impact of the inflow of Na ions can be experienced along the axon
is at the next node of Ranvier, where again there are voltage-gated
Na channels that can be triggered to open if the membrane voltage
becomes positive enough. Thus, the inflowing charge carried by the
Na ions moves in the interior of the axon to the next node of Ranvier,
the membrane is depolarized there, and the voltage-gated Na chan-
nels open. In comes more Na‘, the impact of which will be felt at the
next node of Ranvier as the positive charge moves as a sort of electric
current along the length of the axon. This type of propagation of ac-
tion potential from one node of Ranvier to the next is called saltatory
conduction, from the Latin word saltare, meaning to leap as in a dance.
The action potential leaps from one node to the next, dancing its way
along the axon!
When sodium ions flow into the axon, what actually happens is the
positive charge of the Na pushes against the positive charge of nearby
K (a dominant ion within the cell), as positive charges repel one
another. The pushed-on K then pushes on whatever K* is next to it
and so on down the line: little push, little push, little push, little push.