Figure 5.7. Actions of voltage-gated ion channels during an action potential.To summarize the important concept of a voltage-gated ion chan-
nel, the electrical forces across the membrane (measured by the volt-
age) tug on the amino acids of the channel protein; at certain values of
voltage the protein shifts shape and a channel opens. This allows ions
of specific size and charge to flow through and across the membrane.
Ions will flow from the region where they are more concentrated to
the region where they are less concentrated: Na*, Ca*, and Cl flow
from outside to inside the cell, and K flows from inside to outside the
cell.
Voltage-gated sodium and potassium channels are located along
the entire length of a nerve cell axon. This provides a mechanism
for the propagation of a nerve impulse or signal along the length of
the axon, from the cell body, or soma, down to the axon terminal. It
works like this: once an action potential gets started and voltage-gated
sodium channels open at some location on the axon, sodium ions flow
into the axon and make the region inside the axon at that location
more positive. The Na ions rapidly drift away from where they flow
in and make the nearby regions more positive. This local depolariza-
tion triggers the voltage-gated Na channels in the adjacent axonal
region to open and more Na‘ flows in. These sodium ions then rapidly
drift and make the adjacent region inside the axon more positive
causing the voltage-gated Na* channels in that region to open. This
process is repeated over and over again along the length of the axon—
this is the mechanism by which the action potential moves along the
axon. It is analogous to a crowd of thousands of people doing a wave at
a sport event.
Once an action potential gets started, it moves all the way to the