FoundationalConceptsNeuroscience

Shortly after their initial publication, Great Britain was drawn full
tilt into World War II and both Hodgkin and Huxley devoted the next
several years of their lives to working in war-related activities, such
as the development of radar. Six years went by, at which point they
picked up right where they left off. After several more years of careful
experimental measurements and a lot of clever mathematical reason-
ing, they were able to propose a comprehensive description of what
happens when a neural signal travels along an axon. Their theory pre-
dicted the existence of voltage-gated ion channels years before they
were discovered.
Among the ion-channel proteins found in nerve cell membranes,
two very important kinds, located primarily along the length of
the axon, are sodium and potassium channels that open and close
depending upon the membrane voltage. These are called voltage-gated
channels. Voltage-gated Na channels along an axon are closed when
the membrane potential is at rest and open when the membrane po-
tential reaches about —50 mV. Thus, if some inflow of positive charge
occurs (this happens as a result of signals received from other nerve
cells, discussed in Chapter 6) and the depolarization moves the mem-
brane voltage to —50 mV, then Na channels open and Na (which is
much more concentrated outside the cell) flows into the cell, bringing
with it more positive charge, thereby making the membrane voltage
more and more positive. When the voltage reaches +30 mV, the elec-
trical forces tugging on the sodium-channel protein cause the channel
to close and the flow of Na into the cell ceases.
There are also voltage-gated K channels in the axonal membrane.
These channels are closed when the cell is at rest, and they begin to
open as the voltage becomes more positive. By the time the voltage
reaches +30 mV and the sodium channels are closing, the voltage-
gated potassium channels are rapidly opening and K’ is flowing out