FoundationalConceptsNeuroscience

ing K or Cl channels in a nerve cell at rest produces a hyperpolariz-
ing effect on the cell membrane, as shown by the graph in Figure 5.4.
If an ion-channel protein selective for either sodium or calcium is
opened, then Na or Ca** will flow through the respective channel
from outside the cell to inside the cell, in either case making the inside
of the cell more positive (less negative) relative to the outside. Thus, if
we measure the membrane voltage before, during, and after the tran-
sient opening of sodium or calcium channels, we would see a change
toward a less negative (more positive) value. This represents a de-
crease in the magnitude of the charge difference across the membrane
and is referred to as a depolarization of the cell membrane (Fig. 5.5).
If a voltmeter is used to measure the membrane potential along
a portion of an axon, we will generally observe voltage to be in the
vicinity of the resting membrane potential (approximately -65 mV),
with frequent small deviations above and below the resting potential
(small depolarizations and hyperpolarizations). However, when a
signal passes along a nerve cell’s axon, a striking change in membrane
voltage is observed, called an action potential (Fig. 5.6). These changes
in membrane potential are the result of electrically charged particles
(ions) moving across the membrane. A flow of electric charge is what
defines electricity. Something electrical is happening. Neural signal-
ing is electrical in nature.