FoundationalConceptsNeuroscience

the cell. If we examine a small region near the outside of the nerve cell
membrane and add up all the positive and negative charges there, we
arrive at some value for net electric charge. And if we examine a small
region near the inside of the nerve cell membrane and add up all the
positive and negative charges there, we arrive at some other value for
net electric charge. The net electric charge obtained by doing these
sums differs between the two sides of the cell membrane. This can be
described as a voltage across the cell membrane, where voltage repre-
sents the stored potential energy available to do work.
The voltage across the nerve cell membrane can be measured by
placing one electrode from a voltmeter inside the cell and a second
voltmeter electrode outside the cell. For a human brain cell this will be
about 65 millivolts (mV)—65 thousandths of a volt—with the inside
of the cell being negative relative to the outside. By convention, this
is written as -65 mV and is called the resting membrane potential
or resting voltage of the cell—“resting” because the nerve cell is not
sending a signal.
AA and AAA batteries have potential differences of approximately
1.5 volts between their positive and negative terminals. These batter-
ies are relatively big chunks of material. It is impressive indeed that a
tiny nerve cell has a potential difference of 65 mV across its cell mem-
brane. The voltage across the membrane is a measure of stored energy
that can be used to do work—just as batteries can be used to power
lights and radios, the voltage across the neuronal membrane will be
used to power the transmission of a signal along the cell’s axon. Let’s
see how.