ions responsible for carrying charge cannot
penetrate the plasma membrane’s lipid
bilayer. There is a small build up of anions in
the axoplasm just inside the axolemma,
and an equal build-up of cations in the ECF
just outside the axolemma (Fig.2).
Such separation of positive and negative
charges across the membrane is a form of
potential energy. The membrane potential
can be measured by a voltmeter (Fig. 3) or
oscilloscope.
A resting potential of –70 mV is tiny
compared to the voltages used to provide
electric current in a home (240 V), or even
that of a battery in your TV remote (1.5-
V). Nonetheless, this tiny difference in
charge across the membrane of a neuron is
sufficient to generate a nerve impulse, or
action potential.
The resting membrane potential of a typical
neuron is –70 mV (millivolts). The minus
sign indicates that the inside of the cell is
negative with respect to the outside. Thus,
the resting membrane is polarised.
Tip:To remember the polarity of
resting membrane potential, associate
Negative with iNside and pOsitive with
the Outside.
The membrane is said to be in a state of
polarization any time membrane potential
is other than 0 mV, in either the positive
or the negative direction. If the inner side
of the membrane changes towards positive
(becomes less negative or positive), it is
said to be depolarized. The membrane is
said to be repolarized, if the membrane
returns to resting potential after having been