All animal cells have a voltage across their cell membranes. Neurons and muscle cells can alter this potential and conduct impulses through their membranes, called nerve impulses. This is a comprehensive note on the "resting membrane potential" of a cell membrane, when no impulses are being conducted.

1. Syed Muhammad Khan (BS Hons Zoology)
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RESTING MEMBRANE POTENTIAL
Electrical potentials exist across the membranes of almost all of the cells of the body,
but the neurons and muscle cells are capable of generating rapidly changing
electrochemical impulses and are also capable of transmitting them.
MEMBRANE POTENTIAL
The membrane potential is an electrochemical potential (analogous to the EMF of a
cell) that exists across a membrane of a cell due to a difference in the ion
concentration on either side.
The membrane potential is especially important for the neurons because it is the
principal driving force of the nerve impulse (language/signal of the neuron) which is
a wave of electrochemical changes that travels along the length of the neuron and
involves chemical reactions and movements of ions across the cell membrane.
Figure: Ion concentrations on either side of the cell membrane are responsible for
creating a membrane potential. [Source: Wikimedia, CC-BY-SA]
The nerve impulse is conducted when an action potential exists across the
membrane of the neuron, otherwise, it is said to be in a resting (non-conducting)

2. Syed Muhammad Khan (BS Hons Zoology)
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state and the membrane potential, in that case, is termed as the resting membrane
potential. Our concern is the latter, which is explained as follows:
RESTING MEMBRANE POTENTIAL OF A NEURON
A neuron, in its resting state, does not conduct a nerve impulse and its membrane is
said to be polarized because the fluids on the inner side of the membrane are
negatively charged while those in the exterior are positively charged. This difference
in the electric charge is due to the relative numbers of positive and negative ions
(cations and anions) on either side of the membrane and the relative permeability of
the membrane to these ions.
The membrane potential is measured in millivolts (mV), which is the same unit that
is employed in measuring the EMF of the cell. A millivolt is a 1/1,000 of a volt (the
real unit is volt, mili- is merely a suffix used for our convenience).
Normally, the resting membrane potential is about -70mV which infers that the
potential inside the membrane is 70mV more negative than that of the exterior. The
factors that determine this level are discussed below:
FACTORS INVOLVED IN THE RESTING MEMBRANE POTENTIAL
OF A NEURON
1. Sodium Potassium ATPase Pump
The Sodium ions (Na+) are more highly concentrated in the extracellular fluid,
whereas the Potassium ions (K+) and negative protein ions are more highly
concentrated in the fluid inside the plasma membrane. Naturally, the Na+ and K+ ions
constantly diffuse through ion channels in the plasma membrane along their
respective concentration gradients (Na+ ions tend to move inwards, whereas K+ ions
tend to move towards the outside). The same does not apply to the negative ions
because of their large size.
If these movements were to be unchecked, they would have easily disturbed the
resting membrane potential. These movements are checked by a powerful Sodium
Potassium ATPase Pump present in the membrane of the neuron. This pump
moves the two ions actively (using ATP) against their concentration gradients, i.e:
Sodium ions are forced back out and Potassium ions are pulled inside. For each 3
Na+ ions that are actively transported outside, 2 K+ ions are moved inwards. This

3. Syed Muhammad Khan (BS Hons Zoology)
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causes a net deficit of positive ions in the interior of the membrane, hence causing a
negative potential inside of it.
2. Leakage of Potassium ions
A channel protein called as the potassium “leak” channel in the nerve membrane,
allows the Potassium ions to leak out even in the resting membrane potential. These
leak channels are more permeable to K+ ions as compared to Na+ ions (about 100
times more) and hence they only allow a minute amount of Na+ ions to pass through.
This leakage causes a further drop in the concentration of positive ions inside the
membrane and helps in establishing a resting membrane potential.
Figure: The Sodium Potassium ATPase Pump actively moves Na+ ions back to the
outside and the K+ ions to the inside, using ATP as a source of energy. For every 2
K+ ions that move inwards, 3 Na+ ions are transported outside. The Potassium leak
channels allow K+ ions to leak out even during the resting membrane potential.
These channels also allow a slight amount of Na+ ions to move inwards but they are
about 100 times more permeable to K+ ions. [Source: Wikimedia, CC-BY-SA]
VALUE OF RESTING MEMBRANE POTENTIAL
The average value of a resting membrane potential is -70mV which infers that the
interior of the membrane is 70mV more “negative” than the exterior (thanks to the
actions of the Sodium Potassium ATPase Pump and Potassium leak channels).