Biology Times (^19)
July
depolarized. The membrane is said to be
hyperpolarized, if the inner side of the
membrane becomes more negative than at
resting potential. Depolarizations (-
mV) bring a neuron closer to the threshold,
and hyperpolarizations move the neuron
further away from the threshold.
Factors that contribute to resting
membrane potential
The following factors contribute to the
resting membrane potential of a neuron:
Ionic gradients
There is unequal distribution of ions in
the ECF and in the cytosol (axoplasm).
The concentration of Na+ in the ECF is ten
times to its concentration in the axoplasm.
The concentration of K+ in the axoplasm
is thirty times to its concentration in the
ECF. The principal anion in the ECF is Cl
- .
The principal anions in the axoplasm are
phosphates (e.g. in ATP) and amino acids in
proteins.
Differential permeability
The plasma membrane has more K+ leak
channels than Na+ leak channels and K+
leak channels are more leaky than Na+ leak
channels. This makes the resting membrane
roughly 100 times more permeable to K+ than
to Na+. Consequently, the number of K+ ions
that diffuse down their concentration gradient
out of the cell into the ECF is greater than
the number of Na+ ions that diffuse down
their concentration gradient from the ECF
into the cell. As more and more positive ions
exit, the inside of the membrane becomes
increasingly negative.
Non-diffusible anions
Most anions inside the cell cannot follow K+
out of the cell because they are attached to
non-diffusible molecules such as ATP and
large proteins.
Sodium-potassium pump
The sodium–potassium pump (Na+/K+
ATPase) imports two K+ ions into the cell
for every three Na+ ions it pumps out of the
cell (Fig.4). This helps maintain high K+
concentration inside the cell, and high Na+
concentration outside the cell.
Though all cells display membrane potential,
nerve cells have special mechanisms for
using this potential to transmit information
over long distances.Equilibrium potential
The type of equilibrium in which a
chemical gradient is balanced with an
electrical potential is referred to as an
electrochemical equilibrium. At this
equilibrium, there is no net diffusion of ions
across the membrane. For a cell whose
membrane is permeable to only one ion, the
resting membrane potential will be equal to
the equilibrium potential for that ion.
According to simplified Nernest equation, at
37 o C:Considering that concentration of K+ is 30
times more inside the cell and concentration
of Na+ is 10 times more in the ECF (outside
the cell):
l Suppose the plasma membrane of the
cell is permeable only to K+, then
EK= 62 mV (log 1/30) = -92 mV
l Suppose the plasma membrane of the
cell is permeable only to Na+, then
ENa= 62 mV (log 10/1) = 62 mV
Actually, the axolemma (plasma membrane
of the axon) at rest is permeable to both the
ions but it is 100 times more permeable to K+
than to Na+. So, the resting membrane
potential (-70 mV) is more close to EK value
(-92 mV) than to ENa value (62 mV).
The membrane potential can change from
its resting value when the membrane’s
permeability to particular ions changes.Action Potential
The momentary change (about1millisecond)
in electrical potential on the membrane of a