Page 5 - LN-NEURAL CONTROL & COORDINATION

potential in the neuron.

It occurs in following three steps
Polarisation (Resting Potential)
In a resting nerve fibre (a nerve fibre that is not conducting an impulse), the
axoplasm (neuroplasm of axon) inside the axon contains high concentration of
+
+
K and negatively charged proteins and low concentration of Na .
+
(i) In contrast, the fluid outside axon contains a low concentration of K and a
+
high concentration of Na and thus form a concentration gradient.
(ii) These ionic gradients across the resting membrane are maintained by the
active transport of ions by the sodium-potassium pump, which transports
+
+
3Na out wards and 2K inwards (into the cell).
(iii) As a result, the outer surface of the axonal membrane possesses a positive
charge, while its inner surface becomes negatively charged and therefore, is
polarised.
(iv) The electrical potential difference across the resting plasma membrane is
called as the resting potential. The state of the resting membrane is called

polarised state.
Depolarisation (Action Potential)
When a stimulus of adequate strength (threshold stimulus) is applied to a
+
polarised membrane, the permeability of the membrane to Na ions is greatly
increased at the point of stimulation (site A).
+
(i) This leads to a rapid influx of Na followed by the reversal of the polarity at
that site, i.e., the outer surface of the membrane becomes negatively charged
and the inner side becomes positively charged. The polarity of the membrane

at the site A is thus, reversed and said to be depolarised.
(ii) The electrical potential difference across the plasma membrane at the site
A is called the action potential, another name of nerve impulse.
(iii) At adjacent sites, e.g., site B, the membrane (axon) has positive charge (still
polarised) on the outer surface and a negative charge on its inner surface.
(iv) The stimulated negatively charged point on the outside of the membrane

sends out an electrical current to the positive point next to it. As a result, a
current flows on the outer surface from site B to site A, while on the inner
surface current flows from site A to site B.
This process (reversal) repeats itself over and over again and a nerve impulse is
conducted through the length of the neuron.