neuron that occurs when it is stimulated is
termed action potential, or nerve impulse
(Fig.6). In a resting membrane, the voltage-
gated sodium channels are rest (activation
gate is closed and inactivation gate is open).
Voltage-gated potassium channels are also
closed (Fig.6-a).
When the cell is stimulated, the voltage-gated
channels of K+ open more slowly than those of
Na+. As a result, an action potential begins with
an increase in the membrane’s permeability to
Na+ which depolarizes the membrane, followed
by an increased permeability to K+ which
repolarizes the membrane.
On an oscilloscope, depolarization is
represented as an upward deflection whereas
hyperpolarisation is represented by a
downward deflection (Fig. 6).
Stimulation
When a stimulus depolarizes the axonal
membrane, some voltage-gated sodium channels
are activated (activation gate opens) allowing
influx of Na+ (Fig.6-b). This causes some
depolarization which opens some more voltage-
gated sodium channels allowing more influx Na+.
This positive feedback cycle (Fig.5) opens a large
number of voltage-gated sodium channels
rapidly once the membrane potential reaches
threshold (typically -55 mV).
At threshold potential, both chemical and
electrical gradients favour influx of Na+
(Na+ concentration is more in the ECF
than inside the cell; inside of the cell is
negatively charged)
Depolarising phase (rising phase)
Rapid opening of many voltage-gated sodium
channels causes rapid influx of Na+ resulting
in explosive depolarization. This changes the
membrane potential from -55 to about
+30mV. (Fig. 6).During the rising phase of an action potential,
the membrane potential transiently reverses.If the level of depolarization is less than
the threshold potential, the membrane
potential will normally drop back to resting
levels without further consequences. When
the depolarization is just equal to or above
the threshold potential, an action potential
of equal amplitude is initiated. Thus action
potential is an all-or-none phenomenon.
Firing a gun is an analogy of all-or-none
phenomenon. Applying a very slight
pressure on the trigger is insufficient to fire
the gun. The gun fires only when adequate
pressure is applied to the trigger. The speed
and force of the bullet does not depend on
how hard you pull the trigger. The gun
either fires or it does not fire at all. In this
analogy, the force applied on the trigger
represents the stimulus while the firing of
the gun represents action potential.
Repolarising phase (falling phase)
As membrane potential reaches its peak, the
voltage-gated sodium channels are inactivated
(inactivation gate closes) and voltage-gated
potassium channels open (Fig 6-d). As a
result, Na+ influx slows down and K+ efflux
begins. This causes the membrane potential to