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ACTION POTENTIAL AND ITS STAGES |FINDYOURSELF
ACTION POTENTIAl:
Stimulus:
A stimulus is an external force or event which when
applied to an excitable tissue produces a characteristic response.
Subthreshold stimulus:
A stimulus which is too weak to produce a response is
called a Subthreshold stimulus.
Threshold stimulus:
The minimum strength of stimulus that can produce
excitation is called a Threshold stimulus.
Suprathreshold stimulus:
Stimuli having strengths higher than threshold
stimulus are called Suprathreshold stimuli.
•
Sodium voltage-gated channels: are
fast channels & have 2 gates:
- An
outer Activation gate (closed in resting state)
- An
Inner Inactivation gate (open in resting state)
•
Potassium channels are
slow channels & have only ONE gate.
•
These channels are different from
Sodium & Potassium leak channels.
•
The Sodium-Potassium PUMP is
present separately.
Action
Potential:
An Action
Potential is a self-propagating wave of electro-negativity that passes along
the surface of the axolemma of the nerve fibers.
•
We know that the inside of the nerve
membrane is negative with respect to the outside (RMP=—90 mv)
•
When an effective stimulus (threshold
or suprathreshold) is applied, the electrical charge on the membrane is
reversed: at the active part of the nerve fibre the outside becomes negative as
compared to the corresponding region in the interior. This is called
DEPOLARIZATION and forms the Action Potential.
PHASES OF AN ACTION POTENTIAL:
Phase 1: Depolarization
Phase 2: Repolarization
Phase 3: Hyperpolarization
1.
DEPOLARIZATION: Sodium
(Na) Influx
2.
REPOLARIZATION: Potassium
(K) Efflux
3.
HYPERPOLARIZATION: Leakage
of excess Potassium (K) ions through the slow closing K channels.
4.
RETURN OF THE AP TO THE RMP FROM
HYPERPOLARIZATION: Sodium-Potassium Pump
•
AFTER-DEPOLARIZATION:
The descending limb of the
action potential does not reach the baseline abruptly, but it shows a delay of
several milliseconds. This is due to decreased rate of K efflux at this time.
The excitability & conductivity of the fibre are increased during this
phase.
•
AFTER-HYPERPOLARIZATION:
Leakage of excess Potassium
(K) ions through the slow closing K channels.
Why
does the depolarization not reach the Nernst potential of +66mv for sodium?
There are 2
main reasons. At +35 mv:
•
Sodium
Influx stops because Inactivation gates of Sodium channels close although the
activation gates are open & thus no sodium can enter
•
Potassium
Efflux starts because slow Potassium channel gates open and potassium moves
out.
State of SODIUM channel gates:
•
Resting state:
-
Inactivation gates: OPEN
-
Activation gates: CLOSED
•
Depolarization:
-
Activation gates: OPEN
-
Inactivation gates: OPEN
•
Peak:
-
Inactivation gates: CLOSED
-
Activation gates: OPEN
•
Repolarization:
-
Inactivation gates: OPEN
-
Activation gates: CLOSED
PROPAGATION OF AN ACTION POTENTIAL:
Unmyelinated
Nerve fiber
•
Once
an action potential is initiated at the axon hillock, no further
triggering event is necessary to activate the remainder of the nerve fiber. The
impulse is automatically conducted throughout the neuron.
•
For
the action potential to spread from the active to the inactive
areas, the inactive areas must somehow be depolarized to threshold. This
depolarization is accomplished by local current flow between the area already
undergoing an action potential and the adjacent inactive area
•
This
depolarizing effect quickly brings the involved inactive area to threshold, at
which time the voltage-gated Na channels in this region of the membrane are all
thrown open, leading to an action potential in this previously inactive area.
Meanwhile, the original active area returns to resting potential as a result of
K+ efflux.
