ACTION POTENTIAL:
Dr. Ayisha Qureshi
Assistant Professor
MBBS, MPhil
DEFINITIONS:
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.
REMEMBER:
IMPORTANT:
• 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:
Action Potential:
Definition:
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 on 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
IONIC BASIS OF AN ACTION POTENTIAL:
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
DEFINITIONS:
LATENT PERIOD:
It is the time period between the application of a stimulus and the
start of the response (Action Potential).
DEPOLARIZATION:
When during the transit changes in the action potential, the Potential
difference between the inside of the membrane (-90mv) and
outside (0mv) decreases it is called depolarization ( the tracing will
move upwards in the AP diagram).
REPOLARIZATION:
A return to the resting membrane potential from either direction (i.e.
de- or hyper-polarization) is called repolarization.
HYPERPOLARIZATION: When during the transit changes in the action
potential, the Potential difference between the inside of the
membrane (-90mv) and the outside (0mv) increases it is called
Hyperpolarization.
PROPAGATION OF AN ACTION
POTENTIAL:
Conduction of an Action
Potential in a Myelinated and
an Unmyelinated nerve fibre:
Continuous Conduction
• Occurs in unmyelinated axons.
• In this situation, the wave of de- and repolarization simply
travels from one patch of membrane to the next adjacent
patch.
• APs moved
in this
fashion
along the
sarcolemma
of a muscle
fiber as well.
• Analogous to
dominoes
falling.
Saltatory conduction in a Myelinated Neuron:
• Which do you think has a faster rate
of AP conduction – myelinated or
unmyelinated axons?
Answer: Myelinated axon
Name the events & ions responsible for:
– Depolarization
– Repolarization
– Hyperpolarization OR Undershoot
– Return of the AP from the Overshoot to
the RMP
PROPERTIES OF A NERVE FIBRE:
1. ALL OR NONE LAW
(also called the All or Nothing Law)
On application of a stimulus, an excitable membrane
either responds with a maximal or full-fledged action
potential that spreads along the nerve fiber, or it does not
respond with an action potential at all. This property is
called the all-or-none law.
(This is in direction proportion to the strength of the stimulus applied.)
e.g: This is similar to firing a gun. Either the trigger is NOT
pulled sufficiently to fire the gun (subthreshold stimulus)
OR it is pulled hard enough to fire the gun (threshold is
reached). Squeezing the trigger harder does not produce
a greater explosion, just as pulling the trigger halfway
does not cause the gun to fire halfway.
2a: ABSOLUTE REFRACTORY PERIOD
Definition:
Once an action potential has been generated , the time period during which even a
suprathreshold stimulus will fail to produce a new action potential is called the
Absolute Refractory period.
During this time the membrane becomes completely refractory (‘stubborn’ or
‘unresponsive’) to any further stimulation.
It corresponds to the entire Depolarization phase & most of the Repolarization phase.
Due to Absolute refractory period, one AP must be over before another can be
initiated at the same site. APs cannot be overlapped or added one on top of
another.
• BASIS OF AN ABSOLUTE REFRACTORY PERIOD:
During the depolarization phase of AP, the voltagegated Sodium channels have still NOT reset to
their original position. For the Sodium channels
to respond to a stimulus, 2 events are important:
1. Sodium channels be reset to their closed but
capable of opening position. i.e: inactivation
gates open and activation gates closed.
2. The Resting membrane potential must be reestablished.
2b: Relative Refractory Period
Definition:
Following the absolute refractory period is seen a
period of short duration during which a second
action potential can be produced, only if the
triggering event is a suprathreshold stimulus.
This period is called the Relative Refractory
Period.
It corresponds to the last half of the Repolarization
phase.
• Basis of a Relative Refractory Period:
An action potential can be produced by a
suprathreshold stimulus because of the following
reasons:
1. By the end of the repolarization phase, some Na
channels have reset while some K channels are
also still open.
2. Thus, a greater than normal triggering event
(suprathreshold stimulus) is required to produce
an AP.
What is the significance of the REFRACTORY
PERIOD (both absolute & relative):
1. There is no fusion or summation of the action potentials. This intermittent, ie. Not
continuous conduction of nerve impulses is one of the reasons why a nerve fibre
can respond to continuous stimulation for hours without getting tired. Thus, it
decreases fatigue in a nerve fibre.
2. The Action Potentials are produced separate from each other. So, a new AP is
produced in each part of the nerve fibre. This ensures that the AP does not die out
as it is conducted along the membrane.
3. Only a certain number of Action Potentials can be produced in a nerve fibre
because the interval between any 2 action potentials cannot be shorter than the
Absolute Refractory Period. This prevents fatigue of the nerve fibers and sets an
upper limit on the maximum numbers of AP that can be produced in a nerve fibre
in a given period of time.
4. By the time the original site has recovered from its refractory period and is
capable of being restimulated by normal current flow, the action potential has
been propagated in the forward direction only and is so far away that it can no
longer influence the original site. Thus, the refractory period ensures the one-way
propagation of the action potential down the axon away from the initial site of
activation.
3. COMPOUND ACTION POTENTIAL:
3. Compound Action Potential is seen
in a “nerve trunk” & NOT a nerve fibre:
• An action potential having
more than one peak/spike is
called a Compound action
potential.
CAUSE: A nerve trunk contains
many nerve fibres differing
widely in their excitability &
different speeds of conduction
of AP. Multiple peaks are
recorded with the AP from
fastest conducting nerve fibre
first to be recorded followed
by the slower ones....
MULTIPLE SCLEROSIS:
Multiple Sclerosis:
Going, Going, Gone…..
It is an auto-immune disease in which there is patchy loss of myelin sheath of
nerve fibers leading to disruption of nerve impulse conduction.
Cause: Antibodies mistakenly attack and destroy Schwann cells, thus also
destroying the myelin sheath.
Signs & symptoms:
Patchy destruction of myelin in the CNS→ slow & abnormal conduction of the
nerve impulses in the neurons
↓
1. Visual disturbances
2. Tingling & numbness
3. Muscle weakness & Fatigue
4. Gradual paralysis
5. Bladder & bowel problems
Treatment
• Debilitating disease but not fatal. The quality
of life is affected…. Death may occur when
the paralysis reaches the respiratory muscles
and the person cannot breathe.
• No treatment as yet.
• Only symptomatic treatment.
 Immunosuppressive therapy: as autoimmune disease
so you try to suppress the immune system to prevent
further damage.
 Corticosteroids
 Alternative therapy: as homeopathy, ayurvedic
treatment etc