Action potential is a rapid change in membrane potential that leads to a temporary reversal where the inside becomes positive. Key properties include its abrupt onset, limited amplitude, short duration, compliance with the all-or-none law, self-propagation, and a refractory period. The mechanism involves voltage-gated sodium and potassium channels, resulting in phases of depolarization, repolarization, and hyperpolarization, with specific importance in cardiac muscle's plateau phase for prolonged contraction and refractory periods.

2. • Action potential is abrupt pulse like change in the
membrane potential lasting for a fraction of
second
• During action potential there is reversal of
membrane potential i.e. inside becomes positive
and outside becomes Negative.
• We can see the action potential on cathode ray
oscilloscope

3. Properties of Action Potential
1. Abrupt or sudden in onset
2. Have limited magnitude or amplitude i.e. Inside, the
potential will go to + 35 or + 45 mV and not beyond that.
3. It is of short duration. Duration is in milli seconds.
Duration of spike potential Is 1 -2 milli second. Action
potential with plateau has longer duration i.e. may be up
to 300 m sec

4. Properties of Action Potential
4. It obeys All or None law i.e. if stimulus is sub threshold
it is not produced and when the stimulus is threshold or
supra threshold it will be produced with maximum
amplitude.
5. It is self propagated i.e. once produced in a membrane
it is automatically propagated in both directions.
6. It is not decremented with distance i.e. it will travel
with same amplitude through all the distance.

5. Properties of Action Potential
7. It has refractory period. The period during which the
tissue will not respond to second stimulus after the
application of first stimulus. It could be Absolute and
Refractory.
▫ Absolute no response of tissue what so ever may be the
strength of stimulus example closure of inactivation gate
of sodium channels.
▫ Relative response with higher stimulus than threshold
stimulus

6. Phases of Action Potential
• DEPOLARIZATION: Sudden loss of Negativity
inside the membrane is depolarization.
• REPOLARIZATION: return of negativity inside the
membrane is Repolarization.
• HYPERPOLARIZATION: More Negativity inside
• Resting Membrane Potential

7. Types of Changes in Membrane Potential

8. Mechanism of Action Potential
• Understanding of
▫ Channels Involved
 Voltage gated Sodium Channels
 Voltage gated Potassium Channels
 Sodium Potassium ATPase Pump
▫ Movements of ions
 Concentrations of Sodium and Potassium in ECF and
ICF
 Direction of movement

9. Voltage Gated Sodium Channels
• Two gates: outer side activation gate, inner side
inactivation gate
• In resting state activation gate is closed and
inactivation gate is open

10. Voltage Gated Potassium Channels
• Voltage Gated Potassium Channel: Have one gate
on inner side
• Remains closed at rest

11. Sodium Potassium ATPase Pump

12. Following events occur
1. When stimulus is applied, conformational change starts in the
activation gate of the sodium channel and this leads to the
opening of activation gate causing a little influx of sodium and
when the membrane potential reaches to about -65 mV there
is complete opening of activation gate. This -65 mV is called
threshold for excitation or critical value or Firing level. When
the gates are opened then sodium channels are said to be
activated.
2. With activation of sodium channels there is rapid sodium
influx from ECF leading to the positivity inside and this is
known as DEPOLARIZATION.

13. • 3. Sodium channels remain activated for fraction of a
second and then these channels become inactivated
by the closure of inactivation gate leading to end of
Depolarization.
• 4. After depolarization there is Repolarization
caused by activation of Voltage gated Potassium
channels. When the stimulus is applied there is slow
conformational change in the gate of potassium
channels causing it to open. The potassium channels
are fully activated at the end of depolarization causing
Efflux of potassium ions leading to return of
negativity inside the membrane known as
repolarization.

14. 5. The inactivation gate of sodium channels will not
reopen till the membrane potential becomes near to
RMP and this is the cause of refractory period.
6. The last 30 % of repolarization is slow and this part is
called after depolarization. The cause of after
depolarization is that the potassium ions accumulate on
the outer surface of the membrane to slow down the
rate of further repolarization.

15. 7. The membrane potential after after depolarization
phase becomes more negative known as after
hyperpolarization caused by slow closure of potassium
channels leading to excessive efflux of potassium causing
more negative potential i.e. hyperpolarization. Also
sodium potassium pump is activated all the time which
also contributes to the hyperpolarization.
3 Na ion pump outside and 2K ion pump inside
continuously causing membrane more negative and
causing near to RMP.
8.The spike potential is the portion of action potential
between the threshold of excitation and the start of after
depolarization.

16. Plateau
• Plateau is known as Sustained depolarization.
• In some instances, the excited membrane does not repolarize
immediately after depolarization.
• Duration of depolarization of cardiac muscle is 300 milli sec.
• Plateau phase has got advantages:
• 1. It prolongs the duration of depolarization, AP and
Contraction. It prolongs the refractory period. Cardiac muscle
cannot be tetanized because of this.
• 2. There is influx of calcium into the sarcoplasm from the ECF
which is used for muscle contraction.