Action potential (the guyton and hall physiology)

ACTION POTENTIAL
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

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. 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.
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
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
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
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.

1. ACTION POTENTIAL Source: The Guyton and Hall physiology Maryam Fida (o-1827)

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 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 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 . 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.