1. An action potential occurs when there is a rapid change in the membrane potential of a neuron from a negative resting potential to a positive potential and then back to a negative potential. This is driven by the movement of ions across the cell membrane through voltage-gated ion channels. 2. For an action potential to be initiated, the membrane potential must be depolarized beyond the threshold potential of around -65mV. This opens voltage-gated sodium channels, allowing sodium ions to enter the cell. 3. The influx of sodium ions causes further depolarization, followed by the opening of voltage-gated potassium channels which causes repolarization back to the resting potential and then hyperpolarization below the resting potential.

1. Action Potential
Dr. Sara Sadiq
(MBBS, M.Phil)

2. Action Potential
1. Depolarization
phase
2. Repolarization
phase
3. Hyperpolarization phase
Resting potential
Threshold potential
A rapid change in membrane
potential from –ve value to
+ve and then back to –ve
value due to transport of
ions across cell membrane,
resulting in generation of
impulse

3. Magnitude of stimulus
• Sub threshold stimulus
• Threshold stimulus
 Minimum rise in RMP that can initiate action potential
 -65mv (-70mv to -50mv)

4. Channels in Cell Membrane
1. K-Na leak channels  leak towards conc: gradients
2. Voltage gated Na+ Channels  remain closed at normal RMP
3. Voltage gated K+ Channels  remain closed at normal RMP
4. Slow Ca- Na Channels  only in Cardiac & Smooth muscles

5. Pumps in Cell Membrane
1. Na- K Electrogenic pump
2. Ca+ pump

6. Resting Potential - Both voltage gated Na+ and K+ channels
are closed.

7. Initial Depolarization - Some Na+ channels open. If enough Na+
channels open, then the threshold is surpassed and an action potential is
initiated.

8. Firing Level
More Na+ channels open quickly. K+ channels are still closed.
Change of RMP from -90mv to +35mv

9. Repolarization- Na+ channels self-inactivate, K+ channels are open.
Change of membrane potential from +35mv to again -90mv

10. Hyperpolarization- Excessive K+ outflux until all the channels closed

11. Resting Potential - Both Na+ and K+ channels are closed.

14. Spike Potential
• Initial very large change in membrane potential
is called Spike Potential

15. Latent period
• The interval corresponds to the time it
takes the impulse to travel along the
axon from the site of stimulation to the
recording electrodes.
• Its duration depends upon
1. The distance between the stimulating
and recording electrodes
2. The speed of conduction.

16. "All-or-None" Law
• The action potential fails to occur if the stimulus is
subthreshold in magnitude, and it occurs with constant
amplitude and form regardless of the strength of the
stimulus if the stimulus is at or above threshold intensity.

17. Adaptation
• Slowly rising currents fail to fire the nerve because the
nerve adapts to the applied stimulus, a process called
adaptation.

18. Refractory Period
Period during which a second action potential can not occur in
presence of first action potential
1. Absolute Refractory Period  Stimulus given during depolarization
 Na channel become inactivated & can’t
be reopened
2. Relative Refractory Period  Stimulus given during repolarization
 Na channel have been reversed from state
of inactivation