Action potential

MBBS FIRST YEAR

1. ACTION POTENTIAL & ITS IONIC BASIS Presented by : KAIF Q.

2. ACTION POTENTIAL Definition: An Action Potential is a brief sequential change of membrane potential after application of a threshold stimulus. It can be recorded by a CATHODE RAY OSCILLOSCOPE

3. We know that the inside of the nerve membrane is negative with respect to the outside [POLARISED STATE] (RMP=—90 mv) When an effective stimulus(threshold or suprathreshold ) is applied, the electrical charge on the membrane is reversed ie 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.

4. PHASES OF AN ACTION POTENTIAL: Phase 1: Depolarization Phase 2: Repolarization Phase 3: Hyperpolarization

5. Action Potential

6. DEPOLARISATION LATENT PERIOD: It is the time period between the application of a stimulus and the start of the response (Action Potential) FIRING POTENTIAL: The sharp change observed in the MP which rapidly goes upto 0 and ultimately crosses the ‘0’ line to become positive. SPIKE POTENTIAL: The pointed peak in the graph , attained due to depolarisation and rapid repolarisation

7. IONIC BASIS OF AN ACTION POTENTIAL: DEPOLARIZATION: Sodium (Na) Influx REPOLARIZATION: Potassium (K) Efflux HYPERPOLARIZATION: Leakage of excess Potassium (K) ions through the slow closing K channels. RETURN OF THE AP TO THE RMP FROM HYPERPOLARIZATION: Sodium-Potassium Pump

8. Why does the depolarization not reach the Nernst potential of +65mv 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.

9. REPOLARIZATION: A return to the resting membrane potential from either direction (i.e. de- or hyperpolarization) 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.

10. REPOLARISATION

11. AFTER REPOLARISATION