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.