Membrane Potentials

THE MEMBRANE POTENTIALS ACTION POTENTIAL It is defined as the sequences of events that occur in the membrane potential upon excitation of cell. RESTING MEMBRANE POTENTIAL Inside of nerve is negative and the outside is positive . Resting membrane potential is normally -70 mV UPON STIMULATION OF NERVE, 1. A brief irregular deflection from the baseline is seen, the stimulus artifact. This marks the point of stimulus. 2. Brief latent period: This indicates the time taken by impulse to travel along the axon from the stimulating electrode to the recording electrode. 3. Depolarization Beginning of Depolarization: This is the first manifestation of nerve impulse. It is the reduction in the membrane potential from negative to 0. After initial depolarization of 15 mV, the rate of depolarization increases at a point, called the firing level. With further increase in action potential the depolarization wave reaches up to approximately +35mV. 4. Repolarisation: The impulse is reversed and falls back to the resting level. After 70% completion of repolarisation, the rate of repolarisation falls, and the impulse reaches the resting level more slowly. 5. Afterdepolarisation: Sharp rise and fall are Spike potential of axon and the slower fall at end of the process is called Afterdepolarization. Duration: 4 msec

6. Afterhyperpolarisation: Upon reaching the baseline the potential decrease slightly towards negative. This small but prolonged increase in membrane potential is called after hyperpolarisation. Duration: 35-40 ms

IONIC BASIS OF ACTION POTENTIAL 1. RESTING MEMBRANE POTENTIAL (RMP): The inside of membrane is negative, and outside is positive. K+ maintain the RMP.

2. DEPOLARISATION: It is due to redistribution of ions, leads to increased K+ and Cl- influx.

3. REPOLARISATION: This starts with K+ efflux and decreases in further Na+ influx. K+ efflux and Na+ influx cause net transfer of positive charge out of the cell that serve to complete depolarization.

4. AFTER DEPOLARISATION: At the termination of spike potential, the efflux of K+ is slowed down; a few milliseconds are delayed in restoring membrane potential. This last phase of slow K+ efflux is called after depolarization.

5. AFTER HYPERPOLARISATION: When the after depolarization disappears, the resting membrane potential is achieved but the resting ionic potential is achieved by the active Na+ - K+ pump mechanism.

PROPERTIES OF ACTION POTENTIAL 1. Threshold stimulus: The minimum amount of stimulating current required for a given duration of time for the excitation and conduction of nerve impulse. 2. The relationship between strength of stimulating current and the duration for it must be applied to produce is called strength duration curve. The weakest current strength that can excite a tissue, if allowed to flow through it for an adequate time, is called rheobase. The time for which rheobase must be applied is called utilization time. 3. The length of time, for which the current of double the intensity than rheobase must be applied to produce the stimulus is called Chronaxie. 4. All or none law: Action potential is all or none response. This means that once a minimum threshold is reached, membrane potentials are no longer dependent on the stimulus strength. The action potential fails to occur if the stimulus is subthreshold in magnitude, or it responds to the maximum of its ability. This all or none relationship between the stimulus and the response is called all or none law. 5. Refractory period: if two stimuli of more threshold intensity are applied in succession, to an excitable cell, then after the first stimulus, the cell becomes refractory to the second stimulus for a period of time. This time period is called as the refractory period. Absolute refractory period: time period from the firing level of action potential till the one third of repolarization is completed. It does not respond to any stimulus during this period. Relative refractory period: time period between end of absolute refractory period and the start of after depolarization. Strong stimulus can cause excitation.

6. Conductivity: propagation of wave of depolarization. Action Potential initiated at one side on the excitable cell acts as a stimulus for the generation of an action potential in adjacent area. 7. Accommodation: It as an adaptation of the cell to constant stimulus applied to it, so that it fails to produce action potential on same strength of stimulus. It is due to slower opening and delayed closing of the voltage gated K+ channels.