NERVE IMPULSE (IT'S GENERATION) -

1. It is an electro chemical phenomenon manifested through ionic movement across cell membrane.
2. It was discovered by A. L. Hodykin & A. F. Huxley (1939).
3. Impulse is a change in resting membrane potential of the cell towards positive side, also called as action potential or spike potential.
4. All cells bear positive charge outside (due to Na+) and negativity inside thus membrane remain in polarised state.
5. This potential across the membrane can be measured by putting electrodes inside & outside connected to a galvanometer or potentiometer or a voltameter.
6. When the cell is at normal resting state, the potential in animal cell ranges from - 60 mv to - 90 mv, this is called resting potential (nerve cell has about - 70 mv)
7. Any ext. stimulus reaching the membrane causes change in its permeability and Na+ starts rushing inside which results in the rise of potential towards positive side. This process of charge in potential (action potential) is called as depolarisation.
8. To bring the condition back to normal in nerve cell it is followed by exit of K+  out of the cell which results in resumption of the resting potential known as repolarisation.
9. Since the rate of exit of K+ is very slow while rate of entry of Na+ is very fast, this result in rise & stay of action potential for some time.