Generation of Nerve Impulse

It is basically an electrochemical phenomenon manifested through ionic movement across the cell membrane, discovered by A.L. Hodgkin and A.F. Huxley (1939).
Impulse is a change in resting membrane potential of the cell towards positive side, also called as action potential (or spike potential).
All cells bear positive charge outside (due to Na+) and negativity inside thus membrane remain in polarised state.
Membrane being selectively permeable does not allow Na+ to enter into cell and so it acts as resistance to prevent the flow of current. Hence, membrane shows a particular potential as characteristic of resting state.
This potential across the membrane can be measured by putting electrodes inside and outside connected to a galvanometer (or potentiometer or a voltmeter).
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).

Fig. Changes in the axon during the propagation of a nerve impulse

Any external 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
change of potential (action potential) is called as depolarisation.
To bring the condition back to normal in nerve cell it is followed by exit of K+ ions out of the cell, which results in resumption of the resting potential, known as repolarisation.
Therefore, nerve cell undergoes alternate phases of depolarisation and repolarisation.
Nerve cell is specialised to generate action potential also on its own i.e. without any external stimulus and the site for generation of impulse is only at axon hillock.