We have already mentioned that a nerve impulse is electrical. This electrical charge is generated by the relative distribution of ions on both sides of the cell membrane. If there are more negative ions inside the cell with relatively more positive ions outside, the overall charge will also be negative on the inside and positive on the outside. This state is sometimes referred to as the resting potential. This is the electrical condition a nerve fibre will be in when there is no nerve impulse passing a particular point. Because in this situation there is a negative pole inside and a positive pole on the outside, the cell is described as being polarized. These are the same as the poles of a battery, one of which is negative and the other positive.
However, when a nerve impulse is passing a point on a nerve fibre, the polarity across the cell membrane reverses. There is a change in the permeability of the membrane surrounding the nerve fibre. This allows ions to pass through the cell membrane leading to a change in the relative distribution of positive and negative ions on the two sides of the membrane. The result is the inside becomes positive and the outside negative. This reversal of polarity is referred to as depolarization. After the nerve impulse has passed, the polarity reverts back to being negative inside and positive outside. The process by which resting polarity is restored is termed repolarization. From this it can be seen that a nerve impulse is a wave of depolarisation which passes along a nerve fibre. It is this electrical wave which constitutes the nerve impulse. Changes in polarity necessary for the propagation of the impulse are brought about by changes in the relative distribution and concentration of ions. These concentrations may change because ions are able to pass through the cell membrane in specialised small gaps called ion channels.