The natural current of a nerve, like that of muscle, undergoes a diminution at the moment the nerve is stimulated; this is termed the negative variation. It occurs with any other form of stimulation as well as when an electric shock is used, so it is not dependent on an escape of the stimulating current. In the case of a single stimulation, the negative variation is so rapidly over - lasting only.0005 sec, that the inertia of the needle of the galvanometer prevents the change in the current being indicated. In tetanus, however, it makes a decided impression on the galvanometric needle. The strength of the negative variation depends on the condition of the nerve and the strength of the stimulus; being stronger when the nerve is fresh and irritable and has a good natural current, and when a strong stimulus is applied.
The negative variation of the natural currents passes along the nerve from the point of stimulation in both directions, just as does the nerve impulse; and with a galvanometer the electric change may be traced from the nerve to the muscle. It has also been shown that the negative variation travels along the nerve at the same velocity as the impulse; namely, about 27 metres per second. Further, this rate is said to be influenced in the same way by the passage of a constant current through the nerve (to be presently described) as is the impulse derived from stimulus. These points seem to lead to the belief that the nerve impulse and the negative variation are closely related. This peculiar electric change and its accompanying impulse pass along the nerves as a kind of wave of activity, the speed and duration of which we know to be 27 metres per sec. and.0005 of a sec. respectively; the length of the wave we therefore calculate to be about 18 millimetres.