CHANGES IN NERVE-POTENTIALS PRODUCED BY RAPIDLY REPEATED STIMULI AND THEIR RELATION TO THE RESPONSIVENESS OF NERVE TO STIMULATION

Abstract

The accumulation of potentials during a tetanus and their persistence in the period following it was studied in frog nerve with the aid of a battery-coupled amplifier and a cathode ray oscillograph, so that rapidly changing and slowly changing potentials might be simultaneously and correctly recorded. Spikes separated by more than their relatively refractory periods maintain a constant height above their base level. When more closely spaced, they maintain the height of the 2nd response or progressively decline. The max. of the neg. after-potential tends to increase at first (staircasing). It approaches a steady state higher than the initial value, either directly or after passing through a max. When the spike potentials are added to the residual neg. after-potentials, during an initial period the height of each successive crest is higher than the preceding one, giving the appearance of staircasing. The spike heights measured from the after-potential level are not increased, however. The pos. after-potential is cumulative. Signs of accumulation appear even during a tetanus; and at the end of a tetanus the magnitude of the potential increases with the duration (or frequency) of stimulation until a max. is reached, which may be equivalent to 40% of the spike height. Excitability and conduction velocity are subnormal during the pos. after-potential. Subnormal excitability can promptly be changed to supernormal by the neg. after-potential following a response to a stimulus above threshold. The form of the pos. after-potential indicates that it is dependent on 2 separate processes, of which the first (called P1) has a shorter duration than the second, (P2), and a temp.] coefficient of about 3 for 10[degree].