The influence of diameter of medullated nerve fibres of cats on the rising and falling phases of the spike and its recovery

Abstract

1. The relation of conduction velocity, i.e. fibre diameter (Hursh, 1939a) to certain temporal dimensions of the nerve impulse recorded monophasically was studied in medullated fibres of cats in vivo at temperatures mostly ranging between 21 and 37° C. 2. Contrary to existing belief, it was demonstrated unequivocally that spike duration varies inversely with the conduction velocities of the fibres; so also the durations of the rising and the falling phases (rise-time and fall-time) of the impulse. The fall-time is linearly related to conduction velocity at all recorded temperatures. The rise-time varies steeply with conduction velocity at the lower levels of conduction rate, and very gradually at the higher conduction rates. 3. The spike duration of preganglionic sympathetic fibres is identical with that of somatic medullated fibres with similar conduction velocities. There is therefore little justification for classifying them separately as so-called B fibres. 4. The rate of recovery of spike amplitude following a preceding impulse also varies inversely with conduction velocity, and in the same manner as the absolute refractory period (ARP). In fact the relation of time for 40% recovery of spike amplitude to conduction velocity is identical with the relation of conduction velocity to ARP. The Q₁₀ for 40% recovery of spike amplitude is 4·7 between 13 and 29° C. 5. Rise-time increases exponentially with fall in temperature in all medullated fibres, fast (say 64 m/sec) and slow (say 16 m/sec), the Q₁₀ being 2·5. Fall-time varies exponentially with temperature in slow fibres (Q₁₀ = 3·5); in fast fibres it varies linearly. The Q₁₀ for spike duration is the same in all fibres between 27 and 37° C only, its value being about 3·4. Below 27° C the Q₁₀ depends on the conduction velocity of the fibres. Only in slow fibres does spike duration tend to vary exponentially with temperature. 6. Only abortive spikes are generated during the interval between the end of a preceding spike and the end of the ARP which is about 1½ times spike duration in fast fibres and about twice spike duration in slow fibres.