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

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Abstract
The relation of conduction velocity, i.e. fiber diameter (Hursh, 1939a) to certain temporal dimensions of the nerve impulse recorded monophasically was studied in medullated fibers of cats in vivo at temperatures mostly ranging between 21 and 37[degree]C. Contrary to existing belief, it was demonstrated unequivocally that spike duration varies inversely with the conduction velocities of the fibers; 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 of all recorded temperatures. The rise-time varies steeply with conduction velocity at the lowerlevels of conduction rate, and very gradually at the higher conduction rates. The spike duration of preganglionic sympathetic fibers is identical with that of somatic medullated fibers with similar conduction velocities. There is therefore little justification for classifying them separately as so-called B fibers. The rate of recovery of spike amplitude following a preceding impulse also varies inversely with conduction velocity, and in the same maner as the absolute refractory period (AKP). 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[degree]C. Rise-time increases exponentially with fall in temperature inall medullated fibers, fast (64 m/sec) and slow (16 m/ sec), the Qio being 2.5. Fall-time varies exponentially with temperature in slow fihers (Q10 = 3.5); in fast fibers it varies linearly. The QlO for spike duration is the same in all fibers between 27 and 37[degree]C only, its value being about 3.4. Below 27[degree]C the Q10 depends onthe conduction velocity of the fibers. Only inslowfibersdoes spike duration tend to vary exponentially with temperature. Only abortive spikes are generated during the interval betweentheend of apreceding spike and the end of the ARP which is about 11/2 times spike duration in fast fibers and about twice spike duration in slow fibers.