ELECTRICAL EXCITATION OF MULTIFIBERED NERVES

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Abstract
For a quantitative study of the variations in the electrical excitability of the different elements of multifibered nerves, contraction of the nictitating membrane and of the pilomotors of the tail and inhibition of the heart on electrical stimulation of the nerve supply were recorded in the cat. The distribution of voltage (v) thresholds and duration (t) and the v-t relations for the different fibers (n) were determined. Both v and t thresholds yield curves showing a skewed distribution. The theoretical significance of these curves is discussed. The v-n curves obtained at different durations, and the t-n curves at different voltages, give 2 interchangeable families of curves, which represent graphically the total distribution of thresholds in the nerve. From either of these 2 families a set of v-t curves which covers all the fibers, may be plotted. Accurate v-t curves may be obtained over a wide range of durations by the method of interpolations used. This is especially interesting for autonomic nerves, where previous methods have been quite unsatisfactory. The ratio v/r (r = rheobase) is not constant for a given duration. Its value decreases from the fibers of highest excitability to a minimum at about the mean (50% of the fibers) ; it then increases again to a maximum for the least excitable fibers. Any conventional excitation-time (e.g., chronaxie) is therefore variable for the different fibers, the variations occurring in the same direction as do those of v/r. With logarithmic plotting the v-t curves of the different fibers in a nerve present different slopes. At least 3 parameters are then necessary in the formula of these curves. Hill''s (and consequently Blair''s), Weiss''s and Lapicque''s formulae were tested. All 3 presented systematic deviations, both for short and for long times.