THE RÔLE PLAYED BY THE SIZES OF THE CONSTITUENT FIBERS OF A NERVE TRUNK IN DETERMINING THE FORM OF ITS ACTION POTENTIAL WAVE

Abstract

A correlation was made between the fiber constitution of a mixed nerve and the form of its action potential. Sections of nerves fixed in osmic acid were made from which the cross-sectional areas of the fibers of each diameter could be calculated. Theoretical reconstructions were then made of the action potential waves that the nerve should yield, making use of the following observations and assumptions: (1) In a given nerve trunk the duration of the axon action potential is the same in fibers of all velocities. (2) The wave form was assumed to be triangular but its dimensions were chosen so as to subtend an area equal to the recorded axon potential area, as near as this can be determined. For this purpose, in mammalian nerve the rising phase was taken as 0.2o and the duration as 0.6o. For frog nerve the corresponding values were 0.3o and 0.9o. (3) The rate of propagation is uniform throughout the fiber. (4) The maximum potential developed is the same in all fibers. The fiber would then affect the recording instrument as the internal resistance (cross-section) of the fiber. Electrical potential records were made from the nerves before the histological examination by means of a cathode ray oscillograph, and the actual records were then compared with the theoretical reconstructions. A close resemblance was found between the 2, when the reconstructions were made on the assumption that the velocities of conduction in the axons vary directly as their diameters. The reconstructed action potential behaved, with regard to change of form with conduction and the appearance of secondary waves, in the same manner as the corresponding recorded action potential. The fiber distribution in the spinal roots differed as much as their action potentials. Corresponding to the 3 waves in the sciatic action potential there were 3 distinct accumulations of fibers, while in the phrenic nerve, as one would expect from its simple potential wave, there was but 1 pile.