AXON DIAMETERS IN RELATION TO THE SPIKE DIMENSIONS AND THE CONDUCTION VELOCITY IN MAMMALIAN A FIBERS

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Abstract
From charts showing the distribution of fibers according to size in the saphenous nerve of the cat and the rabbit, reconstructions of the form of the conducted action potentials were made. Good fits were obtained with the recorded action potentials from the same nerves, and the medullated fibers of all sizes were accounted for. From these reconstructions empirical size-velocity curves were prepared. The following evidence was obtained that the medullated fibers of these nerves constitute a homogeneous system and therefore were suitable for study of the variations dependent upon size. a. The spikes all had the same duration within the limits, 0.4 to 0.5 msec. b. The fibers all had the same after-potential system. Success in the reconstructions depended upon the detn. of the fact that the spike size varied as the axon radius, and not as the square of the radius, as previously held. Measurements were made of the axon diams. and the outside diams. of the 675 fibers in a fasciculus of the saphenous nerve, and the ratios of the 2 diams. were plotted against the outside diams. For fibers larger than 8 u the mean ratio was found to be approximately constant. Below 8 u it decreased progressively. With the aid of these data axon-diarn. velocity curves were calculated from the outside-diam. velocity curves. The velocity of conduction was found to be approximately proportional to the axon diam., but a slight curvature in the graph connecting the 2 variables indicated that the velocity changed somewhat less rapidly than it would if the relationship were strictly linear.

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