Excitation and Conduction in Immature Nerve Fibers of the Developing Chick

Abstract

The velocity of impulse transmission in sciatic nerves dissected from fetal chicks has been measured at various stages of development. From day 10 through the adult stage an 80-fold increase in conduction velocity has been observed. The linear velocities of the fastest components are 0.5 m/sec. at day 10, 7–11 m/sec. at day 21, and 50 m/sec. in the adult. In immature nerve prior to myelination (day 10) at least two components of different velocities may be recognized. Spike potentials at day 16 are almost identical in form to the A complex of the adult although the rate of propagation is slower. Potential changes that are believed to be associated with transmission in C fibers are visible at day 16. During development the increase in conduction velocity which was observed was concomitant with the deposition of myelin and when the velocity was increasing most rapidly, the process of myelination was found to be most active. A linear relationship between fiber diameter and CV has been observed only in the juvenile and adult forms. Microscopically the nerve fibers appear to be unmyelinated at day 10-day 14 and at the time of hatching myelination is rather extensive. Before day 15 it was impossible to make a diameter/velocity correlation since measurements with the light microscope are inaccurate below 1µ.