Conduction velocity varies with osmotically induced changes of the area of the axon's profile

The conduction velocity of frog ischiadic nerves incubated in vitro in osmolarities between 220 and 1000 mOsm decreased with the degree of fiber shrinkage. The latter (non-circularity factor) was determined from computer-assisted measurements in freeze-substituted or in chemically fixed fibers. Freeze-substituted normal nerves had a non-circularity factor of 0.91 for fibers of all calibers, which likely reflects the in vivo state of the fiber population. Chemically fixed nerves had a non-circularity factor near 0.68, consistent with previous data. Non-circularity factors decreased with increasing osmolarities of the media, regardless of the type of tissue preparation. Conduction velocity decreased with decreasing non-circularity. Restoration of the nerves to normotonic media increased conduction velocity. The rates of change were accelerated in nerves chemically desheathed with Triton. The decrease in the conduction velocity in osmotically shrunken nerves did not correspond to changes in the absolute refractory period for the propagation of the impulse, used as a sensitive index of non-specific damage. These experimental observations corroborate data from computer simulation of relative sensitivities of nodal and internodal parameters.