Conduction velocity is related to morphological cell type in rat dorsal root ganglion neurones.

Abstract

Combining intracellular recording and dye-injection techniques permitted direct correlation of neuronal soma size with peripheral nerve conduction velocity in individual neurons of the [lumbar] L4 dorsal root ganglion (DRG) of the anesthetized 5-8-wk-old rat. The conduction velocities fell into 2 main groups; those > 14 m/s (A.alpha. and .beta. fibers) and those < 8 m/s (A.delta. and C fibers). Fibers with conduction velocities in the A.delta. range (2.2-8 m/s) in the sciatic nerve between the sciatic notch and the neuronal soma in the DRG often conducted more slowly, that is in the C-fiber range (< 1.4 m/s), in the periphery from the tibial nerve to the sciatic notch. For the fast-conducting myelinated afferents, there was a loose positive correlation between cell size and the conduction velocity of the peripheral axon, whereas a clearer positive correlation existed between neuronal cell size and axonal conduction velocity both for A.delta.- and for C-fiber afferents. The relationship of the cell cross-sectional area (measured at the nucleolar level), to the cell volume for each neuronal soma was similar for the different conduction velocity groups. The somata of the fast-conducting myelinated A.alpha. and A.beta. fibers had a similar mean and range of cross-sectional areas to those of the large light cell population. The somata with A.delta. and C fibers were of a more uniform size and were restricted to the smaller cells within the ganglia. The mean and range of cross-sectional areas of the C cells was similar to those of the small dark cell population. A.delta. somata had a larger mean and range of cell sizes than those of the small dark cell population. The relationships of peripheral axon type to the morphological cell types are discussed.