Unmyelinated and small myelinated axons in rat ventral roots.

The present study is an analysis of the ventral roots in the rat. The smallest myelinated fibers are found in segments T1-L2 and L6-S1. These fibers are regarded as preganglionic efferents and the L6-S1 outflow is more cranial than the sacral parasympathetic outflow in other animals. Large numbers of unmyelinated axons are seen in segments T1-L2 and L6-S1, the same segments that contain the smallest myelinated fibers. This data will be used as a baseline for determining the functional categories of the unmyelinated axons.