The conduction velocities and spinal projections of single renal afferent fibers in the rat

This study was designed to examine the conduction velocities and spinal projections of renal afferent fibers in the rat using electrophysiological techniques. In chloralose-anesthetized rats, we electrically stimulated the peripheral ends of cut, lower thoracic and upper lumbar dorsal roots and recorded and averaged antidromically conducted action potentials in the renal nerves. Of 284 single axons responding to stimulation of ipsilateral dorsal roots T₉-L₁, the majority were activated by stimulating roots T₁₁-T₁₃. No antidromic responses could be elicited by stimulating the contralateral dorsal roots. Afferent fibers were divisible into two groups, distinguished by their conduction velocities: a population of slowly conducting axons, presumably composed of both unmyelinated (0.3-2 m/s, 76%) and thinly myelinated (2-9 m/s, 19%) fibers, and a population of more rapidly conducting, small myelinated axons (12-32 m/s, 5%). Slowly and more rapidly conducting fibers were not differentially distributed among dorsal roots. Postexperimental histological examination of nerves revealed small myelinated axons with diameters appropriate for some, but not for all, of the axons with conduction velocities in the myelinated range. These results indicate that single myelinated and unmyelinated primary afferent axons can be identified by antidromic stimulation in autonomic nerves of rat. They provide the first electrophysiological description of afferent renal nerve fibers in the rat, and they verify the predominantly unmyelinated nature of these fibers.