The distribution and central termination of single cutaneous and muscle unmyelinated fibres in rat spinal cord.

An electrophysiological approach has been used to trace the central course of individual C fibres from a cutaneous (sural) and muscle (gastrocnemius) nerve in anaesthetized rats. Single C fibres were recorded from the central part of cut peripheral nerves and a glass micropipette used to map the spinal cord with low threshold stimuli (5- and 10-microA pulses of 0.25 ms, which are sufficient to activate C axons at distances of 25 to 100 micron) to determine those points from which the recorded fibre could be antidromically driven. Parallel search tracts were made at 50-micron intervals and the location of stimulating sites reconstructed from serial sections of the tissues prepared at the end of experiments. The course of 6 sural and 6 gastrocnemius C afferents has been studied. All afferents gave rise to long running branches within the spinal cord (up to 4 mm in length) and orientated longitudinally. These axons ran in Lissauer's tract or in the dorsal white matter or in lamina I of the spinal cord. At intermittent points along the length of these axons ventrally orientated branches were given off. These branches were generally only a few hundred micron long and appeared to terminate in the superficial dorsal horn. Cutaneous afferents typically had their branches restricted to lamina I and outer lamina II whilst branches from muscle C afferents commonly sent a ventrally directed branch to terminate in inner lamina II or sometimes deeper. It would therefore appear that both individual cutaneous and muscle afferents project extensively within the spinal cord, but that terminations of the former are restricted to more superficial aspects of the dorsal horn than those of the latter.