Electrophysiological evidence for the presence of looping myelinated afferent fibers in the rat ventral root

We obtained neurophysiological evidence for the existence of looping fibers in the rat ventral root. In anesthetized rats, simultaneous recordings of spontaneous activity were made from the lumbar ventral root with two pairs of bipolar recording electrodes. In 6 ventral roots, single unit activity appeared as pairs of spikes with opposite polarity in both the distal and proximal recording electrodes. The timing and polarity of the spikes can only be logically explained by supposing that they are simultaneous recordings from two pairs of electrodes of two action potentials traveling opposite directions in a single myelinated fiber. This explanation was reinforced by recording the activity after sectioning various parts of the ventral root. These data suggest that there are spontaneously active ventral root myelinated fibers that enter the root and then loop back out toward the dorsal root ganglion.