MORPHOLOGICAL AND ELECTROPHYSIOLOGICAL ANALYSIS OF THE PERIPHERAL AND CENTRAL AFFERENT PATHWAYS FROM THE CLITORIS OF THE CAT

Afferent neurons projecting to the clitoris of the cat were identified by WGA-HRP tracing in the S1 and S2 dorsal root ganglia. An average of 433 cells were identified on each side of the animal. 85% and 15% of the labeled cells were located in the S1 and S2 dorsal root ganglia, respectively. The average cross sectional area of clitoral afferent neuron profiles was 1,479 +/- 627 mu m(2). Unilateral transection of the pudendal nerve reduced the number of labeled cells to 1% of that on the control side. Central projections of clitoral afferents were identified in the lumbo-sacral segments (L7-S3) of the spinal cord. HRP labeled fibers were located in the marginal zone on the medial side of dorsal horn and extended into the dorsal half of the dorsal gray commissure. Electrophysiological recordings detected axonal volleys in the pudendal nerve and S1 dorsal root in response to electrical stimulation (threshold, 1-4 V) of the clitoral surface. Estimated axonal conduction velocities at the two sites ranged from 7-27 m/s and 0.6-30 m/s, respectively. Multi-unit recordings from dorsal roots in the lumbo-sacral segments revealed that non-noxious pressure stimulation of the clitoris evoked discharges in the 81 dorsal root. Small increases were also detected in the S2 and L7 roots. Single unit discharges recorded from 1 dorsal roots were activated by electrical stimulation of the clitoral surface at thresholds of 0.6-1.2 V and latencies of 1.5-1.8 ms (estimated conduction velocities of 24-30 m/s). Light constant pressure on the clitoris produced an initial burst of single unit firing (maximum frequencies 170-255 Hz) followed by rapid adaptation and a sustained firing (maximum 30 Hz) which was maintained during the stimulation, Tonic firing increased to an average maximum of 145 Hz at 6-8 g/mm(2) pressure. These results indicate that the clitoris is innervated by mechano-sensitive myelinated afferent fibers in the pudendal nerve which project centrally to the region of the dorsal commissure in the L7-S1 spinal cord.