Effects of acute and chronic midthoracic spinal cord injury on neural circuits for male sexual function. II. Descending pathways

In normal animals, microstimulation of the medullary reticular formation (M RF) has two effects on efferent neurons in the motor branch of the pudendal nerve (PudM). MRF microstimulation depresses motoneuron reflex discharges (RD) elicited by dorsal nerve of the penis (DNP) stimulation and produces l ong latency sympathetic fiber responses (SFR). The midthoracic spinal locat ion of these descending MRF-PudM projections was studied electrophysiologic ally using a variety of acute and chronic lesions. Chronic lesions, in 27 m ature male rats, included dorsal (DHx) or lateral (LHx) hemisections or mod erate/severe contusions (Cx) at spinal level T-8. Behavioral data (sexual r eflex latency, bladder voiding) obtained throughout the recovery period rev ealed a significant impairment of urogenital function for the DHx and sever e Cx groups of animals. Microstimulation-induced PudM-RDs and PudM-SFRs, ob tained in terminal electrophysiological experiments 30 days postinjury in t he same 27 rats (urethan-anesthetized), were tested for a combined total of 1,404 bilateral MRF sites. PudM-RD was obtained for LHx and moderate Cx gr oups of animals but not for DHx or severe Cx groups. PudM-SFRs were obtaine d for LHx, DHx (although significantly weakened) and moderate Cx groups but not for those having received either an over-DHx or a severe Cx injury. Pu dM responses also were tested for 6 MRF sites in six intact control rats bo th before and after various select acute spinal cord lesions. For MRF sites producing a robust PudM-RD and PudM-SFR, acute bilateral lesions confined to the dorsolateral quadrant (DLQ) eliminated the PudM-RD but failed to eli minate PudM-SFRs. A deeper lesion encompassing additional white matter loca ted dorsally in the ventrolateral quadrant (VLQ) was necessary to eliminate PudM-SFRs. Overall, these electrophysiological results provide evidence fo r descending projections conveying information between MRF and the lower th oracic/lumbosacral male urogenital circuitry within the DLQ and the dorsal- most aspect of VLQ at the midthoracic level of spinal cord. The alterations of supraspinal projections observed after chronic injury are likely of imp ortant clinical significance for functional recovery in cases of clinically incomplete spinal cord injury at midthoracic spinal cord.