Cutaneous inputs from perineal region facilitates and modulates spinal locomotor activity and reduces cutaneous reflexes from the foot in spinal cats
It is well known that mechanically stimulating the perineal region potently facilitates hindlimb locomotion and weight support in mammals with a spinal transection (spinal mammals). However, how perineal stimulation mediates this excitatory effect is poorly understood. We evaluated the effect of mechanically stimulating (vibration or pinch) the perineal region on ipsilateral (9-14 ms onset) and contralateral (14-18 ms onset) short-latency cutaneous reflex responses evoked by electrically stimulating the superficial peroneal or distal tibial nerve in seven adult spinal cats where hindlimb movement was restrained. Cutaneous reflexes were evoked before, during, and after mechanical stimulation of the perineal region. We found that vibration or pinch of the perineal region effectively triggered rhythmic activity, unilateral and bilateral to nerve stimulation. When electrically stimulating nerves, adding perineal stimulation modulated rhythmic activity by decreasing cycle and burst durations and by increasing the amplitude of flexors and extensors. Perineal stimulation also disrupted the timing of the ipsilateral rhythm, which had been entrained by nerve stimulation. Mechanically stimulating the perineal region decreased ipsilateral and contralateral short-latency reflex responses evoked by cutaneous inputs, a phenomenon we observed in muscles crossing different joints and located in different limbs. The results suggest that the excitatory effect of perineal stimulation on locomotion and weight support is not mediated by increasing cutaneous reflex gain and instead points to an excitation of central pattern-generating circuitry. Our results are consistent with a state-dependent modulation of reflexes by spinal interneuronal circuits. Significance StatementMechanically stimulating the skin of the perineal region strongly facilitates hindlimb locomotion in mammals following a complete spinal cord injury (SCI). Despite its remarkable effectiveness in promoting hindlimb locomotion in spinal cord-injured mammals, we do not know how this is mediated. The present study provides data on how inputs from the perineal region interact with neuronal circuits that generate locomotor-like activity and reflexes from the foot. A better understanding of how inputs from the perineal region interact with neuronal circuits of the spinal cord could lead to non-invasive approaches to restore walking in people with SCI.