Mechanically stimulating the lumbar region inhibits locomotor-like activity and increases the gain of cutaneous reflexes from the paws in spinal cats

Abstract

Mechanically stimulating the dorsal lumbar region inhibits locomotion and reduces weight support during standing in rabbits and cats. However, how this inhibitory effect from the lumbar skin is mediated is poorly understood. Here we evaluated the effect of mechanically stimulating (vibration or pinch) the dorsal lumbar region on short-latency (8- to 13-ms onset) cutaneous reflex responses, evoked by electrically stimulating the superficial peroneal or distal tibial nerves, in seven adult cats with a low thoracic spinal transection (spinal cats). Cutaneous reflexes were evoked before, during, and after mechanical stimulation of the dorsal lumbar region. We found that mechanically stimulating the lumbar region by vibration or manual pinch abolished alternating bursts of activity between flexors and extensors initiated by nerve stimulation. The activity of extensor muscles was abolished bilaterally, whereas the activity of some ipsilateral flexor muscles was sustained during vibration/pinch. Mechanically stimulating the lumbar region increased ipsilateral and contralateral short-latency excitatory responses evoked by cutaneous inputs, a phenomenon that was generalized to muscles crossing different joints and located in different limbs. Our results indicate that the inhibitory effect on locomotion and weight support is not mediated by reducing cutaneous reflex gain and instead points to an inhibition of central pattern-generating circuitry, particularly the extensor component. The results provide greater insight into interactions between different types of somatosensory inputs within spinal motor circuits. NEW & NOTEWORTHY Vibration or pinch of the lumbar region in spinal-transected cats abolished alternating bursts of activity between flexors and extensors initiated by nerve stimulation. Mechanically stimulating the lumbar region increased ipsilateral and contralateral short-latency excitatory responses evoked by cutaneous inputs in hindlimb muscles. Sensory inputs from mechanoreceptors of the lumbar region do not mediate their inhibitory effect on locomotion and weight support by reducing the gain of short-latency excitatory cutaneous reflexes from the foot.

Cutaneous inputs from perineal region facilitate spinal locomotor activity and modulate cutaneous reflexes from the foot in spinal cats

“…We also classified responses in contralateral muscles as P1 or N1 if the onset was ≤18 ms (Frigon & Rossignol, 2008;Hurteau et al, 2017;Merlet et al, 2020). In spinal cats, the longer-latency response (P2) starting at ~20-25 ms is small or absent (Frigon & Rossignol, 2008;Hurteau et al, 2017;Merlet et al, 2020). In the present study, we observed some P2 responses, in particular in IP and LG muscles, but not enough to perform statistical analyses.…”

“…Interestingly, we reported that pinching the skin of the perineal region was more effective than vibration in decreasing the gain of cutaneous reflexes (Table 4). As the afferents activated by vibration and pinch differ (Merlet et al, 2020), we propose that the greater effect of pinch in decreasing the gain of short-latency cutaneous reflexes was due to the specificity of afferents involved (i.e., mainly cutaneous afferents).…”

“…As previously described (Merlet et al, 2020), cats of the present study were used by Harnie et al (2019) to determine the effects of different interventions on the recovery of hindlimb locomotion after spinal transection. One week after the spinal transection, cats were divided into three groups.…”

“…In contrast, Aδ fibers were highly inhibitory and when mechanically stimulating the lumbosacral skin, the net effect was an inhibition of weight support and locomotion, as shown in cats and rabbits (Frigon et al, 2012;Hurteau et al, 2015;Merlet et al, 2020;Viala & Buser, 1974;Viala et al, 1978). It is possible that Aα, Aβ, and C cutaneous afferents from the perineal region are also excitatory, while…”

“…Studies have shown that some types of somatosensory feedback inhibit locomotion while others facilitate it. For example, mechanically stimulating the lumbar region inhibits weight support and locomotion, or locomotor‐like activity, in chronic spinal cats in various preparations with the hindlimbs free to move or restrained (Frigon et al., 2012; Hurteau et al., 2015; Merlet et al., 2020) and in curarized decerebrate rabbits with intact or transected spinal cords (Viala & Buser, 1974; Viala et al., 1978). In contrast, mechanical stimulation of the skin of the perineal region (scrotum, vulva, base of the tail, and inguinal fold) facilitates hindlimb locomotion in spinal mammals.…”