Cutaneous reflexes are important for rapidly responding to perturbations, correcting limb trajectory and strengthening support. During locomotion, they are modulated by phase to generate functionally appropriate responses. The goal of the present study was to determine if cutaneous reflexes and their phase-dependent modulation are altered with increasing speed and whether this is accomplished at a spinal level. Four adult cats that recovered stable hindlimb locomotion following spinal transection were implanted with electrodes to chronically record hindlimb muscle activity and to electrically stimulate the superficial peroneal nerve to evoke cutaneous reflexes. The speed-dependent modulation of cutaneous reflexes was assessed by evoking and characterizing ipsilateral and contralateral responses in semitendinosus, vastus lateralis and lateral gastrocnemius muscles at four treadmill speeds: 0.2 m/s, 0.4 m/s, 0.6 m/s and 0.8 m/s. The amplitudes of ipsilateral and contralateral responses were largest at intermediate speeds of 0.4 m/s and 0.6 m/s, followed by the slowest and fastest speeds of 0.2 m/s and 0.8 m/s, respectively. The phase-dependent modulation of reflexes was maintained across speeds, with ipsilateral and contralateral responses peaking during the stance-to-swing transition and swing phase of the ipsilateral limb, or mid-stance of the contralateral limb. Reflex modulation across speeds also correlated with the spatial symmetry of the locomotor pattern, but not with temporal symmetry. That cutaneous reflex amplitude in all muscles was similarly modulated with increasing speed, independent of the background level of muscle activity, is consistent with a generalized pre-motoneuronal spinal control mechanism, which could help stabilize the locomotor pattern when changing speed.
When walking, receptors located in the skin that respond to mechanical pressure send signals to our central nervous system to correct the trajectory of the limb and to reinforce weight support. These signals produce different responses, or reflexes, if they occur when our foot is contacting the ground or in the air. This is known as phase-dependent modulation of reflexes. However, when walking at faster speeds we do not know if and how these reflexes are changed. In the present study, we show that reflexes from the skin are modulated with speed and that this is controlled at the level of the spinal cord. This modulation could be important in preventing sensory signals from destabilizing the walking pattern.
The present research was funded by a Discovery Grant (RGPIN-2016-03790) from the Natural Sciences and Engineering Research Council of Canada (NSERC), the Beverly Petterson Bishop Award for Excellence in Neuroscience from the American Physiological Society (APS) and the Arthur C. Guyton Award for Excellence in Integrative Physiology from the APS to A. Frigon. M-F. Hurteau was funded by a doctoral scholarship from NSERC and Y. Thibaudier was funded by a doctoral scholarship from the Fonds de Recherche Nature et Technologies Québec. The authors declare no competing financial interests.