A Spinal Mechanism Related to Left-Right Symmetry Reduces Cutaneous Reflex Modulation Independently of Speed During Split-Belt Locomotion

Task- and phase-dependent reflex modulation during locomotion is well established, but we do not know the signals driving this modulation. To determine whether signals related to left–right symmetry of the locomotor pattern modulate cutaneous reflexes, we stimulated the superficial peroneal nerve in five intact female cats and in four spinal-transected cats (spinal cats, two males and two females) during split-belt locomotion at different left–right speeds. We compared cutaneous reflexes evoked in three ipsilateral and two contralateral hindlimb muscles during split-belt locomotion with those evoked during tied-belt (equal left–right speeds) locomotion at matched speeds of the slow and fast limbs. Our results showed similar phase-dependent modulation of cutaneous reflexes during tied-belt and split-belt locomotion in intact and spinal cats. During tied-belt locomotion in intact cats, an increase in speed significantly increased reflex modulation from minimum to maximum values, whereas in spinal cats, we observed a significant decrease. However, in all muscles of intact and spinal cats, split-belt locomotion significantly reduced reflex modulation compared with tied-belt locomotion independently of which limb was stepping on the slow or fast belt. Additionally, reflex modulation correlated more with spatial left–right symmetry, as opposed to a temporal one, in intact and spinal cats. Our results indicate that signals related to left–right symmetry reduce cutaneous reflex modulation independently of speed via a spinal mechanism. We propose that asymmetric sensory feedback from the left and right legs alters the state of the spinal network, thereby reducing cutaneous reflexes to prevent inputs from destabilizing a potentially unstable pattern.

SIGNIFICANCE STATEMENT When we contact an obstacle during walking, receptors in the skin send signals to the CNS to alter the trajectory of the leg to maintain balance. This response, or reflex, is different when the leg is in the air and when it is contacting the ground. The reflex also differs when we walk at different speeds. Here, we investigated this reflex when the left and right legs were walking at different speeds on a split-belt treadmill in cats. We show that the reflex is smaller during split-belt locomotion compared with when both legs are walking at equal speeds. We propose that the spinal locomotor network controlling walking reduces the reflex response to optimize balance when gait is unstable.

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