The amplitude of interlimb cutaneous reflexes in the leg is influenced by fingertip touch and vision during treadmill locomotion.

Abstract

Light touch at the fingertip has been shown to influence postural control during standing and walking. Interlimb cutaneous reflexes have been proposed to provide a neural link between the upper and lower limbs to assist in interlimb coordination during activities such as walking. In this study, we tested the hypothesis that cutaneous sensory pathways linking the arm and leg will be facilitated if subjects use light touch to assist with postural control during treadmill walking. To test this, interlimb cutaneous reflexes from the median nerve, serving the skin contact region, and radial nerve, serving an irrelevant sensory territory, were tested in the legs of subjects walking on treadmill in an unstable environment. Interlimb cutaneous reflexes were tested while subjects (a) touched or (b) did not touch a stable contact with their fingertip, and while the eyes were either (c) open or (d) closed. Reflexes arising from both nerves were facilitated when vision was removed that was then ameliorated when touch was provided. These changes in reflex amplitude during the eyes closed conditions were mirrored by changes in background muscle activity. We suggest that this facilitation of interlimb reflexes from both nerves arises from a generalized increase in excitability related to the postural anxiety of walking on a treadmill with the eyes closed, which is then restored by the provision of light touch. However, the influence of touch when the eyes were open differed depending upon the nerve stimulated. Radial nerve reflexes in the legs were suppressed when touch was provided, mirroring a suppression in the background muscle activity. In contrast, median nerve reflexes in the leg were larger when touch was provided with the eyes open, despite a suppression of background muscle activity. This nerve-specific effect of touch on the amplitude of the interlimb cutaneous reflexes suggests that touch sensory information from the median nerve was facilitated when that input was functionally relevant.