Cortical contribution to crossed reflexes in walking humans

Abstract

In this pilot study we investigated whether a transcortical pathway mediates the short-latency responses elicited in the contralateral gastrocnemius lateralis (cGL) by ipsilateral posterior tibial nerve (iPTN) stimulation. For this purpose, single pulse sub-threshold (90% active motor threshold) transcranial magnetic stimulation (subTMS) was applied in combination with iPTN stimulation at several interstimulus intervals (ISI) while the subjects (n=6) walked. SubTMS has been shown to reduce cortical output to the target muscle, inducing a suppression in the EMG signal of the voluntary activated muscle. While no differences were observed for the first component to the response, a considerable depression of the second part, referred to as long-latency component (LLC) was observed when the subTMS was timed such that the induced suppression would coincide with this component. These results strongly suggest the cortical control of the LLC in the cGL response. This allows for an enhanced flexibility during unexpected perturbations, ensuring that the appropriate output is produced in accord with the functional demand.