Evidence suggesting that a transcortical reflex pathway contributes to cutaneous reflexes in the tibialis anterior muscle during walking in man

Stimulation of cutaneous foot afferents has been shown to evoke a facilitat ion of the tibialis anterior (TA) EMG-activity at a latency of 70-95 ms in the early and middle swing phase of human walking. The present study invest igated the underlying mechanism for this facilitation. In those subjects in whom it was possible to elicit a reflex during tonic dorsiflexion while se ated (6 out of 17 tested), the facilitation in the TA EMG evoked by stimula tion of the sural nerve (3 shocks, 3-ms interval, 2.0-2.5x perception thres hold) was found to have the same latency in the swing phase of walking. The facilitation observed during tonic dorsiflexion has been suggested to be - at least partly - mediated by a trans cortical pathway. To investigate whe ther a similar mechanism contributes to the facilitation observed during wa lking, magnetic stimulation of the motor cortex (1.2x motor threshold) was applied in the early swing phase at different intervals in relation to the cutaneous stimulation in 17 subjects. In 13 of the subjects, the motor pote ntials evoked by the magnetic stimulation (MEPs) were more facilitated by p rior sural-nerve stimulation (conditioning-test intervals of 50-80 ms) than the algebraic sum of the control MEP and the cutaneous facilitation in the EMG when evoked separately. In four of these subjects, a tibialis anterior II-reflex could also be evoked during walking. In none of the subjects was an increase of the II-reflex similar to that for the MEP observed. In five experiments on four subjects, MEPs evoked by magnetic and electrical corti cal stimulation were compared. In four of these experiments, only the magne tically induced MEPs were facilitated by prior stimulation of the sural ner ve. We suggest that a transcortical pathway may also contribute to late cut aneous reflexes during walking.