Evidence suggesting a transcortical pathway from cutaneous foot afferents to tibialis anterior motoneurones in man.

1. Stimulation of the superficial peroneal or the sural nerve (3 shocks, 3 ms interval, 1 ms duration, 2.5 x perception threshold) evoked a reflex activation of the tibialis anterior muscle at a latency of approximately 70-95 ms in all of nine healthy human subjects. Stimulation of the medial plantar nerve only rarely produced similar effects. The possibility that a transcortical pathway contributes to these late reflex responses was investigated by combining the cutaneous stimulations and a transcranial magnetic stimulation of the contralateral motor cortex. 2. A significant facilitation of short-latency peaks in the post-stimulus time histogram of single tibialis anterior motor units evoked by the transcortical magnetic stimulation was observed in eight out of nine subjects following stimulation of the superficial peroneal or sural nerves at the latency of the long-latency reflex. In contrast such a facilitation was only rarely seen when the medial plantar nerve was stimulated. 3. With the same timing for the stimuli, the superficial peroneal and sural nerve stimulations also produced a significant increase in the short-latency, presumed monosynaptic, facilitation of the tibialis anterior H reflex produced by the brain stimulation. 4. Similar facilitatory effects of the cutaneous stimuli could not be demonstrated when the magnetic stimulation of the cortex was replaced with electrical stimulation, implying that cortical excitability is affected by a conditioning cutaneous stimulation. 5. It is suggested that the long-latency reflexes in the tibialis anterior muscle evoked by activation of cutaneous afferents from the human foot are, at least partly, mediated by a transcortical pathway.