Single low-threshold afferents innervating the skin of the human foot modulate ongoing muscle activity in the upper limbs.

We have shown for the first time that single cutaneous afferents in the foot dorsum have significant reflex coupling to motoneurons supplying muscles in the upper limb, particularly posterior deltoid and triceps brachii. These observations strengthen what we know from whole nerve stimulation, that skin on the foot and ankle can contribute to the modulation of interlimb muscles in distant innervation territories. The current work provides evidence of the mechanism behind the reflex, where one single skin afferent can evoke a reflex response, rather than a population. Nineteen of forty-one (46%) single cutaneous afferents isolated in the dorsum or plantar surface of the foot elicited a significant modulation of muscle activity in the upper limb. Identification of single afferents in this reflex indicates the strength of the connection and, ultimately, the importance of foot skin in interlimb coordination. The median response magnitude was 2.29% of background EMG, and the size of the evoked response did not significantly differ among the four mechanoreceptor classes (P > 0.1). Interestingly, although the distribution of afferents types did not differ across the foot dorsum, there was a significantly greater coupling response from receptors located on the medial aspect of the foot dorsum (P < 0.01). Furthermore, the most consistent coupling with upper limb muscles was demonstrated by type I afferents (fast and slowly adapting). This work contributes to the current literature on receptor specificity, supporting the view that individual classes of cutaneous afferents may subserve specific roles in kinesthesia, reflexes, and tactile perception.