Reflexes in the hand: strong synaptic coupling between single tactile afferents and spinal motoneurones.

We investigated whether the discharge of single afferents could modulate ongoing EMG in muscles acting on the hand. Recordings were made from 129 single low threshold mechanoreceptors via tungsten microelectrodes inserted into the median or ulnar nerves at the wrist in awake human subjects. Reflex modulation of ongoing EMG was observed for 9 of 26 FA I, 1 of 5 FA II, 18 of 39 SA II and 2 ectopically active afferents via spinally mediated, presumably oligosynaptic, pathways. Two reflex responses were found: a single excitatory response which arose from irregularly firing afferents (FA I, FA II), and a cyclic EMG modulation associated with regularly discharging afferents (SA II) and one FA I afferent firing in regular bursts. No strong synaptic coupling could be detected from the discharge of any of the 21 SA I, 2 joint or 34 muscle spindle afferents. These results highlight the important role of certain types of single cutaneous mechanoreceptors in sensorimotor control of the hand.