Short latency crossed inhibitory reflex actions evoked from cutaneous afferents

Although stimulation of cutaneous limb afferents has been shown to evoke crossed extension reflexes in unanaesthetised decerebrate or spinalised animals, here we show that stimulation of cutaneous nerves evokes crossed inhibition rather than excitation of contralateral extensor motoneurones in anaesthetised, spinal cord intact cats. Single pulse stimuli delivered to the saphenous, sural or superficial peroneal nerves evoked IPSPs in a high proportion of contralateral motoneurones including those of knee and ankle extensors. These IPSPs had thresholds of less than 1.5 times the threshold of the most excitable fibres and so large myelinated afferents contributed to them. The relative latencies of IPSPs evoked by stimulation of the contralateral superficial peroneal and sural nerves were longer than those evoked via ipsilateral pathways by approximately 1 ms, suggesting that there are at least three synaptic relays in the crossed reflexes. The IPSPs evoked by stimulation of both ipsilateral and contralateral saphenous nerves had minimal latencies suggesting at least three synaptic delays. Like IPSPs evoked by group II afferents, the frequencies of occurrence of crossed IPSPs evoked by stimulation of cutaneous afferents were significantly reduced after spinal transection and the IPSPs recorded after spinalisation were significantly smaller. These findings are consistent with the recent proposal that dorsal horn neurones, which receive input from cutaneous afferents and contact premotor commissural interneurones may mediate the crossed inhibition.