BILATERAL FACILITATORY AND INHIBITORY SKIN AREAS OF SPINAL MOTONEURONES OF CAT

Abstract

In 4 acute and 18 chronic spinal cats monosynaptic reflex discharges were recorded from single motor axons (fine filaments of ventral roots L6, L7 or S1) supplying the knee and ankle flexor and extensor muscles. The influence of conditioning noxious stimuli to skin on the probability of a test monosynaptic reflex, evoked continuously at 10-25 per second, was determined, and the skin fields of both hindlimbs and pelvic girdle were mapped as facilitatory and inhibitory. The boundaries between the facilitatory and inhibitory areas were not sharp but overlapped to some extent. The size of the facilitatory and inhibitory fields for individual members of a muscle group, for example ankle flexors, sometimes varied widely. For knee extensors the variation was minimal and the motoneurones could be identified by the maps of their facilitatory and inhibitory skin fields. Bilateral conditioning effects were more pronounced when the monosynaptic reflex was evoked from the ankle flexors and extensors. In general, it was found that nociceptive cutaneous stimulation induces bilateral flexion (dorsolateral surface) or extension (ventromedial surface), when the skin area is proximal, and ipsilateral flexion and contralateral extension, when the skin area is distal. It is concluded that the reflex effects resulting from painful stimuli of the skin are protective in character and organized on a functional basis; namely, to withdraw the offended region from the source of noxious stimulation. The most important factor determining the direction of reflex motor action is the locus of skin stimulation. However, exceptions to the general rule occurred in certain motor axons supplying the ankle myotatic unit and indicate the presence of concealed reflexes.