ORGANIZATION OF THE SURAL CUTANEOUS INPUT REGULATING THE DISCHARGE OFTRICEPS SURAE GAMMA-MOTONEURONS IN THE CAT

The organization of the cutaneous afferent influence on the discharge of gamma-motoneurones has been investigated in the decerebrated, spina l cat. gamma-Motoneurone discharges were recorded from cut nerve filam ents. Time and frequency domain analyses were used to reveal the stren gth of coupling between gamma-motoneurone discharge and cutaneous affe rents excited by natural skin stimulation. Time domain analysis (cross -correlation) was also used to reveal the sign (facilitation or inhibi tion) and time course of the cutaneous influence on individual gamma-m otoneurones. Mechanical stimulation of discrete areas of skin within t he sural nerve field caused facilitation or inhibition of individual g amma-motoneurones supplying the gastrocnemius and soleus muscles. In a few cases, a gamma-motoneurone facilitated by stimulation at one site could be inhibited from another location. The effect of cutaneous aff erent stimulation was not evident in the decerebrated cat with intact spinal cord. The intensity of facilitation and inhibition was mapped f or the sural nerve field. Facilitation had a focus of highest intensit y to stimulation applied between the calcaneum and lateral malleolus. The Focus for inhibition was either the same as for facilitation or, m ore frequently, tended to be lateral and dorsal to the calcaneum at th e edge of the sural field. Cutaneous stimulation at the edge of the su ral field could also reduce the coherence between the discharges of ga mma-motoneurones, particularly at low frequencies of association (1-5 Hz), indicating disfacilitation of other sources of afferent input. Th e results reveal a detailed pattern of cutaneous inputs to the fusimot or system that could participate in a wide range of behavioural adjust ments to stretch or contact of the skin at the heel.