Organization of the sural cutaneous input regulating the discharge of triceps surae γ-motoneurones in the cat

The organization of the cutaneous afferent influence on the discharge of γ-motoneurones has been investigated in the decerebrated, spinal cat. γ-Motoneurone discharges were recorded from cut nerve filaments. Time and frequency domain analyses were used to reveal the strength of coupling between γ-motoneurone discharge and cutaneous afferents excited by natural skin stimulation. Time domain analysis (cross-correlation) was also used to reveal the sign (facilitation or inhibition) and time course of the cutaneous influence on individual γ-motoneurones. Mechanical stimulation of discrete areas of skin within the sural nerve field caused facilitation or inhibition of individual γ-motoneurones supplying the gastrocnemius and soleus muscles. In a few cases, a γ-motoneurone facilitated by stimulation at one site could be inhibited from another location. The effect of cutaneous afferent stimulation was not evident in the decerebrated cat with intact spinal cord. The intensity of facilitation and inhibition was mapped for the sural nerve field. Facilitation had a focus of highest intensity to stimulation applied between the calcaneum and lateral malleolus. The focus for inhibition was either the same as for facilitation or, more frequently, tended to be lateral and dorsal to the calcaneum at the edge of the sural field. Cutaneous stimulation at the edge of the sural field could also reduce the coherence between the discharges of γ-motoneurones, particularly at low frequencies of association (1-5 Hz), indicating disfacilitation of other sources of afferent input. The results reveal a detailed pattern of cutaneous inputs to the fusimotor system that could participate in a wide range of behavioural adjustments to stretch or contact of the skin at the heel.