Reflex actions of selective stimulation of sural nerve C fibres in the rabbit

Selective electrical stimulation of the non-myelinated C fibres of the sural nerve in the decerebrated, spinalized rabbit evoked long-latency (76-160 ms), long-lasting (greater than 100 ms) reflex responses in the ipsilateral ankle extensor gastrocnemius medialis (GM). Activation of the same fibres elicited little or no response from the ipsilateral knee flexor semitendinosus (ST). Reflex responses were evoked in both GM and ST muscle nerves by stimulation of the A beta afferents of the sural nerve. The A beta-elicited reflex in GM was enhanced, and that in ST depressed by prior activation of sural nerve C fibres. Strychnine, but not picrotoxin or mecamylamine, blocked C fibre-induced inhibition of the flexor reflex. Pinching the heel with serrated forceps produced an immediate reflex discharge in GM motoneurones, whereas ST responded upon termination of the stimulus. Pinching the second toe evoked reflex activity in ST but not in GM. After strychnine (0.5 mg kg-1), both sets of motoneurones responded simultaneously to stimulation of either the heel or the toe. These data show that sural and other afferent fibres from the heel excite ipsilateral GM motoneurones and inhibit ST reflex responses. One interpretation of these findings is that fine sural afferents activate parallel inhibitory and excitatory pathways, of which the former is sensitive to strychnine and therefore probably mediated by glycine.