MECHANICAL SENSITIVITY OF REGENERATING MYELINATED SKIN AND MUSCLE AFFERENTS IN THE CAT

These experiments describe the responses of myelinated skin and muscle afferent nerve fibres at a neuroma to stretch, local pressure and vib ration in the anaesthetised cat. The sural nerve and the nerve supplyi ng the medial gastrocnemius were studied. Neuroma formation was encour aged by placing the cut end of the nerve in a cuff made of synthetic m aterial (Gore-tex). By 6 days after nerve section, the two nerves cont ained mechanically sensitive afferents. No motor fibres appeared to be mechanically sensitive. Mechanically sensitive sural afferents respon ded to ramp stretch of the nerve, applied at the cuff, with a single i mpulse or brief burst of impulses. The majority of gastrocnemius affer ents responded to stretch with slowly adapting trains of impulses. Man y muscle group II afferents exhibited a steady resting discharge, whil e group I afferents had an intermittent or bursting resting discharge or were silent. Those group I axons which showed resting activity had a low stretch threshold and were probably Ia fibres. Many of the silen t units were also stretch sensitive. It is proposed that the spontaneo usly active units and silent units with low stretch thresholds were Ia fibres, while silent units with high stretch thresholds were Ib fibre s. Both sural and gastrocnemius afferents responded to locally applied vibration. The mean peak response frequency for sural units was 170 H z (+/-70 Hz SD). For gastrocnemius units it was 325 Hz (+/-86 Hz SD). Group I muscle afferents responded to higher frequencies of vibration than group II afferents. In four experiments the nerve was treated at a site a few millimetres proximal to the point of section with the axo nal transport blocker colchicine. Twenty-five millimolar colchicine bl ocked impulse conduction at its point of application. Nevertheless, me chanically sensitive areas developed in the nerve just proximal to the treated region. Ten millimolar colchicine did not block impulse condu ction, but led to dispersion of mechanosensitive areas to more proxima l regions of the nerve. This result suggests that the disruption of or thograde axonal transport by colchicine leads to development of mechan ically sensitive areas in axons further back from their cut ends. Loca l application of the drugs succinyl choline, tetra-ethyl ammonium and gadolinium had no effect on levels of resting activity or on mechanica l sensitivity of afferents in the cuff. The potassium channel blocker 4-aminopyridine, on the other hand, produced an increase in the levels of resting activity and in the stretch responses of afferents. None o f these drugs induced any activity in motor axons: It is proposed that mechanical sensitivity is induced at the sprouting tips of sensory ax ons by a substance or substances transported down the axon from the ce ll body. Such a conclusion implies that some of the response propertie s of normal mechanoreceptors in skin and muscle may be the result of i nfluences exerted by the cell body on the peripheral terminal membrane s. This conclusion has important implications for understanding transd uction mechanisms and the development of somatic receptors, and for in terpretation of receptor responses following nerve section and reinner vation or cross-reinnervation.