SIGNALING OF STATIC AND DYNAMIC FEATURES OF MUSCLE-SPINDLE INPUT BY CUNEATE NEURONS IN THE CAT

1. The capacity of cuneate neurones to signal information derived from muscle spindle afferent fibres about static stretch or vibration of f orearm extensor muscles was examined electrophysiologically in anaesth etized cats. 2. Static stretch (less than or equal to 2 mm in amplitud e) and sinusoidal vibration (at frequencies of 50-800 Hz) were applied longitudinally to individual muscle tendons by means of a feedback co ntrolled mechanical stimulator, and responses were recorded from indiv idual cuneate neurones and from individual spindle afferent fibres. 3. Cuneate neurones sampled were located caudal to the obex and displaye d a sensitivity to both vibration and static stretch of forearm muscle s that was consistent with their input arising from primary spindle en dings. In response to static muscle stretch, they displayed graded and approximately linear stimulus-response relations, and a stability of response level at fixed lengths that was consistent with these neurone s contributing discriminative information about static muscle stretch. 4. In response to sinusoidal muscle vibration the cuneate neurones al so showed graded stimulus-response relations tin contrast to spindle a fferents which at low vibration amplitudes attain a plateau response l evel corresponding to a discharge of 1 impulse on each vibration cycle ). Lowest thresholds were at 100-300 Hz and bandwidths of vibration se nsitivity extended up to similar to 800 Hz. 5. Temporal precision in c uneate responses to muscle vibration was assessed by constructing phas e scatter and cycle histograms from which measures of vector strength could be calculated. Cuneate responses displayed somewhat poorer phase locking land lower vector strengths) than spindle afferent responses to vibration (a reflection of uncertainties associated with synaptic t ransmission). Nevertheless, the remarkable feature of cuneate response s to muscle vibration is the preservation of tight phase locking at fr equencies up to 400-500 Hz, which presumably enables these central neu rones to contribute accurate temporal information for the kinaesthetic sense in a variety of circumstances involving dynamic perturbations t o skeletal muscle.