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In cats anaesthetized with chloralose, evidence has been sought for the projection of information from tendon organs to the sensory receiving areas of the cerebral cortex. Selective stimulation of afferent fibres from tendon organs has been achieved by raising the threshold to electrical stimulation of the fibres from primary endings of muscle spindles. The method uses longitudinal vibration at 200-250 Hz to elicit, over a period of 20 min, one impulse for each excursion of the vibrator from all of the spindles in the test muscle, soleus or medial gastrocnemius. The accumulated post-spike positivities following passage of the impulses are thought to be responsible for the rise in threshold. Segmental monosynaptic reflex testing after a bout of vibration was used to confirm that the residual Group I volley no longer contained impulses from muscle spindles. The volley in response to stimulating the nerve of the test muscle was timed to facilitate the monosynaptic reflex of a synergist. Before vibration 5- to 10-fold facilitation of reflex amplitude could be produced; however, after vibration, if all the spindle primary endings had been effectively engaged by the stimulus, no detectable facilitation remained. This test was found to be sensitive and reproducible. An afferent volley containing only activity of tendon organ afferents evoked small-amplitude potentials from the post-sigmoid gyrus of the contralateral pericruciate cortex. The field was highly localized and lay caudal to the main receiving area for activity from the sural nerve and from afferents of hip flexor muscles. Recordings with tungsten micro-electrodes revealed that the surface-evoked activity took origin in cellular discharges in the internal pyramidal layer of area 3a. Recent psychophysical experiments have provided evidence for a sense of muscle tension, as distinct from a sense of effort, and the tendon organ has been suggested as the likely receptor of origin. Our electrophysiological observations now provide a firm experimental basis for such a proposal.

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