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Work in this field originates from Sherrington's observation (1909) that, when the limb of a decerebrate animal is flexed, there is at first a stretch reflex in the extensor muscle, but at a certain stage the extensor muscle concerned gives way and allows any degree of flexion to be imposed upon it. This was the lengthening reaction or clasp-knife effect which Sherrington described as an autogenetic proprioceptive reflex of inhibitory character. A first attempt at electrophysiological analysis was made by Denny-Brown (1928) who investigated the electromyogram during the tendon jerk, especially with reference to the silent period. He found that the silent period could be 'transmnitted from one extensor muscle to another' and observed that, in preparations in which this occurs readily, 'a stretch of one of these extensors, evoking in it a stretch reflex and then a lengthening reaction, causes partial or even complete inhibition of a stretch reflex present in another. ' Denny-Brown (1928) therefore associated it with the lengthening reaction.The next advance in this field came with the demonstration by Granit (1950) that contraction favours the appearance of the autogenetic inhibition as well as the associated inhibition of extensor muscles operating at neighbouring joints. This showed that receptors in series with the extrafusal fibres were responsible. McCouch, Deering & Stewart (1950) also made experiments with electrical stimulation of the quadriceps tendon and succeeded in obtaining a relatively pure early inhibition which they also ascribed to tendon endings. It was further demonstrated by Granit & Str6m (1951) that on stretching a muscle the onset of autogenetic inhibition had such a short latency that the responsible fibres must be in the group I range. Consequently it could be concluded that Golgi tendon afferents were responsible.These problems were approached in another way by Laporte & Lloyd (1952). They examined the conditioning effect which group I volleys from various-muscles exert on-monosynaptic reflexes. Inthis way it was established

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