The response of 59 muscle afferents to the twitch contraction of the whole medial gastrocnemius muscle and the responses of 68 afferents to the contractions of two or three intramuscular compartments (i.e., muscle fibers innervated by a branch of the muscle nerve) have been analyzed. For 86% of the muscle spindle afferents studied, the twitch of the intramuscular compartment containing their receptors ('test compartment') was a more potent stimulus than the contraction of an immediately adjacent compartment, although in each experiment the force produced by the test compartment was less than that produced by the adjacent compartment. In all the cases tested, the twitch of the test compartment was a more potent stimulus than that of a distant compartment. For six of eight tendon organ afferents studied, the test compartments provided a stronger stimulus than did the twitch of the adjacent compartments, and for all eight afferents the test compartment provided a stronger stimulus than the distant compartment. A majority of the afferent fibers innervating receptors located within a single intramuscular compartment were found clustered in two to four small, contiguous subdivisions of the dorsal roots. The present results clearly demonstrate that muscle receptors are more sensitive to contractions of their 'own' intramuscular compartment than to those of adjacent and more distant regions of a muscle. These results support the proposition that muscle receptors generate a 'sensory partitioning' of their parent muscle and provide the peripheral substrate for an intramuscular localization of stretch reflexes.
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