Simulations reveal large variations in fusimotor action in normal cats: ‘fusimotor set’
Since the advent a decade ago of techniques for the recording from single muscle spindle afferents in behaving cats and monkeys, speculation has mounted that γ-fusimotor neurones might, at least in part, be controlled independently of α-motoneurones during normal movement (Loeb & Marks, ibid.; Schieber & Thach, 1980; review: Prochazka & Hulliger, 1983). This has been based largely on qualitative inferences drawn from spindle afferent firing patterns, as direct recordings from fully identified γ-motoneurones in the awake animal have yet to be obtained. The most compelling qualitative evidence for α-γ-independence has been the frequent observation that spindle afferent firing during movement is not closely coupled to the bursts of α-activity of the receptor -bearing muscles. Furthermore, large variations in Ia stretch-sensitivity, without corresponding changes in α-activity, have been observed in movements of different types, suggesting large variations in fusimotor action (Prochazka & Wand, 1981).
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