A study was made of the change in duration of the pauses in discharge from spindle afferents occurring during a series of isometric twitches of the cat's triceps surae muscle. These were induced by stimulation of the muscle nerve at frequencies of 0.1 to 1.0 Hz. At frequencies of 0.2 Hz and higher, the duration of the pause exhibited by group II and tonic and phasic group Ia afferents decreased progressively until stabilizing within 5 to 15 s. The time course of this change was largely independent of the frequency of stimulation, but the eventual duration in the steady state was strongly dependent on the frequency, being shorter at higher rates. At a given frequency, the pause was shorter at longer muscle lengths. The decrease in pause duration was not accompanied by a parallel change in duration or amplitude of the muscle tension responses, indicating that it arose from some alteration intrinsic to the spindle organ. We suggest that the progressive change in sensory discharge could serve the initial reinforcement of repetitive movements.
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