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University Laboratory of Physiology, Oxford.

1. Autogenetic inhibition from contraction receptors was measured by eliciting contractions of the soleus muscle in the decerebrate cat. Inhibitory feedback was detected when the tension increment f, produced by stimulating motor fibres in the presence of a background reflex contraction, was smaller than the tension d elicited by the same stimulus in the absence of reflex action. Tendon vibration was applied throughout to clamp primary spindle afferents at a constant firing rate, thereby preventing spindle unloading from disfacilitating the reflex contraction. 2. The reduction in tension d--f varied roughly linearly with the size of the tension stimulus f. Feedback gain was proportional to d--f/f, i.e. the ratio of inhibited tension to stimulus tension. It was computed by averaging over several measurements obtained with stimuli of different sizes, and ranged between 0 and 0.88 in ten animals. The average gain, 0.39, implies that voluntary muscle force is reduced by approximately 27% through the direct inhibition of alpha-motoneurones from homonymous contraction receptors. 3. Inhibitory feedback gain did not appear to co-vary with the background reflex contraction. When measured without vibration, however, a positive covariance did emerge, suggesting that this is due to unloading of muscle spindles, either by extrafusal muscle shortening or by inhibition of fusimotor neurones. 4. Inhibited tension varied linearly with the estimated increment in Ib afferent firing. On the assumption that group Ib afferents carried the entire inhibitory signal, inhibitory feedback gain measured with vibration was used to predict the size of the gain if vibration had not been applied. Feedback gain calculated in this way was reduced by still did not vary with reflex tension. 5. In one animal with signs of brain stem trauma, feedback gain was increased to around six. It is argued that inhibitory feedback in the intact animal can rise to comparable values, as a result both of convergence of signals from different muscles and of supraspinal facilitation.

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