The response properties of Golgi tendon organs in cat soleus muscles were studied during physiologically graded contractions provided by crossed extensor and stretch reflexes. Our preparation was the decerebrate cat with intact afferent and efferent innervation, except for a few fine filaments of dorsal roots that were dissected to record from tendon organ afferents. During isometric contractions of the soleus, tendon organ discharge began abruptly at some threshold force and then increased in parallel with increasing force. Occasionally, steplike increases in rate were observed that were not matched by comparable changes in total muscle force. The smooth changes in tendon organ discharge rate and muscle force were attributed to rate modulation of motor units with muscle fibers in series with the tendon organ receptor. The steplike rate increases were attributed to recruitment of motor units with in-series fibers. Tendon organs were recruited at low forces in all preparations. In a group of 37 tendon organs, threshold forces ranged from the lower limit of resolution up to 6 N, with a mean of 1.33 N. Of these tendon organs, 91% had threshold forces less than 13% of the estimated maximal muscle force. The mean discharge rate at threshold was 24 impulses/s. Above threshold, there was a straight-line relationship between discharge rate of individual tendon organs and total muscle force. The slope of the relationship between rate and static force (static sensitivity) ranged from 3.4 to 13 impulses.s-1.N-1, with a mean of 6 impulses.s-1.N-1. Static sensitivity depended to a modest degree on muscle length. The slope was lower and the ordinate intercept was higher at longer muscle lengths. Tendon organ responses during stretch of reflexly activated muscle mimicked total muscle force, both in cases where reflex compensation for muscle yielding during stretch was good and in cases where it was poor. Smooth changes in firing rate that paralleled force were attributed to a combination of muscle elastic properties and motor-unit firing rate modulation. As in the isometric case, steplike changes in rate that were not accompanied by corresponding changes in force were attributed to recruitment of motor units with in-series muscle fibers. The steady-state discharge rate before and after stretch was predicted well by the static sensitivity calculated under isometric conditions. Sensitivity to the rate of force change was shown by tendon organ response in excess of that predicted by the static sensitivity during and immediately following the stretch. The dynamic sensitivity was slightly less than that predicted on the basis of previously published dynamic models. The discharge rate of individual tendon organs provides an accurate measure of the total muscle force despite the restricted spatial sample of the force that each receptor receives. The summed responses from all the tendon organs in a muscle should provide an even better estimate of the total force.
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