CORRELATING RESTING DISCHARGE WITH SMALL-SIGNAL SENSITIVITY AND DISCHARGE VARIABILITY IN PRIMARY ENDINGS OF CAT SOLEUS MUSCLE-SPINDLES

1. In a previous report we proposed that primary endings of cat soleus muscle spindles can be separated into two kinds. One kind, called by us silent endings, at muscle lengths shorter than L(m)-10 (maximum bod y length - 10 mm), fell silent after a 5 mm shortening step. Spontaneo us endings, on the other hand, were able to resume a resting discharge after a brief pause at all muscle lengths down to L(m)-20. This repor t examines further differences between the two kinds of endings. 2. Th ere were consistent differences in the muscle length dependence of the maintained level of resting discharge of the two kinds of endings, me asured after a conditioning contraction or a contraction followed by a shortening step. The resting discharge of spindles with spontaneous e ndings, after both forms of conditioning increased progressively with length. For silent endings, after a conditioning contraction, resting discharge fell slightly at longer lengths. 3. Discharge variability, m easured at a number of muscle lengths, showed a dependence both on mea n interimpulse interval and on spindle type, being higher in silent th an spontaneous spindles. 4. Small signal sensitivity was measured with the use of 1 Hz sinusoidal stretches applied longitudinally to the te ndon. Sine wave amplitude was adjusted to give a 30% depth of modulati on of the resting discharge. Spontaneous endings were consistently les s sensitive to the stretches than silent endings at all muscle lengths . Average sensitivities, measured over a range of lengths between L(m) -4 and L(m)-20 mm were 0.30 imp.s(-1).mu m(-1) for spontaneous endings and 0.66 imp.s(-1).mu m(-1) for silent endings. This difference was s ignificant at the 0.001 level (two-tailed t test). 5. For several spin dles it was confirmed that the small signal measurements had been made within the linear range of spindle responsiveness. 6. Since the avail able evidence suggests that the difference in behavior of silent and s pontaneous endings has a mechanical cause, a number of possible mechan isms is considered. One is a network of elastic fibers around the sens ory ending, which can influence resting discharge, small signal sensit ivity, and discharge variability.