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 body length -10 mm), fell silent after a 5 mm shortening step. Spontaneous 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 report examines further differences between the two kinds of endings. 2. There were consistent differences in the muscle length dependence of the maintained level of resting discharge of the two kinds of endings, measured after a conditioning contraction or a contraction followed by a shortening step. The resting discharge of spindles with spontaneous endings, 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, measured at a number of muscle lengths, showed a dependence both on mean interimpulse interval and on spindle type, being higher in silent than spontaneous spindles. 4. Small signal sensitivity was measured with the use of 1 Hz sinusoidal stretches applied longitudinally to the tendon. Sine wave amplitude was adjusted to give a 30% depth of modulation of the resting discharge. Spontaneous endings were consistently less 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 · μm^-1 for spontaneous endings and 0.66 imp · s^-1 · μm^-1 for silent endings. This difference was significant at the 0.001 level (two-tailed t test). 5. For several spindles it was confirmed that the small signal measurements had been made within the linear range of spindle responsiveness. 6. Since the available evidence suggests that the difference in behavior of silent and spontaneous endings has a mechanical cause, a number of possible mechanisms is considered. One is a network of elastic fibers around the sensory ending, which can influence resting discharge, small signal sensitivity, and discharge variability.