Journal of Neurophysiology, Vol 71, Issue 1 309-316, Copyright © 1994 by APS
Correlating resting discharge with small signal sensitivity and discharge variability in primary endings of cat soleus muscle spindles
J. J. Scott, J. E. Gregory, U. Proske and D. L. Morgan
Department of Physiology, Monash University, Victoria, Australia.
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 Lm -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 Lm -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 Lm -4 and Lm -20 mm were 0.30 imp.s-1.microns -1 for
spontaneous endings and 0.66 imp.s-1.microns -1 for silent
endings.(ABSTRACT TRUNCATED AT 250 WORDS)
