Regularity of muscle spindle receptor discharges at a different muscle length

The regularity of receptor discharges of passive muscle spindles adapted to a different length of the triceps of the calf of narcotized cats was investigated in the range of average frequencies from 5.6 to 40 impulses/sec. The values of the coefficients of variation of the impulse intervals in discharges of the primary endings are distributed in the range from 2.5 to 18.0% and of the secondary from 1.2 to 12%. The discharges of the primary endings are less variable than the discharges of the primary. Among the latter are found receptors with less, moderately, and more regular discharges at any length of the muscle. The shape of the "average impulse interval vs. coefficient of variation" relationship is nonlinear and differs for different units. For samples of receptors having approximately the same variability of the impulse intervals the average values of the coefficients of variation decrease significantly with muscle tension and increase of the rate of impulse flow. In some cases a decrease of the values of the coefficients of variation occurs due to a decrease of the dispersion at the same rate of flow and in other cases both parameters change simultaneously. A decrease of the dispersion of the intervals in receptor discharges apparently reflects an increase of the number of uniformly depolarized endings as their tension increases.