[Identification of primary and secondary muscle spindle endings by the regularity of their discharges]

In two series of experiments on the anesthetized cats' receptors of m. triceps passive muscle spindles, the stretching was performed by flexing the foot in the 1st series whereas in the 2nd one the muscle was stretched by a cut tendon. The secondary endings revealed the 3 per cent maximum of the sharp-peak distribution of variation coefficients (VC) of interspike intervals within the limits of 1.2 to 12.0 per cent. The VC values of the primary endings discharges are distributed relatively regularly within the range of 3--14 per cent, the distribution limits being 2.5--18.0 per cent. The VCs of discharges of both receptor types are overlapping in low frequency of impulses succession. 18 per cent of primary terminals have the same discharge variability as the majority of secondary ones. In intact paw, discharges of over 25/sec frequency are rare, therefore the analysis was performed in respect to low frequencies. The terminals can be with caution identified by their discharge regularity.