Physiological properties of the stretch receptors in the masseter muscle

Abstract

In this study, the physiological properties of the sensory endings of the muscle spindles in the masseter muscle of the cat were analyzed. The relationship between the conduction velocity of the spindle afferents and the response of these endings to muscle stretch and to SCh administration was examined. Further, diameters of the afferent nerve fibers from these endings were calibrated electromicroscopically. The results are as follows:1) The conduction velocity of the spindle afferents ranged from 12m/sec to 72m/sec, showing a bimodal distribution with two peaks between 12-36m/sec and between 36-72m/sec.2) The conduction velocity of the afferents and the dynamic response of the endings to the muscle stretch were significantly correlated (p<0.05).3) The afferents conducting at above 54m/sec and those conducting at below 36m/sec were clssified as the primary and secondary spindle afferents respectively, on the basis of their responsibility to the muscle stretch, vibration and SCh administration.4) The afferents with the intermediate conduction velocity of 36-54m/sec were not classified as either the primary or secondary spindle afferents on the basis of the conduction velocity. The responses to SCh administration and to muscle vibration could be used as criteria for the classification of these afferents.5) The fiber spectrum of the muscle afferents, which remained after destruction of the trigeminai motor nucleus by kainic acid injection, was bimodal with the peaks between 1-5μm and 6-11μm. The majority of the primary and secondary spindle afferents were found to beincluded in the second peak.