Characteristics of the muscle spindle endings of the masticatory muscles in the rabbit under halothane anesthesia

Abstract To explore the response characteristics of muscle spindle units in the masticatory muscles in the rabbit, the responses of muscle spindle units were recorded from the mesencephalic trigeminal nucleus (MesV) under halothane anesthesia during ramp-and-hold stretches. Three firing patterns, initial burst (IB) at the onset of the dynamic phase, negative adaptation (NA) at the end of the dynamic phase and firing during the release (FDR) phase, were observed during muscle stretch. IB was present at higher stretch velocities, FDR at lower stretch velocities. The velocity at which an IB or FDR was present was different from unit to unit. Because, within the range of the velocities of stretch tested, units with NA always showed NA and units without NA never did, all recorded units were divided into two groups on the basis of the existence of NA (NA(+) or NA(−) units). Response characteristics of the two groups were then compared. NA(+) units showed an IB more frequently and FDR less frequently than NA(−) units. NA(+) units had significantly higher dynamic responsiveness and discharge variability than NA(−) units. The conduction velocity of the afferents of NA(+) units was higher than that of NA(−) units. However, distributions of these measurements were not bimodal. These results suggest that NA is the useful criteria to classify the muscle spindle endings in the masticatory muscles in the rabbit under halothane anesthesia.