Fusimotor activity in masseter nerve of the cat during reflex jaw movements.

Publisher Website
Google Scholar
Abstract
Unit recordings were made from the central ends of filaments of the masseter nerve in lightly anesthetized cats. Evidence is presented to show that fusimotor activity may be distinguished from .alpha. motor activity. During reflex cyclic movements induced by intra-oral stimulation, 2 distinct patterns of fusimotor firing emerged. One type of unit increased firing at the beginning and sustained this with little modulation throughout the movements. The other type was strongly modulated approximately in parallel with the .alpha. motor activity. By comparison with records of jaw elevator spindle afferents under similar conditions, the sustained type of action was apparently due to dynamic fusimotor neurons while the modulated type was apparently due to static fusimotor neurones. The patterns of fusimotor activity seen in these rhythmic movements under light anesthesia agree well with the patterns deduced from spindle recordings in the conscious cat during mastication. The importance of looking beyond a simple hypothesis of .alpha.-.gamma. co-activation to explain fusimotor function is emphasized. Tonic dynamic fusimotor activity is probably set at the beginning of a movement to determine the incremental sensitivity of primary endings to stretch. The static fusimotor fibers are activated principally during shortening to help keep both primary and secondary endings active.