Unitary discharges of TMJ mechanosensitive neurons in the rabbit

Objectives: Mechanoreceptors in the temporomandibular joint (TMJ) are considered to play an important role in controlling chewing, based on sensory information about the jaw position and movement. The aim of this study was to investigate the response properties of the mechanosensitive neurons innervating TMJ (TMJ neurons) in the trigeminal ganglion of the rabbit when the condyle was moved passively with a ramp-wave, mechanical stimulator attached to the condyle.Methods: Seven adult female rabbits (Japanese white; 1.9-2.4kg) were used. They were anesthetized with a urethane-chloralose mixture (500 and 40mg/kg, i.v.). The condyle was cut off the ramus of the mandible, and was passively moved rostrally with the stimulator attached to the condyle. The discharges of TMJ neurons sensitive to mechanical stimulation of the condyle were recorded from the left trigeminal ganglion with metal microelectrodes. Results: Eleven TMJ units were recorded in this study. All of them were slowly adapting with tonic activity. Each TMJ unit responded to condyle movement in various directions, but all TMJ units responded to condyle movement in the rostral and ventral directions (jaw opening direction). Of 8 TMJ units the firing frequencies increased as the displacement of the condyle increased. The relationship of the displacement of the condyle and the frequency were expressed with Steven' power law [Power component: 0.55ą0.23 (meanąSD, n=11)]. The firing frequencies of 2 TMJ units increased slightly as the velocity of condyle movement increased. Conclusion: These results suggested that most TMJ neurons carry sensory information about the displacement of the condyle.