Behavior of cutaneous mechanoreceptors recorded in mandibular division of Gasserian ganglion of the rabbit during movements of lower jaw

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Abstract
The activity of cutaneous mechanoreceptors was recorded extracellularly in the mandibular division of the Gasserian ganglion of hemidecerebrate anesthetized rabbits. Hair follicle afferents (54) and skin mechanoreceptor afferents (80) were functionally identified. Their receptive-field characteristics were described and activity patterns recorded while the jaw was displaced by hand in the vertical, horizontal, and anterior-posterior directions, and during masticatory movements. All hair follicle afferents were classified as rapidly adapting, whereas skin afferents could be divided into 2 categories, rapidly adapting and slowly adapting. Rapidly and slowly adapting receptors were found in all regions of the mandibular skin. Only 1 hair follicle afferent and 4 skin afferents fired spontaneously with the jaw at rest. Of the hair follicle afferents 58% (29/50) were activated by imposed displacement of the jaw and all of these tested also discharged during chewing. They were sensitive to movement in all 3 axes and their firing frequency was linearly related to the velocity, regardless of direction. Only 10% (8/80) of the skin afferents were activated by imposed displacement of the jaw if their receptive fields were not directly contacted. Firing frequency was not well related to either the amplitude or velocity of movement. During chewing, the discharge frequency was variable but, in general, firing was restricted to the jaw-closing phase. Most skin afferents that were active during jaw movement had receptive fields close to the corner of the mouth. A possible involvement of cutaneous afferents in kinesthesia and control of jaw movements are discussed. Hair afferents could make specific contributions to the appreciation of movement because their discharge pattern was clearly related to one of its parameters. The lack of activity in most skin afferents, as well as the absence of a strong correlation between the firing frequency of those that were active and the movement parameters, suggest that they do not make specific contributions to the awareness of movement. Hair and skin afferents may also make different contributions to the control of ongoing movements. Skin afferent activity during jaw closure may elicit a reflex reduction in the velocity or duration of this phase. Hair afferent activity may indirectly influence movement via effects on elevator fusimotor neurons.