A tonic vibration reflex evoked in the jaw opening muscles in man

Abstract

Motor effects induced by vibration of the mouth opening muscles were studied in eight subjects. All exhibited a tonic motor response with many characteristics in common with the tonic vibration reflex (TVR) as previously studied in limb muscles and jaw elevators: an involuntary contraction appeared in the jaw openers with gradual onset and gradual decline. The contraction was accompanied by a reciprocal inhibition of the voluntarily activated jaw elevators; it was susceptible to voluntary control—when allowed visual feed-back from a torquemeter all subjects were able to suppress the TVR and keep contraction force in their jaw elevators constant. The results suggest that the jaw openers are supplied with proprioceptors functionally similar to spindle endings in that on sustained activation they can produce tonic autogenic excitation and reciprocal inhibition, i.e. the pattern of the classical tonic stretch reflex. However, the vibration-induced contraction in the jaw openers did not exhibit one of the features typical for the TVR in the jaw elevators. The marked grouping of discharges synchronous with each wave of vibration, seen in gross EMG recordings from jaw elevators, was not seen in any one of the three jaw opening muscles examined, the anterior belly of the digastric, the mylohyoid and one of the infrahyoid muscles. As the vibration-induced synchronization is considered to depend on monosynaptic projections, the results imply that the jaw openers are devoid of such projections and that the TVR is mediated through polysynaptic pathways.