EFFECTS OF INFLAMMATORY IRRITANT APPLICATION TO THE RAT TEMPOROMANDIBULAR-JOINT ON JAW AND NECK MUSCLE-ACTIVITY

An electromyographic (EMG) study was carried out in 40 anaesthetized r ats to determine if the activity of jaw and neck muscles could be infl uenced by injection of the small-fibre excitant and inflammatory irrit ant mustard oil into the region of the temporomandibular joint (TMJ). Injection of a vehicle (mineral oil, 20 mu l) did not produce any sign ificant change in EMG activity. In contrast, injection of mustard oil (20 mu l, 20%) evoked increases in EMG activity in the jaw muscles but not in the neck muscles. The increased EMG activity evoked by mustard oil was reflected in 1 or 2 phases of increased activity. The early E MG increase occurred soon after the mustard oil injection (mean latenc y +/-SD: 3.5 +/- 2.3 sec), peaked within 1 min, and then subsided (mea n duration: 7.5 +/- 5.2 min). The later EMG increase occurred at 14.6 +/- 10.0 min after the mustard oil injection and lasted 14.3 +/- 12.3 min. These excitatory effects of mustard oil on the EMG activity of ja w muscles appear to have a reflex basis since they could be abolished by pre-administration of local anaesthetic into the TMJ region. These results document that TMJ injection of mustard oil results in a sustai ned and reversible activation of jaw muscles that may be related to th e reported clinical occurrence of increased muscle activity associated with trauma to the TMJ.