RESUMO
The muscle was exposed in 12 lightly anaesthetized monkeys so that electromyographic recordings could be made from the superior and inferior heads. The separately recorded activity in each head was initially examined in relation to activity recorded simultaneously in other jaw muscles or to jaw movements produced actively by the animals or passively induced by the experimenter. Subsequently, the reflex effects of stimuli applied to various oral-facial sites were examined. In general, the superior head was active in association with jaw-closing muscle activity and related movements produced by the animal whereas the inferior head was mainly active in jaw-opening. Both heads increased their activity with passively-induced jaw-opening movements, especially if the jaw-opening was combined with a horizontal deviation of the mandible. Both heads showed reflexly-induced excitation and silent periods as a result of stimuli applied to oral-facial sites, the most effective sites being palatal, lingual, labial and buccal mucosa and the teeth, stimulation of which, as well as stimuli applied to mandibular joint and cutaneous afferents, could also reflexly induce silent periods in both heads. The various effects may be related to mechanisms for protection and stabilization of the mandibular joint and masticatory muscles.
