The effect of jaw position on the excitability of two reflexes involving the masseter muscle in man

Abstract

The excitatory monosyntic stretch reflex and the autogenic inhibitory reflex induced by Golgi tendon organ activation were both evoked in the masseter muscle bilaterally by tapping the chin with a reflex hammer. In each subject the reflexes were evoked in five static biting positions; (1) clenching in the intercuspal position, (2) biting on a 6 mm thick wooden stick placed between the anterior teeth, (3) between the right first molars, (4) betvveen the left first molars and, (5) between both first molars. The baseline EMG activity and the changes evoked in it by the chin tap were averaged in a PDP 12 computer and the amplitude of the stretch reflex, the ongoing level of activity, and the length of the inhibitory period were studied. The monosynaptic stretch reflex could not be evoked with the subjects clenching in the intercuspal position but could be evoked in all other positions. When the subjects were biting on one side the reflex on the opposite side was consistently increased by an average of over 100 per cent as compared to the levels obtained while biting bilaterally, while the reflex on the same side was abolished or markedly reduced. This was accompanied by a decrease in the ongoing level of activity on the biting side. The results indicate that, under the conditions of these experiments, static biting in a lateral excursion produces a decrease in the level of masseter muscle activity and stretch reflex excitability on the biting side while enhancing the stretch reflex excitability on the opposite side. The length of the inhibitory period was not markedly affected by changes in lower jaw position.