Gain and threshold of the jaw-jerk reflex in man during isometric contraction

Abstract

The control of mandibular posture has been related to the activity of the anterior temporal muscles, whereas the masseter muscles have been viewed mainly as force producers. However, these groups of muscles, especially in the deep layers, are highly endowed with muscle spindles, so that a difference in function should imply a difference in the reflex sensitivity. By studying the jaw-jerk reflex by means of bipolar surface electromyogram, the reflex sensitivity was determined from relationships between reflex amplitude and jaw displacement from both groups of muscles in eight subjects. At a constant level of background muscle activity, and hence with a constant excitability of the alpha motoneurons, the reflex sensitivity can be determined from these relationships in terms of gain and threshold. In order to account for differences in thickness of the soft tissues overlying the various muscles studied, the reflex amplitude was normalized with respect to the level of maximal voluntary contraction (MVC). In experiments where the inter-electrode distance was 18 mm over both groups of muscles, the reflex gain of the anterior temporal muscles was larger than that of the masseter muscles (P < 0.05). The threshold value did not differ significantly from zero for either group of muscles. Normalization of the reflex amplitude with respect to MVC can be carried out correctly only if the reflexly activated muscle fibres are distributed uniformly within the muscle. In order to gain an insight into this distribution, control experiments were performed with three subjects in which the inter-electrode distance was varied, thus influencing the depth in the muscle from which active muscle fibres were recorded. The reflex gain of the masseter muscle with an inter-electrode distance of 22 mm, which records from deeper layers of the muscle as well as superficial ones, was larger than with a distance of 11 mm (P < 0.01–0.10 in the various subjects). No such differences were found for the anterior temporal muscle. It was concluded that the afferents of the spindles in the jaw-elevator muscles do not project uniformly upon the motoneurons, but involve mainly fibres in the deeper layers of the muscle. The difference in gain found between the masseter and the anterior temporal muscles with the same inter-electrode distance of 18 mm is likely to be due to a larger distance of the reflexly activated muscle fibres in the masseter muscle with respect to the electrodes rather than to a difference in reflex sensitivity between the muscles. Hence, the masseter as well as the anterior temporal muscles could be involved in mandibular stabilization.