A hierarchy of neural control of mastication in the rat

Abstract

In rats anaesthetized with ketamine, rhythmic jaw-opening and jaw-closing movements were induced by palatal stimulation. The two masseter muscles (jaw-closing) and the four digastric muscles (jaw-opening) were fitted with electrodes, which could be used either for electrical stimulation or for recording electromyographic responses. Electrical stimulation of the masseters in the phase when the digastrics were the contracting muscles, caused responses in the digastrics. The amplitude of these responses was dependent on whether the stimulated masseters were active or not. The responses in digastric persisted when contraction of the masseters during stimulation was prevented by dantrolene sodium but they disappeared when the masseteric nerves were blocked with xylocain. The responses in digastric are thus reflexes from stimulating afferent fibres in the masseteric nerves. Likewise, electrical stimulation of the four digastrics in the phase when the masseters were contracting, caused responses in the masseters. The amplitude of these responses, however, was independent of the state of activity of the stimulated digastrics. Furthermore, the responses in masseter disappeared when contraction of the digastrics was prevented by dantrolene sodium; but they persisted when the digastric nerves were blocked with xylocain, provided the digastrics continued to twitch to the electric stimuli. The responses in masseter are thus reflexes in masseter caused by mechanical stretch transmitted from the digastric twitches. In the rhythmic preparation, prevention of contraction of the masseters or digastrics by dantrolene sodium or xylocain leaves the overall frequency and amplitude of the evoked rhythmic activity unchanged. In the quiescent preparation, without rhythmic activity, stimulation of the digastrics still causes the response in the masseters, but the reflex onto digastrics from electrical stimulation of the masseters is inactive. It is proposed that oral activity is controlled by two relatively independent systems. The first “programme-oriented” system generates the basic movements. The second “environment-oriented” system has access to the programme for correcting for perturbations. Muscles poorly supplied with muscle spindles (the digastrics) act supposedly as pure effectors. Corrective alterations in their activity after a perturbation are mainly prompted by muscle receptors in their antagonists.