The effect of infrahyoid-muscle tenotomy on orofacial motor activity in the rat

Abstract

The effects of disturbed proprioceptive feedback from the infrahyoid muscles on the control of chewing and drinking were studied in the unrestrained rat. Rats were trained to lick and to eat in a fixed body position in front of a video camera. After training, the left and right sternohyoid, omohyoid and sternothyroid muscles were cut close to the hyoid bone. EMG-wire electrodes were inserted into the superficial masseter, the anterior and posterior digastic and sternohyoid mucles. EMG-recordings were made at the third, the tenth and the eighteenth day after tenotomy. After about a week the cut ends of the muscles became encapsulated in connective tissue and began to reattach to the hyoid bone; in some animals the sternohyoid muscle reattached to the trachea. In the period before reattachment the mean cycle duration for chewing (but not for drinking) increased. After reattachment the mean cycle duration returned to control values. The contraction patterns after tenotomy were similar to those of the controls during chewing and drinking. The mean burst duration of the sternohyoid increased after tenotomy during chewing but not drinking; it decreased in the digastric muscles and did not change in the masseter. After reattachment of the infrahyoids, the mean burst durations of the digastrics further diminished during chewing whereas that of the sternohyoid dropped below control levels. In the first week after tenotomy, during chewing, the maximum of the sternohyoid burst shifted and became synchronous with the peak of the masseter burst; the shape of the masseter and digastric bursts did not change. After the infrahyoid muscles has reattached the shape of the sternohyoid burst became similar to the control again. Thus proprioceptive information from the infrahyoid muscles may not be integrated in the central programme of motor control but used only in their intrinsic reflex control.