AN ELECTROMYOGRAPHIC STUDY OF WHETHER THE DIGASTRIC MUSCLES ARE CONTROLLED BY JAW-CLOSING PROPRIOCEPTORS IN MAN

Whether in the oral system the digastric muscles (which lack muscle sp indles) are under the control of proprioceptive information from the m asseter muscles (which contain muscle spindles) was investigated by an alysing whether and how the masseters and digastrics showed coordinate d behaviour during a static, forceful bite. Subjects were asked to mai ntain a 100-N force for 15 s with and without visual guidance; bite fo rce exerted, and masseter and digastric electromyograms (EMGs) were re corded. Under visual guidance all subjects co-contracted their digastr ic muslces during the isometric bite. They held the force for a short time, followed by periods with fluctuations (peak-to-peak force amplit ude about 15-20 N). Fluctuations in bite force correlated with the mas seter EMGs, the maximum in the correlogram occurring at about -50 ms w ith the force lagging the masseter. In 75% of the subjects a significa nt periodic component in the masseter and in the force spectra was fou nd at about 4 Hz. This was also seen in the amplitude spectra of the f orces, which showed in 80% of the subjects a significant elevation bet ween 7-10 Hz as well. No correlation between the digastric EMGs and th e bite forces, and between the EMGs of masseter and digastric could be detected. Spectra of digastric EMGs showed no prominent maxima. When subjects were deprived of visual feedback, maxima at -50 ms in the cro ss-correlation functions of the masseters and the forces were reduced considerably; periodicities of +/- 250 ms disappeared. In the force am plitude spectra the 3-5 Hz components became much smaller or were not significant; elevations between 7-10 Hz were not affected, however. Cr oss-correlation of the digastrics and forces, and of the masseters and digastrics showed no significant correlation whatsoever in the absenc e of visual feedback. Thus, the digastrics (although co-contracting du ring the bite) do not contribute actively to the control of an isometr ic bite; they do not receive information from jaw-closing muscle recep tors during an isometric bite. Large amplitude fluctuations in the bit e force arise from the jaw-closing muscles. These fluctuations may be caused by both resonances of high-order feedback loops (involved in de coding the target force engram), and from visual feedback.