PATTERNS OF JAW MUSCLE-ACTIVITY DURING VOLUNTARY CHEWING

Mastication is controlled by central rhythm and burst generators, but whether this control is similar under automatic and voluntary chewing is not clear. Our first aim was to determine whether increased chewing frequency led to a proportional decrease in both the burst duration ( the time of muscle activity) and the interburst interval (time of inac tivity) of each muscle or to a minimal decrease in the burst duration and a substantial decrease in the interburst interval. Our second aim was to determine whether these two manifestations of control differed between the opening and closing muscles. Electromyograms from digastri c, medial pterygoid, masseter and anterior temporalis muscles were obt ained during gum chewing by five subjects at four frequencies. A nearl y fourfold decrease in total cycle duratin (TCD) with increased chewin g frequencies was accompanied by minimal changes in burst duration in the digastric muscle but proportional changes in the jaw closing muscl es. The onset latency of the digastric muscle as a percentage of TCD d ecreased and became negative (i.e. occurred before opening started) as the TCD decreased. The onset latencies of the jaw closing muscles rem ained roughly 10-20% of the TCD for the three lower chewing frequencie s. Control of the opening muscle appears to differ from control of the closing muscles.