Electromyographic Activity of Masticatory Muscle in Sheep.

The masseter muscle of the Australian Merino sheep consists of three layers, superficial (MS), middle (MM) and deep (MD). We studied the EMG activities of jaw elevator muscles including the three layers of the masseter muscle during mastication. The EMGs were recorded with a bipolar electrodes of stainless wire which was coated with enamel, and from ten muscles as follows, temporal muscle (T), MS, MM, MD and medial pterygoid muscle superficial fiber (MP) on both sides. Masticatory movements were induced by feeding. Three types of feed were given ; grasses as a fibrous type, oats as an elastic type and sheep nuts as a solid type. In T, MD and MM, the discharge volumes of the EMGs on the working side were larger than those on the balancing side, but in MS and MP, the durations on the working side were smaller than those on the balancing side. The starting times of activities of these muscles in mastication were in sequence, MD, MM and MS on the working side, MD, MS and MM on the balancing side. The starting time of MD on the balancing side in mastication arose earliest. Therefore, the activity patterns of these muscles reveal which is the working side and which is not. It is recognized that the activities of the three layers of masseter muscle differed from one another in mastication.