Neuromuscular control mechanisms in human mastication

Studies of the masticatory process show that besides the rhythmic character of the chewing cycles much variability in the movement trajectories can be observed. This is related to differences in food texture, but even in chewing one type of food there are considerable variations between chewing cycles in movement trajectories, cycle time and muscle activity of cycles. This indicates that sensory information in actual cycles plays a role in the control of jaw muscle activity during chewing. Notwithstanding the differences between cycles a force always has to be built up to overcome the food resistance or to fragment the particles to smaller ones. Before the next cycle starts, the bite force has to stop. Muscle spindles and periodontal mechanoreceptors are important in the jaw muscle response to loading. This was demonstrated in experiments with anesthetized rabbits where chewing movements were evoked by cortical stimulation and a hard object was placed between opposing molars. In our own observations in humans, where a resisting force disturbed cyclic open-close movements in the closing phase (simulated chewing), the role of muscle spindles and periodontal receptors was confirmed. When jaw closing suddenly accelerates after a decrease in load, such as during unloading of a static bite force, the muscle force quickly decreases. This jaw muscle response, preceding the neuromuscular response, is mainly due to intrinsic muscle properties. Although there is some understanding of the main processes that control muscle force during chewing, the fine tuning of the muscle activity and many details of the control are still object of investigation.