Patterns of jaw reflexes induced by incisal and molar pressure stimulation in relation to background levels of jaw-clenching force in humans

Patterns of jaw reflexes induced by periodontal stimulation were examined in ten adults. Surface electromyograms (EMGs) from the masseter and anterior temporal muscles were recorded when pressure stimulation was applied to either an incisor or a molar. Reflex responses to periodontal pressure stimulation varied, depending on the background levels of jaw-clenching force that preceded stimulation (background clenching force, BCF). At low BCF, excitatory reflexes were elicited from the jaw-closing muscles and jaw-clenching force. However, the magnitude of excitatory reflexes varied with the location of the stimulated tooth along the dentition. While excitatory reflexes were induced equally in the masseter and temporal muscles during incisal stimulation, stronger excitatory reflexes were induced in the temporal muscle than in the masseter muscle during molar stimulation. At high BCF, inhibitory reflexes in the jaw-closing muscles and jaw-clenching force were elicited in eight subjects (group A) during periodontal stimulation. However, excitatory reflexes in the muscles and force were elicited in the remaining two subjects (group B). In the subjects of group A, stronger inhibitory reflexes were elicited in the temporal muscle than in the masseter muscle, and jaw-clenching force also decreased during both incisal and molar stimulation. In the subjects of group B, the magnitude of excitatory reflexes decreased with increases in BCF.