MECHANORECEPTORS AROUND THE TOOTH EVOKE INHIBITORY AND EXCITATORY REFLEXES IN THE HUMAN MASSETER MUSCLE

1. The reflex responses evoked in the human masseter muscle by controlled mechanical stimulation of an incisor tooth were examined electromyographically. The stimuli were (slow) pushes and (brisk) taps of about 0-5-3 N peak force, applied orthogonally to the labial surface. 2. The brisk taps elicited a short-latency inhibitory reflex that was often followed by an excitatory peak, as has been described earlier. The inhibition increased as the taps became stronger. 3. Slow pushes evoked a long-latency, primarily excitatory response. The excitation increased with stronger, faster rise-time pushes; however, with the stronger stimuli, the short-latency inhibitory response often became evident before the onset of the excitation. 4. The reflex responses to 3 N pushes and 2 N taps were abolished when the receptors around the tooth were blocked with local anaesthetic, indicating that the response was elicited from receptors located within the periodontal area. 5. Prolonged, rapid-onset displacements evoked a complex reflex response that combined the characteristics of the taps and the pushes. 6. The most likely explanation for the different responses evoked by the pushes and taps is that the patterns of afferent activity elicited by the slow and fast tooth displacements activated different interneuronal pathways to motoneurones. 7. The inhibitory response to taps is essentially a protective reflex which probably serves to reduce the activity of the jaw-closing muscles when one bites unexpectedly on hard objects. It is suggested that the excitatory response may contribute to the muscle activity required to hold food between the teeth during chewing, or may act as a load compensation reflex to control chewing force.