The Masseter Inhibitory Reflex
One such region affected is the region in the brain stem controlling the masseter inhibitory reflex (MIR). The MIR is a protective reflex located in the mesencephalic nucleus of the pons. An example of the MIR at work is when one is chewing a soft food and encounters a hard object in the food. Immediately, chewing stops, the object is removed, and chewing slowly resumes. The MIR has two distinct phases or suppressions of muscle activity termed excteroceptive suppression 1 (ES1) and excteroceptive suppression 2 (ES2).
Figure 1 demonstrates the ES1 or first phase of the MIR. It is activated by stretch receptors in the periodontal ligament of the teeth (as the hard food forces the teeth into the alveolar socket, the stretch receptors activate) and the mandibular nerve of the mandible (the mandible bends or twists and is detected by sensory nerves in the mandible).
The ES1 phase is 10 ms in length and acts unilaterally, only on the side affected. This is an instantaneous response, exactly what is needed when chewing, to protect the teeth, muscles and TMJ from injury.
Figure 2 demonstrates the second phase, ES2.
In the ES2 second phase, Golgi receptors in the masseters send signals to inhibit contraction of the masseters bilaterally. This phase takes slightly longer to occur, but lasts 40-50 ms, much longer than the ES1 phase. This phase offers even more protection by inhibiting muscle contraction bilaterally (click to enlarge).
The combination of the ES1 unilateral phase immediately followed by the bilateral ES2 phase effectively provides excellent protection when chewing, from hard objects that could cause injuries.
Sleep-Related Bruxism and the MIR:
In SRB, the MIR is suppressed, reducing this protection and allowing damage to the teeth, jaws, muscles and TMJ.
The following EMG tracing demonstrates exactly how SRB affects the MIR (click to enlarge).
This suppression of the masseter inhibitory reflex has been shown to be present in the majority of SRB patients. Uninhibited contraction of the masseter explains the damage to teeth and dental work as well as tooth sensitivity and headaches seen in these patients.