The Masseter Inhibitory Reflex

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).

Figure 1 The ES1 or First Phase of the MIR: Stimulation occurs in the PDL and mandibular nerve. The signal travels through the Gasserion ganglion, to the trigeminal sensory nucleus, on to the 1st inhibitory nucleus which suppresses the trigeminal motor nucleus briefly inhibiting contraction of the masseter.

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.

Figure 2: The ES2 phase of the MIR occurs after the ES1. It is activated by Golgi stretch receptors in the masseter muscle. The signal passes through the Gasserion ganglion, bilaterally to both trigeminal sensory nuclei, and onto both second inhibitory nuclei lower in the pons An inhibitory signal is sent bilaterally and both masseter muscles are inhibited.

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).

Figure 3: Normal EMG Tracing of the MIR. in the tracing, the ES1 and ES2 phases are clearly seen following stimulation of the MIR. The muscle activity stops briefly during each phase. The reflex is activated repeatedly until the stimulus is removed.

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).

Figure 4: Sleep-related bruxism’s effect on the MIR: In mild sleep-related bruxism, the ES1 phase is fairly normal however the ES2 phase is shortened to 15-20 ms, reducing the effectiveness of the reflex. In Severe sleep-related bruxism, the ES1 phase is reduced and the ES2 phase is completely absent. This allows the masseter to contract with 100% of its bite force during SRB events.



















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.