Timothy S Miles, Stanley C Flavel, & Michael A Nordstrom
Research Centre for Human Movement Control, The University of Adelaide, Adelaide, SA 5005, Australia timothy.miles@adelaide.edu.au

https://doi.org/10.36866/pn.56.31

Ever wondered why your teeth don’t bang together when you run or jump, or even when you nod your head violently? We all know that this does not happen, and that it would be very uncomfortable if it did. Recent studies have demonstrated that the mechanisms that maintain mandibular posture under static conditions are different from those that restrain it when the head is moving.

The mandible, of course, hangs below the skull, hinged at the temporomandibular joints and supported against gravity by soft tissues including the muscles of the face, the jaw-closing muscles and various ligaments. Consider first how it is supported when its owner is sitting or standing quietly with the head upright in the so-called rest position or postural position of the mandible. This position is important in clinical dentistry and maxillofacial surgery because it is reproducible within a millimetre or two, and therefore can be used to establish the normal face height in situations such as the construction of artificial dentures in patients who have no remaining natural teeth. If the dentures are made too high, the teeth will click together when the subject speaks and eventually pain may develop in the masticatory muscles. If the dentures are too low, the face will have an unattractive, collapsed appearance. Surgeons also use the rest position of the mandible as a reference point for establishing the height of the face when they are repairing the facial bones following injury, or repositioning the maxilla and/or mandible for cosmetic reasons.

It has often been postulated that, when the head is still, the mandible is supported in its rest position by a stretch reflex in the jaw-closing muscles, i.e. the mass of the mandible stretches muscle spindles in these muscles, which send signals to the brain to excite the motor neurones innervating the jaw-closing muscles, pulling the jaw upwards again (see Woda et al. 2001 for review). A problem with this notion is the fact that it is extremely difficult to demonstrate any activity in the jaw-closing muscles when the mandible is in its rest position. We have recently shown that there is an extremely low level of activity in the jaw-closers at rest, but that this is not the result of a stretch reflex. In fact, the muscle activity is pulsatile and alternates with pulses of low-level activation of the jaw-openers (Jaberzadeh et al. 2003).

How, then, is the position of the mandible maintained in the more challenging situation in which the head moves up and down during walking and running? Figure 1 shows that in a subject who walks briskly (1.4 m.s-1), not only does the head move up and down with every step, but the mandible moves up and down relative to the rest of the skull (i.e. the jaws open and close with each step). The downward mandibular movements must stretch the jaw-closing muscles; however, there is no burst of activity in the jaw-closer electromyogram (EMG) with each step that would indicate that the subsequent upward jaw movement was the result of a stretch reflex. This is seen clearly when the EMG signal before and after each landing is averaged for hundreds of steps (upper panels of Fig. 2).

Not surprisingly, the head movements are more vigorous when subjects run. The greater impact of landing causes the mandible to move more briskly downwards relative to the skull. This movement triggers a burst of EMG in the masseter which is evident in both the raw data in Fig. 1 and the averaged data in the lower panels of Fig. 2. This muscle activity then causes the mandible to move briskly upwards again. When subjects run uphill, they land on their toes. This gentler landing results in a slower, smaller movement of the mandible, which is often insufficient to trigger a reflex response. However, during downhill running, they must land on their heels, and the larger mandibular movement that results always triggers a reflex response in the jaw-closers.

To eliminate the possibility that the EMG response was the result of a vestibular reflex, our subjects also hopped down from a step and landed on one heel, both with their teeth clenched together to prevent their jaws from moving when they landed and without clenching. If the reflex arises from the vestibular system, it should still be present when the teeth are clenched. Landing on the heel (which feels pretty uncomfortable, incidentally) evokes strong reflex activation of the jaw-closing muscles when the teeth are unclenched. However, clenching the teeth abolishes the reflex (Miles et al. 2004). Thus we can be confident that the reflex is not vestibular, and, given its latency, is a stretch reflex.

The amplitude of this reflex response does not increase linearly with the amplitude or velocity of stretch. Instead, the amplitude of the EMG increases during brisker jaw movements even when the maximum amplitude and/or velocity of downwards jaw movement decreases or remains constant. This is because the downward movement of the mandible stretches the jaw-closing muscles and evokes reflex muscle excitation which then restrains the downward jaw movement. That is, when the stretch is sufficient to evoke reflex EMG activity, the muscle activation then prevents further downward jaw movement. Stronger stretches evoke more muscle excitation which result in smaller maximal downward jaw movement.

Thus, at rest, the mandible is supported not by reflex activity but by the visco-elasticity of the soft tissues in the masticatory system. This mechanism is sufficient to support the mandible during walking. However, the brisker downward movements of the mandible that occur when one lands on one’s heel during running evoke stretch reflexes in the jaw-closers that actively maintain the posture of the mandible. Even during running, the mandible usually moves up and down less than a millimetre, and the teeth do not crash together. This is a unique demonstration of how a stretch reflex operates to maintain posture under entirely natural conditions.

Acknowledgement

This work was supported by the Australian National Health & Medical Research Council.

References

Jaberzadeh S, Brodin P, Flavel SC, O’Dwyer NJ, Nordstrom MA & Miles TS (2003). Pulsatile control of the human masticatory muscles. J Physiol 547, 613-620.

Miles TS, Flavel SC & Nordstrom MA (2004). Control of human mandibular position during locomotion. J Physiol 554, 216-226.

Woda A, Pionchon P. & Palla S (2001). Regulation of mandibular postures: mechanisms and clinical implications. Crit Rev Oral Biol Med 12, 166-178.