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Jaw reflexes are of different kinds, including the jaw jerk reflex, the myotactic reflex, the jaw closing and opening reflex. This paper will aim to describe each in turn, with the clinical relevance of the normal and abnormal reflexes along with published scientific research in dental journals that has focused on the mechanism of these reflexes. These reflexes, apart from signifying integrity of neuronal pathways and brainstem centers, also play a modulatory role during normal oral function. As such they may be abnormal in disorders of mastication associated with pain. (Maillou) However, the same author proved in 2008 that temporomandibular disorders did not cause a reduction in the amplitude of those reflexes that are protective.
The jaw jerk is an abnormal response to a stimulus given to the area of the chin. It can be demonstrated in patients with damage to the area of the brain that coordinate motor activity of the trigeminal nerve (cranial nerve V). In these patients, if the mouth is opened half way and the jaw muscles are relaxed, tapping the chin at a downward angle with a rubber hammer. An abnormal response would be if the jaw deviates towards the side of the lesion, or the jaw-jerk is exaggerated and very brisk with lesions affecting the pyramidal pathways above the 5th nerve motor nucleus, especially if the lesions are on both sides. This is also called the chin reflex, chin-jerk reflex or the mandibular reflex. It is performed in people who have suspected damage to the trigeminal nerve and show signs of it.
In normal people this reflex is usually absent or very slight. It tests the monosynaptic pathway, coordinated by the trigeminal nerve, the afferent being taken by the sensory neurons to the mesencephalic nucleus, which relays to the motor nucleus. This in turn innervates the masseter muscle the contraction of which causes jaw closing. The periodontal ligament mechanoreceptors play a part in the jaw jerk as the sensory organs. This is susceptible to modulation by remote noxious stimuli. A study done in 2001 by Lobbezzo et al demonstrated that ‘periodontal mechanoreceptors on the ipsilateral side have an inhibitory effect on the jaw-jerk reflex amplitude in the anterior temporalis muscle’.
The myotactic reflex is another form of jaw reflex that depends on stretching of muscle fiber. This reflex is not monosynaptic and consists of an arc of two neurons. The afferent fibers are present in the muscle spindle and relay to the spinal cord through the posterior nerve roots. Collaterals from these fibers make direct synaptic contact with motor neurons supplying the muscle that lies next to the spindle fibre receptors.
Another reflex is the digastric jaw opening reflex in reaction to tooth tapping. An experiment in 1987 by Collier was done to determine the pathway taken to the brain by periodontal afferents responsible for this reflex. They found that sectioning the trigeminal sensory roots of anesthetized rate resulted in elimination of the ipsilateral jaw opening reflex but had no effect on the stretch response of the jaw closing muscles.
During feeding, feedback from oral receptors is important in the control f mastication so as to avoid overloading of food. These constitute the jaw closing reflexes. They are a response to stimuli in and around the oral cavity. However they are subject to modulation by nociceptive stimuli and stress. This was demonstrated in a study conducted in 2007 by Cadden. Electrical stimulation of the upper lip results in inhibition of messeteric activity by reflex. This reflex is reduced when there is noxious stimulation of other parts of the body surface of deeper tissues. Experimental stress induction and other psychological manipulations also result in the diminishing of this reflex although to a less significant level.
A similar study was done on human subjects that showed that stimulation of remote nociceptive, but not non-nociceptive, deep somatic nerves can affect jaw reflexes in humans. (Maillou 1997) This is a manifestation of the phenomena known as diffuse noxious inhibitory controls which act through inhibitory paths that originate in the medulla and produce post-synaptic inhibition of intermediate neurones in the trigeminal nuclei.
Jaw opening and the trigemino-hypogossal reflex can be illustrated by innocuous as well as noxious stimuli. In a study done in 1999 on anesthetized cats by Ono et al, electrical activity was recorded from the anterior digastric, genioglossus, and styloglossus muscles during swallowing which was done by dripping water on the tongue or repeated stimulation of the superior laryngeal nerve. Jaw opening was illicited by stimulation of the lingual nerve. It was found that low threshold stimulation was significantly suppressed during swallowing presumably by a central mechanism. This explains the phenomena that the jaws cannot be fully open while swallowing. Jaw opening reflexes have also been used to see compare soft tissue oral surgery procedures via scalpel versus laser. This study, done in 2005 by Zeredo et al demonstrated that YAG laser surgery is less painful and thus would require less anesthesia and sedation as compared to standard scalpel surgery as reflex amplitudes (marker of pain) were smaller during the laser surgery.
References
Maillou, P. & Cadden, S. W. (2008) The Effects of a Maximal Jaw Clench on an Inhibitory Jaw Reflex. J Oral Rehabil, 35, 415-23.
Lobbezoo, F., Verheij, J. G. & Naeije, M. (2001) Influence of Periodontal Receptors on the Jaw-Jerk Reflex Amplitude in Man. Eur J Oral Sci, 109, 40-3.
Collier, T. G. & Lund, J. P. (1987) The Effect of Sectioning the Trigeminal Sensory Root on the Periodontally-Induced Jaw-Opening Reflex. J Dent Res, 66, 1533-7.
Cadden, S. W. (2007) Modulation of Human Jaw Reflexes: Heterotopic Stimuli and Stress. Arch Oral Biol, 52, 370-3.
Maillou, P. & Cadden, S. W. (1997) Effects of Remote Deep Somatic Noxious Stimuli on a Jaw Reflex in Man. Arch Oral Biol, 42, 323-7.
Maillou, P. & Cadden, S. W. (2007) Characteristics of a Jaw Reflex in Humans with Temporomandibular Disorders: A Preliminary Report. J Oral Rehabil, 34, 329-35.
Ono, T., Ishiwata, Y., Kuroda, T. & Nakamura, Y. (1999) Suppression of Jaw-Opening and Trigemino-Hyspoglossal Reflexes during Swallowing in the Cat. J Dent Res, 78, 1720-6.
Zeredo, J. L., Sasaki, K. M., Yozgatian, J. H., Okada, Y. & Toda, K. (2005) Comparison of Jaw-Opening Reflexes Evoked by ER:Yag Laser versus Scalpel Incisions in Rats. Oral Surg Oral Med Oral Pathol Oral Radiol Endod, 100, 31-5.
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