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Can neck injury disturb eating behaviour?
Per-Olof Eriksson
Department of Odontology,
Clinical Oral Physiology, Umeå University,
Center for Musculoskeletal Research, Gävle University, Sweden.

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THE ANSWER IS YES !
THE JAW AND THE NECK SENSORIMOTOR SYSTEMS ARE FUNCTIONALLY LINKED
 
A line of evidence from neuroanatomical and neurophysiological studies in animal have demonstrated close connections between the trigeminal and the neck neuromuscular systems. In man, a functional coupling between the cranio-mandibular and the cervical spine regions is suggested by intimate anatomical and biomechanical relationships, by findings of reflex activities in the neck muscles following electrical stimulation of trigeminal nerve branches, and from observations of simultaneous activation of jaw and neck-shoulder muscles during mandibular movements.
 
The earliest reflex found in the human embryo is the trigemino-neck reflex, which consists of contraction of neck muscles elicited by light touch of the perioral region. Even spinal reflexes to the jaw have been demonstrated. Thus, previous studies in animal and man indicate a close functional linkage between the jaw and the neck regions. However, systematic studies in man of integrated mandibular and head-neck behaviour during natural jaw function have until the last fifteen years been lac ing.
 
A NEW CONCEPT FOR NATURAL JAW FUNCTION. BY DEFINITION,
JAW FUNCTION COMPRISES INTEGRATIVE JAW-NECK BEHAVIOUR.
 
In a series of studies in man, using optoelectronic wireless technique for 3-D movement recording and electromyography, we have shown concomitant and well coordinated mandibular and headneck movements during single as well as rhythmic jaw opening-closing tasks. We have also demonstrated that such movements are invariant in nature. The findings have led us to propose a new concept for natural jaw function. In this concept, "functional jaw movements are the result of jointly activated jaw as well as neck muscles, leading to simultaneous movements in the temporomandibular, atlanto-occipital and cervical spine joints".
 
We have also suggested from our results that these jaw and head-neck movements have neural commands in common and are preprogrammed. Furthermore, our findings combined with observations in ultrasonographic studies on human fetal yawning that mouth opening-closing is accompanied by head extension-flexion movements, respectively, indicate that a functional connection between the jaw and the neck in natural jaw function is innate. Thus, based on results from studies in animal and man natural jaw function by definition includes integrative jaw-neck behaviour.
 
WHAT IS THE SIGNIFICANCE OF FUNCTIONAL COUPLING
BETWEEN THE JAW AND THE NECK?
 
Connections between the trigeminal system and neck motoneurones are of importance for head withdrawl reactions in all species. Any sudden or unexpected stimulus in the oro-facial region leads to fast head aversion, hence paralleling the flexor reflex of the limbs. Such connections are also likely to be critical for coordinating jaw and head-neck motions, in timing of jaw opening and closing with head-neck movements during daily activities such as eating and communication.
 
Free neck movements are a prerequisite for natural maximal gaping. A reduced head extension ability may limit the three-dimensional space for the mandibular movement, due to impingement of the mandible with suprahyoid and airway structures. A connection between the jaw and the neck motor systems is probably of importance to allow simultaneous mandibular and head-neck movements, aimed at optimising both the magnitude of the gape and the positioning of the gape in space. From an evolutionary perspective, such a mechanism, to optimally direct the jaw motor system, i.e. the mouth or the gape, in basic actions such as feeding, attack and defence behaviour, is probably of great survival value, e.g. during catching of a pray.
 
NATURAL JAW FUNCTION CAN BE DISTURBED FOLLOWING NECK
INJURY!
 
Trauma to the neck from a motor vehicle accident or some other type of head-neck trauma, generally called "whiplash trauma or injury", may lead to a condition comprising a number of symptoms and signs in the neck and head termed "Whiplash-Associated Disorders", WAD. Previous studies have indicated that such head-neck trauma can result in pain and dysfunction in the jaw-face region, or "Cranio-Mandibular Disorders", CMD.
 
Our studies on integrated jaw-neck motor control in healthy subjects and in WAD patients have led to an explanatory model for the development of CMD in subjects who have met with whiplash injury. Thus, given that natural jaw actions require a healthy state with unrestricted motion not only of the temporomandibular joint, but also of the atlanto-occipital and cervical spine joints, it can be assumed that an injury to or disease of any of these three joint systems might derange natural jaw motor control. Furthermore, such a functional impairment would be reflected by disturbed jaw-neck behaviour, which could be detected by recording and analysing concomitant mandibular and head-neck movements during natural jaw actions. We have recently tested this hypothesis by studying integrative jaw-neck function in patients suffering from WAD and pain and dysfunction in the jaw-face. The results showed an association between neck trauma and disturbed jaw-neck function, indicating that coordinated jaw-neck motor control during single as well rhythmic jaw opening-closing movements indeed can be disturbed following neck injury. This means that jaw behaviour such as gaping, chewing, swallowing, i.e. eating, and speech can be disturbed in response to trauma to the neck. Consequently, neck injured patients should be offered assessment and management also for pain and dysfunction in the jaw-face. In addition, the WAD concept should be revised to encompass also symptoms and signs in the jaw and face sensorimotor system. It should also be noted that since the jaw and the neck are functionally linked in both health and disease, treatment including neck fixation, by definition, will impair natural jaw function. In case of such treatment, patients should be informed about this drawback. Finally, it can be proposed that an appropriate term for the clinical condition comprising both jaw-face and head-neck pain and dysfunction would be "Cervico-Cranio-Mandibular Diso ders, CCMD".