Center for Musculoskeletal
Research, Gävle University,
Sweden.
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".
