ORGANIZATION AND FUNCTION OF JAW MUSCLES
Confluent Motor Behaviors
A distinguishing feature of the jaw musculature is the number
of completely different motor activities in which the same muscles
may be involved at one time. For instance, we may talk while we
are eating, stop to swallow some food, and simultaneously cease
respiration for a moment; then we resume chewing, talking,
breathing, and swallowing. In the absence of disease we carry out
these actions smoothly, independently, and almost without a
thought. Some oral motor behaviors are shown in Table I below:
The suckling reflex in infants is completely necessary for
survival, and is well developed at birth. This normally changes
to the adult pattern, although failure to do so may result in a
tongue-thrust pattern of swallowing (Straub, 1960). Mastication
itself is a complex, coordinated series of movements, during
which the food particles must be lubricated and reduced to an ingestible size. At the same time, solid foods are shaped into a
bolus by stereotyped tongue movements, while the tongue itself is
protected from the sharp cusps of the teeth. At the appropriate
time the bolus is moved to the posterior pharynx, where the swallowing reflex is initiated.
Table I. Types of oral motor behaviors
Ingestion
Infant suckling
Drinking
Mastication
Preparation of food for swallowing
Swallowing
Biting
Gripping with teeth
Attack/defense
Communication
Speech
Song
Facial expressions
Yawning
Respiration
Apnea during swallowing
Sneeze, cough
Earlier in our history, the teeth were no doubt necessary
for combat and self-defense. Nowadays they continue to serve an
important function of biting through hard objects such as apples
and pizza crusts. In addition, the jaws and one arm are often used
in synchrony, to tear a bite from a piece of food such as meat or
a hard roll.
While all this behavior is going on at the dinner table, we
also talk with each other. Our teeth, lips and tongues are positioned in exactly the right way to produce the sounds of our language, while expired air vibrates the vocal cords. We may occasionally break into a song, where sustained tones and articulated sounds are combined in an esthetically pleasing way. The
facial muscles may be used at the same time, to communicate happiness or sympathy, approval or disgust, without saying a word.
We may also yawn at times, either to clear the eustacean tubes or
to signal a lack of interest.
The muscles of the face and jaw are also intimately involved
with respiration. An irritant breathed into the nasopharynx may
cause us to sneeze, or something in the throat may bring on a
cough. During these events all speech, mastication and swallowing
cease. Conversely, when we swallow, because the airway and food
channels are crossed, breathing stops; this is the apnea of
deglutition.
In summary, all of the above oral motor functions are confluent and use many of the same muscles for different purposes at
the same time. In this and the following chapters we shall examine some of the details of how this is accomplished.
Kinesiology of the Head and Mandible
The movements of mastication, such as opening and closing the
jaw, and lateral and protrusive movements of the mandible, are
carried out against the cranium, which itself is suspended on the
vertebral column. This is diagrammed in Figure 1. The cranium is
balanced like a see-saw, with neck and spinal muscles in back and
the mandibular elevators and depressors in front. The jaw-closer
muscles attach directly between the mandible and cranium.
Openers, however, attach between the mandible and hyoid bone, and
between the hyoid and the shoulder girdle. (Other anterior
muscles also attach directly between the cranium and shoulder
girdle, such as the sternocleidomastoids.)
One result of this arrangement is that jaw reflexes can be
shown to include coactivation or inhibition of neck muscles.
When the jaw-opening reflex is stimulated by electrical shock to
the lip or gingiva, for instance, the posterior neck muscles are
excited at the same time (Sartucci et al, 1986), and anterior cervical muscles may be inhibited (Browne et al., 1989). These actions serve to maintain the head in an upright position. Since an
important function of the elevator muscles (temporal and masseter) is to maintain the jaw in rest position against the force
of gravity, the activity of these muscles is strongly dependent
on posture (Lund et al., 1970). This is discussed further in
Chapter 13.
A feature of the temporomandibular joint which is unique in
the body is that it is diarthroidal, or bicondylar. Both sides of
the mandible work more or less together, unlike the peripheral
limbs. The joint also has a sliding capacity, as each condyle can
move backward and forward in the glenoid fossa. The mandible
usually is held about in the midline, but can be rotated several
degrees to either side. In that case the joint on one side acts
as a fulcrum for rotation of the other side.
The suspension of the cranium on the cervical spine and the
possible movements of the mandible with respect to the cranium
involve three different classes of levers, shown in Figures 3, 4
and 5. The first class lever is one in which the fulcrum is
placed between the load and the applied force. The efficiency,
defined as the ratio of load lifted to applied force, depends on
the spacing of each from the fulcrum. In Figure 3, the weight, W,
acts to make the head fall forward in the absence of muscle tone.
The head does tend to fall forward when the body is in an upright
position, and the postcervical muscles are considerably more
powerful than the anterior
ones.
The second class lever is one where the load is placed between the fulcrum and the applied force. This type is highly efficient. As shown in Figure 4, the crushing force exerted by the
elevator muscles in the molar and premolar regions results from
the action of this type of lever, where the opposing temperomandiblar joint serves as the fulcrum. Since most chewing is
unilateral, this type of lever undoubtedly functions to increase
the masticatory forces in
humans.
In a third class lever the force is applied between the
fulcrum and the load, as shown in Figure 5. This is the least efficient arrangement to produce lifting force, but also the
fastest for a given rate of application of force. This lever system produces the incisive closing movements of the teeth; the
masseter muscle provides the applied force and the temperomandibular joint on the same side is the fulcrum. While inefficient
for biting, this type of lever may be indispensable for the rapid
movements of
speech.
The lever systems of the mandible are also found in the extremities, but the terminology is different. The closer muscles
flex the jaw, but they are the equivalent of limb extensors, because they resist the force of gravity. The openers act to extend
the jaw, but are like the limb flexors; that is, they protect
against harmful contacts or other noxious stimuli.
A principal difference between the movements of the mandible
and the extremities is that, approximately at the rest length of
the closer muscles, all movement stops abruptly as the teeth come
together. This "hard stop" has no counterpart in other joints of
the body, and necessitates a protective reflex to prevent damage
to the tooth surfaces. Moreover, when the teeth come together in
occlusion no lateral or anterior-posterior movement is possible.
This event is repeated hundreds of times a day as we swallow excess saliva from the oral cavity, and has profound effects on the
development of the muscles and of the occlusion itself.
The Muscles of Mastication
The muscles which serve to move the mandible with respect to
the rest of the head may be divided into four categories as
indicated in Table II. The jaw closers consist of the masseter,
temporalis and medial pterygoid muscles.
It is by the combined action of the suprahyoids and infrahyoids
(and the simultaneous inhibition of the closers) that the jaw is
opened.
As shown in Figure 1, the hyoid bone does not articulate with any
other bone; it is moved with respect to the cranium only by the
musculature. Lateral movements of the mandible are accomplished by
alternate contraction of the lateral pterygoid muscles. Protrusion
involves the simultaneous use of both lateral pterygoids.
It can be seen in Table II that the main innervation of the
masticatory muscles is from the trigeminal (V), facial (VII) and
cervical spinal (CI-CIII) nerves. The fifth nerve innervates both
openers and closers, and the seventh and cervical nerves only
openers.
Table II. Muscles of mastication and motor nerves
Cranial Nerve
Jaw Closers
Masseter
Temporal
Medial pterygoid
V
V
V
Jaw Openers
Suprahyoid
Ant. digastric
Post. digastric
Mylohyoid
Geniohyoid
V
VII
V
CI
Infrahyoid
Sternothyroid
Sternohyoid
Thyrohyoid
Omohyoid
CII, CIII
CI-CIII
CI
CI-CIII
Lateral Movements
Lateral pterygoid (unilateral)
V
Protrusion
Lateral pterygoid (bilateral)
V
Histochemistry of Muscle Fiber Types
The peripheral limb muscles are specialized according to
their functions: Muscles which typically exert low forces for long
periods of time have a higher proportion of Type I fibers, as
shown in Table III. Muscles which are used for continuous work at
high intensity have more Type IIA fibers, and muscles which exert
large forces in short bursts of activity have more Type IIB
fibers. This system was proposed by Dubowitz (1960) and standardized by Brooke and Kaiser (1970). In the leg muscles, the slow
fibers are typically used to resist gravity, in a continuous manner, while the fast IIB fibers in the arm muscles come into play
while throwing a ball. The proportion of IIA fibers in limb
muscles is increased by endurance training. Type I and Type IIA
fibers are the most resistant to fatigue.
Table III. Properties of different muscle fiber types
Designation Myosin Mitochondria Capillary
Speed
ATPase
density
low
high
high
slow
high
Type IIA
high
high
high
fast
high
Type IIB
high
low
low
fast
low
Type I
circulation
Fatigue
resistance
The myosin (or myofibrillar) ATPase is necessary for rapid
contraction of the muscle fibers. Mitochondrial enzymes permit
aerobic metabolism and increase resistance to fatigue. A high
density of capillary microcirculation assists with aerobic metabolism and confers a red color to the muscle.
By staining for myosin ATPase, it is possible to distinguish
between Type I and Type II fibers and measure the diameters of
the different fiber types, as shown in Figure 8. This shows the
biceps muscle, in which Type I fibers (lightly-staining) make up
about 37% and Type II (mostly IIB) (darkly-staining) about 63%
(Brooke and Engle, 1960). Type I fibers have a diameter of about
64 m, and Type II about 72 m.
In the masseter muscle, however (Fig. 9), the Type I fibers are
about 40-50 m, and the Type IIB only 20 m. There are also some
intermediate-staining fibers, which look like Type II but whose
function is unclear (Rowlerson, 1990). Interestingly, the Type I
fibers outnumber the Type II in this muscle (Eriksson and
Thornell, 1983). This is probably related to the fact that, with
the torso in an upright position, the masseter continually opposes
the force of gravity.
The typical jaw opener muscle, such as the digastric, shows
a fiber composition almost indistinguishable from that of an arm
or hand muscle (Eriksson et al., 1982). This may be correlated
with the absence of contraction of the digastric except during
depression of the mandible.
We shall see further correlation of these fiber types with
motoneuron sizes, and the roles of different muscles in jaw
reflexes and mastication in the following chapters.