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Muscles of Mastication
Anatomically the muscles of mastication can be divided into
simple and complex muscles (Hannam 1994,1997). The lateral
pterygoid and the digastric muscles are counted among
the simple muscles. These muscles work through a favorable
lever arm relative to the joint and so do not have to
produce a great deal of force to bring about functional
mandibular movements. The parallel muscle fibers in these
muscles have their sarcomeres arranged in series, and these
are responsible for the adequate muscle contraction. During
contraction, the diameter of each muscle increases and is at
its greatest near the midpoint of the muscle.
In contrast, the complex muscles include the temporal,
masseter, and medial pterygoid muscles with their many
aponeuroses and varying sizes. During function the aponeuroses
can shift and become deformed (Langenbach et al.
1994). The muscle fibers in this group run obliquely and
increase their angle to one another during contractions. A
complex muscle can produce a force of approximately
30-40 N per cm2 of cross-section (Korioth et al. 1992, Weijs
and van Spronsen 1992). The orientation of the muscle
fibers and their facultative activation during various
mandibular movements is one of the reasons that muscle
symptoms can be reproducibly provoked by loading in one
certain direction but not in others. Although there are recur-
Function
Movement Stabilization
Progress! ve adaptation
HypD-/hypertonicity»hypo-/hypertrophy
Muscle shortening
Regressive adaptation
Inflammation Rupture
Ossification
Muscles of Mastication 33
rent principles in muscle architecture (Hannam and McMillan
1994), variations in the areas of muscle attachment and
differences in intramuscular structure have an effect on
craniofacial development (Eschler 1969, Lam et al. 1991,
Tonndorf 1993, Holtgrave and Muller 1993, Goto et al. 1995).
The motor units in the muscles of mastication are small and
seldom extend beyond the septal boundaries (Tonndorf and
Hannam 1997). The "red" muscle fibers (with higher myoglobin
content) contract slowly. They maintain postural
positions and are slow to become fatigued. The "white"
fibers (with lower myoglobin content) have fewer mitochondria
and can contract more rapidly, but they fatigue
sooner because of their predominantly anaerobic
metabolism. The muscles of mastication are composed of
varying mixtures of fibers of types I, IIA, IIB, IIC, and IM
(Mao et al. 1992, Stal 1994).
Next, the four chewing muscles proper (temporal, masseter,
medial pterygoid, and lateral pterygoid) and the suprahyoid
and perioral musculature will be described in preparation
for the clinical examination that will be addressed later.
77 Muscles of mastication
Drawing of the muscles of mastication.
In the narrowest sense these
include only the temporal, masseter,
medial pterygoid, and lateral
pterygoid muscles. The suprahyoidal
musculature is also shown
here because it is of interest in the
diagnosis and treatment of functional
disturbances. The sternocleidomastoid
muscle is not included
here because it belongs to the musculature
of the neck.
78 Function and structural
adaptation of the muscles of
mastication
Antagonistic muscle activity serves
not only to execute mandibular
movements, but also helps stabilize
the joints. Functional demands can
bring about changes in tonus,
response to stimuli, and muscle
length. Adaptation depends upon
the combination of fibers present.
Chronic overloading may lead to inflammation,
ruptures, or ossification.