Craniofacial Muscles

7 Motor Control of Masticatory Muscles
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7.2.1 Masticatory Muscles Mechanics
The generation of mandibular movement is brought about by active jaw muscle
tensions and passive soft tissue tensions. The anatomy of the jaw muscles is extraordinarily
complex (e.g., Hannam 1994 ; Hannam et al. 2008 ) . The motor units within
the masseter, temporalis, and medial pterygoid muscles (the jaw-closing muscles)
are arranged in a highly complex manner within each muscle. For example, masseter
muscle fi bers on the whole do not run from the zygomatic arch to the ramus but
rather there are small compartments of short fi bers divided by aponeurotic sheaths
and arranged in a so-called pennate (compartment) manner (Fig. 7.1 ). Therefore,
when motor units on one side of a compartment contract, forces can be generated at
an angle (the pennation angle) to the long axis of the muscle, with a force vector
(i.e., magnitude and direction of force) at an angle to the force vector that would be
generated if muscle fi bers passed directly from the zygomatic arch to the ramus
without pennation. These complexities of muscle- fi ber architecture, together with
selective activation of certain motor units within the muscle, provide a wide range
of directions with which forces can be applied to the jaw and thereby contribute to
the enormous range and sophistication of jaw movements that are possible. When
generating a particular movement of the jaw, the sensorimotor cortical regions that
drive voluntary movements are not organized in terms of speci fi c muscles to activate.
Rather, they send a command signal to activate those motor units, in whatever muscles
are available, that are biomechanically best suited to generate the force vector
(i.e., magnitude and direction of force) required for that particular jaw movement
(e.g., Widmer et al. 2003 ) .
7.2.2 Masticatory Motor Function Mechanics
The active and passive tensions mentioned above generate a range of jaw motions
and stresses, strains and forces throughout the various components (e.g., teeth, temporomandibular
joints [TMJs], bone) of the masticatory system. Anatomical and
functional studies in experimental animals and humans have documented some of
these variables (e.g., Herring 2007 ) . Because some of these approaches are highly
invasive, mathematical modeling has been used to clarify structure–function relationships.
These models can range from relatively simple 2D static analyses that
tend to focus on peak bite force, to more complex 3D models based on rigid body
mechanics, rigid body meaning that the jaws undergo no deformation. Some models
are becoming very sophisticated (Hannam et al. 2008 ; Curtis 2011 ) . For example,
Artisynth ( http://www.magic.ubc.ca/artisynth/pmwiki.php ; Hannam et al. 2008 ) is
a 3D biomechanical computer simulation that models the vocal tract and upper
airway and is capable of articulatory speech synthesis. The accuracy with which
these models re fl ect normal function, however, is dependent on the sophistication
and range of variables (e.g., muscle size, muscle site, muscle angulation, muscle