Craniofacial Muscles

13 Tongue Biomechanics and Motor Control
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enough sizes to fi t through the pharynx and esophagus. All of the tongue muscles
are active in manipulating the food into the correct position for the teeth. The SG
and the PG lifts the posterior tongue to close off the pharynx while the intrinsic
muscles manipulate the bolus. If the food moves too quickly to the pharynx or a
tongue depressor depresses the tongue and stimulates the oral pharynx, the gag
re fl ex is stimulated. The vagus nerve senses the stimulation and makes a monosynaptic
connection with the hypoglossal nerve.
The pharyngeal phase propels the bolus to the esophagus while coordinating
inhibition of respiration, closing the palatopharyngeal isthmus, and constricting the
larynx. The genioglossus and the anterior digastric may be active initially, followed
by the mylohyoid, stylohyoid, and geniohyoid, working to lift the hyoid anteriorly
(Ono et al. 2009 ) . Finally, the posterior tongue is depressed to form the anterior wall
of the pharynx as the pharyngeal muscles take over in the effort to propel the bolus
toward the esophagus.
It is also important to consider the role of the mandible during eating and speech.
Obviously, several of the muscles attach to the mandible and so need the stability of
the bone on which to move the tongue. The temporomandibular joint facilitates the
synchronicity of anteroposterior movement of the mandible and tongue but limits
horizontal movement. The hyoid, on the other hand, moves with the tongue to produce
large amplitude horizontal movements.
There has been a lot of exploration of a masticatory CPG focused on the medial
bulbar reticular formation, with most of the studies using in vitro isolated mammalian
brainstem preparations (Katakura et al. 1995 ; Nakamura et al. 1999 ) . It became
apparent that sucking rhythm generators coupled with cranial nerves V, VII, and XII
were initially necessary to coordinate suckling during late gestation and infancy
(see Barlow 2009 for review). At birth, the system has to develop an interaction
between the swallow and a protection of the airway. Once again, sensory experience
is important during this critical period to develop a brainstem pattern for swallowing
pro fi ciency. As the infant develops, the tongue begins to discern the size of
the food bolus and how to move it to the back of the tongue for swallowing. The
more mature patterns of mastication involve the rhythmic opening and closing of
the jaws but rhythmical bursts also occur in hypoglossal motoneurons (Dellow and
Lund 1971 ) .
13.5.4 Speech
Manipulation of the tongue during speech is highly variable depending on the language,
acoustic feedback, and learned pattern of speech. Therefore the movements
of the muscles are extremely complex and only somewhat follow a pattern. Models
of the tongue and vocal tract have been enhanced by computer power. Speci fi cally,
the models have tried to relate muscle recruitment and tongue shape (Perkell and
Zandipour 2002 ) . More recently 3D biomechanical models of the tongue and oral
cavity have provided a view of the activity of parts of muscles rather than whole