Craniofacial Muscles

10 Motor Control and Biomechanics of Laryngeal and Pharyngeal Muscles
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Other muscles that dilate the upper pharynx include the levator veli palatini
and the hyoid muscles (mylohyoid, thyrohyoid, hyoglossus, and geniohyoid)
(Table 10.2 ). The levator veli palatini elevates the velum, closing off the velopharyngeal
port to prevent entry of the bolus into the nasal cavity during swallowing
and air fl ow during all speech sounds except the nasal sounds (/m/,/n/,/ng/). Elevating
the velum also serves as an upper airway dilator as it opens the hypopharynx, allowing
an increased opening for air fl ow into the hypopharynx.
During swallowing, changes in upper airway shape and pressures assure rapid
and complete passage of the bolus (food or liquid) through the oral cavity, hypopharynx,
and upper esophageal sphincter into the esophagus. Oral transit starts with
closure of the lips and jaw to prevent spillage, collection of the bolus into one mass,
and a stripping motion of the tongue blade against the roof of the mouth to move the
bolus backwards into the posterior pharynx. Prior to passage of the bolus through
the pharynx, the hyoid moves forwards and upwards to dilate the hypopharynx to
allow entry of the bolus while closing the upper airway by tucking the larynx forwards
and upwards under the epiglottis. The epiglottis becomes inverted over the
entry to the laryngeal vestibule by posterior action of the tongue over the hyolaryngeal
complex as it elevates under the tongue, squeezing the epiglottis downwards
(Fig. 10.1a ). Elevation of the hyo-laryngeal complex involves contraction of
the suprahyoid muscles pulling the hyoid forward and upward (geniohyoid,
mylohyoid, hyoglossus). Simultaneous contraction of the thyrohyoid is required to
elevate the thyroid cartilage to the hyoid as the hyoid is raising to close the laryngeal
vestibule reducing the risk of bolus entry into the airway (Fig. 10.1a ). Passage of the
bolus through the pharynx begins with pressure of the posterior tongue towards the
posterior pharyngeal wall while dilating the anterior pharynx though anterior elevation
of the hyo-laryngeal complex. Subsequent sequential squeezing actions of the
pharyngeal constrictors from superior to middle and inferior push the bolus downwards.
Opening of the upper pharyngeal sphincter is achieved through two actions,
anterior–superior movement of the larynx stretching the cricopharyngeus (Fig. 10.1b )
and re fl exive relaxation of the cricopharyngeus.
10.5 Pharyngeal Innervation
The human pharyngeal constrictor muscles have two distinct layers, an outer fast
layer innervated by the vagus and a slow acting inner layer innervated by the
glossopharyngeal nerve (Mu and Sanders 2007 ) . Mu and Sanders hypothesized that
the slow acting inner layer maintains stiffness in the pharyngeal wall for dilation for
respiration while the fast outer wall is essential for rapid and forceful contraction to
increase pressure on the bolus and push it downwards in the hypopharynx towards
the upper esophageal sphincter during swallowing. The inferior pharyngeal constrictor
also has a two layered structure but has rostral and caudal compartments
with the caudal portion having immunohistological characteristics similar to the
cricopharyngeus muscle. For that reason, it has been proposed that the inferior