Craniofacial Muscles

9 Structure and Function of the Laryngeal and Pharyngeal Muscles
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normal swallow (Burnett et al. 2003 ). When stimulation was applied bilaterally to
the thyrohyoid alone approximately 50% of normal laryngeal elevation and 80% of
normal speed of elevation was found (Burnett et al. 2003 ). Regarding temporal
factors, thyrohyoid activation was closely linked with laryngeal rise and fall with its
activation occurring an average of only 52 ms prior to laryngeal elevation (Burnett
et al. 2005 ) .
Electrical activity of the strap muscles during swallowing as measured with surface
EMG demonstrated different average electrical activity depending on swallowing
parameters (Vaiman et al. 2004 ) . For instance, electrical activity during continuous
drinking of 100 mL of water was the smallest in magnitude, followed by activity
during a single saliva swallow (Vaiman et al. 2004 ) . However, electrical activity during
single water swallows and fi xed 20 mL water swallows was stronger and similar
to one another in magnitude (Vaiman et al. 2004 ) . Interestingly, electrical activity of
the infrahyoid area has been found to decrease with age (Vaiman et al. 2004 ) .
9.2.3 Summary
Extrinsic muscles of the larynx contribute to laryngeal position during swallowing
and pitch change by altering the distance among laryngeal cartilages. Extrinsic
muscles are vital to promoting airway protection through laryngeal elevation and
exhibit different levels of electrical activity depending on age and the type of swallow.
Reports about the correspondence between pitch lowering/raising and extrinsic
muscular activity are con fl icting. Likewise, methodologies for examining strap
muscle functional properties vary widely from manipulating muscle position in
cadavers to use of electrical stimulation in human or animals or use of EMG in
humans only. Thus, while the role of the extrinsic musculature is broadly known,
the speci fi c function of each individual extrinsic muscle is not completely clear.
9.3 Muscles of the Pharynx
There are many muscles of the pharynx throughout the extent of the nasopharynx,
oropharynx, and the hypopharynx that have a primary role in swallowing function.
For instance, muscles within the nasopharynx (tensor palatine, levator palatini) function
to elevate the soft palate and close the nasopharynx to the bolus, while altering
pharyngeal pressures to receive and propel the bolus. Movement of the pharyngeal
wall toward the soft palate as well as elevation and anterior displacement of the larynx
during the swallow is accomplished by muscles such as the mylohyoid, palatopharyngeus,
salpingopharyngeus, and stylopharyngeus (Table 9.1 ). Increased velopharyngeal
pressure relative to baseline results from these actions and has been recorded
during a swallow using high resolution manometry (Hoffman et al. 2010 ) . In combination
with reduced hypopharyngeal pressure during the swallow, a pressure gradient
is established that works in favor of bolus fl ow toward the esophagus.