Craniofacial Muscles

158 L.A. Vinney and N.P. Connor
9.3.1 Structure, Function, and Muscle Fiber Types
In general, the majority of the pharyngeal muscles (Table 9.1 ) receive motor
innervation from the pharyngeal plexus of the vagus nerve (CNX). There are a few
exceptions, including the tensor veli palatine (innervated by the medial pterygoid
nerve, a branch of the mandibular nerve branching from the trigeminal CNV),
stylopharyngeus (the only muscle receiving motor innervation by the glossopharyngeal
nerve CNIX), and palatopharyngeus (innervated by the pharyngeal plexus of
the vagus CNX and the spinal accessory nerve CNXI). Although these are commonly
noted innervation patterns at the most basic level, reports vary as to the
speci fi c branches of the cranial nerves supplying certain pharyngeal muscles (Hixon
et al. 2008 ) . We will highlight one such controversy related to the cricopharyngeus
later in this section.
Mylohyoid muscle fi ber characteristics in human samples were reported in one
recent study as having a hybrid composition of unusual myosin properties, such as
embryonic, neonatal, a -cardiac, and slow-tonic, in combination with typical skeletal
muscle myosins including type I, Type IIA, and Type IIX (Ren and Mu 2005 ) .
Notably, these hybrid fi bers comprised 84% of the total number of fi bers analyzed
in adult human samples. Thus, the muscle fi ber type composition in the mylohyoid
muscle is unique and distinct from skeletal muscles found in the extremities and is
more similar to those found in other cranial muscles. Ren and Mu ( 2005 ) interpreted
their fi ndings as suggestive of specialization of this muscle for chewing, swallowing,
and breathing, and the need for postural stability and endurance during these
critical functions.
9.3.2 The Upper Esophageal Segment
The terms cricopharyngeus (CP) and upper esophageal sphincter (UES) are often
used interchangeably, but the CP muscle is really one muscular component of the
UES; thus, these terms are not synonymous. The CP along with the inferior pharyngeal
constrictors and inferior cervical esophageal muscle make up the UES.
Although there has been some controversy over the CP’s role in UES function
(Goyal et al. 1993 ) , the CP is typically thought to facilitate the contraction and
relaxation patterns that lead to the pressure changes exhibited by the UES as a
whole. In fact, the CP is the only component of the UES which contracts and relaxes
during tasks associated with UES opening, including swallowing, emesis, and
belching (Belafsky 2010 ; Kahrilas 1997 ) . The terminology UES can be used interchangeably
with the pharyngoesophageal segment (PES) (Belafsky 2010 ) . Both
refer to the same area of the esophagus, although the UES is de fi ned as the “2–5-cm
high-pressure zone located between the pharynx and esophagus” that is “measured
by manometry” (Belafsky 2010 ) . On the other hand, “the PES refers to the anatomic
components that make up the high-pressure zone” (Belafsky 2010 ) .