Craniofacial Muscles

12 Tongue Structure and Function
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unit, and formation of whole-wavelength S shapes requires two. As discussed,
motor unit territories have been localized only coarsely—to anterior, middle, or
posterior regions and to left/right sides, but this localization is all that is required to
achieve the many tongue shapes used during routine behavior. It is also possible that
motor unit territories are much smaller and could allow either sharper ( Z -like) or
more serpentine shapes.
12.5 Tongue Muscle Fiber Biochemistry
12.5.1 Background
Studies in appendicular and cranial muscles demonstrate a relationship between
MyHC isoform and muscle fi ber contractile properties. For example, among muscle
fi bers that uniformly express one of the four conventional MyHC, shortening velocity
progresses from slowest to fastest in the order: MyHCI, MyHCIIA, MyHCIIX,
MyHCIIB. Although shortening velocity is one of the most dramatic differences
among fi bers, MyHC isoform is highly correlated with a range of specializations
that include calcium kinetics, glycolytic capacity, and mitochondrial content.
Experimental models indicate that fi ber type is plastic and can be altered to meet
functional demands, most notably duty cycle, suggesting that muscle protein expression
is regulated to provide speci fi c contractile or metabolic properties. Muscle fi ber
contractile diversity can additionally be achieved by hybridization of multiple
MyHC in single fi bers, thereby creating fi bers with intermediate properties in proportion
to the prevalence of constituent MyHC.
In mammals, including human appendicular muscles, single extrafusal fi ber contractile
diversity is typically achieved by homogeneous expression of MyHCI,
MyHCIIA, MyHCIIB, or MyHCIIX and only limited hybridization of MyHC isoforms
(primarily MyHCII) (as opposed to single intrafusal fi bers found in muscle
spindles). Head and neck muscles diverge from this appendicular norm in two
respects. In addition to the four MyHC isoforms expressed in limb and body skeletal
muscle, some adult head and neck muscles express developmental isoforms
MyHCembryonic and MyHCneonatal and additional isoforms MyHCalpha-cardiac,
MyHCextraocular, MyHCmasticatory, and MyHCslow tonic, which are absent from
normal appendicular muscles. Further, some head and neck muscles contain many
hybrid fi bers, including fi bers with MyHCI-MyHCII hybridization and MyHC
hybridization where single fi bers contain mixtures from all three categories of
MyHC isoforms. MyHC hybridization is most extensive in masticatory and extraocular
muscles where single fi bers may contain fi ve MyHC (Yu et al. 2002 ; McLoon
et al. 2011 ) .
Many head and neck muscles, but not tongue muscles, undergo a different
developmental path than axial and appendicular musculature. Limb, body, and
tongue muscles originate from the embryonic somites and are dependent on
Pax3/7 for differentiation (Buckingham et al. 2003 ) . Head and neck muscles
largely originate from the branchial arches of the embryonic somitomeres and