Craniofacial Muscles

12 Tongue Structure and Function
221
and MyHC phenotype disparity might re fl ect differential patterns of MU recruitment
between intrinsic and GG tongue muscles. Further studies with equivalent IHC
methods are required to con fi rm MyHC phenotype differences between intrinsic
and extrinsic muscles.
Few studies have investigated MyHC phenotype or fi ber type with respect to
extrinsic muscle architecture. By ATPase, a greater prevalence of Type II vs. Type I
fi bers was reported in dog anterior-oblique compared to horizontal GG regions (Mu
and Sanders 1999 ) . In the human, the oblique GG contains relatively more MyHCIIA
and less MyHCI than the horizontal GG (Daugherty et al. 2012 ) . In contrast, greater
prevalence of faster isoforms was reported in the posterior vs. anterior GG of the rat
(Volz et al. 2007 ) . Greater prevalence of “faster” myosin isoforms in anterior vs.
posterior intrinsic tongue muscles of the human and macaque is demonstrated by
ATPase and separation SDS-PAGE, with speci fi c type/MyHC prevalence varying
by region and muscle (dePaul and Abbs 1996 ; Stal et al. 2003 ; Granberg et al. 2010 ) .
A general anteroposterior disparity in “fast” (anterior) vs. “slow” (posterior) fi ber
composition might re fl ect relatively greater participation of anterior tongue regions
for feeding and oral transport tasks and of posterior tongue regions for respiration
and maintenance of airway patency.
By IHC only occasional fi bers are positive for the developmental MyHC isoforms,
MyHCalpha-cardiac, and MyHCslow tonic in macaque and human tongue muscles
(0–3% of total fi bers in any individual; Sokoloff et al. 2010 ; Granberg et al. 2010 ) .
Thus, mammalian tongue muscles studied to date differ from some head and neck
muscles, which have appreciable expression of developmental and additional MyHC
isoforms.
The bases for the limited expression of the developmental and additional nonlimb
skeletal muscle MyHC in the human tongue are not known. IHC studies of the
human HG and SG suggest that MyHCneonatal is primarily localized to fi ber endings,
possibly re fl ecting fi ber remodeling at the myotendinous junction (Sokoloff
et al. 2010 ) . Appendicular muscles may also express MyHCneonatal at fi ber terminations
(Rosser et al. 1995 ) , and expression of MyHCneonatal at fi ber terminations
may account for low levels of MyHCneonatal reported in many head and neck muscles
by PCR and IHC (see also Tellis et al. 2004 ) . The absence of signi fi cant levels
of developmental MyHC suggests however that persistent muscle fi ber remodeling
is not a feature of human tongue muscles, even in very old age.
12.5.3 Capillarization and Oxidative Metabolism
of Tongue Muscles
In appendicular muscles, there is a general relationship between fi ber type/MyHC
phenotype, capillarization (capillary number/mm 2 fi ber), and oxidative metabolism
such that type I fi bers tend to have a higher capillarization and higher oxidative
capacity than type II fi bers. However, capillarization, mitochondrial density, and
oxidative capacity of muscle fi bers are highly plastic and change with age and use.