Craniofacial Muscles

17 Comparison of the Craniofacial Muscles: A Unifying Hypothesis
327
Table 17.2 Myosins and muscle contractile properties
Muscle type Myosins
Unloaded shortening
velocities (Vo) (ML/s)
Twitch contraction
times (ms)
Limb 4
a
2.2–3.7 14–22
Tongue 8, but majority are same ?
b
8–33
as limb
Laryngeal 6
a
4–5 3–7 c
Facial 4
d
8.5–33
Masticatory 8 including masticatory
e
2.2 13–32
speci fi c
Extraocular ³ 9 including EOM speci fi c 19 f 5.27
ML/s muscle lengths per second
? indicates no one has published this measurement in tongue muscle
a
Larsson and Moss ( 1993 ) and Sciote et al. ( 2003 )
b
(Sokoloff ( 2000 )
c
Hinrichsen and Dulhunty ( 1982 )
d
Lindquist ( 1973 )
e
Toniolo et al. ( 2004 )
f
Asmussen et al. ( 1994 ) and McLoon et al. ( 2011 )
superfast MyHC in the jaw-closing muscles (Hoh 2002 ) and the EOM-speci fi c
MyHC in the EOM and variably in the laryngeal muscles (Wieczorek et al. 1985 ;
Shiotani and Flint 1998 ; Toniolo et al. 2005 ) . These diverse MyHCs are combined
in single fi bers, and this complexity of MyHC isoform co-expression results in
myo fi bers with a greater range of shortening velocities (Morris et al. 2001 ; Sciote
et al. 2002 ; McLoon et al. 2011 ) . There is also a mismatching of myosin light chains
(MLC) with the various MyHC isoforms (Stål et al. 1994 ; Bergrin et al. 2006 ; Bicer
and Reiser 2009 ) . Thus, the craniofacial muscles demonstrate that one motor neuron
does not connect to myo fi bers of only one MyHC or MyLC isoform, but rather
controls the contraction characteristics of groups of myo fi bers with different MyHC
compositions (Kwa et al. 1995 ) . This is hypothesized to result from their distinct
embryological origins.
The masticatory, laryngeal, and extraocular muscles in particular display
extremely fast shortening velocities, the fastest of any mammalian skeletal muscles
(Table 17.2 ; Close and Luff 1974 ; Asmussen et al. 1994 ) , although it should be
pointed out that the masticatory muscles also have myo fi bers that are slower than
those seen in limb muscle (Morris et al. 2001 ) . EOM and laryngeal muscles also
contain slow tonic MyHC along a portion of their length (Jacoby et al. 1989 ; Hoh
2005 ) . As a group, the craniofacial muscles are fatigue resistant (Asmussen and
Gaunitz 1981 ; Fuchs and Binder 1983 ; Prsa et al. 2010 ) . Fatigue resistance in these
muscles is associated with the unusual co-expression of succinic dehydrogenase
(SDH) and glycerophosphate dehydrogenase (GPDH) in single myo fi bers, enzymes
involved in oxidative and glycolytic energy pathways, respectively (Asmussen et al.
2008 ) . These complex fi ber types and specialized contractile characteristics increase
the functional repertoire of these muscles, presumably critical for their ability to
adapt quickly to changing physiological needs.