Craniofacial Muscles

330 L.K. McLoon and F.H. Andrade
17.6 Aging
Aging also affects the craniofacial muscles differently than limb skeletal muscles.
Again, while some alterations from normal have been described, relative to the
signi fi cant atrophy that can be seen in aging skeletal muscle, the craniofacial muscles
remain relatively normal. We will give some examples (Table 17.3 ), but this is
not meant to be a complete summary of the work in this fi eld.
As might be expected from the differential susceptibility to disease, aging affects
each craniofacial muscle group differentially. Extraocular muscles again show the
fewest age-related changes both morphologically and functionally (Yang and
Kapoula 2008 ; Valez et al. 2012 ) , although some changes in connective tissue density
and an increase in mitochondrial defects occur (McMullen et al. 2009 ) . Masseter
also shows constancy in muscle structure and function during aging (Norton et al.
2001 ) . Laryngeal muscles display fewer age-related changes compared to limb
muscle, but still show elevation in connective tissue, some increased variation in
myo fi ber cross-sectional area, changes in neuromuscular junction structure (Connor
et al. 2002 ), plus less force and slower shortening velocity (Kersing and Jennekens
2004 ; McMullen and Andrade 2006, 2009 ) . In facial muscles, while the skin undergoes
signi fi cant loss of elastin, the facial muscles themselves are normal (Lee et al.
2011 ) . Aging changes in tongue muscles are quite interesting; aging retrusive tongue
muscles do not show a decrement in overall contractile speed or force production,
but signi fi cant changes in forces is seen in aging protrusive tongue muscles (Nagai
et al. 2008 ; Connor et al. 2009 ) . The basis for this difference in unclear. However,
all of the changes in craniofacial muscles are relatively minor compared to those
seen in aging limb skeletal muscle where signi fi cant atrophy, fat conversion, fi brosis,
and denervation changes can occur.
17.7 Conclusions and Future Studies
It is clear from the collective chapters in this book that one cannot draw conclusions
about structure and function of craniofacial muscles from the study of limb skeletal
muscles. Their behavior cannot be predicted by studies of limb skeletal muscle;
often they change in diametrically opposed ways in the presence of myogenic signaling
factors (Tzahor et al. 2003 ) or during aging (Monemi et al. 1999 ) . The craniofacial
muscles appear to represent one end of the continuum of skeletal muscle
types in the body, with the relatively more homogeneous soleus at one end and
extraocular and laryngeal muscles at the other. Except for tongue muscles, from
their onset craniofacial muscles are derived from non-segmented, non-somitic
cranial mesoderm, with different genes controlling their formation compared to
those that direct the formation of somitic muscle. The mature muscles each display
a relatively unique set of contractile and metabolic proteins, which are mirrored in
the collective physiological and contractile differences seen in the craniofacial