Craniofacial Muscles

8 Masticatory Muscle Response to Neuromuscular Diseases and Speci fi c Pathologies
135
analysis is complicated by confounding factors in an aging population, including
decreased nutrition and altered hormones, and aging of joints and teeth (Hatch et al.
2001 ) . Studies suggest that masticatory muscles undergo changes in myosin composition
and synapse remodeling, but the extent to which this impacts the functioning
of the muscles themselves is unclear.
The masticatory muscles have anatomically distinct regions, and age-related
changes are often region-speci fi c. The super fi cial and posterior regions of masticatory
muscles are primarily composed of fast myo fi bers in order to generate the
quick force needed when biting. The deep anterior aspects of the muscles contain
more slow-type motor units, which likely allow for fi ne control of muscle force for
chewing and biting behaviors. In the aging masticatory muscles, pronounced phenotypical
changes occur which includes changes in fi ber type and myosin heavy
chain (MyHC) isoform composition. These changes are muscle and region speci fi c.
The masseter muscle has a large number of slow myo fi bers containing MyHC type 1.
With aging, myo fi bers expressing the fast MyHC isoforms increase in number with
a proportionate loss of myo fi bers expressing the slow MyHC isoform. This is interesting
and contrasts with aging limb muscle, e.g., the biceps brachii, which shows a
shift towards a slower phenotype with aging (Monemi et al. 1999a, b ) . Further, aged
masseter muscle increases the expression of fetal MyHC and hybrid fi bers which
co-express more than one MyHC isoform (Monemi et al. 1996, 1999a ) . In the pterygoids,
a similar trend occurs. The pterygoids do not have many fi bers that are solely
fast and express MyHC type IIA, but the number increases signi fi cantly in aging
muscle (Monemi et al. 2000 ) . In addition to changes in MyHC isoform expression,
cross-sectional area of the masticatory muscles may also decrease with aging
(Newton et al. 1987 ; Monemi et al. 1999b ) . It is unknown if the changes in MyHC
isoform content are due to re-innervation by other motor neurons or due to changes
in the activity pattern of the original innervating motor neurons.
Animal studies have suggested that changes may occur in the innervation itself.
The nerve innervating the masseter in aged cats has decreased axon diameters and
disrupted myelin, which corresponds to a decreased velocity of action potentials
(Chase et al. 1992 ) . Aged mice also show decreased axon diameter and nerve terminal
areas (Elkerdany and Fahim 1993 ) . Speci fi c changes are seen in the neuromuscular
junctions of aged mice, including nerve terminal area, perimeter, and length
decreases, while branching of the nerve increases. These changes suggest increased
degeneration and regeneration of the nerve terminals in aging masticatory muscles
in these rodents.
How the potential changes in nerve and myo fi ber phenotype potentially correlate
with changes in use of the masticatory muscles with aging is unclear. Many studies
have demonstrated that elderly adults do not lose masticatory function with age
(Hatch et al. 2001 ) and maintain adaptability of their muscles (Peyron et al. 2004 ) .
Bite force decreases with aging (Bakke et al. 1990 ; Hatch et al. 2001 ) , but could be
due to other components of the masticatory system. The literature is mixed on this
issue. For example, EMG activity changes in aged masticatory muscles. The transition
from adult to elderly may show no (Peyron et al. 2004 ) or only a slight effect in
masticatory muscle EMG values (Cecílio et al. 2010 ) . Other studies have shown