Craniofacial Muscles

12 Tongue Structure and Function
223
age-related muscle decline, although it does not mitigate motoneuron loss or prevent
sarcopenia. Hypoglossal motoneurons also receive inputs from numerous central and
peripheral sources, and it is possible that this rich synaptic milieu supports hypoglossal
nucleus motoneurons during dysfunction in any one projection system.
12.7 Conclusions
The architectural and neural specialization of the tongue re fl ects its unique lack of
skeletal constraints. Deformations of the tongue during oromotor behaviors are varied
and are not well described by activation of classically de fi ned muscles. Tongue
muscle architecture is complex and tongue motor units occupy limited territories,
enabling localized contraction of fi bers with different orientation that is needed to
achieve the dimensional control required by the muscular hydrostat model. The
extent to which the nervous system actually uses the fi ne-grained control structure
during routine behavior is not yet clear.
The tongue appears to be composed of conventional skeletal muscle fi bers with
speci fi c structural and control adaptations that re fl ect an unusually high degree of
daily activity and the absence of skeletal constraints on motion. In many species
tongue muscles are comprised principally of two conventional MHC isoforms. In
the rat, tongue motor unit contraction times are similar to those of other fast appendicular
motor units but produce much less force likely re fl ecting a low number of
muscle fi bers per motor unit.
Compared to appendicular muscles, tongue motor units have high duty cycles,
whether constitutively active to maintain airway patency or phasically active during
respiration or swallowing. Highly localized and small motor units may require high
fi ring rates for meaningful force production. Persistent activation of many motor
units may require high mitochondrial content and capillarization and protect tongue
muscles from typical aging dysfunction.
Acknowledgements We thank Audrey Jernigan for illustrations. This work was supported by
grant DC005017 from the National Institute on Deafness and Other Communication Disorders .
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