Craniofacial Muscles

208 A. Sokoloff and T. Burkholder
The most obvious contrast between conventional skeletal muscle systems and
the tongue is the lack of rigid structural elements. Tongue movement is a consequence
of deformation of the constant-volume soft-tissue tongue body, movement
of the tongue body relative to the head and neck by tongue muscles with extrinsic
attachment, and movement of attached head and neck structures (e.g., mandible or
hyoid bone). Changes in tongue shape are enabled by arrangement of tongue muscle
fi bers in multiple axes and innervation by hypoglossal nucleus motoneurons
(approximately 6,000 hypoglossal nucleus motoneurons in the rat; 15,000 in the
human; Arvidsson and Aldskogius 1982 ; O’Kusky and Norman 1995 ) . Tongue
movements are accompanied by spatially diverse tongue body deformations indicating
spatially complex patterns of muscle fi ber and motor unit activation (motor
unit, MU: a motoneuron and the muscle fi bers it innervates).
12.2 Tongue Muscular Anatomy
12.2.1 Muscle Anatomy and Innervation: Classical Description
In the appendicular musculature, muscles are de fi ned anatomically as discrete populations
of muscle fi bers which are substantially separate from other fi ber populations.
Activation of contractile material anywhere within many appendicular muscles
has a similar mechanical action due to the constraints imposed by discrete muscle
attachments and by skeletal and ligamentous structures. Although considered as
separate entities, muscles frequently share common tendons, such as the gastrocnemius
and the soleus, and are linked by intermuscular connective tissue that limits
their mechanical independence (Maas et al. 2001 ) . Some muscles with large distributed
attachments may have very divergent regional functions, however, and there
can even be some variability in mechanical action within fusiform muscles (Chanaud
et al. 1991 ; Carrasco and English 1999 ) .
Nonetheless, locomotion and other tasks of the appendicular musculature are well
described by rigid segments connected at discrete joints, and simplifying contractile
material into discrete units has greatly facilitated the analysis of locomotion and understanding
of the control structures involved. Treating appendicular muscles as functionally
discrete structures is an illustrative simpli fi cation that facilitates analysis.
The musculature of the tongue is classically divided into extrinsic muscles,
which have their origins on bony structures outside the tongue body, and intrinsic
muscles, whose fi bers exist entirely within the tongue body. 1 The extrinsic muscles,
genioglossus (GG), hyoglossus (HG), palatoglossus (PG), and styloglossus (SG)
1
Dissection, histological, and MRI investigations have produced detailed descriptions of tongue
muscle organization in relatively few mammal species (primarily cat, dog, human, and rat). Primary
descriptions in the human are those of Abd-El-Malek ( 1939 ) and Gaige et al. ( 2007 ) for the adult
and Barnwell ( 1977 and related) for the fetal tongue. For organization of tongue musculature in
non-mammals see Herrel et al. ( 2001 ) and Nishikawa et al. ( 1999 ) .