Craniofacial Muscles

230 M.S. Shall
13.2 Biomechanics
Dr. Sokoloff described the anatomy of the muscles in the previous chapter; Figure 1
in that chapter illustrates their orientation. The body of the tongue is composed of
three pairs of “intrinsic” muscles and four pair of “extrinsic” muscles. By de fi nition,
the vertical, transverse, and longitudinal intrinsic muscles originate on muscle fi bers
and insert on other muscle fi bers or the connective tissue within the body of the
tongue. The extrinsic genioglossus (GG), styloglossus (SG), hyoglossus (HG), and
palatoglossus (PG) muscles originate on bone (genial tubercle of the mandible, the
styloid process, the hyoid bone, and the lateral palate, respectively) and insert onto
the base of the body of the tongue. There has been debate on the intrinsic/extrinsic
terminology since muscles from both groups interdigitate and work together on
many actions.
The jaw and hyoid positions must be part of the pattern that strategically places
the entire tongue for the appropriate muscle movements for speech or mastication.
The masseter muscle is important to position the mandible and is examined thoroughly
in Chaps. 6 , 7 , and 8 . As the jaw opens to accept food, the mylohyoid and
geniohyoid contract to move the hyoid bone forward relative to the mandible to
elevate and close the larynx. Like the GG, the geniohyoid originates on the genial
tubercle of the mandible, but inserts on the hyoid bone rather than the tongue. The
mylohyoid forms the fl oor of the oral cavity from the mandible to the hyoid bone.
13.3 Motor Activation by the Hypoglossal Nerve
Hypoglossal (cranial nerve XII) motoneurons are dedicated to providing innervation
to all the tongue muscles except for the palatoglossus, which is innervated by
the vagus nerve (cranial nerve X). Most of the innervation details of the tongue
muscles have been discovered by retrograde labeling techniques from injections
into various regions of the tongue (McClung and Goldberg 1999, 2000, 2002 ;
Sokoloff 1993 ; Sokoloff and Deacon 1992 ) . The hypoglossal nucleus, located close
to the midline of the medulla is subdivided into two major compartments, which
have been most thoroughly studied in rats (Uemura-Sumi et al. 1988 ; Sokoloff
1993 ; Aldes 1995 ; McClung and Goldberg 1999, 2000 ) (Fig. 13.1 ).
The motoneuron projections from the ventral portion of the nucleus travel
through the medial branch of the hypoglossal nerve to supply the GG muscle, a
protrusor muscle. Aldes ( 1995 ) found that some hypoglossal motoneurons in the
ventral aspect of the hypoglossal nucleus innervate the vertical and transverse intrinsic
muscles, implying shared functions of intrinsic muscle fi bers with the GG muscle.
The motoneurons innervating the geniohyoid muscle mostly originate in the
lateral accessory subcompartment of the hypoglossal nucleus and travel in the
medial branch of the hypoglossal nerve. A small number of the motoneurons supplying
the geniohyoid arise from the medial subnucleus of the caudal hypoglossal
nucleus and pass through the upper root of the ansa cervicalis before jumping on