Craniofacial Muscles

244 Z.-J. Liu
Fig. 14.2 Stimulation of hypoglossal motor system. Dots indicate the locations of hook electrodes
for stimulations. HG hypoglossal nerve trunk; LB hypoglossal nerve lateral branch; MB hypoglossal
nerve medial branch; GG genioglossus; SG styloglossus; LN lingual nerve, LF lingual frenum;
CP circumvallate papillae; FP follicle papillae
geniohyoid, and intrinsic tongue muscles) whereas the lateral branch supplies motor
output to the tongue retractor complex (styloglossal and hypoglossal muscles)
(McClung and Goldberg 2000 ; Fuller and Fregosi 2000 ; Yoo and Durand 2005 ) .
Electrical stimulations to its trunk, medial, and lateral branches (Fig. 14.2 ) widen
the posterior tongue body by increasing its dorsal and ventral width (~8–10%).
However, tongue body shortening and thickening (~4–9%) are evoked by the trunk
and lateral branch stimulations. In contrast, stimulation to the medial branch lengthens
the sagittal dimension of the tongue body (~7–9%) along with moderate tongue
protrusion. On the other hand, stimulation to the major tongue protrudor, the genioglossus,
results in the widening of the anterior tongue body and thinning of the
posterior tongue body, along with tongue body lengthening. All of these deformations
have the effect of dilating the upper airway in the ventral and lateral pharyngeal
wall, thereby maintaining upper airway patency. Because the stimulations to
the tongue protrudor (medial branch and genioglossus) and retractor (lateral branch
and styloglossus) show opposite directions in the tongue deformation, the notion
that coactivation of both complexes has an effect on maintaining upper airway patency
may not be true. While the tongue manipulations result in signi fi cantly larger
changes in width, length, and thickness than those by electrical stimulations to the
hypoglossal motor system, the changes by stimulations are surprisingly similar to
each other no matter which nerve trunk, branches, or muscles is stimulated. This
similarity may imply that at least in the hypoglossal motor system, the supramaximal
activation of a nerve branch could produce motor output strength analogous to
that caused by the tetanic contraction of its innervated muscle (Liu et al . 2006 ) .
14.2.2 Tongue Kinematics During Function
As illustrated in Fig. 14.1 , it has been veri fi ed that the implantation of a number of
crystals (2 mm in diameter) into the tongue has no signi fi cant functional impairment