Craniofacial Muscles

212 A. Sokoloff and T. Burkholder
12.2.3.1 Muscle Fibers with Primarily Vertical Orientation
Muscle Fiber Architecture
Fibers with primarily vertical orientation are present in all regions of the tongue
body and contributed mainly by GG and V with additional contributions from HG,
PG, SL, and T. 2 GG fi bers are absent in the anterior tongue of the rodent and cat but
are present in anterior human tongue (Abd-El-Malek 1938, 1939 ; Hellstrand 1980 ;
Gaige et al. 2007 ) . In most species the anterior-most GG fi bers ascend vertically to
the dorsum near the midline with fi bers with progressively posterior insertion radiating
obiquely in a fan shape such that the deepest fi bers travel horizontally to the
tongue root (Figs. 12.1 and 12.2 ). In some species, oblique (anterior-dorsal fi bers)
and horizontal (ventral fi bers) divisions of the GG are recognized. In the posterior
tongue, GG fi bers occupy the majority of the medial volume, displacing V fi bers to
the lateral periphery. Although nominally vertical, V fi bers often have a dorsomedial
to ventrolateral orientation which would tend to form an A/P trough upon contraction.
In rodents, V fi bers may originate and insert in the lateral epithelium
forming a lateral arc of fi bers (Fig. 12.3 ). V fi bers are organized into laminae orthogonal
to the A/P axis of the tongue body. V fascicles alternate with T fascicles and
course through dorsal and lateral-ventral longitudinal fi bers to insert in epithelium
or connective tissue (CT) investing fascicles of other muscles (Fig. 12.2 ). At the
tongue tip, fascicles of V are less coherent and alternation with T irregular.
Fibers with vertical orientation are additionally contributed by HG fi bers that
course dorsally or antero-dorsally to the dorsum, and by PG fi bers that descend in
the lateral body and mingle with fi bers of SG (Fig. 12.1 ). The posterior-most fi bers
of SL originate near the hyoid bone and also course vertically, deep to the posterior
dorsum. In some species, T fi bers originating from the dorsal and ventral limits of
the medial septum course vertically to the dorsal and ventral epithelium, respectively
(Fig. 12.3 ). Vertical fi ber components are thus contributed by extrinsic and
intrinsic muscles and muscles classically considered retrusors and protrusors.
Innervation
GG and V fi bers extend for much of muscle origin to insertion length. V and vertical/
oblique GG fascicles appear to have a single MEP in the middle third, indicative of
in-parallel fi ber organization (Fig. 12.2 ). However, more horizontal GG fascicles
may contain two or more MEP zones (Mu and Sanders 2010 ) . The extent to which
multiple MEPs along GG fascicle length re fl ect offset of muscle fi bers, branching
of a single terminal nerve to innervate multiple MEPs on a single fi ber or innervation
2
Most tongue muscles have complex architecture the details of which may differ substantially
between species (e.g., minimal lateral longitudinal muscle fi bers in the anterior cat tongue,
Hellstrand 1980 ) . Cross-species differences in tongue muscle organization have not been studied
in detail and can offer insights into neuromuscular bases of tongue movement.