Craniofacial Muscles

12 Tongue Structure and Function
217
12.3.2.2 Motor Unit Physiology
Physiological investigations, primarily in the rat, demonstrate that tongue motor
units are predominantly nonfatiguing and are similar to appendicular motor units
with respect to speed of contraction (Gilliam and Goldberg 1995 ; Sokoloff 2000 ;
for similar fi ndings in cat see Hellstrand 1981 ) . These fi ndings correlate with muscle
biochemistry; most rat tongue muscle fi bers have a moderate-to-high oxidative
capacity and are composed of conventional “fast” MyHC isoforms. However, rat
tongue motor units produce 100–1,000-fold less force than rat appendicular motor
units (Gilliam and Goldberg 1995 ; Sokoloff 2000 ) . Motor unit force is determined
primarily by the number and cross-sectional area of constituent muscle fi bers
(Totosy de Zepetnek et al. 1992 ) . Although studies are limited, the cross-sectional
area of rat tongue muscle fi bers appears to be one to four times less than the crosssectional
area of equivalently typed fi bers in rat neck and appendicular muscles
(e.g., Oliven et al. 2001 ; Matsumoto et al. 2007 ) . This suggests that, compared to
most appendicular motor units, tongue motor units comprise many fewer fi bers.
12.3.2.3 Motor Unit Anatomy and Localization
Direct anatomical evidence of tongue motor unit location, fi ber architecture, and
fi ber number (i.e., innervation ratio, IR) is lacking. Independent activation of anterior
vs. posterior regions of the GG during speech indicates localization of at least
some tongue motor units with respect to the A/P tongue axis in humans (Baer et al.
1988 ) . Localization of motor units with respect to the A/P tongue axis has also been
demonstrated physiologically in the rat. Following intra-axonal activation of individual
motor units, 65/105 SL-IL motor units and 41/42 GG, T and V motor units
were localized by EMG to either anterior, middle, or posterior tongue body regions
(Fig. 12.4 ).
Studies have not described motor unit IR, the dorso-ventral and medio-lateral
extent of motor unit territories and whether motor unit territories respect muscle
architecture divisions. Estimated motor unit innervation ratios of less than 25 in the
rat SG and GG suggest that motor unit territories may be circumscribed in the coronal
plane as well (Sutlive et al. 2000 ) .
We saw above that muscle fi bers of all orientations are found in most regions of
the tongue and that traditional division of contractile material into discrete, anatomically
de fi ned volumes does not simplify description of tongue motion and does
not facilitate understanding. The ability to voluntarily change the local curvature of
the tongue indicates that the nervous system controls sub-volumes of contractile
material and our goal is to describe tongue structure in a way that clari fi es both the
deformations and their control. Although data on anatomical localization and distribution
of motor unit territories are limited, data reviewed above indicate that motor
units span more than one T/V laminae but are spatially restricted. Some inferences
can also be made from observed behavior. Imaging data indicate that the tongue
deformation gradient is relatively low frequency, and a high-resolution fi nite