Craniofacial Muscles

216 A. Sokoloff and T. Burkholder
be organized into multiple pools independent of muscle region or compartment
identity (Herrmann and Flanders 1998 ) .
In the human GG, grouping of tongue motor units appears to be related to motor
unit location in some tasks but independent of motor unit location in others. During
speech tasks, GG motor units are selectively activated in anterior vs. posterior
regions (Baer et al. 1988 ) which may represent selective motor unit activation by
virtue of compartment membership (for example in anatomically de fi ned oblique
vs. horizontal GG regions) or by virtue of the speci fi c effect of motor unit activation
on tongue deformation. During wakeful respiration however, multiple groups of GG
motor units can be de fi ned by activity patterns recorded from the same electrode
and thus the same muscle region (Saboisky et al. 2006 ) . Whether differing respiratory
GG motor unit pools are grouped by virtue of metabolic, contractile, or other
motor unit properties are not known.
Above we reviewed the muscle architecture that constrains the geometry of
tongue deformation. The capacity of the nervous system to use this geometry is
dependent on the organization of motor units and the ways in which the nervous
system can combine and regulate their activation. Although evidence is limited, we
next review contractile features of tongue motor units that re fl ect molecular composition
of constituent muscle fi bers and anatomical features that determine the speci fi c
effect of motor unit activation on tongue deformation.
12.3.2 Tongue Motor Unit Organization and Activation
12.3.2.1 MEP Morphology and Muscle Fiber Innervation
Synapses of head and neck muscles show some differences from conventional
appendicular muscles. Two MEP morphologies have been described in human
tongue muscles, plate-like “en plaque” MEPs typical of appendicular systems and
grape-cluster-like “en grappe” MEPs present in many head and neck muscle systems
(Oda 1986 ; Perie et al. 1997, 1999 ; Slaughter et al. 2005 ; Mu and Sanders
2010 ) . A third MEP pattern described in extraocular muscles, which consists of
small terminal boutons distributed along the muscle fi ber length (Oda 1986 ) is not
present in the tongue. Functional and molecular correlates of en grappe MEP morphology
in human tongue muscles are not known. In human tongue, laryngeal, and
suprahyoid muscles, the absence of appreciable slow/tonic myosin heavy chain
(MyHC) precludes a relationship between MyHCslow tonic and en grappe MEP
morphology (Sokoloff et al. 2007 ) .
As discussed above, some tongue muscle fi bers have two MEPs. In extraocular
muscles, multiple MEPs per muscle fi ber may re fl ect innervation of a single fi ber by
multiple nerves (Chiarandini and Stefani 1979 ; Oda 1986 ) . SL fi bers with dual
MEPs appear to be singly innervated (Slaughter et al. 2005 ) , an organization similar
to human laryngeal muscle fi bers (Perie et al. 1997 ) . It is not known whether other
tongue muscle fi bers with dual MEPs are multiply or singly innervated.