Craniofacial Muscles

9 Structure and Function of the Laryngeal and Pharyngeal Muscles
153
Fig. 9.5 Wide fi eld fl uorescent image (×4 objective) of a 50- m m-thick transverse section from a rat
larynx showing the distribution of motor endplates in the thyroarytenoid (TA) muscles. Motor
endplates are labeled with Alexa Fluor 488 conjugated alpha-bungarotoxin (Molecular Probes/
Invitrogen, Eugene, OR) which has a high af fi nity to acetylcholine receptors in skeletal muscle.
Note the difference in the distribution between the horizontal endplate band in the lateral TA compared
to the diffuse distribution along the length of the medial TA. Courtesy of Aaron Johnson
in cat and human specimens was similar. That is, while most skeletal muscle appears
to have a distinct endplate band at the midbelly of the muscle, the TA muscle
appeared to have widely distributed endplates throughout the length and width of
the muscle in both cats and humans (Rosen et al. 1983 ) . We have also observed this
widespread distribution of endplates in rat medial TA in our laboratory (Connor
et al. 2002 ) , but not in lateral TA (Fig. 9.5 ) where motor endplates were con fi ned to
an endplate band and clustered together as is observed in limb muscle.
Endplate band clustering of NMJs has also been shown for CT muscle (Rosen
et al. 1983 ) . Thus, the medial TA muscle appears to be richly supplied with NMJs in
a complex geometry that is unique to this muscle when compared with other laryngeal
and limb muscles, perhaps to serve the sensorimotor control needs of this critical
muscle of the airway and voice. The abundance of NMJs is reduced with aging in
the TA and PCA muscles, thus supporting that aging putatively affects the NMJs in
a manner similar to that seen with denervation (Connor et al. 2002 ; McMullen and
Andrade 2009 ) . However, there are alternative explanations for these fi ndings, such
as muscle fi ber atrophy and/or reductions in muscle fi ber size or length.