Craniofacial Muscles

154 L.A. Vinney and N.P. Connor
9.1.5 Summary
The intrinsic laryngeal muscles represent the fi nal common path for the fi nely tuned
actions that allow for breathing, airway protection, and phonation. Muscle fi ber type
morphology and mitochondrial density likely subserve the unique functions of these
muscles. While the innervation patterns of these muscles are considered well known,
con fl icting reports exist regarding the IA and TA. Anatomic and physiologic studies
of the intrinsic laryngeal muscles have been performed in humans and cadaveric
models, as well as in animal models. There appear to be some species differences,
but there are more similarities than differences across species. For instance, in the rat
larynx there has been note of a muscle not found in humans, the alar cricoarytenoid
muscle (Inagi et al. 1998 ) . Because the larynx and pharynx are dif fi cult to access in
humans, the use of animal models in research concerning muscles of larynx and
pharynx is necessary. Thus, some species differences must be tolerated but considered
when making interpretations relative to human muscle anatomy and physiology.
Clearly, the larynx has unique muscular components including a large density of
mitochondria as well as a large percentage of hybrid, slow tonic, and fast twitch
fi bers. These muscle fi ber types contribute to the specialized functions of the larynx.
Differing fi ber types found in human larynx compared to other species may be
linked to differences in body size and functional differences (i.e., speech in humans).
Controversy still remains as to whether adult or infant human vocal fold musculature
contains EO MyHC, but this fi nding would support the rapid contraction speeds
of muscles like the TA. As knowledge about laryngeal muscle fi bers increase, further
conclusions may be drawn about their in fl uence on laryngeal movement in normal
and voice-disordered populations. Thus far, it is clear that the variable composition
of different laryngeal muscles suits each to differing task requirements. It is not
clear whether particular laryngeal adductor muscles predominate in speci fi c phonatory
or airway protection tasks, and how different muscle characteristics may exert
in fl uence on voice disorders.
9.2 Extrinsic Muscles of the Larynx
9.2.1 Role in Voice
The extrinsic laryngeal muscles contribute to voice production and modulation by
allowing for modi fi cations in laryngeal posture. While the intrinsic muscles of the
larynx have connections solely within the larynx, the extrinsic muscles have an
external origin or insertion. For example, the sternothyroid and the thyrohyoid muscles,
while not located wholly within the larynx, function to control the vertical
position of the larynx and are active up to 200 ms prior to phonation onset (Shipp
1975 ) . One example of vertical position change of the larynx was provided by
Shipp, speci fi cally, the superior laryngeal movement observed during glissando