Craniofacial Muscles

9 Structure and Function of the Laryngeal and Pharyngeal Muscles
151
neural impulses with gradually rising tension and slow tension decline after these
impulses cease (Han et al. 1999 ) . The unique characteristics of slow tonic fi bers
were hypothesized to be an adaptive mechanism that allows for human vocalization
speci fi c to speech (Han et al. 1999 ) . Slow tonic fi bers would allow for precise adduction
of the vocal folds without fatigue; however, the quick adjustments that are made
during vocalization (such as beginning to phonate immediately after a breath or
singing staccato notes) may not be served well by them. Perhaps hybrid fi bers or
fast contracting MyHC are responsible for rapid phonatory tasks and airway protection,
while slow tonic fi bers decrease the possibility that continuous phonation will
result in fatigue. Therefore, the high concentration of hybridized fi bers, fast contracting
fi bers, and STFs as well as fi ber types that are still under investigation
contribute to the functional properties of the adult larynx.
MyHC composition is reportedly unique in infant laryngeal musculature relative
to that found within limb skeletal muscle (Perie et al. 2000 ) . Speci fi cally, the persistence
of fetal (IIF) MyHC was present in 7-month-old human PCA and TA muscles.
Fetal isoforms have not been reported in adult laryngeal muscles. In most cases,
adult MyHC isoforms replace fetal MyHC during fetal development. For instance,
in limb muscles, slow and fast MyHC isoforms typically replace fetal isoforms
between prenatal months 6 and 9, and are gone very soon after birth occurs.
However, fetal MyHC persist in some muscles through adulthood such as masseter
and extraocular muscles (Wieczorek et al. 1985 ; Soussi-Yanicostas et al. 1990 ) .
The presence of fetal isoforms well after birth in laryngeal muscles may indicate
“a delayed maturation of the human laryngeal neuromuscular system” in comparison
to limbs, “and could be related to the progressive development of these muscles
during the fi rst few years of life” (Perie et al. 2000 ) . The idea of an immature neuromuscular
system persisting into infancy is further supported by the fi nding that up
until 7 months, TA, PCA, IA, and CT muscles exhibit motor end plates innervated
by more than one axon mixed with those only innervated by a single axon (Perie
et al. 1999 ) . In adults, only uni-neuronal innervations patterns have been found in
these muscles.
Additionally, an unknown isoform was discovered in infant larynx. This isoform
was found to have a similar mobility and, hence, molecular weight as 2-month-old
rat MyHC IIL during electrophoresis. Like in rats, the concentration of this isoform
in human infant was higher in the TA vs. the PCA muscles. The presence of this
isoform in infants may be associated with their differing functional needs. Perhaps
conversion of this isoform to its adult form occurs as phonation evolves for use in
speech (Perie et al. 2000 ) .
9.1.4 Nerve–Muscle Connections
A detailed discussion of concepts related to the motor unit can be found in any basic
physiology textbook (Widmaier et al. 2004 ) . An excellent review of muscles of
interest in this chapter was written by Palmer ( 1989 ) . In general, a motor unit is a