Craniofacial Muscles

10 Motor Control and Biomechanics of Laryngeal and Pharyngeal Muscles
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The LCA muscle primarily received branches from the ipsilateral RLN; only in
a few cases fi bers from the eSLN were identi fi ed as innervating this muscle
(Maranillo et al. 2005 ) , and the numbers of branches from the RLN to the LCA
varied from 1 to 8. The thyroarytenoid muscle also varied in the numbers of branches
from the RLN (1–6, with a mean of 1.6) and arose either from the branch to the
PCA, IA, or LCA. Only 4.6% of larynges received innervation from another nerve
than the RLN, the source being the eSLN (Maranillo et al. 2005 ) .
Interlaryngeal variation in RLN branches to the intrinsic laryngeal muscles
complicates the interpretation of the bases for reductions in vocal fold motion in
abduction and adduction. It also impacts the outcome of attempts to selectively
stimulate individual laryngeal muscles or to reinnervate particular intrinsic laryngeal
muscles for breathing (PCA), voice (TA, LCA) or for closure airway protection
during swallowing (IA, TA).
In spasmodic dysphonia, abnormal bursting of motor neuron fi ring patterns leads
to voice disruptions during speech (Ludlow et al. 2008 ) . Currently, anastomosis of
the RLN branch(es) to the TA to the ansa cervicalis is used to permanently denervate
the TA muscle from axons in the RLN which produce spasms in these patients (Berke
et al. 1999 ) . This procedure identi fi es the adductor branch of the RLN going to the
TA muscle and prevents reinnervation of the TA muscle by the RLN. On the other
hand, following laryngeal denervation, consideration of the RLN branching pattern
may be considered for selective reinnervation of the PCA and IA to restore volitional
abduction and adduction of the vocal folds, respectively (Kwak et al. 2010 ) .
10.3 Central Nervous System Control of the Laryngeal
Motor Neurons
The laryngeal muscles are innervated by motor neurons in the ipsilateral nucleus
ambiguus in the medulla (Davis and Nail 1984 ) with the CT motoneurons located
more rostrally while the PCA, TA, and LCA motoneurons are located more caudally.
Injury to the motor neurons in the nucleus ambiguus affects the ipsilateral
laryngeal muscles except the interarytenoid which may have some bilateral innervation.
Vagal or recurrent nerve injury results in at least short term unilateral vocal fold
paralysis. However, central nervous system injury rarely results in unilateral vocal
fold movement impairment except when there is damage to the laryngeal motor
neurons or disruption of the pre-synaptic input to the motor neurons in the brain
stem as in lateral medullary stroke or Wallenberg syndrome (Kim et al. 2000 ;
Aydogdu et al. 2001 ) . Cortical lesions do not result in unilateral vocal fold paralysis
suggesting that there is bilateral supramedullary input to the laryngeal motor neurons
in the medulla. However, unilateral vocal fold movement reduction (bowing)
can occur in Parkinson disease (Hanson et al. 1984 ) on the same side as limb involvement
suggesting more laterality in control above decussation in the substantia nigra
and medulla, regions involved early in Parkinson disease (Braak et al. 2003 ) .