Craniofacial Muscles

10 Motor Control and Biomechanics of Laryngeal and Pharyngeal Muscles
179
genetic mutations resulting in neural transport abnormalities (Benson et al. 2010 ;
Landoure et al. 2010 ) . Brain stem strokes in the lateral medullary region, referred to
as Wallenberg syndrome, disrupt pre-synaptic input to the laryngeal motor neurons
in the nucleus ambiguus, resulting in a unilateral vocal fold paralysis for voice and
swallowing. In addition, lateral medullary lesions often result in a serious disturbance
in swallowing patterning as the integrity of the brainstem central pattern
generator for swallowing is affected on one side (Kim et al. 2000 ; Aydogdu et al.
2001 ) . Disorders and diseases of the basal ganglia can interfere with the timing of
muscle patterning and level of muscle activation. Most prominent are those that
accompany Parkinson’s disease particularly as the disease progresses (Dickson and
Grunewald 2004 ) to involve regions well beyond the substantia nigra (Braak et al.
2003 ) impacting the precision of recruitment of laryngeal muscles for rapid voice
onset and offset during speech (Gallena et al. 2001 ) .
Given the important contribution of cortical mechanisms in the left hemisphere
for speech and voice production, mechanisms of enhancing left hemisphere cortical
control while suppressing interfering brain mechanisms in the right hemisphere
have recently received a great deal of attention. Using transcranial magnetic stimulation
at slow rates which are inhibitory to motor function over the right hemisphere
has been found to induce recovery in a few patients with motor speech disorders
(Martin et al. 2009a, b ; Hamilton et al. 2010 ) .
For the elicitation of swallowing, sensory stimulation seems to enhance the elicitation
of re fl exogenic swallowing, perhaps at the brain stem level. However, recent
studies using functional magnetic resonance imaging now suggest that the application
of sensory stimulation in the oropharynx can enhance cortical activation not
only in the somatosensory regions but also in regions of the cortex that are active for
the volitional control of swallowing in normal humans (Lowell et al. 2008 ; Soros
et al. 2008 ) , indicating that sensory stimulation may be useful in up-regulating cortical
motor control mechanisms involved in the control of both the laryngeal and
pharyngeal muscles for automatic and volitional swallowing.
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