Craniofacial Muscles by Linda K. McLoon & Francisco Andrade

Author:Linda K. McLoon & Francisco Andrade
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

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).

Transcranial magnetic stimulation has been used in human to map the cortical region controlling the laryngeal muscles (Khedr and Aref 2002; Rodel et al. 2004). Both groups reported bilateral muscle response latencies of approximately 10.8 ms in the CT and TA muscles, respectively, when the primary motor cortex was stimulated unilaterally while the left and right TA muscles had latencies of 11.7 ms and 10.7 ms, respectively. These latencies are difficult to explain given the difference in length of the RLN which innervates the TA muscle and eSLN which innervates the CT. Further no difference in latency was noted between responses in the right and left TA muscles despite the significant length differences due to the longer course of the left RLN which descends below the aortic arch on the left. In fact, previous research in both dogs and human has shown a 3 ms latency difference in latency of TA response between the right and left sides (Atkins 1973) and similar latency differences between the left and right sides were found when transcranial magnetic stimulation was applied over the mastoid where the vagus emits from the skull (Sims et al. 1996). A 2.3 ms latency was found between the right and left TA muscles, a 4.86 latency between the TA and CT on the left and 1.6 ms difference on the right. Given the cortical to muscle latencies of 10.7 which are less than a millisecond later than the peripheral responses, it is likely that these reports (Khedr and Aref 2002; Rodel et al. 2004) include direct nerve responses as the magnetic field at the cortex induced peripheral nerve responses as has been found in other cranial nerve using TMS (Benecke et al. 1988). A more careful TMS study using more focal coils is needed to examine the cortico-bulbar pathway to the laryngeal muscles in humans.

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