The American Speech-Language-Hearing Association (ASHA) provided a summary article on laryngeal innervation, and we thought it would be of interest to you (http://goo.gl/kWJ8Qu). The larynx or “voice box” is incredible, and it’s pretty amazing that things work so well in the first place for this finely tuned mechanism. The density of motor input to the larynx is greater than any other anatomical structure. The larynx is used throughout the lifespan for a variety of tasks even beyond voice, including breath holding, airway protection, swallowing, coughing, and bearing down. Much of this is regulated by the vagus nerve, or cranial nerve X.
For our interests here, the 2 branches of the vagus nerve that are most relevant are the recurrent laryngeal nerve (RLN) and the superior laryngeal nerve (SLN). The right RLN branches off the vagus once it exits the skull base, travels down towards the collar bone, and then back up the neck inserting into the larynx. The left RLN branches off the vagus once it exits the skull base, travels down around the aorta in the chest, and then back up to the neck inserting into the larynx. The SLN does not travel into the chest once it branches off the vagus. They are all long nerves in vulnerable locations. And as such, are susceptible to injury due to virus, surgical trauma, physical trauma, cancer, and thyroid disease just to name a few.
The RLN controls the muscles (thyroarytenoids, posterior cricoarytenoids, transverse arytenoids, and interarytenoids) that open and close the vocal folds, and tense the vocal folds such as for lowering pitch or increasing loudness. The SLN controls the muscles (cricothyroids) which lengthen the vocal folds, such as for raising pitch. So, an injury to the RLN, will cause weak voice because the vocal folds cannot close against one another to produce voice; and often higher pitch develops because the muscles which raise pitch are still working as the SLN was not injured. Or the alternate would be an injury to the SLN which also causes weak but monotone voice, because of the inability to vary vocal fold length and subsequently pitch. There can be injury to one or both of the SLN or RLN, and there can be injury to both the SLN and RNL at the same time.
Here is a diagram of the vagus nerve referenced in the article http://goo.gl/2bm9FB.
Contact Voicetrainer LLC at info@voicetrainer.com or 202-580-6646 to schedule an appointment if you are struggling with your voice due to neurological injury or some other insult.
~LPV
