Otolaryngology head and neck surgery is perhaps the sole specialty that will evaluate and treat all of the disorders of the laryngopharynx. Yet we, like most specialists, are ill-prepared to handle the complex and subtle interplay of laryngeal events that occur in concert with background respiration. The larynx and pharynx work to allow for continuous respiration, lung volume control, and safe swallowing, as well as episodic but essential cough, sneeze, vomit, and gag. Coupled with all of these activities are vocalizations that include laughing and crying, as well as phonation associated with speech, a unique human laryngeal use. Most of these laryngeal-based events are occurring in parallel and below the level of our conscious mind, although fine control of the larynx during speech and singing is certainly a skilled and volitional behavior. These unique reflexes, central pattern generators, and laryngeal volitional acts use many common neural pathways, share common motor nuclei, and are acted on by nearly every part of the brain. Many of the articles in this supplement will provide detailed information about these reflexive and central pattern generator-driven behaviors. As such, we will focus our discussion on the thyroarytenoid muscle (musculus thyroarytenoideus [TA]), as this is a well-studied muscle that will allow for reasonable support for our central thesis that the glottis (represented by the TA) is uniquely charged with a diverse set of life-sustaining tasks, but also skilled functions, many of which occur simultaneously and share common neuromuscular substrates. Further, compromise of this muscle at an anatomic or neurologic level can disrupt many, if not all of these functions.
The TA muscle is a very small muscle with its origin on the inner surface of the lower third of the internal aspect of the thyroid cartilage and its insertion on the anterior medial surface of the arytenoid cartilage. It has a very unique medial surface where it interdigitates with a fibroelastic band of nonmuscular tissue, the vocal ligament (). Like most muscles, it works in concert with a group of associated, antagonistic, and supportive muscles. The TA motor innervation is the recurrent laryngeal nerve off the vagus nerve, with its motor neurons arising from the nucleus ambiguus. Although studied primarily in animals, the TA is represented dorsally in the nucleus ambiguus, and TA cells can show co-labeling with tracings from the lateral cricoarytenoid (musculus cricoarytenoideus lateralis) and interarytenoid (arytenoideus) muscles.1–4
In humans, these bilateral motor nuclei reside in a region of the mid to upper medulla and receive both excitatory and inhibitory input from the brainstem central pattern generators for respiration, cough, and swallow, as well as the less common gastro-respiratory events of sneezing, gagging, belching, and vomiting.5,6
They are also modulated by motor pathways from the cortex for volitional tasking, such as phonation. These motor nuclei also may have direct and indirect connections to the limbic system.7,8
On the sensory side, along with a variety of pulmonary and pharyngeal regions, the nucleus tractus solitarius receives laryngeal sensory input from both the recurrent and superior laryngeal nerve branch of the vagus.9,10
The sensory innervation includes mucosal mechanoreceptors and chemoreceptors, and deep articular and muscular mechanoreceptors.10–12
During this review, we have given ourselves some latitude to include other muscles that are closely aligned in these functions, such as the lateral cricoarytenoid muscle and interarytenoids, as they are often times targeted in therapies and associated with the functions discussed below.
Basic anatomy of thyroarytenoid and associated laryngeal muscles.
The nucleus ambiguus acts as the final arbitrator of TA movement and interconnections with the medulla and other central nervous system structures are extensive. In a rat model, these were mapped using a pseudorabies virus.8
As shown in , there is remarkable complexity of the TA in terms of sensorimotor control. These data also demonstrate the relatively early connections to 2 striatal-like limbic structures, the central nucleus of the amygdala and the dysgranular insula.8
The central nucleus of the amygdala is an area of the limbic system believed to be responsible for generating or modulating autonomic and somatic responses to aversive stimuli in humans, and the insula is known to be involved in responses to new aversive stimuli, particularly orofacial stimuli. In humans, limbic connections have been postulated for controlling respiratory muscles,13
and laryngeal functions, such as laugh or cry, can be intact in individuals with ventral pontine infarction or “locked in syndrome,” even with the absence of volitional vocal control for speech or respiratory tasks.14
Transneuronal spread of PRV after inoculation into the thyroarytenoid muscle.
Comprehensively describing the functions and neural connections of the TA muscle is well beyond the scope of this article, but its myriad of activities such as respiration, cough, swallowing, and phonation are all represented in its associated neuronal connections. It is equally important to note that this muscle is controlled at various levels, from reflexive to highly skilled. For example, the laryngeal adductor reflex is elicited with sensory stimulation to the laryngopharynx or electrical stimulation of the superior laryngeal sensory nerve and causes contraction to the TA.15–18
Although somewhat controversial, the timing and magnitude of this reflex is often used to inform clinicians about the integrity of airway protection during the swallow,19–22
and is affected by conditions such as Parkinson Disease11
and gastroesophageal reflux,23
and anatomic modifications such as tracheotomy.24
At a more complex level, the TA is a target of several central pattern generators related to swallowing and respiration.25–32
For a review of the seminal work in this area, see Jean et al, 2001.27
At a volitional level, the TA is highly active during phonated segments of speech.33,34
Further, control of the TA is considered a highly skilled fine sensorimotor act for trained singers, such as opera performers.35,36
There are also differences in neural control when moving from a simple reflexive event like spontaneous swallow to a volitional swallow.37