Laryngeal Structure and Spaces
The larynx is situated in the neck, inferior to the oral cavity and anterior to the pharynx (Box 1.27). Simplistically, the larynx is a tube that opens into the pharynx and connects the oral, nasal, and pharyngeal regions to the lungs. While this is a convenient arrangement for respiration, it requires special physiologic consideration and alteration during swallowing. Namely, as food or liquid enters through the common area of the pharynx, the airway entrance requires protection. During swallowing, when the bolus passes through the pharynx, airway protection is engaged. Airway protection is achieved by closing off the airway entrance and moving the airway entrance out of the path of the oncoming bolus. While the physiologic actions will be discussed later in this text, the anatomy required to achieve these goals is noted here.
Box 1.27 Image Gallery: Gallery of laryngeal images
Consider the larynx as a tube composed of ligaments, muscles, cartilages, and a mucous membrane. The entrance to the larynx is in the pharynx and may be referred to as the laryngeal inlet, aditus, or vestibule. Individually, the laryngeal cartilages have some degrees of freedom of movement with respect to each other based on their configuration, joints, and attached intrinsic musculature. Further, the larynx as a whole has great degrees of freedom due to the fact that it has no bony attachment to the skeleton. That is, the larynx is suspended from the hyoid bone, which is attached to the skeletal system via muscle and tissue membrane only (Box 1.27).
Laryngeal Muscles
The structure within the larynx allows for the internal diameter (opening) of the laryngeal tube to be decreased down to zero during the swallow. This is achieved with the help of intrinsic laryngeal muscles. There are five intrinsic laryngeal muscles–thyroarytenoid, cricothyroid, lateral cricoarytenoid, posterior cricoarytenoid, and interarytenoid. Of these five, three are important in glottal closure, and therefore, airway protection. The interarytenoid (IA) muscle goes from the muscular process of one arytenoid to the other. Upon contraction, the IA muscle adducts the arytenoids, closing the posterior glottis. The lateral cricoarytenoid (LCA) muscle is an adductor muscle that draws the arytenoids forward and medial. This results in glottal closure particularly at the anterior aspect of the glottis. The thyroarytenoid (TA) muscle goes from the thyroid cartilage, directly below the thyroid notch, to the vocal process of the arytenoid. When contracted, the TA muscle draws the arytenoid cartilages forward, thereby shortening and thickening the vocal folds. They are all innervated by the recurrent laryngeal nerve of CN X.
Clinical Note
Airway protection can be physiologically challenged if any of the following occur:
- Reduced glottal closure
- Reduced tightening of the laryngeal vestibule (aryepiglottic rim)
- Reduced laryngeal elevation
- Poor timing of glottal closure
Clinical Note
In a healthy individual there is redundancy in airway protection during the swallow —vocal folds, tightening of the aryepiglottic folds, epiglottic inversion, and superior anterior position of the hyo-laryngeal complex.
The cricothyroid (CT) muscle, another intrinsic laryngeal muscle, is a vocal fold tensor that serves as an agonist to the TA muscle. When contracted, in combination with the TA muscle, the folds are tensed. This may help in closing the glottis and protecting the airway during the swallow. Further, the CT muscle aids laryngeal elevation by providing closer approximation of the thyroid and cricoid cartilages. It is innervated by the superior laryngeal nerve of CN X.
The posterior cricoarytenoid muscle (PCA) is an intrinsic laryngeal muscle that serves as an abductor (opens the glottis) and, therefore, is not of interest when discussing airway protection. However, the fact that this muscle disengages when the swallow occurs may provide passive biomechanical support for airway closure.
The movement of the larynx (or, more importantly, the airway entrance) out of the pathway of the bolus flow, is largely attributed to extrinsic laryngeal muscles, namely the floor-of-mouth/suprahyoid muscles and longitudinal pharyngeal muscles. The extrinsic laryngeal muscles important in airway protection (primarily the suprahyoid muscles) pull the hyoid in a superior-anterior (up and forward) direction during the swallow. Consider the anatomical linkage between the larynx and the hyoid; this connection obliges the larynx to follow hyoid movement. Extrinsic laryngeal muscles include the stylohyoid, anterior belly of the digastric, geniohyoid, and mylohyoid muscles. The hyoglossus and thyrohyoid muscles may also be involved in the elevation of the larynx during swallowing.
The stylohyoid (SH) muscle goes from the styloid process of the temporal bone to the hyoid bone. It is innervated by the mandibular branch of CN VII. When contracted, it elevates and retracts, or pulls back, the hyoid and, consequently, the larynx. During the swallow, the posterior engagement on the larynx is opposed by muscles with anterior attachments, thus allowing the larynx to move in a superior and anterior direction.
As noted earlier, the digastric muscle is divided into two bellies — anterior and posterior. The anterior belly of the digastric muscle (ABD) goes from the digastric fossa on the posterior surface of the mental symphysis to the fibrous loop on the lesser cornu of the hyoid. When contracted, the anterior belly pulls the hyoid bone up and forward. The posterior belly of the digastric (PBD) is less involved in the swallow process because it courses from the mastoid process to the fibrous loop on the hyoid bone, thereby pulling the larynx in a posterior direction, an action not desired during swallowing. The anterior belly is innervated by CN V whereas the posterior belly is innervated by CN VII.
The geniohyoid (GH) muscle goes from the posterior surface of the mental symphysis of the mandible to the superior border of the hyoid bone. When contracted, it can either elevate and protract the hyoid, or depress the mandible. It is innervated by C1. The fibers of the mylohyoid (MH) muscle fan across the mandible and terminate on the hyoid bone. It pulls the hyoid in an anterior/superior direction. The hyoglossal (HG) muscle goes from the superior border of the greater cornu of the hyoid bone to the sides of the tongue. When contracted, if the tongue is stabilized, it elevates the hyoid in a superior and anterior direction. It is innervated by CN XII. The thyrohyoid (TH) muscle can be an elevator or a depressor. It goes from the thyroid to the hyoid. When the thyroid is free of inferior muscular constraints, it will pull the larynx closer to the hyoid bone.
Laryngeal Spaces
The juxtaposition of the laryngeal structures results in spaces important to swallowing (Box 1.28). Inside the laryngeal tube, the ventricle is the space between the true and false folds. The glottis is the space between the right and left true vocal fold. The sub-glottis is the air column below the true vocal folds (below the glottis).
Box 1.28: Laryngeal spaces noted in a coronal cut from the posterior view.