|Pharynx and Larynx
The PHARYNX is a fibromuscular tube which extends from the base of the skull to the lower border of the cricoid cartilage (at which point it becomes the esophagus). Portions of the pharynx lie posterior to the nasal cavity (nasal pharynx), oral cavity (oral pharynx) and larynx (laryngeal pharynx). The muscular walls of the pharynx are comprised of an outer layer made up of 3 circularly disposed muscles, the CONSTRICTORS. The constrictors are logically named (from superior most to inferior most) the SUPERIOR, MIDDLE and INFERIOR PHARYNGEAL CONSTRICTORS. Figure 1 below shows the arrangement and attachments of the pharyngeal constrictors. Note that the inferior edges of the superior and middle constrictors nestle inside the superior edges of the next lower constrictor. (This arrangement has been described as being similar to 3 stacked flower pots.)
Figure 1. Side view illustrating the 3 constrictors of the pharynx and their attachments. From Basmajian, Grant's Method of Anatomy, p. 501.
The inner muscular layer of the pharynx is made up of 3 small longitudinally oriented muscles: the STYLOPHARYNGEUS, PALATOPHARYNGEUS, and SALPTNGOPHARYNGEUS. Figure 2 and 3 illustrate the arrangement and attachments of these longitudinal muscles, as well as their relationship to the circularly disposed muscles of the pharynx. Note that the STYLOPHARYNGEUS takes origin from outside of the pharynx, from the styloid process, and gains access to the interior of the pharynx by passing between the superior and middle constrictors. The PALATOPHARYNGEUS takes origin, as the name suggests, from the soft palate, and runs down the length of the pharynx, intermingling with fibers of the stylopharyngeus below the inferior border of the superior constrictor. The SALPINGOPHARYNGEUS is usually considered as a slip of palatopharyngeus which has a separate origin from the posterior part of the cartilage at the opening of the auditory tube (salpinx=tube, hence the name is logical).
Figure 2. (On left.) View from behind the pharynx. Part of the posterior wall of the pharynx has been cut away (jagged edge) to expose the interior of the pharynx. This figure illustrates the relationship between the longitudinal and circular muscles making up the muscular walls of the pharynx.
Figure 3. (On right.) View from behind the pharynx. On the left side the constrictors have been left in their normal anatomical position, so that the relationship between the STYLOPHARYNGEUS muscle and the superior and middle constrictors can be clearly visualized. On the right side the pharynx has been opened, so that the relationship between the PALATOPHARYNGEUS, SALPINGOPHARYNGEUS and the constrictors can be clearly visualized.
ACTIONS OF THE PHARYNGEAL MUSCLES
During swallowing, successive contraction of the superior, middle, and inferior constrictor muscles helps to propel the bolus (ball) of food down into the esophagus. In addition, contraction of the 3 longitudinal muscles of the pharynx helps to raise the pharynx, effectively aiding it in engulfing the bolus of food.
In between acts of swallowing, the lowest fibers of the inferior constrictor are thought to act as a sphincter, guarding the entrance to the esophagus and preventing the entry of air into the digestive system.
The LARYNX is an apparatus made up of cartilage, ligaments, muscles, and mucous membrane, which guards the entrance to the lower respiratory passages (trachea, bronchi, and lungs) and houses the vocal cords. Figure 4 shows the major cartilaginous and ligamentous structures which make up the larynx. (Note that the hyoid bone is not usually considered as part of the larynx and is included in this figure strictly as an aid to orientation.) Before you consider the muscles associated with the laryngeal apparatus, pay very careful attention to the arrangement of the structural components shown in Figure 4, and realize that three major types of movement are possible:
1. MOVEMENT AFFECTING THE TENSION OF THE VOCAL CORDS. Since the vocal cords are attached to the arytenoid cartilages posteriorly and the thyroid cartilage anteriorly, regulation of vocal cord tension (and therefore pitch of the voice) is accomplished primarily by pivoting the thyroid cartilage forward or backward at the cricothyroid joint (Figure 51.
2. MOVEMENTS WHICH INCREASE OR DECREASE THE SIZE OF THE OPENING BETWEEN THE VOCAL CORDS (ABDUCTION OR ADDUCTION OF THE VOCAL CORDS). This action is best appreciated by imagining the larynx viewed from above, as is shown semi-schematically in Figure 6. Realize: I. That the vocal cords on each side are covered with a mucous membrane, so that when they are abducted, relatively little air can pass either between them or around them. 2. The bases of the two arytenoid cartilages are L-shaped. The medial process of each is called the vocal process, and the vocal cords attach to it. The lateral process of each is called the muscular process, and most of the muscles which act to abduct or adduct the vocal cords attach to it. Abduction of the vocal cords may be accomplished either by externally rotating the arytenoid cartilages on a pivot located at the angle of the "L" (center panel, Figure 6), or by sliding the 2 arytenoid cartilages apart slightly (right panel, Figure 6). Realize that there is only 1 muscle which has an abductor action on the vocal cords, although there are several which act to adduct the cords.
3. MOVEMENTS WHICH CLOSE OFF THE ENTRANCE TO THE LARYNX, AS IN SWALLOWING. Figure 7 below indicates with arrows the movements of the epiglottis and arytenoid cartilages which take place during swallowing to block off the entrance to the larynx, to prevent food from entering the larynx. Basically, the epiglottis is pulled down, and the arytenoid cartilages are pulled forward, toward the epiglottis. Realize that two other mechanisms which are independent of laryngeal movements per se also help to keep food from entering the larynx during swallowing. First, the larynx is raised, along with the pharynx, by the contraction of the longitudinal muscles of the pharynx. Second, the epiglottis also acts partly as a passive barrier to the movement of food, so that food tends to be funneled to either side of the larynx, into the PIRIFORM RECESSES and from there into the esophagus. Refer to Figure 9-56 in your atlas, which shows the pharynx opened up to reveal the interior of the pharynx, as seen from behind. Locate the epiglottis, piriform recesses, and esophagus, and trace the path of a bolus of food from the back of the tongue into the esophagus.
Figure 4. (On left) Side view illustrating the major cartilaginous and ligamentous structural features of the larynx. Shaded structures are imagined as seen through the thyroid cartilage or hyoid bone.
Figure 5. (On right) Schematic drawing of the larynx as seen from the side to show how movement of the thyroid cartilage at the cricothyroid joint affects the tension of the vocal cords. From Basmajian, Grant's Method of Anatomy, p. 530.
Figure 6. Schematic drawing of the larynx as seen from above showing the thyroid cartilage, the arytenoid cartilages, and the vocal cords. From Basmajian, Grant's Method of Anatomy, p. 532.
Figure 7. Schematic drawing of the larynx as seen from the side. Arrows indicate the direction of movement of the epiglottis arytenoid cartilages which take place cluring swallowing.
OTHER STRUCTURAL FEATURES OF THE LARYNX
Refer to Figures 9-78 and 9-79 in your atlas to appreciate several additional structural features which have been intentionally omitted from the previous discussion. These features (described below) are not particularly important to understanding movements of the larynx, but you will find it somewhat difficult to understand the appearance of the larynx as seen through a laryngoscope or on dissection, until you appreciate that:
1. Above and parallel to the vocal ligament or vocal cord on each side is a second ligament called the VESTIBULAR or VENTRICULAR LIGAMENT (Atlas Figure 9-78, right side). This ligament is not actively moved during vocalization, and is therefore referred to as a "false vocal cord." Nonetheless, this ligament (or the fold of mucous membrane which covers it) is a prominent feature when the larynx is examined with a laryngoscope.
2. The entire larynx is covered with a mucous membrane, which is given different names in its different parts, depending on the structures it covers over or runs between. (See Atlas Figure 9-78, left side.)
3. The membrane running between the epiglottis and the arytenoid cartilages, called the QUADRANGULAR MEMBRANE, is reinforced by a small cartilage which is embedded in it, the CUNEIFORM CARTILAGE.
4. On each side, another small cartilage, called the CORNICULATE CARTHAGE, caps the arytenoid cartilage superiorly.
N.B. Do not waste any time during the dissection seeking the cuneiform and corniculate cartilages, as finding them grill not significantly advance your understanding of the larynx.
PARTS OF THE LARYNGEAL CAVITY
The boundaries formed by the vestibular and vocal folds (see left side of Atlas Figure 9-78) divide the cavity of the larynx into three parts. These are illustrated schematically in Figure 8 below:
1. The VESTIBULE, which is the area above the vestibular folds.
2. The VENTRICLE, which is a pocket-like recess between the vestibular and vocal folds on both sides.
3. The INFRAGLOTTIC CAVITY, which is the area below the vocal folds.
Figure 8. Schematic diagram (coronal section) illustrating the 3 parts of the laryngeal cavity. Modified slightly from Basmajian, Grant's Method of Anatomy, p.532.
There are two other terms you must know:
1. The GLOTTIS is the aperture formed by the vocal folds (analogous to the iris of the eye).
2. The RIMA GLOTTIDIS is the opening between the vocal folds (analogous to the pupil of the eye). Realize that the size of the rime glottidis is a major factor in determining how much air can enter the trachea and lungs.
MUSCLES OF THE LARYNX
Since you presumably already understand the possible movements of the larynx, understanding the actions of the laryngeal muscles should now be a relatively straightforward matter. The muscles will be presented in functional groups.
1 . MUSCLES WHICH AFFECT THE TENSION OF THE VOCAL CORDS. The CRICOTHYROID MUSCLES (illustrated in Figure 5) are the muscles primarily responsible for regulating the tension of the vocal cords. The small VOCALES muscles, which lie parallel and inferior to the vocal cords (see Atlas Figure 9-74) control fine adjustment of vocal cord tension.
2. MUSCLES WHICH OPEN AND CLOSE THE RIMA GLOTTIDIS (ABDUCT AND ADDUCT THE VOCAL CORDS).
Abductor: The posterior cricoarytenoid is the only abductor of the vocal cords. The actions of the horizontal and vertical fibers of the posterior cricothyroid muscle are illustrated in the center and right panels of Figure 6.
Adductors: The major abductors of the vocal cords are the LATERAL CRICOARYTENOID, THYROARYTENOID, and ARYTENOIDEUS MUSCLES. The disposition of these muscles is illustrated in Figures 9 and 10 and in the left panel of Figure 6.
3. MUSCLES WHICH CLOSE OFF THE VESTIBULE OF THE LARYNX, AS HAPPENS DURING SWALLOWING. The muscles which are mainly responsible for closing off the vestibule are the ARYEPIGLOTTICUS, the THYROEPIGLOTTICUS, and the THYROARYTENOID. Look at Figures 9 and 10 and try to understand how contraction of these muscles would exert a sphincter-like action on the vestibule to the larynx, drawing the epiglottis downward and backward, and the arytenoid cartilages forward, while keeping the vocal cords adducted.
Figure 9. (On left.) Side view with part of the thyroid cartilage removed to show the muscles of the larynx. From Basmajian Grantts Method of Anatomy p. 531.
Figure 10. (On right.) View from behind to show the muscles of the larynx. On the left side the posterior cricoarytenoid muscle has been removed. From Basmajian, Grant's Method of Anatomy, p.531.
BLOOD SUPPLY TO THE LARYNX AND PHARYNX
Blood is supplied to this region by the ascending pharyngea] arteries (pharynx only) and branches of the superior and inferior thyroid arteries (pharynx and larynx).
NERVE SUPPLY TO THE LARYNX AND PHARYNX
1. MOTOR INNERVATION OF THE MUSCLES OF THE PHARYNX AND LARYNX arises mainly from the CRANIAL DIVISION of the ACCESSORY NERVE (XI). Note, however, that these axons TRAVEL with those of the VAGUS NERVE, and are indistinguishable from it (see Figure 1 1).
Exception: The STYLOPHARYNGEUS MUSCLE receives its motor innervation from the GLOSSOPHARYNGEAL NERVE (TX) (see Figure 12). Note that this is the ONLY muscle innervated by this nerve.
Most of the muscles of the larynx receive their innervation via the RECURRENT LARYNGEAL BRANCH of the vagus nerve (see Figure 11 ).
Exception: The CRICOTHYROID MUSCLE, which receives its innervation via the EXTERNAL LARYNGEAL NERVE.
2. SENSORY INNERVATION OF THE LARYNX AND PHARYNX.
Pharynx: via the GLOSSOPHARYNGEAL NERVE (see figure 12).
Larynx: via branches of the VAGUS NERVE (see Figure 11). Above the vocal folds the sensory innervation of the larynx is via the INTERNAL LARYNGEAL NERVE. Below the vocal folds it is by way of branches of the RECURRENT LARYNGEAL NERVE.
3. PARASYMPATHETIC INNERVATION OF THE LARYNX AND PHARYNX is mainly by way of branches of the vagus nerve.
Figure 11. (On left.) Distribution of the branches of the vagus nerve and the fibers of the cranial division of the accessory nerve which travel with it.
Figure 12. (On right.) Distribution of the branches of the glossopharyngeal nerve.
SPECIAL LYMPHATIC TISSUE OF THE PHARYNX
Realize that the pharynx contains a "ring" of specialized lymphatic tissue designed to prevent the entry of pathogens into the digestive and respiratory tracts. This specialized lymphatic tissue is known as "tonsils" and is organized into three groups: 1. nasopharyngeal tonsils (adenoids), located in the nasal pharynx; 2. palatine tonsils (tonsils), located between the palatoglossal and palatopharyngeal folds in the oral pharynx; and 3. lingual tonsils, located on the posterior surface of the tongue. The approximate location of these three groups of lymphoid tissue is indicated in Figure 13.
Figure 13. Semischematic mid-sagittal section through the nose, mouth, end pharynx to illustrate the approximate position of the three groups of "tonsils".