Pharynx is 12–14 cm in its vertical length and extends from the base of skull to the upper border of upper esophageal sphincter (UES) [14]. It can be divided into three segments: nasopharynx that extends from the base of skull to the soft palate, oropharynx extending from soft palate to the pharyngoepiglottic fold, and hypopharynx from the pharyngoepiglottic fold to the UES (Figure 2). It is predominately a muscular structure although epiglottis, arytenoid, cuneiform, corniculate, and cricoid cartilage form part of the anterior wall. Furthermore, hyoid and thyroid bones provide attachment to some of its muscles. Muscles of pharynx can be broadly viewed as intrinsic and extrinsic; the former constitutes the superior, middle, and inferior pharyngeal constrictors along with thyropharyngeus (oblique) and cricopharyngeus (horizontal fibers). A triangular area of relatively scanty muscle fibers exists between the oblique and horizontal muscle fibers (Killian's triangle), which is the site of Zenker diverticulum. The latter is seen in patients with difficulty in swallowing, who have impaired relaxation or opening of the UES [15,16]. Extrinsic muscles of the pharynx can be categorized into three subgroups: Group 1—elevators and tensors of palate (levator veli palatini, tensor veli palatini, and palatoglossus); Group 2—geniohyoid, mylohyoid, stylohyoid, thyrohyoid, diagastric, stylopharyngeus, and palatopharyngeus, which cause superior and anterior displacement of the larynx during swallowing; Group 3—aryepiglottic, thyroarytenoid, and oblique arytenoids muscles, which close the laryngeal inlet. These muscles are supplied by the branches of cranial nerves V (trigeminal), VII (facial), IX (glossopharyngeal), X (vagus), ansa cervicalis, and XII (hypoglossal). Pharyngeal muscles are richly innervated, with a nerve–muscle fiber innervation ratio of 1:2 to 1:6, as compared to 1:2000 for human gastrocnemius muscle [14] and 1:9 for extraocular eye muscles, [17] which is important for the “fine” control required for its function.
FIGURE 2
Musculature of pharynx and upper esophageal sphincter. (Source: Netter medical illustration with permission of Elsevier.).
A lateral radiograph of the oropharyngeal region shows an air column in the nasopharynx, oropharynx and upper part of the laryngopharynx (up to the lower border of laryngeal opening) [14]. While the posterior wall of pharynx is smooth, the anterior wall is irregular because it is formed by the posterior nasal aperture, soft palate, oral cavity, tongue, valleculae, epiglottis, laryngeal vestibule, and posterior surface of cricoids cartilage. Pharynx is in contact with four different cavities, nasal, oral, laryngeal, and esophagus and serves two major functions, breathing and swallowing. Upon deglutition, pharynx becomes a swallow structure from a breathing structure that has distinct anatomy (Figure 3). The characteristics of pharynx during swallow are closure of palate along with elevation and closure of the laryngeal inlet. A swallow can be divided into four phases: (1) preparatory, (2) oral, (3) pharyngeal, and (4) esophageal. Oral and pharyngeal phase is also referred to as the oropharyngeal phase. During the preparatory phase, food bolus remains in the oral cavity, masticated and mixed with saliva, sized and shaped, and positioned on the dorsum of tongue. Sequential or peristaltic contraction of the tongue against the hard and soft palates generates peristaltic pressure wave that pushes the bolus into pharynx. With pharyngeal phase starts the involuntary phase of swallow. Elevation of soft palate during oropharyngeal phase seals the nasopharynx to prevent nasal regurgitation. Contraction of suprahyoid muscle causes elevation and forward movement of the larynx, pharynx, and UES that result in approximation of larynx against the epiglottis. At the same time, contraction of intrinsic laryngeal muscles results in closure of laryngeal inlet to provide additional airway protection mechanism [18]. Laryngeal closure is of paramount importance during swallowing. A three-tier system exists to protect the airway from swallowed contents. Three-tier system starts from the distal to the proximal end of larynx, (1) adduction of true vocal cords and arytenoids, (2) vertical approximation of closed arytenoids to the base of epiglottis, and (3) descent of epiglottis to cover the closed glottis thereby closing the laryngeal vestibule [5]. Larynx is elevated 2 to 3 cm during a swallow and it sits under the base of tongue at the height of its excursion, a key step in the airway protection mechanism. Temporal sequence of swallow related events in the pharynx and larynx, crucial to the airway protection mechanism, is shown in Figure 4 [19]. A normal oropharyngeal phase consists of complete transfer of oral contents into the esophagus without any entry into the laryngeal inlet.
FIGURE 3
The oropharyngeal swallow as imaged by videofluoroscopy and reconstructed in three dimensions with computer graphics. From left to right, each horizontally arranged group of images contains the three-dimensional reconstruction of the pharyngeal cavity (more...)
FIGURE 4
Bolus transit through the pharynx and across the upper esophageal sphincter (UES) begins and ends while the vocal cords are at maximal adduction. TB-O, onset of tongue base movement; SH-O, onset of superior hyoid movement; SM-O, onset of submental myoelectrical (more...)
Movement of the head end of bolus in the pharynx and pharyngeal peristalsis are distinct and two separate events. Thrust caused by tongue peristalsis (and gravity in the upright position) propels the head end of bolus rapidly into the pharynx. On the other hand, pharyngeal peristalsis that follows the tail end of bolus clears the bolus from the pharynx relatively slowly (approximately 1 second). Closure of nasopharynx and larynx has already occurred when the head end of the bolus enters pharynx. Mylohyoid is the first muscle to be activated when swallowing [2], and it is followed by contraction of other suprahyoid muscles [20,21] (Figure 5). Upper esophageal sphincter (UES) relaxes 0.3 seconds after the onset of mylohyoid muscle contraction and prior to the arrival of bolus head. Increase in bolus volume delays the onset of pharyngeal contraction without affecting timing of closure of nasopharynx and larynx, and UES relaxation. Certain aspects of swallow reflex are modifiable, such as the duration of UES opening and excursion of hyoid bone that increase with increase in the bolus volume [22,23]. Amplitude of pharyngeal peristalsis may also increase with bolus volume [19], suggesting that not all parts of swallow cascade are stereotypically programmed. Literature suggests that a negative pressure wave develops in the cervical esophagus with swallowing that provides pressure gradient for the movement of bolus head into the esophagus [24–26]; however, it is not clear if it is a recording artifact. With the arrival of bolus in the pharynx, there is small increase in the pharyngeal pressure (bolus pressure), which likely provides the force that distends and opens the UES. A bolus volume increase from 1 to 20 ml increases bolus pressure from 5 to 17 mm Hg [27]. Increase in the bolus pressure is reflective of outflow resistance caused by a poor relaxation or low compliance of the UES and is seen in normal subjects with aging and patients with oropharyngeal dysphagia who have cricopharyngeal bar and Zenker's diverticulum [16].
FIGURE 5
This figure illustrates the EMG responses of the three UES closure muscles and many of the superior hyoid UES opening muscles during a dry swallow ad lib feeding of canned food in a chronically instrumented dog. Note that the three UES closing muscles (more...)
Pharyngeal peristalsis follows the tail end of bolus and is the slower part of the bolus transport mechanism across the pharynx. The speed of peristalsis is approximately 15 cm/sec in the pharynx and it takes 1 second for the contraction wave to travel from the top of pharynx to the UES. Pharyngeal contraction waves are axially and circumferentially asymmetric, most likely related to the complex anatomy of the closed segment. Amplitude of pharyngeal contraction in normal subjects ranges from 100 to 150 mm Hg. Amplitude increases and duration decreases from proximal to the distal location in the pharynx [27]. Outflow resistance related to the UES dysfunction, as may occur with aging, cricopharyngeal bar, and Zenker's diverticulum are key modulators of pharyngeal peristalsis, pharyngeal contraction amplitude, and duration [18].
