[58, 59]. The oropharynx is the upper portion of the pharynx, between the soft palate and the tongue, extending to the valleculae and tip of the epiglottis, and to the nasopharynx superiorly. The remainder of the pharynx, the hypopharynx, extends from the valleculae to the bottom of the cricoid cartilage. The pyriform sinuses are situated laterally between the insertion of the inferior constrictor and the lateral wall of the thyroid cartilage; they end at the cricopharyngeus muscle and add to the axial and radial asymmetry of the pharynx. The hyoid bone forms the attachment location for a number of muscles that raise and lower the larynx. The upper esophageal sphincter, formed primarily by the cricopharyngeus muscle, operates closely with the pharynx.
Masticating, forming the food bolus, and positioning it on the tongue for the swallow involves the lips, teeth, hard and soft palates, floor of the mouth, mandible and tongue, and associated muscles. During mastication, the posterior tongue is elevated and the soft palate pulled down against the tongue to prevent spillage into the pharynx, although some leaking of oral content does occur [60]. Jaw‐closing and jaw‐opening muscles of mastication coordinate with each other and shift to the swallowing reflex when mastication ends and swallowing begins, likely under central control [61]. Voluntary triggering of swallows occurs >100 milliseconds before the onset of the swallowing reflex, while this interval is <50–100 milliseconds in reflexive swallows [1]. Once the swallow is initiated, the bolus is rapidly thrust into the pharynx by the tongue, following which the involuntary oral, pharyngeal, and esophageal stages of the SPG‐controlled program are initiated. This results in a series of rapid and highly coordinated events that take less than a second to complete: closure of the nasopharynx, elevation of the entire soft pharynx and the UES to protect the airway, opening of the UES, and a propulsive contraction to clear the pharyngeal content. Respiration is halted to temporarily switch the pharynx from a respiratory to an alimentary configuration [2, 54, 62]. Oropharyngeal dysphagia and/or aspiration can occur if any portion of the process fails or becomes abnormal, such as with neurologic or muscle disorders [63–65]. The timing of the symptom within a second of swallow initiation is often a clue that pathophysiology lies in the oropharyngeal stage of swallowing.
Figure 5.3 shows the multitude of muscles involved in different functions of the pharyngeal swallow. Figures 5.4 and 5.5 illustrate the timing of some of the events and the typical X‐ray pattern seen as they occur. Videofluoroscopic examination is the most common method of assessing oropharyngeal swallowing [66]. Computed imaging techniques used to assess tongue muscle function are not practical clinical tools [67, 68]. High‐resolution manometry (HRM) provides precise evaluation of pharyngeal and UES physiology and pathophysiology (Figure 5.6). The clinical value of these measurements continues to be investigated.
Propulsion of the bolus
A liquid bolus moves quickly along the pharynx from tongue thrust and gravity, prior to the slower peristaltic contraction of the three pharyngeal constrictor muscles [69, 70]. The larger the bolus, the quicker it moves; tongue force and shape change to accommodate bolus size [71]. Solid boluses, in contrast, rely on peristaltic contraction for passage and clearance. Pharyngeal peristalsis starts within less than 0.5 s, with the pressures showing radial and axial asymmetry [72, 73]. The 1 s timing of the process to clearance of either liquid or solid is not altered much by bolus consistency or size [71, 74, 75], and pharyngeal peristaltic velocity remains constant.
Protection of the nasal cavity
As the bolus enters the pharynx, the soft palate moves upward and backward to meet concurrent upper constrictor muscle contraction, producing a ridge in the posterior pharyngeal wall called Passavant’s ridge [67, 76]. These actions close the nasopharynx. The contraction of the posterior pharyngeal wall proceeds distally through the pharynx, and the palate remains elevated until the larynx descends [71].
Protection of the airway
Elevation and anterior movement of the larynx and tongue shorten the pharynx [77] through several muscles acting on the hyoid bone, thyroid cartilage, and tongue (see Figure 5.3). A larger bolus is associated with a longer period of laryngeal elevation. The epiglottis is pulled downward while the arytenoid cartilages tilt forward to close entry to the laryngeal vestibule. The false and true vocal cords approximate, closing entry to the trachea. Repositioning of the larynx and opening of the laryngeal vestibule occur after UES closure and aftercontraction, which follows laryngeal contraction. Respiration temporarily ceases during swallowing, most often during expiration, starting immediately prior to a swallow, and lasting 1 1.5 s [54, 62].
Upper esophageal sphincter
Anatomy and innervation
The UES is a 2–4 cm zone of high pressure at the distal end of the pharynx. It is part of the inferior pharyngeal sphincter, intimately associated with the musculature, innervations, and SPG‐programmed events of the oropharyngeal stage of swallowing. The UES is asymmetrical, with pressures higher anteriorly and posteriorly (Figure 5.7). The cricoid cartilage forms the anterior wall. The lateral and posterior walls are formed by the lower portion of the cricopharyngeus muscle, which loops around the esophageal inlet at the location of maximum pressure and inserts anteriorly into the inferolateral margins of the cricoid lamina (Figure 5.8) [78, 79]. The lower‐pressure areas at either end of the high‐pressure zone consist of the lower end of the inferior constrictor muscle proximally and the circular esophageal muscle distally. A triangular space exists between the cricopharyngeus muscle and the lower border of the inferior constrictor muscle (Killian triangle), which is where a Zenker’s diverticulum can form [80].
Figure 5.3 Muscular activity during the pharyngeal stage. A schematic view of the muscles and their actions during the early and late parts of the pharyngeal stage of the swallow.
Source: Jones [69] with permissions of Springer Nature.
Figure 5.4 Volume‐induced (1 and 20 mL) modification of timing of events during the pharyngeal swallow. Each horizontal bar depicts the time each one of the oropharyngeal “valves” is moved into position to facilitate bolus passage and protect the airway. Events at the onset and offset of these moves bear a fixed time relationship to each other regardless of bolus volume. This fixed relationship is shown by referencing onset events from time 0, counting forward, and offset events from time 0, counting forward or backward. The middle portion of the events (shaded bar) defines the volume effect. Regardless of bolus size, the entire series of events occurs in about a second or less.
Source: Kahrilas et al. [75] with permissions of Elsevier.
The UES consists predominantly of slow‐twitch type striated muscle, with abundant connective and elastic tissue [5981–83] that allow maintenance of resting tone even when excitatory neural input is lost. Of the three UES muscular elements, the inferior constrictor receives its innervation from the pharyngeal branches of the vagus, the cricopharyngeus from both the pharyngeal and recurrent laryngeal (RLN) branches, and the upper striated muscle esophagus from the RLN [84]. These serve both motor and sensory functions. Motor fibers terminate