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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 as motor endplates and release acetylcholine acting on nicotinic receptors for muscle contraction [85]. The motor nuclei for all three muscles are in the nucleus ambiguus with somatotopic organization [86]. Sympathetic neural supply is also present, but its role in motor events is unclear [80].