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Mittal RK. Motor Function of the Pharynx, Esophagus, and its Sphincters. San Rafael (CA): Morgan & Claypool Life Sciences; 2011.

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Motor Function of the Pharynx, Esophagus, and its Sphincters.

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Upper Esophageal Sphincter

Upper esophageal sphincter (UES) has also been referred to as the inferior pharyngeal sphincter because it is located at the lower end of pharynx and guards the entrance into the esophagus. It has two major functions: (1) to prevent air from entering into the esophagus during breathing and (2) to prevent reflux of esophageal contents into the pharynx to guard airway aspiration. It is best recognized functionally as a high-pressure zone that extends 3–4 cm in its vertical extent. Anatomically, it is located behind the cricoid cartilage but extends both above and below it. Even though it is generally agreed that cricopharyngeus is a major contributor to the UES high-pressure zone, thyropharyngeus (part of inferior pharyngeal constrictor) and cervical esophagus also contribute to it in its proximal and distal extents, respectively. Simultaneously, pressure and fluoroscopic imaging studies show that the peak pressure of the UES high-pressure zone is located above the cricopharyngeus muscle [28] (Figure 6). Furthermore, cricopharyngeus is only 1 cm in width but the UES pressure zone is 3–4 cm long. Accordingly, a surgical incision of 5–6 in length [29], which extends over inferior pharyngeal constrictor, cricopharyngeus, and cervical esophageal muscle, is required to completely ablate the UES pressure (as measured by the Sleeve sensor in the humans [30]). Fibers of thyropharyngeus are placed obliquely (pars obliques) and cricopharyngeus horizontally (pars profundus) to form the UES (Figure 7). Muscle fibers of the cricopharyngeus have both slow (oxidative) and fast (glycolytic) type fibers, even though slow ones predominate [3133]. Slow fibers most likely contribute to the tonic and fast fibers to the phasic contractions that are involved in rapid reflex contractions of the UES high-pressure zone. Forty percent of the muscle mass is contributed by the collagen and elastic tissue (endomysial tissue) [31,32], and it is felt that the UES is functionally quite compliant even though noncompliant cricoid cartilage forms its anterior extent. Cricopharyngeus originates from the cricoid cartilage, loops around the pharynx in a C or “horse shoe shape manner,” and is inserted back into the cricoid cartilage (unique muscle that has origin and insertion into the same structure). Most skeletal muscles generate maximal force at what is referred to as the optimal muscle length. However, the in vivo operational length of the UES muscle is significantly shorter than its optimal length (1.7 times) [34]. As a result, the greater the diameter of the manometry probe the greater the measured UES pressure. Muscle spindles are absent from the UES muscle but Golgi-tendon-like structure, through which motor neurons may monitor muscle tone, is present [35].

FIGURE 6. Data compiled from three different studies of the position of the UEHPZ with respect to individual pharyngeal muscles based on combined manometric and videofluoroscopic studies.

FIGURE 6

Data compiled from three different studies of the position of the UEHPZ with respect to individual pharyngeal muscles based on combined manometric and videofluoroscopic studies. Note that the peak pressures of the UEHPZ in humans at rest with the head (more...)

FIGURE 7. Trajectory of UES with swallow and belch.

FIGURE 7

Trajectory of UES with swallow and belch. Open circles indicate UES opening observed by videofluoroscopy. Although hyoid bone movement during swallowing was invariably upward, forward, and counterclockwise, its movement during belching was mainly anterior (more...)

UES is innervated by the glossopharyngeal, branches of vagus, ansa cervicalis, and sympathetic nerves (from cervical ganglion). The vagus nerve, through its pharyngeal, superior laryngeal and recurrent laryngeal nerve branches, is the major motor nerve of the UES. All these nerves form a pharyngeal plexus before penetrating into the muscle fibers. Nerve cell bodies of the vagal efferent fibers are located in the nucleus ambiguus. It is not clear if there are any special structures that form the sensory nerve ending in these regions, afferent nerves travel to nodose and jugular ganglion cells and from there go on to the nucleus tractus solitaries (NTS), which in turn communicates through the reticular formation of brain stem or directly to the motor neurons of the nucleus ambiguus. Sympathetic nerves supply the mucosal gland and blood vessels in the region and probably carry some sensory information. Acetylcholine acting through the nicotinic receptors located on the motor nerve plate is the major neurotransmitter of the UES muscles. However, other neuropeptides, calcitonin gene-related peptide, neuropeptide Y, substance P, vasoactive intestinal polypeptide, and galanin are present in the region, their function is probably related to the control of blood flow [28,36].

UES pressure is distributed predominantly in the anterior–posterior directions; lateral pressures are about 33% of the anterior–posterior ones. In addition to circumferential asymmetry, there is also axial asymmetry of the UES pressure. Peak anterior pressure is located more cranially than the peak posterior pressure. Laryngectomy decrease UES pressure asymmetry [37,38]. Normal range of UES pressure is quite large, 30–200 mm Hg (side-hole manometry or solid-state transducer) and 30–110 mm Hg (sleeve sensor). Therefore, measurement of resting pressure is not a useful parameter in the clinical studies. UES pressure is extremely labile; it is higher with rapid pull-through than station pull-through technique of pressure measurement, decreases with decrease in the wakeful state, and almost disappears with sleep [39] and anesthesia. Psychological stress and anxiety [40] also increase UES pressure significantly, and with aging, there is a decrease in UES pressure and its compliance [27]. Inspiration and phonation augment UES pressure [41]. A number of aerodigestive protective reflexes are operative in the UES. (1) The pharyngoglottal reflex, which is part of the gag reflex and results in increase in the UES pressure with pharyngeal stimulation. It can be elicited by injection of tiny amounts of water just above the UES [42]. (2) The esophago-UES reflexes can be either excitatory or inhibitory. Distension of the esophagus with a balloon or air causes reflex contraction of the UES (proximal distension greater than the distal). Rapid injection with air or a long cylindrical balloon causes UES relaxation which is important in belching [43]. It appears that rapidity of pressure change in the esophagus associated with gastroesophageal reflux is the major determinant whether UES relaxes or contracts in response to distension of the esophagus. Air reflux into the esophagus, especially in the upright position is associated with UES relaxation. On the other hand, liquid reflux, associated with slow increase in the esophageal pressure induces reflex UES contraction [44]. Same may be true for regurgitation and vomiting where pressure increases quite rapidly in the esophagus to cause UES relaxation. Findings with regards to the effect of acid in the esophagus on UES have provided contradictory results [45,46]; even though latest word is that it has no significant effect [44].

Swallow-induced relaxation of the UES lasts for 0.32–0.5 seconds, depending upon the bolus volume, directly related to the bolus volume [47]. Two distinct events are responsible for the swallow-induced relaxation of UES: (1) cessation of tonic discharges of the motor neurons of nucleus ambiguus and (2) anterior and superior lift of the hyoid, cricoids, and UES by the contraction of suprahyoid muscles. Cessation of motor neuron discharges causes UES relaxation which is seen as the cessation of EMG activity in the cricopharyngeus and thyropharyngeus muscles. A residual UES pressure of 10–15 mm Hg [48], following cessation of the EMG activity in these muscles, is because of the viscoelastic properties of muscle and surrounding structures. The residual UES pressure is ablated by a forceful superior (2.5 cm) and anterior (0.75 cm) stretch exerted on the UES by contraction of suprahyoid muscles (geniohyoid and mylohyoid), which results in the UES opening. Extent of UES opening is related to bolus volume and bolus pressure. UES during a swallow is described as a grabber because it ascends to grab the bolus and then descends with it. Physical therapy used to strengthen suprahyoid muscles (Mendelsohn maneuver) improves UES relaxation and opening function in patients with dysphagia related to the UES relaxation and opening dysfunction [18]. UES relaxation and opening also occurs during belching but the trajectory of movement of cricoid cartilage and UES is different from the swallow. With belching, the UES moves mostly in the anterior direction (not in the oral direction) related to the contraction of infrahyoid muscles [49] (Figure 7), suggesting that different set of muscles are activated during these two events.

Copyright © 2011 by Morgan & Claypool Life Sciences.
Bookshelf ID: NBK54282
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