The Motility of the Gastrointestinal Tract
James Christensen, M. D.
Peer Review Status: Internally Peer Reviewed
Introduction
The accessibility of the pharynx and esophagus accounts for the fact that we know so much more about the motility of these parts of the gut than we do about any of the others. As organs with essentially one function, the rapid transfer of swallowed material to the stomach (and the corollary purpose of keeping it there), their motor operation differs considerably from that of the stomach, the small intestine, and the colon. The motor mechanisms in the pharynx and esophagus operate only on demand rather than continuously and autonomously. Also they produce essentially stereo-typical rather than highly variable behavior. They cause only forward bulk transit rather than mixing or microflows. This chapter summarizes what we know about this biological pumping system.
Behavior of the Pharynx and Esophagus at Rest
This part of the gastrointestinal conduit contains different kinds of nerve and muscle in its two parts yet it behaves as a unit, one that comes into action only on demand. Except in the brief period just after the initiation of a swallow, the pharynx and esophagus remain in a relatively steady state, in a state of equilibrium.
Striated Muscle Regions. The striated musculature of the pharynx, organized into the three pharyngeal constrictors, exhibits a stable level of contraction or tone just sufficient to keep the organ in its normal conical configuration without occluding the lumen. This minimizes resistance to the passage of air in normal respiration. A constant low level of tonic contraction also characterizes the parapharyngeal musculature, the striated muscles that share their innervation with the pharynx. They contract more forcefully in swallowing to straighten the angle between the pharynx and esophagus, thus enhancing flow into the gastrointestinal tract.
The cricopharyngeus muscle (the upper esophageal sphincter) maintains a stable tonic contraction. This is just strong enough to fully occlude the conduit by compressing the pharyngoesophageal junction against the cricoid cartilage, producing a crescentic closure. The axial length of the occluded segment is about 2 to 4 centimeters. The force of the occluding contraction falls a little during deep sleep and in general anesthesia. It fluctuates with respiration, and it rises in the Valsalva maneuver, during esophageal acidification, and with esophageal distension.
The force of closure of the upper esophageal sphincter is usually estimated clinically in terms of the pressure as it is recorded by a standardized technique, esophageal manometry, from a small constantly perfused catheter with a distal lateral opening located within the occluded segment. In such a system, the pressure required to keep water flowing through the small opening reflects the force of contraction of the circular muscle layer. The recorded pressure varies with the direction that the lateral opening faces because of the crescentic shape of the lumen. The normal sphincter rarely produces an intraluminal pressure greater than about 130 millimeters of mercury relative to atmosphere. The presence of the catheter in the lumen, as small as it is, excites the muscle to contract, so this measurement must necessarily be somewhat artificial. The actual resting pressure in the lumen should be the same as that in the substance of the muscle, and most of the time this must remain below the vascular perfusion pressure of the tissue. Thus, the sphincter muscle must shorten just enough to occlude the lumen without raising pressure in the tissue enough to compromise blood flow, but it contracts in response to efforts to lengthen it.
The striated muscle of the rostral one-third of the esophagus has no tone. Whatever movements it exhibits in repose arise passively as a part of the motions of respiration. Intraluminal pressure in this part of the organ, essentially the same as intrathoracic pressure, varies at rest between a minimum of —15 (in inspiration) and a maximum of +5 (in expiration) millimeters of mercury, according to the respiratory effort.
Smooth Muscle Regions. The smooth muscle of the caudal two-thirds of the esophagus also remains flaccid at rest, devoid of tone. No differences in behavior at rest distinguish the smooth muscle segment of the esophageal body from that of the striated muscle segment.
In contrast, the circular muscle layer at about the level of the esophagogastric junction exhibits a stable level of contraction sufficient to occlude the lumen over an axial length of 2 to 3 centimeters. This tonic contraction defines the lower esophageal sphincter. The contraction wrinkles the thick mucosa at this level so that the fully occluded lumen takes on a stellate configuration.
The force of the contraction in the lower esophageal sphincter varies over time, with the amount of fat in the diet, with smoking, with the ingestion of alcohol, and with the oxygen supply of the muscle. As measured by the technique of intraluminal manometry, the intraluminal pressure in the sphincter also varies with the precise position of the lateral opening at the tip of the catheter. It varies with the radial position because the variation in forces created by the wrinkles in the mucosa and produced by the right diaphragmatic crus increase the pressure recorded. The pressure registered is actually that which is necessary to overcome the resistance to transit of water through the distal lateral opening in the perfused catheter. This is greater when the catheter opening lies directly against a fold of the mucosa or the firm diaphragmatic muscle than it is when the opening faces a crevice in the folded mucosa.
Manometrically recorded intraluminal pressure in the lower esophageal sphincter also varies with the exact position of the catheter opening along the axis of the conduit. The sphincter does not exert the same force of contraction throughout all of its length. Force is probably highest at about the middle of that segment. For this reason, most investigators measure pressure by a "pull-through" technique, in which the catheter is pulled slowly and continuously through the sphincter while the pressure is measured. The highest reading is taken as definitive.
The normal resting pressure recorded by conventional manometry in the lower esophageal sphincter generally lies between 10 and 40 millimeters of mercury above atmospheric pressure. As described above for the upper esophageal sphincter, this pressure partly represents an artifact from the reflex contraction of the muscle induced by the presence of the catheter in the lumen. The actual intraluminal pressure in the sphincter must be the same as that within the tissue of the sphincter, and this cannot exceed the vascular perfusion pressure of the tissue. The circular layer of muscle in the sphincter probably shortens just enough to occlude the lumen without much raising pressure in the tissue, at the same time resisting efforts to stretch it.
Behavior of the Pharynx and Esophagus in Swallowing(Figure 7.1)
Striated Muscle Regions. Voluntary efforts that include the closure of the jaws, the elevation of the tongue against the palate, and the contraction of the oropharynx, initiate a swallow. This set of movements triggers a series of involuntary events. They include contraction of the posterior part of the tongue, further contraction of the pharyngeal muscles, contraction of the parapharyngeal muscles, relaxation of the tonic contraction of the upper esophageal sphincter, and the initiation of a peristaltic contraction of the esophageal body.
The involuntary apposition of the soft palate to the posterior pharyngeal wall, lasting almost a second and producing a pressure of about 160 millimeters of mercury, initiates pharyngeal peristalsis. This moving front of contraction passes through the pharyngeal constrictors in sequence, traversing the pharynx and hypopharynx at about 15 centimeters per second to reach the upper esophageal sphincter in about one second. The hypopharyngeal contraction lasts 0.3 to 0.5 second and generates an intraluminal pressure of 200 millimeters of mercury. At about the time these involuntary events begin, the parapharyngeal muscles contract beginning with the mylohyoid. The other suprahyoid muscles follow in a fixed order, with the whole complex lasting about 0.5 second. These movements produce a moving occlusion of the pharyngeal lumen, a straightening of the pharyngoesophageal conduit produced by the rostral and anterior displacement of the larynx, and the closure of the nasal, oral, and pharyngeal openings of the pharynx.
The relaxation of the upper esophageal sphincter, necessary to reduce the resistance to transit across the region, begins about 0.2 second after the initiation of the swallow and lasts up to 0.8 second. The muscle then contracts forcefully for another second or so before relaxing to the force of closure that characterizes the resting state.
The contraction of the upper esophageal sphincter that follows its relaxation, in effect, moves into the esophageal body as a progressive front of contraction, proceeding at a velocity of about 3 centimeters per second. The force of contraction declines slightly as it advances, reaching a nadir at about the level where the muscle becomes smooth muscle. No other functional change marks the level of this transition. Presumably, the longitudinal layer of striated muscle in this part of the esophagus contracts in a swallow as well.
Smooth Muscle Regions. The front of contraction that moves along the esophageal body composed of smooth muscle at first increases its velocity, achieving a maximum of 5 centimeters per second before it declines to about 2.5 centimeters per second just above the lower esophageal sphincter. The magnitude of the intraluminal pressure induced by this peristaltic contraction, as measured by esophageal manometry, varies following the same general pattern along the conduit as the velocity does. They both rise slightly from a nadir at the junction of the two kinds of muscle and then both decline slightly with the approach to the lower esophageal sphincter. The force of the peristaltic contraction also varies with the age of the subject, the volume of the bolus, the temperature of the bolus, the intraabdominal pressure, and other factors. As a result, normal values for the force of the peristaltic contraction vary widely. Most authorities accept anything between about 20 and about 120 millimeters of mercury as normal.
The constant tonic contraction of the muscle of the lower esophageal sphincter begins to abate about 2 seconds after the initiation of a swallow, so that its relaxation starts well after the peristaltic contraction has commenced its progress down the esophageal body. The relaxation of the sphincter muscle lasts for 5 to 10 seconds. A transient contraction to a force about double that of the resting tone of the sphincter follows this relaxation, after which the force falls gradually, reaching the resting level after about 2 sec. The timing of this transient hypercontraction after the relaxation makes it appear to be an extension of the peristaltic contraction that sweeps the esophageal body.
Considerable axial displacement of markers placed along the esophagus occurs with a swallow. This motion must represent contraction of both the outer longitudinal muscle layer and the powerful longitudinal muscle of the mucosa. As these two longitudinal muscle layers of the esophagus contract after a swallow, a segment of the esophageal body just above the diaphragm must elongate to compensate for the esophageal shortening, for the longitudinal muscle layers do not pull the cardia up into the thorax as they contract. The failure of this compensatory mechanism would account for the formation of hiatus hernias.
Underlying Neural and Muscular Physiology
The complex, yet stereotyped, pattern of pharyngeal, parapharyngeal, and esophageal contractions in swallowing represents a coordination of these movements that is exerted by a governing system outside the conduit itself. The dominance of such an extrinsic executive mechanism also explains why the pharynx and esophagus show far less autonomy in their operation than do the other parts of the gut.
Central Organization of Swallowing. The swallowing center, which becomes functional as early in fetal development as 12 weeks, lies in the reticular substance bilaterally, 1 to 3 millimeters dorsal to the superior pole of the inferior olive. It receives several inputs besides those from cortical and subcortical sources, as evidenced by the initiation or facilitation of swallowing by mandibular movement or position, by the elevated position of the tongue, and by pharyngeal mucosal stimulation. Its links to the respiratory and vomiting centers accomplish the necessary coordination of swallowing with the events controlled by those other centers. The output targets of the swallowing center include many brainstem nuclei. The pattern of discharge of the swallowing center must be constant, for the various inhibitions and excitations of the muscles participating in swallowing occur in a fixed sequence.
Control of the Pharynx, Parapharyngeal Muscles, Upper Esophageal Sphincter, and Rostral (Striated Muscle) Part of the Esophageal Body. These striated muscles exhibit tonic contraction at rest. A striated muscle fiber exists in total submission to its cholinergic somatic motor neuron. Thus, the magnitude of the tonic contraction of this striated musculature reflects the level of tonic discharge of the responsible motor neurons. The size of the motor unit in this muscle approximates the size of that in the extraocular muscles, less than about ten muscle fibers. From this, it seems clear that the contraction and relaxation of the striated muscle in the different parts of the pharyngoesophageal conduit represent the coordinated behavior of motor neurons within the motor nuclei of the glossopharyngeal and vagus nerves. In all of these muscles, swallowing first inhibits the constant tonic contraction that characterizes the resting state before the major contraction occurs. Thus, a motor neuron in any of these nuclei must exhibit a constant low level of discharge at rest. After the neuron receives a signal from the swallowing center, it must first cease its tonic discharge before it discharges maximally. The neurons receive these signals from the swallowing center in a fixed sequence that corresponds to the craniocaudal sequence of the motor units in the organ.
This scheme explains the full sequence of events. It accounts for both the relaxation of the upper esophageal sphincter, distinguished by a high rate of motor nerve discharge in the resting state, and the hypercontraction that follows it. It explains the progressive nature of the peristaltic contraction of the striated muscle of the esophageal body, where the tonic contraction of the resting state is either absent or too low to detect.
Control of the Caudal (Smooth Muscle) Part of the Esophageal Body and Lower Esophageal Sphincter. The anatomy of esophageal smooth muscle and of the visceral nerves that supply it make a motor unit organization impossible. The foundation of peristalsis and of sphincter function in this region lies in the general properties of smooth muscle and of autonomic nerves, qualities quite different from those of the somatic nerves and striated muscle of the more rostral parts of the pharyngoesophageal conduit. The vagal discharge that occurs with a swallow only triggers the peristaltic contraction in this region. The contraction itself, including its force and its progression along the organ, represents the operation of an intramural mechanism.
The flaccidity of the smooth muscle part of the esophageal body at rest reflects both the quiescence of its regulatory innervation and the absence here of those properties of smooth muscle that lead to tone or spontaneous contractions. The presence of the latter characteristics, however, producing a myogenic contraction, defines the lower esophageal sphincter. The innervation of the esophageal body and the lower esophageal sphincter is the same. Only the myogenic tonic contraction of the sphincter muscle distinguishes between them. A relative hypersensitivity to hormones and to agents that activate smooth muscle through membrane mechanisms accompanies this myogenic tone in the sphincter. Changes in the levels of circulating hormones released in eating account for the variations in sphincter tone observed with diet. Sphincter tone declines with the ingestion of fats, the result of an inhibitory reflex that arises in chemoreceptors of the duodenal mucosa. Sphincter tone declines with smoking and alcohol ingestion, but these changes remain unexplained.
Two kinds of motor nerves, excitatory nerves that release acetylcholine and inhibitory nerves that release nitric oxide, account for all events that occur in smooth muscle of the esophagus in a swallow. Contractions of both the longitudinal muscle layer and the mucosal muscle seem to result from the release of neuronal acetylcholine alone. The excitation of nerves that release nitric oxide in swallowing explains both the relaxation of the lower esophageal sphincter and swallow-induced contractions of the esophageal body. The magnitude of peristaltic contractions in this part of the esophagus partly reflects the release of acetylcholine. Their timing (their organization into a progressing event), however, represents the effect of nitric oxide. How these two neurotransmitters interact in this process remains one of the great challenges in the field.
Esophageal peristalsis can also be excited reflexly in the smooth muscle segment (but not in the striated muscle part), by transient distension of the organ, both in situ and in vitro. This indicates the presence of complete sensorimotor reflex pathways within the organ wall. The responsible stretch receptors may well be the intraganglionic laminar endings in the myenteric plexus, described in Chapter III.
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