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Author: C.E. Pope ♦ Download Chapter ♦ Download Lecture Slides & Notes

D. Esophageal Physiology

The esophagus has certain physiological differences from the rest of the gastrointestinal tract. For instance, it contains both striated and smooth muscle which suggests the possibility of separate control mechanisms. Unlike the rest of the gastrointestinal tract which maintains a more or less constant state of activity, the esophagus, except for its upper and lower sphincters, remains quiescent until it is called upon to transport gas, fluid or solids. Therefore, it does not possess (or need) a basic electrical rhythm. In the upper sphincter (striated muscle) electrical measurements show a constant series of spikes that is inhibited by a swallow. Spike activity is also recorded from the striated and smooth muscle of the esophageal body during a peristaltic contraction. In humans, no spike activity is recorded from the closed lower esophageal sphincter even though muscular tone is maintained.

One way to explore the physiology of the esophagus is to list the diagnostic modalities available and then see what they reveal about normal esophageal activity. Most of the transport function of the esophagus has been studied by x-ray.

A bolus can drop to the level of the LES by gravity.

In recumbent position, peristaltic wave pushes bolus through esophagus & into stomach. 		With the subject upright, a bolus (lump of food or volume of fluid) of barium suspended in water can be seen by x-ray to drop rapidly by gravity to the area of the lower esophageal sphincter; here it will often hesitate until the peristaltic wave coming behind it reaches the same area. When the patient is recumbent, the barium merely distends the esophagus and the peristaltic wave is necessary to push the barium into the stomach. This allows more leisurely study of the action of the esophagus; x-ray examination in the prone, or even head-down position, should always be employed when the motor function of the esophagus is being evaluated.

Impedance measurements document fluid and gas transport.

Measurement of impedance can also document transport of both fluid and gas through the esophagus. This technique utilizes two circumferential ring electrodes on a catheter. A very weak alternating current is applied across the electrodes, and the impedance ( an analog of resistance in a direct current circuit) is measured. When the esophagus is distended by an electrolyte-containing fluid, the increased ease of current flow through the solution causes a drop in impedance. Conversely, if the esophagus is distended by swallowed or regurgitated air, current flow is impeded and the impedance rises. Pairs of electrodes up and down a catheter can document the passage and direction of fluid and gas.


Pharyngeal muscles rapidly “inject” into esophagus

Peristalsis then propels bolus more slowly into stomach. 		Another method of studying volume transport in the esophagus is by monitoring the passage of a radioactively labeled fluid along the esophagus by means of an externally located gamma camera. This allows a more quantitative description of bolus passage. The transport of a bolus of liquid when the subject swallows in a supine position is rapid initially. This initial rapid phase probably represents the injection phase by the pharyngeal muscles. The peristaltic wave then propels the radioactive fluid at a less rapid rate until it enters the stomach. Abnormalities in transport can be shown either by failure of bolus propulsion, or by to-and-fro movements of the bolus within the esophagus.

Relative luminal pressures
UES: "+40 mmHg"
Body: "-5 mmHg"
LES: "+20 mmHg"
Stomach: +5 mmHg 		To study the circular muscle function of the esophagus, registration of intraluminal pressures (esophageal manometry) is used. Manometrically the esophagus behaves like a flaccid sausage-shaped balloon within the enclosed thoracic cavity. Pressure recording devices placed within the esophageal lumen usually show a resting pressure of 5 mm of mercury below that of the atmosphere. Inspiration makes both intraesophageal and intrathoracic pressure more negative. Pressure within the stomach is usually 4 to 5 mm of mercury higher than that of the atmosphere and inspiration raises this. Thus, there is approximately a 10 mm of mercury pressure gradient between the stomach and the esophagus. Since gastric contents do not normally regurgitate into the esophagus, there must be some structure which separates the two organs. Indeed, manometric evidence of such separation exists. By slowly withdrawing a pressure recording catheter from the stomach into the esophagus, a zone of 2-4 cm in the lower end of the esophagus is observed where the pressure rises to 15 or 20 mm of mercury above gastric pressure. It is not the pressure which separates the two cavities; the rise in pressure merely signifies the fact that the catheter tip has been pulled into a zone where mucosa closed over the tip and caused the pressure level to rise. The height of the pressure recorded is a very rough estimate of the strength of the sphincter. Sphincter pressures are usually expressed in millimeters of mercury greater than gastric pressure. The band of elevated pressure results from the contraction of the specialized muscle of the lower esophageal sphincter, around the manometric catheter. This contraction is maintained by excitatory nerves releasing acetylcholine. When it is necessary for the sphincter to relax during the passage of food or fluid, special inhibitory nerves releasing nitric oxide (NO) are activated. In addition to the basic strength of the contracted lower esophageal sphincter, other factors are important in maintaining sphincter competence. These include the length of the sphincter, the intra-abdominal position of the sphincter, and contraction of the crural fibers of the diaphragm on the sphincter zone during lifting or straining. There are also passive mechanisms that help to maintain separation between esophagus and stomach. The most important is a valve-like structure best appreciated by viewing the gastroesophageal junction from below with a retroflexed endoscope. This structure, the gastroesophageal flap-valve even separates esophagus from stomach in cadavers (no active muscular contraction is present!!). The flap valve is usually absent or deformed in patients with reflux disease.

Swallowing causes a Pressure wave

1° peristalsis = peristaltic wave initiated by swallow

2° peristalsis = peristaltic wave initiated by suddenly stretching esophageal wall (usually caused by reflux)

3° contractions = spontaneous, non-propulsive 		In the body of the esophagus, swallowing is associated with an initial wave of muscular inhibition (mediated by nitric oxide) followed by a sequential activation (mediated by acetylcholine) of longitudinal and circular muscles. Several recording tips in the body of the esophagus show that a pressure wave crosses the recording tips sequentially (Fig. 3). This type of recording is the manometric equivalent of peristalsis (the stripping emptying wave) observed radiologically. This wave can be stimulated either by swallowing air or a small amount of saliva (dry swallow), or by the ingestion of fluid or a solid bolus. Saliva does not “wash” food down; it lubricates the bolus and the esophageal walls. This type of peristalsis is termed primary peristalsis. A peristaltic wave may also be initiated by sudden stretching of the esophageal wall. This can be done experimentally by inflating a balloon in the upper portion of the esophagus, or, more commonly, by reflux of air or fluid from the stomach. Peristalsis which is stimulated in this manner is termed secondary peristalsis. Simultaneous contractions which occur spontaneously without a deglutition or reflux of material are called tertiary contractions. They do not have a transport function.



Figure 3

1. Manometric Events in UES & LES




Deglutition causes both sphincters to relax. UES pressure is restored within 1-2 sec. LES pressure is restored once peristaltic wave passes LES -- ~ 7 sec 		Characteristic manometric events also occur in both the upper and lower esophageal sphincters. As is diagrammed in Fig. 4, deglutition causes an abrupt fall in the high resting pressure maintained in both sphincters. Correlative radiologic studies demonstrate that material passes through the sphincter area during this period of decreased sphincter tension. In the case of the upper sphincter, the high resting pressure is rapidly restored within 1 2 seconds after swallowing. The period of relaxation of the lower esophageal sphincter is much longer (7-8 seconds), lasting from the moment of deglutition until the peristaltic wave has traveled down the length of the esophagus. Recently it has been appreciated that there are episodes of lower sphincter relaxation which are not associated with either swallowing or esophageal distension. These are called transient lower esophageal sphincter relaxations (tLESR). These tLESRs are more common when the stomach is distended. These relaxations normally allow ingested air to be vented as a belch. Episodes of esophageal reflux of gastric contents can occur during these unguarded moments.



Figure 4

2. Description of Esophageal Bolus Transport







Larynx, cricoid pulled up 		Let us now return to that bolus of food which was left suspended in the oropharynx at the beginning of this section and study the act of swallowing from its initiation. After the bolus is placed in the mouth and chewed, swallowing is initiated either voluntarily or automatically. The tongue is sequentially pressed to the roof of the mouth from anteriorly to posteriorly, displacing the bolus posteriorly and inferiorly. The constrictor muscles of the pharynx have a rapidly propulsive sequence which forces the bolus towards the cricopharyngeus. If the bolus is viscous, the duration of the pharyngeal contraction is prolonged. The larynx and cricoid cartilages are pulled superiorly and anteriorly, thus physically opening the upper esophageal sphincter. At the same moment, the intrinsic tone of the cricopharyngeal muscle is reflexly inhibited to allow opening of the upper esophageal sphincter. Once the bolus has passed the cricopharyngeus which rapidly closes, it drops by gravity to the lower esophageal sphincter area (liquid bolus). A solid bolus must reach the lower esophageal sphincter area by being pushed by the peristaltic wave. When either type of bolus reaches the lower esophageal sphincter, it must wait there until the propulsive forces of the peristaltic wave push it through the sphincter. Although the smooth muscle of the lower esophageal sphincter relaxes immediately on swallowing, the sphincter does not actually open until the cohesive forces which keep the esophageal mucosa in close apposition are overcome by the weight of the bolus or by the forces of peristalsis. In addition to the relaxation of the smooth muscle of the lower esophageal sphincter, the crural fibers of the diaphragm are also inhibited to allow passage of the bolus. This process is unidirectional in humans; ruminants have the ability to reverse this process to return the cud for subsequent mastication and re-swallowing.


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