Ganong's Review of Medical Physiology, 23rd Edition

472
SECTION V
Gastrointestinal Physiology

at the junction with the stomach (intrinsic sphincter). Fibers
of the crural portion of the diaphragm, a skeletal muscle, sur-
round the esophagus at this point (extrinsic sphincter) and ex-
ert a pinchcock-like action on the esophagus. In addition, the
oblique or sling fibers of the stomach wall create a flap valve
that helps close off the esophagogastric junction and prevent
regurgitation when intragastric pressure rises.
The tone of the LES is under neural control. Release of ace-
tylcholine from vagal endings causes the intrinsic sphincter to

contract, and release of NO and VIP from interneurons inner-

vated by other vagal fibers causes it to relax. Contraction of

the crural portion of the diaphragm, which is innervated by

the phrenic nerves, is coordinated with respiration and con-

tractions of chest and abdominal muscles. Thus, the intrinsic

and extrinsic sphincters operate together to permit orderly

flow of food into the stomach and to prevent reflux of gastric

contents into the esophagus (Clinical Box 28–1).

FIGURE 28–3
Movement of food through the pharynx and upper esophagus during swallowing. (a)
The tongue pushes the food bo-
lus to the back of the mouth.
(b)
The soft palate elevates to prevent food from entering the nasal passages.
(c)
The epiglottis covers the glottis to
prevent food from entering the trachea and the upper esophageal sphincter relaxes.
(d)
Food descends into the esophagus.

Hard palate Soft palate

Tongue

Glottis

Trachea Esophagus

(a) (b) (c)

Epiglottis

Food

Upper

esophageal

sphincter

(d)

Pharynx

FIGURE 28–4
Esophagogastric junction.
Note that the lower esophageal sphincter (intrinsic sphincter) is supplemented by the crural por-
tion of the diaphragm (extrinsic sphincter), and that the two are anchored to each other by the phrenoesophageal ligament.
(Reproduced with
permission, from Mittal RK, Balaban DH: The esophagogastric junction. N Engl J Med 1997;336:924. Copyright © 1997 by the Massachusetts Medical Society. All rights reserved.)

Costal part

Crural part

Diaphragm

Internal

External

Lower

esophageal

sphincter

Phrenoesophageal

ligament

Sling fibers

Squamocolumnar Stomach

junction

Longitudinal muscle

Circular muscle