Human Physiology, 14th edition (2016)

622 Chapter 18

and an outer longitudinal layer of smooth muscle. Contrac-

tions of these layers move the food through the tract and

physically pulverize and mix the food with digestive enzymes.

The myenteric plexus ( Auerbach’s plexus ), located between

the two muscle layers, provides the major nerve supply to

the entire GI tract. It includes fibers and ganglia from both the

sympathetic and parasympathetic divisions of the autonomic

nervous system.

Serosa

The outer serosa completes the wall of the GI tract. It con-

sists of areolar connective tissue covered with a layer of simple

squamous epithelium, and is continuous with the mesentery

and visceral peritoneum.

Regulation of the

Gastrointestinal Tract

The GI tract is innervated by the sympathetic and para-

sympathetic divisions of the autonomic nervous system. As

discussed in chapter 9, parasympathetic nerves in general

stimulate motility and secretions of the gastrointestinal tract.

The vagus nerve is the source of parasympathetic activity in

the esophagus, stomach, pancreas, gallbladder, small intestine,

and upper portion of the large intestine. The lower portion of

the large intestine receives parasympathetic innervation from

spinal nerves in the sacral region. The submucosal plexus and

myenteric plexus are the sites where parasympathetic pregan-

glionic fibers synapse with postganglionic neurons that inner-

vate the smooth muscle of the GI tract.

Postganglionic sympathetic fibers pass through the submu-

cosal and myenteric plexuses and innervate the GI tract. The

effects of the sympathetic nerves reduce peristalsis and secre-

tory activity and stimulate the contraction of sphincter muscles

along the GI tract; therefore, they are antagonistic to the effects

of parasympathetic nerve stimulation.

Autonomic regulation, which is “extrinsic” to the gastro-

intestinal tract, is superimposed on “intrinsic” modes of regu-

lation. The gastrointestinal tract contains intrinsic sensory

neurons that have their cell bodies within the gut wall and

are not part of the autonomic system. These help in the local

regulation of the digestive tract by a complex neural network

within the wall of the gut called the enteric nervous system,

or enteric brain (discussed in section 18.6). Regulation by

the enteric nervous system complements paracrine regulation

by molecules acting locally within the tissues of the GI tract,

as well as hormonal regulation by hormones secreted by the

mucosa.

In summary, the digestive system is regulated extrinsically

by the autonomic nervous system and endocrine system, and

intrinsically by the enteric nervous system and various para-

crine regulators. The details of this regulation will be described

in subsequent sections.

Submucosa

The relatively thick submucosa is a highly vascular layer of
connective tissue that serves the mucosa. Absorbed molecules
that pass through the columnar epithelial cells of the mucosa
enter into blood and lymphatic vessels of the submucosa. In
addition to blood vessels, the submucosa contains glands and
nerve plexuses. The submucosal plexus ( Meissner’s plexus )
( fig. 18.3 b ) provides a nerve supply to the muscularis mucosae
of the small and large intestine.

Muscularis

The muscularis (also called the muscularis externa ) is respon-
sible for segmental contractions and peristaltic movement
through the GI tract. The muscularis has an inner circular

Figure 18.2 The organs of the digestive system. The
digestive system includes the gastrointestinal tract and the
accessory digestive organs.

Oral cavity

Teeth

Tongue

Sublingual

gland

Parotid

gland

Pharynx

Submandibular

gland

Esophagus

Stomach

Common

bile duct

Pancreas

Transverse

colon

Descending

colon

Sigmoid colon

Rectum

Anal canal

Diaphragm

Liver

Gallbladder

Duodenum

Ascending

colon

Cecum

Appendix

Small

intestine

Anus