Chapter 23
The Digestive System
855
23
The Serosa
Te
serosa
, the outermost layer of the intraperitoneal organs,
is the
visceral peritoneum
. In most alimentary canal organs, it is
formed of areolar connective tissue covered with
mesothelium
,
a single layer of squamous epithelial cells (see Figures 4.8a and
4.3a, respectively).
In the esophagus, which is located in the thoracic instead
of the abdominopelvic cavity, the serosa is replaced by an
adventitia
(ad
0
ven-tish
9
e-ah), ordinary fibrous connective
tissue that binds the esophagus to surrounding structures.
Retroperitoneal organs have
both
a serosa (on the side facing
the peritoneal cavity) and an adventitia (on the side abutting
the dorsal body wall).
Enteric Nervous System
of the Alimentary Canal
As we noted earlier, the alimentary canal has its own in-house
nerve supply, staffed by the so-called
enteric neurons
(
enter
5
gut), which communicate widely with one another to regulate
digestive system activity. Tese semiautonomous enteric neu-
rons constitute the bulk of the two major
intrinsic nerve plexuses
(ganglia interconnected by unmyelinated fiber tracts) found in
the walls of the alimentary canal: the submucosal and myenteric
nerve plexuses (Figure 23.6).
Te
submucosal nerve plexus
occupies the submucosa
whereas the large
myenteric nerve plexus
(mi-en-ter
9
ik; “intes-
tinal muscle”) lies between the circular and longitudinal mus-
cle layers of the muscularis externa. Enteric neurons of these
plexuses provide the major nerve supply to the GI tract wall
and control GI tract motility (motion). Control of the patterns
of segmentation and peristalsis is largely automatic, involving
pacemaker cells and local reflex arcs between enteric neurons in
the same or different organs.
Te enteric nervous system is also linked to the central ner-
vous system by (1) afferent visceral fibers and (2) sympathetic
and parasympathetic branches (motor fibers) of the autonomic
nervous system that enter the intestinal wall and synapse with
neurons in the intrinsic plexuses. Hence, extrinsic controls—
exerted by autonomic fibers via long reflex arcs—also regulate
digestive activity (see Figure 23.4). Generally speaking, para-
sympathetic inputs enhance digestive activities, whereas sym-
pathetic impulses inhibit them.
But the largely independent enteric ganglia are much more
than just way stations for the autonomic nervous system. In-
deed, the enteric nervous system contains over 100 million neu-
rons, more than the entire spinal cord.
Check Your Understanding
9.
Name the layers of the alimentary canal from the inside out.
10.
Jerry has been given a drug that inhibits parasympathetic
stimulation of his digestive tract. Should he “eat hearty” or
temporarily refrain from eating, and why?
For answers, see Appendix H.
The Mucosa
Te
mucosa
, or
mucous membrane
—the innermost layer—is
a moist epithelial membrane that lines the alimentary canal lu-
men from mouth to anus. Its major functions are to:
Secrete
mucus, digestive enzymes, and hormones
Absorb
the end products of digestion into the blood
Protect
against infectious disease
Te mucosa in a particular region of the GI tract may perform
one or all three of these functions.
More complex than most other mucosae in the body, the
typical digestive mucosa consists of three sublayers: (1) a lin-
ing epithelium, (2) a lamina propria, and (3) a muscularis
mucosae. Except for that of the mouth, esophagus, and anus
where it is stratified squamous, the
epithelium
of the mu-
cosa is a
simple columnar epithelium
rich in mucus-secreting
cells. Te slippery mucus it produces protects certain diges-
tive organs from being digested by enzymes working within
their cavities and eases food passage along the tract. In the
stomach and small intestine, the mucosa also contains both
enzyme-synthesizing and hormone-secreting cells. In such
sites, the mucosa is a diffuse kind of endocrine organ as well
as part of the digestive organ.
Te
lamina propria
(
proprius
5
one’s own), which underlies
the epithelium, is loose areolar connective tissue. Its capillaries
nourish the epithelium and absorb digested nutrients. Its iso-
lated lymphoid follicles, part of
MALT
(the mucosa-associated
lymphoid tissue described on p. 759), help defend us against
bacteria and other pathogens, which have rather free access to
our digestive tract. Particularly large collections of lymphoid
follicles occur within the pharynx (as the tonsils) and in the
appendix.
External to the lamina propria is the
muscularis mucosae
,
a scant layer of smooth muscle cells that produces local move-
ments of the mucosa. In the small intestine, this muscle layer’s
tone throws the mucosa into a series of small folds that im-
mensely increase its surface area.
The Submucosa
Te
submucosa
, just external to the mucosa, is areolar con-
nective tissue containing a rich supply of blood and lymphatic
vessels, lymphoid follicles, and nerve fibers which supply the
surrounding tissues of the GI tract wall. Its abundant elastic
fibers enable the stomach to regain its normal shape a±er tem-
porarily storing a large meal.
The Muscularis Externa
Surrounding the submucosa is the
muscularis externa
, also
simply called the
muscularis
. Tis layer is responsible for
segmentation and peristalsis. It typically has an inner
circu-
lar layer
and an outer
longitudinal layer
of smooth muscle
cells (see Figures 4.9c and 23.6). In several places along the
tract, the circular layer thickens, forming
sphincters
that act
as valves to control food passage from one organ to the next
and prevent backflow.
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