The Digestive System
, the outermost layer of the intraperitoneal organs,
. In most alimentary canal organs, it is
formed of areolar connective tissue covered with
a single layer of squamous epithelial cells (see Figures 4.8a and
In the esophagus, which is located in the thoracic instead
of the abdominopelvic cavity, the serosa is replaced by an
e-ah), ordinary ﬁbrous connective
tissue that binds the esophagus to surrounding structures.
Retroperitoneal organs have
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
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).
submucosal nerve plexus
occupies the submucosa
whereas the large
myenteric nerve plexus
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
Name the layers of the alimentary canal from the inside out.
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 innermost layer—is
a moist epithelial membrane that lines the alimentary canal lu-
men from mouth to anus. Its major functions are to:
mucus, digestive enzymes, and hormones
the end products of digestion into the blood
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
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.
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
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
External to the lamina propria is the
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.
, 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
simply called the
. Tis layer is responsible for
segmentation and peristalsis. It typically has an inner
and an outer
of smooth muscle
cells (see Figures 4.9c and 23.6). In several places along the
tract, the circular layer thickens, forming
as valves to control food passage from one organ to the next
and prevent backﬂow.