Layers of the GI tract

▪ The mucosa, or inner lining of the GI tract, is a mucous membrane. It is
composed of (1) a layer of epithelium in direct contact with the contents of
the GI tract, (2) a layer of connective tissue called the lamina propria, and
(3) a thin layer of smooth muscle (muscularis mucosae).

▪ The submucosa consists of areolar connective tissue that binds the mucosa
to the muscularis. It contains many blood and lymphatic vessels that
receive absorbed food molecules. Also located in the submucosa is an
extensive network of neurons known as the submucosal plexus. The
submucosa may also contain glands and lymphatic tissue.

▪ The muscularis of the mouth, pharynx, and superior and middle parts of the esophagus
contains skeletal muscle that produces voluntary swallowing. Skeletal muscle also forms the
external anal sphincter, which permits voluntary control of defecation. Throughout the rest of
the tract, the muscularis consists of smooth muscle that is generally found in two sheets: an
inner sheet of circular fibers and an outer sheet of longitudinal fibers. Involuntary contractions
of the smooth muscle help break down food, mix it with digestive secretions, and propel it
along the tract. Between the layers of the muscularis is a second plexus of neurons—the
myenteric plexus.
▪ Those portions of the GI tract that are suspended in the abdominopelvic cavity have a
superficial layer called the serosa. As its name implies, the serosa is a serous membrane
composed of areolar connective tissue and simple squamous epithelium (mesothelium). The
serosa is also called the visceral peritoneum because it forms a portion of the peritoneum. The
esophagus lacks a serosa; instead only a single layer of areolar connective tissue called the
adventitia forms the superficial layer of this organ.