The layers of the GI tract are also known as tunics. There are four of them, and they run all the way from the esophagus to the anal canal. Each layer of each tunic is created by specialized tissue, and this tissue is designed to perform specific functions that are necessary in the process of digestion. The innermost layer of the gastrointestinal tract is known as the mucosa. The three remaining layers in order from innermost to outermost are the sub-mucosa, the musclularis, and finally the serosa.


The mucosa is an absorbent lining that can be identified along the lumen. In addition to being able to absorb nutrients, it also secretes a type of mucous along the GI tract. This layer is created by simple columnar epithelial cells and is aided and supported by a slight layer of connective tissue known as the lamina propria. The mucosa helps fight the spread of infectious diseases via the lamina propria, as lymph nodules exist within the lamina propria. The muscularis mucosae, which is slight layers of paper-thin muscle tissue, can be found just outside the lamina propria. This layer of muscle tissue assists in the necessary involuntary action of churning and undulating to assist food down the gullet. The specialized goblet cells within the mucosa secrete mucous throughout nearly the entire gastrointestinal tract.


The sub-mucosa is considerably thicker, and is there to serve the vascular requirements of the mucosa. When the mucosa absorbs the necessary nutrients (in the form of molecules) it passes them onto the sub-mucosa via the columnar epithelial cells. The nutrients enter either the blood vessels or the lymph ductules that belong to the sub-mucosa in order to reach their final destination. Glands and nerve plexuses also reside within the sub-mucosa. Meissner’s plexus, which is in the sub-mucosa, is able to provide the necessary autonomic innervation to the muscularis mucosae.


Image: Tunics


The basic function of the tunic muscularis is to provide the gastrointestinal tract with segmented contractions and peristaltic contractions. This is attainable as the tunic muscularis is created by an inner circle of smooth muscle layered by an outer length wise layer of smooth muscle. When these muscles contract, food is literally smashed and pulverized as it is undulated through the digestive system. Digestive enzymes enter the mix to help assist with the food pulverization.

Between the two layers of smooth muscle one can find the myenteric plexus. This plexus is the pathway for the single most important nerve pathway in the gastrointestinal tract. Within the myenteric plexus, nerve fibers and ganglion from the autonomic nervous system provide the gastrointestinal tract with sympathetic and parasympathetic innervation.


The serosa is the outer layer, or the wall, of the gastrointestinal tract. Created by connective tissue and simple squamous epithelium, the wall becomes the protective coating for the action that occurs inside the GI tract. The serosa simultaneously binds the GI tract, holding it together under sometimes significant pressure. The simple squamous epithelium doubles as a continuation of the visceral peritoneum, as the visceral organs known as the retroperitoneal organs are devoid of serosa.
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