Tunics refer to the layers found on the GI tracts. There are four tunics, and they extend from the esophagus to the anal canal. Each tunic is lined with specialized tissues that are important to digestion. The gastrointestinal tract’s most internal tunic is the mucosa. The other three tunics are (from the next most interior to the exterior) the sub-mucosa, the muscularis and the serosa.

The Mucosa

The mucosa is a lining that is situated along the lines of the lumen. It secretes a certain kind of mucous on the GI tract and is also capable of absorbing nutrients. This is formed by simple columnar epithelial cells, supported by connective tissues referred to as lamina propria. The lamina propria is home to lymph nodules - which help the body fight diseases and infections. Just outside this tissue is the layers of very thin muscle tissue known as muscularis mucosae. It regulates the involuntary actions of undulating and churning to let food pass doen the gullet properly. The mucosa has goblet cells which continually secrete mucous all over the whole gastrointestinal tract.

The Sub-Mucosa

The sub-mucosa is thicker than the mucosa, and it exists to assist the mucosa in its functions. This is where the nutrients absorbed by the mucosa (now in molecular forms) is delivered through the columnar epithelial cells. To get to the final receptacle in the body, the nutrients either use the blood vessels or the lymph ductules of the sub-mucosa. The second tunic also houses the nerve plexuses and the glands. Meissner’s plexus in the sub-mucosa provides innervation to the muscularis mucosae.

The Tunic Muscularis

The muscularis is the tunic that is in charge of providing segmented contractions and peristaltic contractions to the gastrointestinal tract. This is done because the muscularis is formed by smooth muscle topped with a wise layer of more smooth muscles. These muscles contract, and food then is undulated all over the digestive system. The pulverization of the food gets easier, too, with the help of digestive enzymes.  

Between the layers of the muscularis is the myenteric plexus. This is where the most necessary nerve pathway in the gastrointestinal tract is conducted. Nerve fibers and the autonomic nervous system’s ganglion in the myenteric plexus supply sympathetic and parasympathetic innervaion to the gastrointestinal tract.

The Serosa

As the outermost tunic, the serosa is the gastrointestinal tract’s wall. This tunic protects all the actions occurring inside the GI tract, and it’s formed by simple squamous epithelium and connective tissues. The serosa also makes sure that when the GI tract goes through a strong pressure, it holds itself together. The serosa’s simple squamous epithelium is a continuation of the visceral peritoneum, since the latter does not have serosa.

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