GASTROINTESTINAL TRACT LAYERS

The GI tract contains four layers: the innermost layer is the mucosa, underneath this is the submucosa, followed by the muscularis propria and finally, the outermost layer - the adventitia. The structure of these layers varies, in different regions of the digestive system, depending on their function.

1. GASTROINTESTINAL TRACT LAYERS

2. GASTROINTESTINAL TRACT LAYERS JAI NARAIN VYAS UNIVERSITY, JODHPUR ASSISTANT PROFESSOR:- ASHWIN SINGH CHOUHAN DEPARTMENT:- PHARMACOLOGY E-mail:- [email protected]

3. JNVU PHARMACY, JODHPUR BASIC STRUCTURE The gastrointestinal tract is a muscular tube lined by a special layer of cells, called epithelium. The contents of the tube are considered external to the body and are in continuity with the outside world at the mouth and the anus. Although each section of the tract has specialised functions, the entire tract has a similar basic structure with regional variations.

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5. JNVU PHARMACY, JODHPUR The wall is divided into four layers as follows: MUCOSA The innermost layer of the digestive tract has specialised epithelial cells supported by an underlying connective tissue layer called the lamina propria . The lamina propria contains blood vessels, nerves, lymphoid tissue and glands that support the mucosa. Depending on its function, the epithelium may be simple (a single layer) or stratified (multiple layers ). Areas such as the mouth and oesophagus are covered by a stratified squamous (flat) epithelium so they can survive the wear and tear of passing food. Simple columnar (tall) or glandular epithelium lines the stomach and intestines to aid secretion and absorption. The inner lining is constantly shed and replaced, making it one of the most rapidly dividing areas of the body! Beneath the lamina propria is the muscularis mucosa. This comprises layers of smooth muscle which can contract to change the shape of the lumen.

6. Layers of Tissue Within the Mucosa Since the mucosa is the innermost layer within the GI tract, it surrounds an open space known as the lumen. Food, mucus, and digestive juices pass through the lumen, and the mucosa comes in direct contact with digested food ( chyme ). The mucosa is made up of three layers: The epithelium is the innermost layer and it is responsible for most digestive, absorptive, and secretory processes. The lamina propria is a layer of connective tissue that is unusually cellular compared to most connective tissue. The muscularis mucosae is a thin layer of smooth muscle and its function is still under debate.

7. The mucosae (singular: mucosa) are highly specialized in each organ of the gastrointestinal tract in order to deal with different digestive tract conditions. The most variation is seen in the epithelium tissue layer of the mucosa. In the esophagus , the epithelium is stratified, squamous , and non-keratinizing, for protective purposes. In the stomach. the epithelium is simple columnar, and is organized into gastric pits and glands to deal with secretion. In the small intestine, the epithelium (particularly the ileum) is specialized for absorption, with villi and microvilli increasing surface area. JNVU PHARMACY, JODHPUR

8. SUBMUCOSA The submucosa surrounds the muscularis mucosa and consists of fat, fibrous connective tissue and larger vessels and nerves. At its outer margin there is a specialized nerve plexus called the submucosal plexus or Meissner plexus. This supplies the mucosa and submucosa . The Muscularis The muscularis in the stomach differs from that of other GI organs in that it has three layers of muscle instead of two. Under these muscle layers is the adventitia—layers of connective tissue that are continuous with the omenta . JNVU PHARMACY, JODHPUR

9. JNVU PHARMACY, JODHPUR MUSCULARIS EXTERNA This smooth muscle layer has inner circular and outer longitudinal layers of muscle fibres separated by the myenteric plexus or Auerbach plexus. Neural innervations control the contraction of these muscles and hence the mechanical breakdown and peristalsis of the food within the lumen.

10. The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics , and nerves that branch into the mucosa and muscularis externa . It contains Meissner’s plexus, an enteric nervous plexus, situated on the inner surface of the muscularis externa . In the gastrointestinal tract, the submucosa is the layer of dense irregular connective tissue or loose connective tissue that supports the mucosa. It also joins the mucosa to the bulk of underlying smooth muscle ( fibers running circularly within layer of longitudinal muscle). Blood vessels, lymphatic vessels, and nerves (all supplying the mucosa) will run through here. Tiny parasympathetic ganglia are scattered around forming the submucosal plexus (or Meissner’s plexus) where preganglionic parasympathetic neurons create synapses with the postganglionic nerve fibers that supply the muscularis mucosae . JNVU PHARMACY, JODHPUR

11. Muscularis The muscularis is responsible for the segmental contractions and peristaltic movements in the gastrointestinal (GI) tract . The gastrointestinal (GI) tract is composed of four layers of tissue, known as tunics. Each layer has different structures and functions. From the inside out they are called the mucosa, submucosa , muscularis externa , and serosa . JNVU PHARMACY, JODHPUR

12. Structure of the Muscularis Externa The muscularis externa is responsible for segmental contractions and peristaltic movement in the GI tract. These muscles cause food to move and churn together with digestive enzymes down the GI tract. The muscularis externa consists of an inner circular layer and a longitudinal outer muscular layer. It should not be confused with a thin layer of muscle known as the muscularis mucosa, which lies within the submucosa , a layer of tissue adjacent to the muscularis externa . The muscularis mucosa is made up of smooth muscle, and is most prominent in the stomach. Within the muscularis externa , the circular muscle layer prevents food from traveling backward, while the longitudinal layer shortens the tract. The layers are not truly longitudinal or circular, rather the layers of muscle are helical with different pitches. The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch. JNVU PHARMACY, JODHPUR

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14. The coordinated contractions of these layers is called peristalsis. Between the two muscle layers is the myenteric or Auerbach’s plexus, which controls peristalsis. Peristaltic activity is regulated by these nerve  cells, and the rate of peristalsis can be modulated by the rest of the autonomic nervous system. The thickness of muscularis externa varies in each part of the tract. In the colon, for example, the muscularis externa is much thicker because the feces are large and heavy, and require more force to push along. The outer longitudinal layer of the colon thins out into three discontinuous longitudinal bands known as tiniae coli (bands of the colon). This is one of the three features helping to distinguish between the large and small intestine. JNVU PHARMACY, JODHPUR

15. Occasionally in the large intestine (two to three times a day), there will be mass contraction of certain segments, moving a lot of feces along. This is generally when one gets the urge to defecate. The pylorus of the stomach has a thickened portion of the inner circular layer: the pyloric sphincter. Alone among the GI tract, the stomach has a third layer of muscularis externa . This is the inner oblique layer, and helps churn the chyme in the stomach JNVU PHARMACY, JODHPUR

16. JNVU PHARMACY, JODHPUR SEROSA/MESENTERY The outer layer of the GIT is formed by fat and another layer of epithelial cells called mesothelium . The outer lining of organs and body cavities of the abdomen and chest, including the stomach. This layer of visceral peritoneum invests the organ to a varying extent depending on the particular segment of the organ and its mesenteric attachments.

17. JNVU PHARMACY, JODHPUR The Serous Membrane In anatomy, the serous membrane (or serosa ) is a smooth membrane that consists of a thin connective tissue layer and a thin layer of cells that secrete serous fluid. Serous membranes line and enclose several body cavities, known as serous cavities, where they secrete a lubricating fluid to reduce friction from muscle movements. Serosa is not to be confused with adventitia, a connective tissue layer that binds together structures rather than reduces friction between them.

18. JNVU PHARMACY, JODHPUR Each serous membrane is composed of a secretory epithelial layer and a connective tissue layer underneath. The epithelial layer, known as mesothelium , consists of a single layer of avascular flat nucleated cells (simple squamous epithelium) that produce the lubricating serous fluid. This fluid has a consistency similar to thin mucus. These cells are bound tightly to the underlying connective tissue. The connective tissue layer provides the blood vessels and nerves for the overlying secretory cells, and also serves as the binding layer that allows the whole serous membrane to adhere to organs and other structures. For the heart, the surrounding serous membranes include: the outer, inner, parietal pericardium, and visceral pericardium ( epicardium ). Other parts of the body may also have specific names for these structures. For example, the serosa of the uterus is called the perimetrium .

19. JNVU PHARMACY, JODHPUR The pericardial cavity (surrounding the heart), pleural cavity (surrounding the lungs) and peritoneal cavity (surrounding most organs of the abdomen) are the three serous cavities within the human body. While serous membranes have a lubricative role to play in all three cavities, in the pleural cavity it has a greater role to play in the function of breathing. The serous cavities are formed from the intraembryonic coelom and are basically an empty space within the body surrounded by a serous membrane. Early in embryonic life, visceral organs develop adjacent to a cavity and invaginate into the bag-like coelom . Therefore each organ becomes surrounded by a serous membrane—they do not lie within the serous cavity. The layer in contact with the organ is known as the visceral layer, while the parietal layer is in contact with the body wall.

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