There are four junctions in the GI tract that are characterized by abrupt changes in the mucosal lining: the gastro-esophageal junction, the gastro-duodenal junction, the ileo-cecal junction, and the recto-anal junction. In Physiology, you will study the movement of food through the GI tract in detail. For now, it is important to understand a few basic principles and to relate them to the histological structure of the digestive system. In the first portion of the GI tract, from the oral cavity to the upper esophagus, food moves by voluntary muscular action.
In the following portion, from the lower esophagus through the large intestine, food moves by peristalsis. Material moves through the rest of the tract by mass movements, which transport tract contents over long distances.
The motility of the GI tract is accomplished via a diffuse neuroendocrine system that involves the autonomic nervous system and hormones. The enteric nervous system is composed of several groups of nerve cells:. With this understanding of the basic structural features of the GI tract, we will now take a more focused look at different portions of the tract. Be sure you understand the function of each portion of the tract before you study the structure in detail in the Laboratory Slides.
Food enters the digestive tract in the oral cavity, where it is masticated into particles on which digestive enzymes can act more efficiently. In the mouth, food particles are mixed with saliva, which lubricates them and initiates their digestion. The salivary glands will be discussed in detail in the next laboratory. The tongue is a muscular organ covered by oral mucosa that manipulates the food and contains the sensory organs for taste. The taste buds will be discussed in detail in the Laboratory on Sensory Systems.
The esophagus is a muscular tube that transports food from the pharynx to the stomach. It is lined by a stratified squamous epithelium and has a prominent muscularis mucosa and thick muscularis externa. The muscularis externa of the esophagus is unique in that it transitions from striated to smooth muscle over the length of the tube. The esophagus ends in the gastro-esophageal junction. The stomach is the site where food is mixed with gastric juice and reduced to a fluid mass called chyme.
The layers of the stomach wall follow the basic plan described above. The gastric glands are the basic structure of the stomach wall and can be thought of as tiny pits, or indentations, lined by epithelial cells. Gastric glands are structured as a gastric pit that opens into the lumen, followed by an isthmus, neck, and base.
There are several types of cells that are important in producing stomach secretions:. The three different regions of the stomach are distinguished on the basis of the histological characteristics of the mucosal glands:. The muscularis externa of the stomach is notable because it contains an additional muscular layer. 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. The coordinated contractions of these layers is called 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 inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch.
The coordinated contractions of these layers is called 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. General Structure of the gut wall : General structure of the gut wall—the muscularis externa is labeled circular muscle and longitudinal muscle here. 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. Serosa consists of a secretory epithelial layer and a thin connective tissue layer that reduce the friction from muscle movements. 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. Layers of stomach lining : The serosa is labeled at far right, and is colored yellow. 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.
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. Privacy Policy. Skip to main content.
Digestive System. Search for:. Layers of the Alimentary Canal.
0コメント