Physiology Topics
The gastrointestinal track is a continuous tube that extends from the mouth to the anus. Organs in this tube include the mouth, pharynx, esophagus, stomach, small intestine and large intestine. Accessories organs include the teeth, tongue, salivary glands, liver, gallbladder and pancreas.
Overall, the GI track has six functions: food digestion, secretion, mixing and propulsion, digestion, nutrient absorption and defecation. Secretion includes saliva, stomach acid, buffers, and digestive enzymes.
The wall of the GI track from the esophagus to the anus
has the same basic structure, composed of four layers: mucosa, submucosa, muscularis,
and serosa. The mucosa is the inner most layer and
is composed of epithelial cells, connective tissue and smooth muscle. The
submucosa consists of connective tissue and portions of the enteric nervous
system (submucosal plexus). The muscularis of the
upper part of the GI track (mouth, to middle esophagus) is made of skeletal
muscle (voluntary contractions), while the rest is made of smooth muscle (involuntary
contractions). The serosa is the superficial layer made, up of connective tissue
and epithelium.
Sight, smell, or taste perceived by the central nervous system (CNS), or even thought of food will stimulate the cerebral cortex and the hypothalamus, which in turn send parasympathetic signals through the medulla along the vagus (X) nerve. This impulses reach the submucosal plexus, where acetylcholine (Ach) signaling increases the secretion of gastric juices, gastrin (a hormone that stimulates digestion in the stomach), and increases peristalsis (involuntary movement of smooth muscle) in the stomach.
After food has reached the stomach, stretch receptors and chemoreceptors (detect distention and chemical content, respectively) in the stomach wall signal back to the submucosal plexus and CNS to increase gastric secretions and peristalsis. As the food travels out of the stomach and into the small intestine, stretch receptors and chemoreceptors in the intestinal wall signal in a similar way for the end of digestion in the stomach and start of secretion of pancreatic and intestinal juices and hormones and of peristalsis in the small intestine.
Continue to "Upper GI Track " or take a quiz: [Q1] [Q2].
Need more practice? Answer the review questions below.
1- List the organs in the GI tube
2- List the accessory organs of the GI track
3- List the functions of the GI track.
4- Describe the structure of the GI track.
5- List the main secretions of the GI tract.
6- Which parts of the GI track can and which cannot produce voluntary contractions, and why?
7- What kind of signals and tissues (organs or systems) are involved in the start of activity in the stomach?
8- What kind of signals and tissues are involved in the start of activity in the small intestine?