Lecture 40 - Digestive System: Stomach - Gastric Secretions, Gastric Motility, and Emptying Flashcards
What are the 4 distinct subregions and 3 muscle layers of the stomach?
Subregions:
1. Cardia
2. Fundus
3. Body
4. Pylorus (pyloric antrum and pyloric canal)
Muscle layers:
1. Longitudinal layer
2. Circular layer
3. Oblique layer
What is the point of having different zones of the stomach?
It has different mucosa specializations
Ex. The cardia secretes more mucus, the fundus and body contribute more digestive secretions
What are sphincters made up of and what is their function in the stomach?
Sphincters are thickenings of the circular smooth muscle layer that can be controlled independently from the rest of the muscle layer
The 2 sphincters in our stomach control the entrance of boli and the exit of chyme to and from the stomach. Under normal healthy conditions, sphincters are contracted (closed) by default, and must receive signals to relax (open)
Name 5 functions of the stomach
- Temporary food storage of ingested food (and liquid)
- Secretion of gastric juice contributes to chemical digestion
- Churning movements in the smooth muscle layers contribute to physical digestion
- Denaturing and destruction of many potential pathogens, contributing to protection
- Secretes hormones and neural reflexes that alter function of other parts of the digestive tract, contributing to coordination
What factor contributes to the stomach being highly expandable? How does it know to stretch?
Rugae - they are wrinkles and folds that allow the mucosal layer to stretch
When empty, a typical human stomach is <150 mL in volume. When fully expanded, it can hold up to 1-1.5 L
The current state of the stomach’s volume is sensed by mechanosensitive stretch receptor neurons within the stomach
What is the organization of gastric mucosa?
It is a simple columnar epithelium organized into pits
The mucosal surface is further folded into numerous microscopic gastric pits, lined by mucus-secreting epithelial cells. Deep to the gastric pits are gastric glands, where several types of epithelial cells produce a variety of secretions
What is the average lifespan of a gastric epithelial cell? What is the function of mucus in the stomach?
The average lifespan is <1 week. Mucus is alkaline, and offers some protection against the acidic environment
Where are parietal cells located and what is their function?
Parietal cells are located within gastric glands, which secrete HCl to create a highly acidic environment in the gastric lumen (pH ~1.5). They also secrete water, chloride ions, H+ ions, and a glycoprotein called “Intrinsic Factor”
What is the function of chief cells and where are they located?
They are within gastric glands and they secrete zymogen pepsinogen and is converted to the active enzyme pepsin (peptidase) within the acidified gastric lumen
What is a zymogen?
Many digestive enzymes, especially those that break down proteins, are secreted as zymogens (inactive precursors, also called pro-enzymes) so that the enzymes don’t digest the cell that synthesized them
What is gastrin and G cell?
G cells are located in pyloric glands and secrete gastrin (peptide hormone) into the underlying tissue (and bloodstream), not into the gastric lumen
Gastrin is a small peptide hormone/paracrine factor which signals through metabotropic plasma membrane receptors
How can gastric secretion be enhanced?
By the presence of partly digested protein (oligopeptides) and elevated pH (reductions in H+, more basic)
What type of reflex does gastrin participate in and what is the result?
Gastrin acts in local reflexes as a paracrine factor, ie. in short reflexes within the stomach wall. It enhances the activity of chief cells and parietal cells by acting on their receptors, increasing their secretion of pepsinogen and HCl (respectively)
What mechanisms regulate gastric activity and what are its 3 phases?
Gastric activity is regulated by:
- Feedforward mechanisms - signals from proximal regions of the tract changes the activity of more distal regions before the ingested material reaches those regions
- Feedback mechanisms - signals from downstream (more distal) parts of the digestive tract having effects on more proximal segments
Gastric activity can be separated into 3 phases:
1. Cephalic phase
2. Gastric phase
3. Intestinal phase
Describe the cephalic (from the head) phase of gastric activity
The CNS stimulates and enhances gastric secretion in preparation for the reception of food
The brain can initiate or enhance the rate of all gastric secretions and gastric motility via the vagus nerve (CN X, containing parasympathetic fibres)
Chemo- or mechano-receptors in the oral cavity, pharynx, and esophagus are sensors which can enhance vagal activity in the stomach