Lecture 40 - Digestive System: Stomach - Gastric Secretions, Gastric Motility, and Emptying

Question 1

What are the 4 distinct subregions and 3 muscle layers of the stomach?

Answer 1

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

Question 2

What is the point of having different zones of the stomach?

Answer 2

It has different mucosa specializations

Ex. The cardia secretes more mucus, the fundus and body contribute more digestive secretions

Question 3

What are sphincters made up of and what is their function in the stomach?

Answer 3

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)

Question 4

Name 5 functions of the stomach

Answer 4

1. Temporary food storage of ingested food (and liquid)
2. Secretion of gastric juice contributes to chemical digestion
3. Churning movements in the smooth muscle layers contribute to physical digestion
4. Denaturing and destruction of many potential pathogens, contributing to protection
5. Secretes hormones and neural reflexes that alter function of other parts of the digestive tract, contributing to coordination

Question 5

What factor contributes to the stomach being highly expandable? How does it know to stretch?

Answer 5

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

Question 6

What is the organization of gastric mucosa?

Answer 6

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

Question 7

What is the average lifespan of a gastric epithelial cell? What is the function of mucus in the stomach?

Answer 7

The average lifespan is <1 week. Mucus is alkaline, and offers some protection against the acidic environment

Question 8

Where are parietal cells located and what is their function?

Answer 8

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”

Question 9

What is the function of chief cells and where are they located?

Answer 9

They are within gastric glands and they secrete zymogen pepsinogen and is converted to the active enzyme pepsin (peptidase) within the acidified gastric lumen

Question 10

What is a zymogen?

Answer 10

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

Question 11

What is gastrin and G cell?

Answer 11

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

Question 12

How can gastric secretion be enhanced?

Answer 12

By the presence of partly digested protein (oligopeptides) and elevated pH (reductions in H+, more basic)

Question 13

What type of reflex does gastrin participate in and what is the result?

Answer 13

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)

Question 14

What mechanisms regulate gastric activity and what are its 3 phases?

Answer 14

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

Question 15

Describe the cephalic (from the head) phase of gastric activity

Answer 15

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

Question 16

Describe the gastric phase (from the stomach) of gastric activity

Answer 16

Local neural and hormonal reflexes respond to stretch and luminal stomach contents to enhance gastric secretion when the bolus reaches the stomach

Chemo- or mechano-receptors within the stomach wall act as sensors for the gastric phase

Overall, these responses work to enhance the secretions of the stomach to help deal with the material inside the gastric lumen

Question 17

What do gastrin and increased gastric sensory neuron activity enhance?

Answer 17

• the rate of gastric gland secretions
• the frequency and amplitude of gastric muscle contractions

Question 18

What is the function of the pyloric sphincter?

Answer 18

It regulates the entrance of chyme into the small intestine during gastric emptying. The pyloric sphincter is closed when ingested material first enters the stomach, and it can remain so for 1.5-2 hours after

When the meal is sufficiently digested, the pyloric sphincter relaxes for brief periods, allowing chyme to pass into the small intestine

Question 19

Describe the intestinal (from the intestine) phase of gastric activity

Answer 19

Gastric activity and the rate of gastric emptying is controlled by changes detected within the lumen of the duodenum by local sensory receptors (chemo-, mechano-)

Overall, these responses act to inhibit the secretions and motility of the stomach to slow down the rate of gastric emptying and slow the influx of chyme into the intestine

Question 20

T/F? Mixing waves and gastric secretion are only inhibited by intestinal hormones

Answer 20

False - Mixing waves and gastric secretion are inhibited by both intestinal hormones and neural reflexes

Both intestinal hormones and enhanced duodenal sensory neuron activity inhibit the frequency and amplitude of gastric muscle contraction. Intestinal hormones inhibit the rate of gastric gland secretions

Question 21

What 2 factors stimulate feedforward short reflexes and what is the result?

Answer 21

Stomach stretching and the release of gastrin stimulate feedforward short reflexes, which accelerate activity further down the gastrointestinal tract

These reflexes generally act to enhance the throughput of the gastrointestinal tract, freeing up space for stomach contents to move forward