Motor Functions of the Stomach

GIT system

MOTOR FUNCTIONS OF THE STOMACH

Objectives of our lecture:

• what are the motor functions of stomach?
• What is stomach emptying ?

The motor functions of the stomach are threefold:

(1) storage of large quantities of food until the food can be processed in the stomach, duodenum, and lower intestinal tract;
(2) mixing of this food with gastric secretions until it forms a semifluid mixture called chyme; and
(3) slow emptying of the chyme from the stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.

Anatomically, the stomach is usually divided into two usually major parts: (1) the body and (2) the antrum.
Physiologically, it is more appropriately divided into (1) the “orad” portion, comprising about the first two thirds of the body, and (2) the “caudad” portion, comprising the remainder of the body plus the antrum.

MIXING AND PROPULSION OF FOOD IN THE STOMACH

As long as food is in the stomach, weak peristaltic constrictor waves, called mixing waves, begin in the mid to upper portions of the stomach wall and move toward the antrum about once every 15 to 20 seconds. These waves are initiated by the gut wall basic electrical rhythm, consisting of electrical “slow waves” that occur spontaneously in the stomach wall.

As the constrictor waves progress from the body of the stomach into the antrum, they become more intense, some becoming extremely intense and providing powerful peristaltic action potential–driven constrictor rings that force the antral contents under higher and higher pressure toward the pylorus.

Also, as each peristaltic wave approaches the pylorus, the pyloric muscle often contracts, which further impedes emptying through the pylorus. Therefore, most of the antral contents are squeezed upstream through the peristaltic ring toward the body of the stomach, not through the pylorus. Thus, the moving peristaltic constrictive ring, combined with this upstream squeezing action, called “retropulsion,” is an exceedingly important mixing mechanism in the stomach

Chyme. After food in the stomach has become thoroughly mixed with the stomach secretions, the resulting mixture that passes down the gut is called chyme. The degree of fluidity of the chyme leaving the stomach depends on the relative amounts of food, water, and stomach secretions and on the degree of digestion that has occurred. The appearance of chyme is that of a murky semifluid or paste .

Hunger Contractions.

Besides the peristaltic contractions that occur when food is present in the stomach, another type of intense contractions, called hunger contractions, often occurs when the stomach has been empty for several hours or more. These contractions are rhythmical peristaltic contractions in the body of the stomach.

When the successive contractions become extremely strong, they often fuse to cause a continuing tetanic contraction that sometimes lasts for 2 to 3 minutes. Hunger contractions are most intense in young, healthy people who have high degrees of gastrointestinal tonus; they are also greatly increased by the person’s having lower than normal levels of blood sugar.

STOMACH EMPTYING

Stomach emptying is promoted by intense peristaltic contractions in the stomach antrum. At the same time, emptying is opposed by varying degrees of resistance to passage of chyme at the pylorus.


Role of the Pylorus in Controlling Stomach Emptying.
The distal opening of the stomach is the pylorus. Here the thickness of the circular wall muscle becomes 50 to 100 percent greater than in the earlier portions of the stomach antrum, and it remains slightly tonically contracted almost all the time. Therefore, the pyloric circular muscle is called the pyloric sphincter.





Despite normal tonic contraction of the pyloric sphincter, the pylorus usually is open enough for water and other fluids to empty from the stomach into the duodenum with ease. Conversely, the constriction usually prevents passage of food particles until they have become mixed in the chyme to almost fluid consistency. The degree of constriction of the pylorus is increased or decreased under the influence of nervous and hormonal signals from both the stomach and the duodenum

REGULATION OF STOMACH EMPTYIN

The rate at which the stomach empties is regulated by signals from both the stomach and the duodenum. However, the duodenum provides by far the more potent of the signals, controlling the emptying of chyme into the duodenum at a rate no greater than the rate at which the chyme can be digested and absorbed in the small intestine. .

Effect of the Hormone Gastrin on Stomach Emptying.

stretching of the stomach wall and the presence of certain types of foods in the stomach—particularly digestive products of meat—elicit release of the hormone gastrin from the G cells of the antral mucosa. This has potent effects to cause secretion of highly acidic gastric juice by the stomach glands.

Gastrin also has mild to moderate stimulatory effects on motor functions in the body of the stomach. Most important, it seems to enhance the activity of the pyloric pump. Thus, gastrin likely promotes stomach emptying.

Powerful Duodenal Factors That Inhibit Stomach Emptying (Inhibitory Effect of Enterogastric Nervous Reflexes From the Duodenum).
When food enters the duodenum, multiple nervous reflexes are initiated from the duodenal wall. These reflexes pass back to the stomach to slow or even stop stomach emptying if the volume of chyme in the duodenum becomes too much.

These reflexes are mediated by three routes:

(1) directly from the duodenum to the stomach through the enteric nervous system in the gut wall,
(2) through extrinsic nerves that go to the prevertebral sympathetic ganglia and then back through inhibitory sympathetic nerve fibers to the stomach
(3) probably to a slight extent through the vagus nerves all the way to the brain stem, where they inhibit the normal excitatory signals transmitted to the stomach through the vagi.


All these parallel reflexes have two effects on stomach emptying: First, they strongly inhibit the “pyloric pump” propulsive contractions, and second, they increase the tone of the pyloric sphincter. The types of factors that are continually monitored in the duodenum and can initiate enterogastric inhibitory reflexes include the following:

1. Distention of the duodenum

2. The presence of any irritation of the duodenal mucosa
3. Acidity of the duodenal chime
4. Osmolality of the chyme.
5.The presence of certain breakdown products in the chyme, especially breakdown products of proteins and, perhaps to a lesser extent, of fats

The enterogastric inhibitory reflexes are especially sensitive to the presence of irritants and acids in the duodenal chyme, and they often become strongly activated within as little as 30 seconds. For instance, whenever the pH of the chyme in the duodenum falls below about 3.5 to 4, the reflexes frequently block further release of acidic stomach contents into the duodenum until the duodenal chyme can be neutralized by pancreatic and other secretions.

Breakdown products of protein digestion also elicit inhibitory enterogastric reflexes; by slowing the rate of stomach emptying, sufficient time is ensured for adequate protein digestion in the duodenum and small intestine.

Summary of the Control of Stomach Emptying

Emptying of the stomach is controlled only to a moderate degree by stomach factors such as the degree of filling in the stomach and the excitatory effect of gastrin on stomach peristalsis. Probably the more important control of stomach emptying resides in inhibitory feedback signals from the duodenum, including both enterogastric inhibitory nervous feedback reflexes and hormonal feedback by CCK

These feedback inhibitory mechanisms work together to slow the rate of emptying when (1) too much chyme is already in the small intestine or (2) the chyme is excessively acidic, contains too much unprocessed protein or fat, is hypotonic or hypertonic, or is irritating. In this way, the rate of stomach emptying is limited to the amount of chyme that the small intestine can process.

Write True or False:

• Activation of the salivary sympathetic nerves inhibits salivary secretion
• The myenteric plexus lies between the longitudinal and circular smooth muscle layers of the gut wall.
• Gastric acid is secreted by parietal cells of the gastric glands in response to hormonal stimulation.
• Gastric emptying is inhibited by the enterogastric reflex.
• The most vigorous mixing movements in the stomach take place in the antral region.