Motor Functions of the Stomach pdf - D. Noor

بسم هللا الرحمن الرحيم

Lecture -4- Medical Physiology (GIT system)
2

stage Dr. Noor Jawad

MOTOR FUNCTIONS OF THE STOMACH

Objectives of our lecture:

1. what are the motor functions of stomach?
2. 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, and

(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.

Thank you

References: Guyton and Hall textbook of medical physiology,
thirteen edition.