Git

Lec.6 GIT 19/4/2016

Gastric Motility

Objectives

Physiological anatomy
Motor functions of stomach
Storage
Mixing
emptying

Anatomically, the stomach is usually divided into two major parts: (1) the body and (2) the antrum.The stomach can be divided Physiologically into two regions based on patterns of motility: the orad or proximal region is responsible for accommodating of an ingested meal, comprising about the first two thirds of the body, the caudad” portion, or distal region is responsible for the contractions that lead to mixing and propulsion into the duodenum ,comprising the remainder of the body plus the antrum
The motor functions of the stomach are: 1.storage of food up to 0. 8 to 1 liters.2. Mixing of bolus with gastric juice which is called chym.
3. Slow emptying of chym from stomach into duodenum.
1. Storage function of the stomach
Solid food remains in the stomach about 3 – 4 hours while liquid about 1--2.5 hours. Stomach has a capacity of about one liter or more.

As food enters the stomach, it forms concentric circles of the food in the orad portion of the stomach, the newest food lying closest to the esophageal opening and the oldest food lying nearest the outer wall of the stomach. Normally, when food stretches the stomach, a “vagovagal reflex” from the stomach to the brain stem and then back to the stomach, reduces the tone in the muscular wall of the body of the stomach so that the wall bulges progressively outward, accommodating greater and greater quantities of food up to a limit in the completely relaxed stomach of 0.8 to 1.5 liters. Accommodation is made possible by the active relaxation of the gastric smooth muscle, a process referred to as receptive relaxation which is mediated by a vagovagal reflex..


2. Mixing and propulsive of food in the stomach
mixing of the food with gastric secretion until it forms a murky semifluid or paste called chyme.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 -20 seconds . Contractions of the caudad area of the stomach are initiated BER generated by pacemaker cells known as the Cajal cells.. These BER occurs 3 to 4 times per minute. As the peristaltic contractions progress from the body to antrum portions of the stomach , they become more intense constrictor rings that force the antral contents under higher and higher pressure toward the pylorus .These constrictor rings also play an important role in mixing stomach contents . The opening of the pylorus is small enough that only a few millimeters or less of antral contents are expelled into the duodenum with each peristaltic wave.
Chyme
After food in the stomach has become thoroughly mixed with the stomach secretions, the resulting mixture that passes down the small intestine 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 had occurred. The appearance of chyme is that of murky semifluid or paste.

Hunger contractions

In addition to the peristaltic contractions that occur when food is present in the stomach, another type of intense contractions called Hunger Contractions occur when the stomach has been empty for several hours or more. They are rhythmical peristaltic contractions in the body of stomach. Person sometimes experiences mild pain in the pit of stomach called hunger pangs which usually do not begin until 12 -24 hours after the last ingestion of food , in starvation they reach their greatest intensity in 3-4 days. They are associated with the sensation of hunger and to be an important regulator of appetite .They are most intense: 1. in young healthy people who have high degree of gastrointestinal tonus and.2. By the person's having hypoglycemia. Why?
3. Gastric Emptying
There is a 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 .Stomach emptying is promoted by intense antral peristaltic contractions of the stomach which is called pyloric pump, and opposed by varying degrees of resistance to passage of chyme at the pylorus (The distal opening of the stomach). The peristaltic waves in the stomach, providing two actions, mixing and pumping actions.
At the distal part of the stomach the thickness of the circular wall muscle becomes 50 to 100 per cent 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.
Pyloric pump and the tone of the pylorus, is under the control of nervous and humoral reflex signals from both the stomach and the duodenum.

Regulation of Gastric Motility and Emptying

The rate at which the stomach empties into the duodenum depends on the type of food ingested. Food rich in carbohydrate leaves the stomach in a few hours. Protein-rich food leaves more slowly, and emptying is slowest after a meal containing fat. The rate at which the stomach empties is regulated by signals from both the stomach and the duodenum.

A. Gastric Factors That Promote Emptying: these factors increases the pyloric pump and inhibit activity (decrease the tone) of pylorus.
1. Neural control 2.hormonal control
1. Neural: In general, the greater the volume, the more rapidly the contents empty. Stretching of the stomach wall does elicit local myenteric reflexes in the wall that greatly accentuate activity of the pyloric pump and at the same time inhibit the pylorus.. Liquids empty more rapidly than solids, and solids must be reduced in size to particles of 2 - 3 mm or less for emptying to occur.
2. Hormonal: release of hormone gastrin: stomach wall stretch and the presence of certain types of foods in the stomach—particularly digestive products of meat—elicit release of a hormone called gastrin from the antral mucosa. This has potent effects to cause secretion of highly acidic gastric juice. But Gastrin 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, it, too, probably promotes stomach emptying.

B. Duodenal Factors That Inhibit Gastric Emptying

1. Enterogastric Nervous Reflexes from the duodenum. When food enters the duodenum, multiple nervous reflexes are initiated from the duodenal wall that 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 two 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. All these reflexes first strongly inhibit the “pyloric pump” propulsive contractions, and second, they increase the tone of the pyloric sphincter. The factors in the duodenum and that can initiate enterogastric inhibitory reflexes, are:
1. The degree of distention of the duodenum 2. The presence of any degree of irritation of the duodenal mucosa 3. The degree of acidity of the duodenal chyme. When pH of the chyme in the duodenum falls below about 3.5 to 4 .4. The degree of osmolality of the chyme. The most rapidly emptying substance is isotonic saline. Both hypo- and hypertonic saline empty more slowly. 5. The presence of certain breakdown products in the chyme, especially breakdown products of proteins and perhaps to a lesser extent of fats.
2. Hormonal -Role of Fats and the Hormone Cholecystokinin (CCK)
There are hormones released from the duodenum to the stomach inhibit stomach emptying. The stimulus for releasing these inhibitory hormones is mainly fats entering the duodenum. On entering the duodenum, the fats causes the release of several different hormones from the duodenal and jejunum .In turn, the hormones are carried by way of the blood to the stomach, where they inhibit the pyloric pump and increase the strength of contraction of the pyloric sphincter. These effects are important because fats are much slower to be digested than most .The most potent hormones is 1. (CCK).. CCK can inhibit gastric emptying when excess quantities of chyme, fatty chyme, enter the duodenum from the stomach. 2. Secretin 3.gastric inhibitory peptide (GIP).
Since fats are particularly effective in inhibiting gastric emptying, some people drink milk, cream, or even olive oil before a cocktail party. The fat keeps the alcohol in the stomach for a long time, where its absorption is slower than in the small intestine, and the intoxicant enters the small intestine in a slow, steady stream so that—theoretically, a sudden rise of the blood alcohol to a high level and consequent embarrassing intoxication are avoided.