In this article we will discuss about the four main types of movements that are seen in stomach.
Type # 1. Movements of the Fundus and Body:
A peristaltic wave follows the basic electrical rhythm (B.E.R.) of stomach. It starts in the longitudinal muscle of greater curvature as a pacemaker and passes over the body, antrum and pylorus at a rate of 3 per minute and the frequency and rate of progress are influenced by nervous and humoral factors. Rate of progress is 1 cm per sec. in the muscle of body and the duration of wave at a point is 1.5 sec, which is accelerated in the antrum to 3 – 4 cm per sec.
The excitative wave spreads from longitudinal to circular muscle. Consequently irregular band of contraction, 2 cm wide, moves as excitation wave over the body and the antrum following B.E.R. The wave deepens as the digestion proceeds and duration of wave increases.
Type # 2. Movements of Pylorus:
The terminal antrum and the pylorus contract with the arrival of wave in their region known as systolic contraction. Due to the viscous property of gastric contents and pressing of gastric contents into the terminal part of antrum by the peristaltic wave, the antral pressure rises which overcomes the pressure barrier in the pylorus and thus antral contents passes into the duodenum. The passage of chyme into the duodenum is stopped suddenly by the contraction of pylorus.
Since the contraction of last antral portion continues and the pressure rises to about 10 – 25 mm of Hg, the antral contents are forcibly moved in oral side and not in aboral side, i.e., in the stomach, as the pylorus (pyloric sphincter) prevents it. Thus the reflux of chyme helps in mixing the food with gastric juices. Finally the terminal part of antrum and pylorus relax and continue to relax till the next peristaltic wave reaches the antrum.
Cause of the Movements:
Excised and isolated stomach when placed in the warm saline shows rhythmic movements. These contractions are believed to be myogenic because they occur even after the complete division of all nervous connections. However, the present state of knowledge indicates that although the movements in the two parts of the stomach may be mostly myogenic in nature, yet the perfect co-ordination between the tonic contraction of the left half and the peristaltic movement of the right half depends on the integrity of the local nervous system [Auerbach’s plexus.]
Role of the Vagus and the Sympathetic:
Stomach is supplied by both vagus and sympathetic. Although they are not essential for the genesis of the gastric movements, yet, they exert profound influence upon them. Stimulation of vagus does not give uniform results. On the whole, the effect depends on the pre-existing tone of the stomach.
When the tone is low, vagus causes contraction, when the tone is high—relaxation. The effect of sympathetic stimulation is also not uniform. It is generally believed that sympathetic inhibits gastric movements and constricts the sphincter, while vagus stimulates movements and relaxes the sphincter.
Role of Hormones and Other Chemicals:
Adrenaline has the same effect as sympathetic. Pituitrin stimulates gastric contractions. Insulin (as well as low blood sugar) increase gastric movements. Enterogastrone inhibits movements. High gastric acidity is generally associated with a hypermotile stomach, whereas low acidity with a hypomotile stomach.
Type # 3. Pyloric Sphincter:
In the fasting condition the pyloric sphincter remains relaxed. When food enters, the sphincter closes. It then opens at intervals. The time of opening varies with different conditions. For instance, with water and liquid diet, the sphincter almost immediately opens; but with a mixed diet the opening starts at least 15 minutes (average half an hour).
The time of opening depends on:
(a) The normal motility of the stomach,
(b) Quality and consistency of the food, and
(c) Stage of digestion and possibly some other factors.
The sphincter is regulated by the pH of the gastric and duodenal contents. High acidity on the gastric side opens it, whereas that on the duodenal side closes it. As gastric secretion precedes the acidity rises and when, it goes up to the critical level, the sphincter opens and gastric contents are pushed into duodenum.
Entry of acid chyme in the duodenum raises the acidity of the duodenal contents, consequently the sphincter closes. Thus it goes on alternately, till the stomach is empty. But this view cannot be accepted as a whole. Since, people suffering from achylia, do not show absence of normal activity of pyloric sphincter in general.
ii. Intraluminal Pressure:
According to some physiologists, opening of the pyloric sphincter depend on adequate pressure. When the peristaltic wave in the pylorus becomes sufficiently strong, the sphincter is forced to open and thus stomach empties into the duodenum. On the other hand, when the antiperistalsis in the duodenus becomes strong, the sphincter is also opened and duodenal regurgitation takes place. Although this view is widely accepted, yet it is seen that in those subjects whose pylorus has been excised the sphincter behaves almost normally.
iii. Amino Acid, Glucose, Fat or Fatty Acid Concentration:
The proteose-peptone concentration of the gastric content is supposed to control the sphincter. As gastric digestion proceeds, the proteose-and-peptone concentration rises and when it arrives at a certain level, pylorus opens.
Some quantity is thereby expelled into the duodenum and thereby fall in concentration rises again due to further protein digestion and the cycle repeats. Similar reaction of sphincter is seen in case of glucose but reciprocal reaction occurs in case of fat or fatty acid.
iv. Osmotic Pressure:
The osmotic pressure of the gastric content is another factor that controls the sphincter. When the osmotic pressure of the gastric content approaches that of the saline constituent of the plasma, the sphincter opens. As digestion proceeds, number of dissolved particles increase, so that osmotic pressure rises. When it goes up to the requisite level, the sphincter opens.
Gastrin increases tonus whereas enterogastrone or urogastrone inhibits tonus, so intraluminal pressure increases and decreases respectively. It may be concluded that the opening and closure of the pyloric sphincter depend on several factors.
Type # 4. Emptying of Stomach:
The type of foodstuffs and their consistency determine the emptying time of stomach. Water almost immediately passes out; liquid food is emptied more rapidly than solid. Larger meals have necessarily longer emptying time.
A small amount is being evacuated out at interval of every 20 seconds. The emptying of the stomach is regulated by the accumulation of evacuated substances in the intestine. Fats are retained for the longest time, because they inhibit gastric movements. Fats act through the hormone enterogastrone released into the blood from the mucous membrane of the intestine.
Carbohydrates leave early. Proteins are intermediate. Products of protein digestion act mainly through the vagi-enterogastric reflex. In animals mechanical irritation or distention of the duodenum is followed by inhibition of peristalsis and this is due to enterogastric reflex mediated also through the vagi. With a normal mixed diet the stomach completely empties in about three and a half hours. With a rich fatty diet about four and a half hours.