VPB 121: Veterinary Physiology - II (2+1)

GASTRIC MOTILITY

Mechanism of gastric motility

• Function of the stomach is to regulate the flow of food to the small intestine  at a controlled rate .
• The proximal region (fundus) near the oesophageal end serves storage function and the distal region serves grinding and sieving function. 
• Muscular activity of the fundus shows weak continuous contraction, but capable of relaxation without  changing  intraluminal pressure as the food enters the stomach . Thus, the fundus serves as a food storage area and not involved much in the mixing of food.
• The body of the stomach serves as a mixing site of the gastric juice with the food.
• The distal stomach, the antrum shows intense slow wave of muscular contractions.
• It acts as a gastric pump and regulates the propulsion of food through the pyloric sphincter into the duodenum.

 

• The antral contractions also show reverse peristalsis of the pyloric contents,  thus delay the passage of solid particles from the stomach.
• The peristaltic waves movement of the stomach begins near the middle of the body of the stomach and runs towards pylorus.
• The sphincter gets closed only when a peristaltic wave reaches it.
• Bile and duodenal contents flow into the stomach through the opened sphincter. 
• As gastric digestion proceeds, the peristaltic wave becomes stronger and small quantities of gastric contents are carried into the pylorus.
• As the peristaltic wave comes near the pylorus, the pylorus constricts and blocks the exit of gastric contents excepts those particles smaller than 2mm in size.
• Larger particles are retropelled back into the corpus for further mixing and regrinding.
• Thus, the distal stomach serves to propel the food for mixing and grinding and acts as gastric pump, regulating the rate of gastric emptying (View animation).
• Termination of pyloric peristaltic wave follows as duodenal contraction  to carry the ingesta along the duodenum.
• Reverse peristaltic waves are also noticed in the stomach.
• The rate of passage of solids is regulated by the rate at which the solids are broken down to small particles, which in turn is controlled by the motility of distal stomach.
• Greater the motility of antrum, faster the material is broken down.
• Liquids leave the stomach more quickly than solids and it is less dependent on antral motility.
• Emptying of solid material depends on fat content. Low fat meals leaves the stomach in 3 to 4 hours. 

Regulation of gastric motility 

• The stomach muscle possess a high degree of automaticity. 
• Gastric motility is  regulated by the vagi and sympathetic nerves.
• Stimulation of vagus increases peristaltic activity of the distal stomach, but suppresses muscular activity of the proximal stomach. 
• Anticipation of eating increases vagal activity. 
• Entry of food into the stomach provokes peristaltic contractions,  through reflex arcs involving, receptors in stomach mucosa, vagus nerves and vagal nucleus in medulla.
• Sympathetic nerves inhibit peristalsis.
• Gastrin increases gastric motility, whereas secretin and CCK suppress gastric motility.

Enterogastric reflex

• The enterogastric reflex involves both extrinsic and intrinsic nervous system and endocrine system.
• This reflex regulates gastric emptying by gastric motility.  These reflexes have their afferent receptors in duodenum  which are activate by low pH, high osmolality, and presence of fat. 
• Vagus is the extrinsic nerves stimulation of which brings about increased gastric motility, whereas inhibition of  vagus nerve  causes a decrease in  gastric motility.
• Intrinsic nerves with their receptors and motor nerves also alter gastric motility through interconnections between stomach and duodenum.
• Osmotic pressure and acidity of duodenal contents are detected by duodenal chemoreceptors which alter the vagal activity. Increase in osmotic pressure and decrease in acidity depresses gastric motility.

Endocrine control of gastric motility

• The endocrine system controlling the gastric motility include secretin, produced by low duodenal pH; CCK released in response to fat in duodenal contents and GIP release in response to carbohydrates. These hormones enter into the blood, carried to stomach and inhibit gastric motility. 

Hunger 

• Hunger or urge to eat is a discomfort or pain localised in the epigastric region caused by a need for food.
• The feeling of hunger is marked by rhythmic contraction related to motor activity of the stomach which is determined by low blood sugar level.
• Hunger contractions appear even before the stomach has completely emptied itself.
• Hunger sensations mainly act through the hypothalamic hunger center, which determines food intake and the satiety center, which regulates satisfaction of eating.