Additional information | Gastrointestinal system | Physiology

Zollinger -Ellison Syndrome: It is due to gastrin producing tumor or tumors in the duodenum or pancreas. A gastrin level 10 times the upper limit of normal (approximately 1000 pg/mL) plus a gastric pH below 2.0 is diagnostic of Zollinger-Ellison syndrome. It presents as recurrent, multiple, refractory peptic ulcers, diarrhea and steatorrhea. Steatorrhea is due to inhibition of pancreatic lipase by increased acidity. Secretin stimulation test causes serum gastrin levels to rise in Zollinger-Ellison syndrome although opposite effect will be seen in healthy individuals.
Action of gastrin: Gastrin acts through two mechanisms to increase gastric HCl production. It binds to parietal cells causing increased expression of H+/K+ ATPase which increases H+ secretion into the stomach. Gastrin also stimulates enterochromaffin cells to produce histamine. Histamine binds to H2 receptors on parietal cells to stimulate H+ secretion. End result is more HCl production and secretion. Gastrin also promotes proliferation of parietal and enterochromaffin cells.
Incretins: They are GIT hormones that are secreted in response to food intake and stimulate secretion of insulin. GIP and GLP 1 are the known incretins. Together, they are responsible for the incretin effect: a two- to three-fold higher insulin secretory response to oral as compared to intravenous glucose administration. In subjects with type 2 diabetes, this incretin effect is diminished or no longer present.
Vomiting reflex: Vomiting or emesis is the forceful retrograde expulsion of gastric contents from the body. Nausea is the unpleasant sensation that precedes vomiting. Retching (“dry heaves”) is the simultaneous contraction of the abdominal muscles and muscles of inspiration that may occur with vomiting. The vomiting center (VC) is located in the dorsolateral medulla adjacent to the ascending reticular activating system and the medullary centers controlling cardiovascular and respiratory reflexes. It receives afferents from the cortex, limbic system, hypothalamus, vestibular centers, gut, and other viscera. The chemoreceptor trigger zone (CTZ), located in the floor of the fourth ventricle, provides specific receptors for circulating toxins in the blood and cerebrospinal fluid and relays this information to the VC. The afferent information is processed by the nucleus of the tractus solitarius. Effect is proximal gastric relaxation and a giant retrograde contraction of the intestine, mediated by the vagus nerve, leading to vomiting.
Defecation reflex: The anal canal is guarded by two sphincters - external and internal anal sphincters. The internal sphincter is supplied by parasympathetic pelvic nerves , has smooth muscle and is involuntary. The external sphincter is supplied by the somatic pudendal nerve, has striated muscle and is voluntary. Accumulation of fecal material activates stretch receptors in the rectum which spread through the myenteric plexus to initiate peristaltic waves in the descending and sigmoid colon and the rectum. Relaxation of the internal anal sphincter follows and the feces move towards the anus . Impulses are conducted to the defecation center in the sacral spinal cord which in turn increases peristalsis in descending colon, the sigmoid colon and the rectum through the parasympathetic pelvic nerves. The external anal sphincter remains contracted till it is relaxed voluntarily, allowing defecation. The puborectalis muscle relaxes, straightening out the rectum and facilitating defecation.
Enterohepatic circulation: Most of the bile salts that are secreted into the intestine are reabsorbed into the portal vein and carried back to the liver. That way bile salts are recycled. Bile salts exert negative feedback on the synthesis of bile acids by the liver by inhibiting enzyme cholesterol 7alpha-hydroxylase. Recirculation of bile salts to the liver also has a “choleretic” effect where it stimulates further biliary secretion.