Digestive System Regulation Quiz: Gut Hormones

Nitric oxide releasing NANC fibers are postganglionic parasympathetic neurons that do not use acetylcholine as their neurotransmitter. Instead, they release nitric oxide, which diffuses into smooth muscle cells and activates guanylate cyclase. This increases cyclic GMP levels, leading to smooth muscle relaxation. Clinically, this mechanism explains lower esophageal sphincter relaxation and corpus cavernosum vasodilation. Unlike adrenergic fibers, NANC fibers are cholinergic-independent pathways.

Gastrin is classified as a hormone because it is secreted into the bloodstream and acts on distant target cells. It originates from G cells in the gastric antrum and binds to CCK-B receptors on parietal cells. Hormones differ from enzymes, antibodies, and neurotransmitters in their endocrine mechanism of action. Gastrin increases hydrochloric acid secretion and mucosal growth through systemic circulation, confirming its hormonal classification.

Secretin is released when acidic chyme lowers the duodenal pH below approximately 4.5. S cells detect hydrogen ion concentration and respond by secreting secretin into circulation. This creates a negative feedback loop aimed at neutralizing acidity. By sensing luminal pH rather than nutrients or distention, secretin functions primarily as a regulator of acid-base balance in the small intestine.

Motilin is secreted during fasting and initiates the interdigestive migrating myoelectric complex every ninety to one hundred twenty minutes. This coordinated wave of depolarization clears residual food and secretions from the stomach and small intestine. Activation occurs via cholinergic pathways. The migrating complex prevents bacterial overgrowth and prepares the gut for the next meal cycle.

Gastrin directly stimulates parietal cells through CCK-B receptors, activating phospholipase C and increasing intracellular calcium. This signaling cascade enhances hydrogen ion secretion via proton pumps. Gastrin also stimulates enterochromaffin-like cells to release histamine, which further increases acid production. Through dual direct and indirect mechanisms, gastrin serves as the primary hormonal driver of gastric acid secretion.

Secretin stimulates bicarbonate secretion from pancreatic ductal cells by increasing intracellular cyclic AMP. This bicarbonate neutralizes acidic chyme entering the duodenum, raising luminal pH toward neutrality. It also promotes bicarbonate release from the biliary system. By buffering hydrogen ions, secretin prevents mucosal injury and optimizes pH conditions for pancreatic enzyme activity.

GIP, released from K cells in response to carbohydrates, fats, and proteins, enhances glucose-dependent insulin secretion from pancreatic beta cells. It binds to GIP receptors, increasing intracellular cyclic AMP and amplifying insulin release when glucose levels rise. This incretin effect accounts for higher insulin responses after oral glucose compared to intravenous glucose, demonstrating its endocrine regulatory role.

Ghrelin, produced primarily in the stomach during fasting, stimulates the hypothalamic appetite center by increasing neuropeptide Y activity. Plasma ghrelin levels rise before meals and decrease after feeding. It also stimulates growth hormone secretion via pituitary receptors. By linking energy deficiency to hunger signaling, ghrelin plays a central role in energy homeostasis and meal initiation.

Somatostatin acts as a universal inhibitory regulator in the gastrointestinal tract. Released from D cells when gastric pH falls, it suppresses gastrin secretion and directly inhibits parietal cell acid production. It also reduces pancreatic enzyme release and intestinal motility. Through paracrine and neurocrine mechanisms, somatostatin maintains acid balance and prevents excessive digestive activity.

Protein in the stomach stimulates gastrin secretion through both direct and neural pathways. Amino acids activate G cells, while gastric distention triggers vagal stimulation via gastrin-releasing peptide. This coordinated response increases acid secretion proportional to protein load. In contrast, low pH inhibits gastrin through somatostatin release. Therefore, protein presence represents the strongest physiological stimulus for gastrin secretion.