GIT Physiology: Hormonal Regulation and Small Intestine Motility

Vocabulary flashcards covering GI hormones, small intestine motility types, pacemaker cells, and motility reflexes from Lectures 4 and 5.

Enteric endocrine system

The collective group of hormone-secreting epithelial cells (endocrinocytes) located in the stomach and small intestine, making the gastrointestinal tract the largest endocrine organ in the body.

Gastrin

A major hormone synthesized in G cells of the gastric antrum that stimulates gastric acid secretion, promotes gastric mucosal growth, increases gastric motility, opens the pyloric sphincter, and relaxes the ileocecal sphincter; its secretion is inhibited when luminal pH falls below $3$.

Pentagastrin

The sequence of $5$ C-terminal amino acids in gastrin that preserves its biological activity and is identical to the $5$ C-terminal amino acids of cholecystokinin.

Zollinger-Ellison syndrome

A severe condition caused by hypergastrinemia, typically resulting from gastrinomas (gastrin-secreting tumors), characterized by gastric and duodenal ulceration due to excessive and unregulated gastric acid secretion.

Somatostatin

A paracrine hormone secreted by D cells of the pylorus that continuously acts on adjacent G cells to inhibit the release of gastrin.

Cholecystokinin (CCK)

A hormone secreted by duodenal mucosal epithelial cells and enteric nervous system neurons in response to partially digested fats and proteins; it stimulates pancreatic enzyme release, gallbladder contraction, and bile salt secretion.

Secretin

A hormone released by the small intestine in response to duodenal acidification that stimulates the pancreas and bile duct cells to release a bicarbonate-rich fluid to neutralize acid.

Gastric Inhibitory Peptide (GIP)

A hormone secreted by mucosal epithelial cells in the upper small intestine that inhibits gastric motility and acid secretion and enhances insulin release (also termed glucose-dependent insulinotropic peptide).

Vasoactive Intestinal Peptide (VIP)

A potent vasodilator structurally related to secretin that induces smooth muscle relaxation, stimulates water secretion into pancreatic juice and bile, and inhibits gastric acid secretion and intestinal absorption.

VIPomas

Tumors arising from pancreatic islets or nervous tissue that secrete excessive quantities of VIP, resulting in chronic, watery diarrhea.

Segmentation contractions

The primary type of small intestinal mixing contraction where approximately $2\,\text{cm}$ of the wall contracts for $5$ to $6\,\text{seconds}$ at a frequency ranging from $12\,\text{times/min}$ in the duodenum to $8\,\text{times/min}$ in the ileum.

Interstitial cells of Cajal (ICC)

Pacemaker cells located within the wall of the intestine that control the frequency of segmentation contractions.

Intestinal transit time

The duration required for chyme to travel from the proximal to the distal end of the small intestine, typically taking $2$ to $4\,\text{hours}$.

Peristaltic reflex

An enteric nervous system reflex initiated by 5-HT (serotonin) release from enterochromaffin cells, leading to circular muscle contraction behind a food bolus and circular muscle relaxation ahead of the bolus to propel chyme at about $1\,\text{cm/min}$.

Migrating Motor Complex (MMC)

A pattern of peristaltic contractions during interdigestive (fasting) states occurring every $1\frac{1}{2}$ to $2\,\text{hours}$ over $20$ to $30\,\text{cm}$ segments, traveling at $6$ to $12\,\text{cm/min}$ to sweep residual undigested materials and secretions into the colon.

Gastroileal reflex

A reflex triggered by gastric distension after meals that increases vagally-mediated ileal peristalsis and causes gastrin-mediated relaxation of the ileocecal sphincter, delivering contents into the large intestine.

Ileogastric reflex

A motility reflex in which distension of the ileum causes reflex inhibition of gastric motility.

Intestinointestinal reflex

A motility reflex in which overdistension of a single intestinal segment causes relaxation of smooth muscles throughout the remaining segments of the small intestine.