Unit 11 Must Know GI SS

Ch. 36: Structure and Function of the Gastrointestinal System 

Colonic Motility and Defecation

  • Colonic motility, defecation, and the pudendal nerve 

  • Types of movements in the colon: 

  1. Segmental mixing movements: occur within compartments called haustra → haustral churning: filling and expelling contents to ensure that all portions of the fecal matter are exposed to the intestinal surface

  2. Propulsive mass movements: large segments of the colon contract as a unit, moving the fecal contents forward as a unit; contractions last 30 seconds, followed by a 2-3 minute period of relaxation, a series lasts only for 10-30 minutes and may occur only several times per day ~ the mass movements usually initiate defecation

  • Normal stool is 75% water, 25% solid matter; colonic transit time of 24-48 hours

  • Defecation is controlled by the action of two anal sphincters: 

  1. Internal: several cm long; circular thickening of smooth muscle; lies just inside the anus

  2. External: composed of striated voluntary muscle; surrounding the internal sphincter

  • Controlled by the nerve fibers in the pudendal nerve that is part of the somatic nervous system and under voluntary control

  • To prevent involuntary defecation, the external anal sphincter is under the conscious control of the cortex

Gastrointestinal Hormones

  • GI hormones and their MOAs

  • Gastrin 

    • Produced by G cells in the antrum of the stomach 

    • Stimulates gastric acid secretion

    • Has a trophic (or growth-producing) effect on the mucosa of the small intestine, colon, and acid-secreting area of the stomach


  • Ghrelin

    • Produced by endocrine cells in the mucosal layer of the fundus (stomach)

    • Displays potent growth hormone-releasing activity

    • Stimulatory effect on food intake and digestive function while balancing energy expenditure

  • Secretin 

    • Secreted by S cells in the mucosa of the duodenum and jejunum 

    • Inhibits gastric acid secretion 

    • released in response to duodenal pH, which stimulates the pancreas to secrete large quantities of fluid with high bicarbonate concentration

  • Cholecystokinin (CCK)

    • Secreted by I cells in the intestinal mucosa 

    • Provides appetite control 

      • Deficiencies → poor appetite control

    • CCK’s primary function is the stimulation of pancreatic enzyme secretion

      • Potentiates action of secretin, increasing the pancreatic bicarbonate response to low levels of secretin; stimulates biliary secretion of fluid and bicarbonate; and regulates gallbladder contraction and gastric emptying

  • Incretin Effect producing hormones: increase insulin release after an oral glucose load. ≈ 90% of the incretin effects are from: 

  1. GLP-1 (released from the L cells in the distal small bowel)

  • GLP-1 can exert other metabolically beneficial effects, including suppression of glucagon release, slowing of gastric emptying, augmenting of net glucose clearance, and decreasing appetite and body weight

  1. GIP (released by K cells in the upper gut, mainly the jejunum)

  • Increased levels of GLP-1 and GIP can ↓ blood glucose levels by ↑ insulin release in a glucose-dependent manner

Salivary Secretions

  • Saliva and its actions 

  • Saliva is secreted by the salivary glands 

    • Secretion is regulated by ANS: parasympathetic stimulation ↑ flow; and sympathetic stimulation ↓ flow

    • Salivary glands consist of the parotid; submaxillary; sublingual; and buccal glands 

  • Functions: 

    • Protection and lubrication:

      • mucus which protects the oral mucosa and 

      • Lubricates food to make it easier to swallow 

    • Protective antimicrobial action: 

      • cleans the mouth; contains the enzyme lysozyme (antibacterial and antimicrobial actions)

    • Promotes digestion of dietary starches: 

contains ptyalin and amylase (enzymes), which initiate the digestion of dietary starches (also lipids)