Defecation Reflex Mechanism

Defecation Reflexes

• There are two types of defecation reflexes
• Intrinsic reflex mediated by the local enteric nervous system in the rectal wall • Distention of the rectal wall initiates afferent signals through the myenteric plexus • Initiates peristaltic waves in the descending colon, sigmoid, and rectum, forcing feces toward the anus
• External anal sphincter is consciously and voluntarily relaxed at the same time of defecation
• Internal anal sphincter is relaxed by inhibitory signals from the myenteric plexus

Secretory Functions of the GIT

• Secretory glands have two primary functions: • Secrete digestive enzymes in most areas of the GIT • Provide mucus for lubrication and protection of all parts of the GIT
• Basic mechanisms of stimulation of GIT glands: • Contact of food with the epithelium stimulates secretion • Parasympathetic stimulation increases secretion rate • Sympathetic stimulation has a dual effect

Esophageal Secretions

• Entirely mucous and provide lubrication for swallowing
• Main body of the esophagus is lined with many simple mucous glands

General Principles of Gastrointestinal Function

• The alimentary tract provides the body with a continual supply of: • Water • Electrolytes • Vitamins • Nutrients
• Achieving this requires: • Movement of food through the GIT • Secretion of digestive juices and digestion of the food • Absorption of water, electrolytes, vitamins, and digestive products • Circulation of blood through the GI organs to carry away the absorbed substances • Control of all these functions by local, nervous, and hormonal systems

Physiologic Anatomy of the GI Wall

• Characteristics of GIT wall: • Layers from outside towards the lumen:
  • Serosa
  • Longitudinal muscle layer
  • Circular muscle layer
  • Submucosa
  • Mucosa

Electrical Activity of Gastrointestinal Smooth Muscle

• Slow waves: • Slow, undulating changes in the resting membrane potential • Intensity usually varies between 5 and 15 millivolts
• Spike potentials: • True action potentials that occur automatically when the resting membrane potential becomes more positive than about −40 millivolts • Normal resting membrane potential in the smooth muscle fibers of the gut is between −50 and −60 millivolts
• Factors that depolarize the membrane: • Stretching of the muscle • Stimulation by acetylcholine • Stimulation by several GI hormones
• Factors that hyperpolarize the membrane: • Effect of NE or EP on the fiber membrane • Stimulation of the sympathetic nerves that secrete mainly NE at their endings

Small Intestine Function

• Digestion: • Enzymes are intracellular (disaccharidases and aminopeptidases)
• Absorption: • Completed in small intestine • Large surface area (as large as a tennis court area) • Site where most digestion and absorption take place
• Secretion: • Secretion of digestive enzymes by the brush border of the villi • Enterocytes of the mucosa contain digestive enzymes that digest specific food substances while they are being absorbed through the epithelium
  • Several peptidases for the splitting of small polypeptides into AA
  • Disaccharidases that break disaccharides into monosaccharides
  • Small amounts of lipases for the digestion of fat