Medicine Core Block - Physiology - The Digestive System - UCLan

Digestion and the Nervous system

• The phases of digestion include: 

  • Ingestion
  • Movement
  • Mechanical
  • Absorption
  • Elimination

• Components: Parasympathetic, Sympathetic, and Enteric nervous systems as well as the gut peptides, Paracrine, and Hormones all work together to induce peristalsis, mobility, secretion, and absorption.

• Automatic Nervous System: 

  • Involves extrinsic nerves which result in long reflexes and external stimuli
  • Associated with the CNS
  • Parasympathetic stimulates digestion while sympathetic nerves inhibit digestion.

• Enteric Nervous System:

  • Contains all the elements of a nervous system 
  • Involves intrinsic control which results in short reflexes and internal stimuli
  • There is communication between parasympathetic and sympathetic but it is autonomous
  • Two-well-organized neural plexuses: myenteric plexus which is above the submucosal plexus.

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Digestive System

• Mouth: Teeth mechanically break down food into small pieces. Tongue mixes with saliva (contains amylase, which helps break down starch)

• Epiglottis: Flap-like structure at the back of the throat that closes over the trachea preventing food from entering it. It is located in the Pharynx

• Esophagus: Approximately 20 cm long. The main function is the secretion of music as well as the movement of food using a movement called peristalsis.

• Stomach: J-shaped muscular bag that stores food and mixes it with digestive juices that contain enzymes that break down proteins and lipids, in addition to acid (HCl) that kills bacteria. Food found in the stomach is called Chyme

• Small Intestine: Small intestine is roughly 7 meters long, and the lining of the intestine walls has finger-like projections called villi to increase surface area. These villi are covered in microvilli which further increases the surface area for absorption. Nutrients from the food pass into the bloodstream through the small intestine walls.

• Large Intestine: About 1.5 meters long, accepts what small intestines don’t absorb. Contains the rectum which is short-term storage that holds feces before it is expelled. Some of its functions include bacterial digestion (fermentation of carbohydrates), absorption of water and its concentration as well as the elimination of wastes.

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Accessory Organs In Digestion

• Liver: Breaks down many biological molecules as well as stores vitamins and iron, it may also destroy old blood cells and poisons however its most important aspect is bile production.

• Gall Bladder: Stores and concentrates bile from the liver, and releases it into the small intestine. Fatty diets can cause gallstones to be formed.

• Pancreas: Produces digestive enzymes to digest fats, carbohydrates, and proteins. It regulates blood sugar by producing insulin.

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Enteric Nervous System (ENS)

• Myenteric Plexus: Between longitudinal and circular layers of muscle involved in the control of digestive tract motility.

• Submucosal: Located between the circular muscle and the luminal mucosa, can sense the environment of the lumen and regulates gastrointestinal blood flow and epithelial cell function.

• Enteroendocrine cells: Related to hormonal control by the production of about 22 hormones and paracrine and is present within the mucosa. However, they are single cells scattered about in the GI tract, densely packed secretory vesicles. They can sense luminal contents such as chemical, osmotic, and pH and thus can release hormones and paracrine to help fix any issues.

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Motility

• Phasic Contractions: Short-lasting contractions that control the movement of material in the small intestine.

  • Peristalsis: Waves of contraction of about 20 cms. Food is moved distally along the tract in adjacent segments
  • Segmented: Waves of contraction of about 10 cms. Food is moved forward and then backward in nonadjacent segments.

• Tonic Contractions: Long-lasting contractions that result in the closing of the sphincter, it is a controlled movement of material through the tract in an ordered sequence of events that through ‘compartmentalization’ ensures processes are complete before passing to the next area.

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