Motility (Gastrointestinal)

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Motility

Objectives

  • General principles
    • Understand the major differences between striated muscle and gut smooth muscle (enteric nervous system, slow wave)
    • understand the function of GI tract sphincters
  • Esophagus
    • Know the functional anatomy of the esophagus (UES, striated body, smooth muscle body, LES)
    • Understand the central (CNS) and peripheral (enteric nervous system) control of swallowing and peristalsis.
    • know the clinical significance of LES dysfunction (reflux vs achalasia)
  • Stomach
    • Know the main functions of gastric motility
    • Understand the mechanisms of receptive relaxation and gastric emptying of liquids and solids
    • understand that gastric peristalsis is a function of intrinsic smooth muscle properties
    • know the main determinants of the rate of gastric emptying
  • Small bowel
    • Know the main functions of small bowel motility
    • Understand that small bowel peristalsis is a function of intrinsic smooth muscle properties (slow wave and phase lag)
    • know the function of fasting on small bowel motor activity (migrating myoelectric complex)
  • Colon
    • Know the main types and function of colonic motility (segmentation, peristalsis, mass movement)
  • Rectum
    • know the mechanism of fecal continence

General principles

Striated vs smooth muscle

Striated muscle Smooth muscle
Very small fibres organized in bundles Long fibres organized in muscle units
Muscle fibres within bundles connected by gap junctions Fibres independent of each other except by common innervation by one motor neuron
Innervated by fibres of nervous plexuses (the enteric nervous system): a potentially independent nervous system that can act without the CNS Innervated by motor neurons: entirely dependent on CNS
Muscle fibres have no motor endplate: efferent fibres from plexuses liberate mediators near the cell surface Each fibre has a motor endplate
Muscle fibre potential displays spontaneous variations at rest (independent of nervous activity): referred to as the slow wave Membrane potential constant at rest
Active tension always present No active tension without action potential

GI tract sphincters

  • The gastrointestinal tract is separated into functional units by muscular sphincters
  • These prevent flow of material in the wrong direction and regulate the rate of aboral movement

Esophagus

Stomach

Small bowel

Functions

  • Motility of the small intestine is organized to optimize the process of digestion and absorption of nutrients
  • Contractions of the small intestine perform at least three functions:
    1. mixing of ingested material with digestive secretions and enzymes
    2. circulation of luminal contents to facilitate contact with the intestinal mucosa
    3. propulsion of material in a net aboral direction
  • Understand that small bowel peristalsis is a function of intrinsic smooth muscle properties (slow wave and phase lag)

migrating myoelectric complex (MMC)

  • aka interdigestive myoelectric complex (IDMEC)
  • a sequence of electrical activity found in the intestine during fasting (> 6 hours)
  • The cycle begins with action potentials in the stomach and the accompanying peristaltic wave
  • The wave of electrical and contractile activity consists of an activity front which is followed by a period of rest
  • The MMC migrates down the intestine to the ileum
  • As one complex finishes in the ileum another begins in the stomach
  • The duration of each cycle is approximately 100 - 120 min
  • Administration of food disrupts this pattern
  • The MMC is thought to play a role as an "intestinal housekeeper" removing dead cells, bacteria, undigested food etc. from the small intestine

Colon

  • Contractions of the large intestine are organized to allow for optimal absorption of water and electrolytes as well as net aboral movement of contents

Rhythmic segmentation

  • The primary form of motor activity observed in the colon
  • As a consequence of this lack of regular propulsive movement, the colon is able to store its contents to allow for maximal dessication
  • Occasionally, segmental contractions are organized in an Dictionarycom:aboral direction so that propulsion over short distances takes place

Mass movement

  • Most propulsion occurs during a characteristic sequence termed mass movement
  • The coordinated contractile activity that sweeps intraluminal material in an Dictionarycom:aboral direction
  • This type of activity normally occurs 1-3 times per day

Rectum

  • Fecal continence is maintained via the actions of the internal anal sphincter (involuntary control) and the external anal sphincter (voluntary control).

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