Large Intestine Motility and Defecation

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Large Intestine Motility and Defecation
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Objectives

• Describe the functions of the colon
• Describe the movement of fecal matter through the
  colon to the rectum
• Describe the duodenal and gastrocolic reflexes
• Explain the defecation reflexes and the factors
  that initiate and regulate them
• Describe secretion in the large intestine
• Describe absorption in the GI tract

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Movement of Chyme into the Cecum

• Ileocecal sphincter feedback control
• Peristalisis in the ileum
• Reflex control from the cecum
• Distension of the cecum
• Irritants in the cecum (e.g. inflamed appendix)
• Control
• Via myenteric plexus
• Prevertebral sympathetic ganglia

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Functions of the Colon

• Absorption of water and electrolytes (proximal)
• Storage of fecal matter (distal)

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Absorption in the Large Intestine

• Absorption of sodium and chloride creates an
  osmotic gradient for absorption of water
• Can absorb 5-8 liters of fluid and electrolytes
  daily
• Aldosterone greatly potentiates sodium absorption

• Most of the water and electrolytes in chyme that
  enters the colon is absorbed
• Active absorption of sodium, creating a gradient
  for absorption of chloride

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Fluid and Electrolyte Absorption Role of
Transporters
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Absorption of Sodium
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Ingestion, Secretion and Absorption in the GI
Tract
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Movement of the Colon

• Normally sluggish
• Two types
• Mixing movements (Haustrations)
• Haustrations bulging of the large intestine
  into baglike sacs as a result of circular and
  longitudinal muscle contraction
• Propulsive movements (Mass movements)

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Mixing Movements - Haustrations

• Large circular constrictions (similar to
  segmentations in the small intestine)
• 2.5 cm of circular muscle contracts along with
  the longitudinal muscle
• Haustrations
• Up to 90 seconds long (with peak activity at
  approx. 30 sec)
• Increases exposure to fecal material and enhances
  absorption
• Responsible for some of the propulsion in the
  cecum and ascending colon

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Propulsive (Mass) Movements

• Modified peristalisis
• Constrictive ring occurs in response to distended
  or irritated point in the colon
• 20 or more cm of colon distal to the constrictive
  ring contract together (as a unit)
• Fecal material is thus propelled further down the
  colon
• For 30 sec the contraction is forceful, then
  relaxation for 2-3 minutes
• Followed by another mass movement

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Mass Movements

• Series of mass movements persists for 10-30 min
• Mass movements propel chyme from the cecum to the
  sigmoid colon
• Occur 1 to 3 daily, especially during the first
  hour following a meal
• When a mass of feces is forced into the rectum,
  there is a desire to defecate

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Movement of the Colon
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Reflexes Affecting Mass Movements

• Gastrocolic reflex stimulatory
• Distention of the stomach
• Duodenocolic reflex - stimulatory
• Distention of the small intestine
• Both reflexes transmitted by autonomic nervous
  system
• Mass movements also initiated by irritation in
  the colon (e.g. ulcerative colitis)

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• Most of the time the rectum is empty
• Due to weak functional sphincter (20 cm from
  anus, at junction of the sigmoid colon and the
  rectum)
• Sharp angulation of the rectum

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Defecation

• When mass movement forces feces into the rectrum
• Immediate desire to defecate
• Reflex contraction of rectrum
• Relaxation of anal sphincter
• Approx. 80 to 200 mL of fecal matter expelled
  daily

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Defecation

• Tonic constriction by
• Internal anal sphincter (circular smooth muscle)
• External anal sphincter (striated voluntary
  muscle)
• Controlled by pudendal nerve (somatic NS)
• Voluntary control

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Mader, 5th edition
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Defecation Reflexes

• Intrinsic reflex
• Local enteric nervous system (myenteric plexus)
• Distension of rectal wall
• Peristaltic waves in descending colon, sigmoid
  and rectum
• Relaxation of internal anal sphincter (inhibitory
  action of the myenteric plexus)
• Weak when functioning alone
• Parasympathetic defecation reflex
• Involves sacral segments of the spinal cord
• Pelvic nerves

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Defecation and the Parasympathetic Nervous System

• Signals transmitted into spinal cord
• Reflex back to descending colon, sigmoid, rectum
  and anus (pelvic nerves)
• Intensify peristalisis
• Relax internal anal sphincter

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Defecation

• Relaxation of the external anal sphincter is
  voluntary
• Defecation reflexes can be purposely activated
• Forcing fecal particles into the rectum to cause
  new reflexes
• Not as effective as the natural reflexes

• From the spinal cord, defecation signals also
  have the following effects
• Taking deep breath
• Closure of glottis
• Contraction of abdominal wall muscles
• Relaxation and movement of pelvis floor downward

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Secretions of the Large Intestine

• Crypts of Lieberkuhn, but no villi
• Mainly mucous cells
• Mucus
• Bicarbonate ions
• Regulated by tactile stimulation and local
  reflexes to the mucous cells

• Mucus in the large intestines
• Protects intestinal wall against excoriation
• Adherent medium for fecal particles
• Protects wall from bacterial activity (within the
  feces)
• Provides barrier against acidity

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Secretion of Water and Electrolytes

• Irritation
• Eg. Bacterial infection (enteritis)
• Mucosa secretes large amount of water and
  electrolytes
• Secretion of mucus
• Rapid movement of fecal particles, i.e. diarrhea

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Composition of Feces

• Three-fourths water
• One-fourth solid matter
• 30 dead bacteria
• 10-20 fat
• 10-20 inorganic matter
• 2-3 protein
• 30 undigested roughage (e.g. bile pigment,
  sloughed epithelial cells)

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References

• Guyton and Hall, 2006. Textbook of Medical
  Physiology. Chapter 63, 64, 65.