Gastrointestinal Motility and Peristalsis

Intestinal Motility Patterns

• Ileus is an atonic/paralytic bowel that is pathological

• Ileus typically occurs post-op after bowel surgery

• Ileus is caused by disordered autonomic control (increased SNS tone) and metabolic factors

• There is increased inhibitory effector neuron activity and decreased cholinergic effector neuron activity in Ileus

• Spasm affects intestinal motility, caused by unrelenting circular smooth muscle contraction due to absence of inhibitory neuron activity

• Hirschsprung's disease can cause spasm due to absence of ENS in the colon section causing circular muscle contraction to dominate

• Congenital caudial mesenteric innervation failure to meet with sacral innervation

• Segmentation is a normal motility pattern that slows digesta passage, induced by activity of enkephalinergic interneurons

• Segmentation involves alternating segments where inhibitor effectors are OFF (circular smooth muscle contracts) and cholinergic effectors are ON

• Segmentation slows passage and mixes content while allowing for aboral progression of digesta

• Peristalsis is the main propulsive pattern for digesta passage

• During feeding, digesta is sensed and local ENS action occurs (GVA>CNS>PS action)

• During fasting, the CNS directs MMC activity

• At or aboral to a bolus, longitudinal smooth muscle contracts (+ACh) which "pulls back the gut wall"

• Inhibition of circular smooth muscle contraction (+VIP/NO/ATP) opens the lumen

• Oral to a bolus, longitudinal smooth muscle relaxes (-ACh) while circular smooth muscle contracts (-VIP/NO/ATP)

• Inhibitors are turned off, reducing lumen diameter and impeding back flow

Large Intestine Motility Patterns

• Pelvic flexure is located between ventral/dorsal proximal colon, the small colon is distal

• Pacemaker activity is driven by Interstitial Cells of Cajal.

  • Stationary pacemaker activity initiates at the same site with a fast rate
  • Duodenum moves content away from the pylorus
  • some species have colonic activity (mid-colon, equine pelvic flexure)
  • Shifting pacemaker activity (ICC affected by extrinsic nerves) affects the large intestine
  • It initiates at different sites based on content, with Ll sites changing to move solid content aborally

• Large intestine functions include microbial digestion (cecum and proximal colon) and net reabsorption of electrolytes and water (distal colon)

• Large intestinal motility patterns slow digesta passage allowing more time for both microbial digestion and/or electrolyte/water absorption

• Segmentation in intestines affects dogs, cats and cattle, acting the same as it does in the small intestine.

• Haustral movements affect humans, horses and pigs

• Haustral movements involve taenia, with longitudinal smooth muscle arranged in bands

  • Haustrations include circular muscle contractions at intervals that causes the digesta to bulge the bowel wall outward while taenia contract
  • Haustral movements slow digesta passage, but aboral progression still occurs
  • Inhibitor neurons are OFF at intervals

• Reverse peristalsis originates at midcolon in most species, slowing aboral digesta flow

  • Midcolonic pacemaker initiates both reverse (oral direction) and forward peristalsis (aboral direction)

• Horse cecal retropulsion occurs at the pelvic flexure (mid-proximal colon)

• Dog cecal retropulsion has no colonic reverse peristalsis and act with fast transit

• Cecal retropulsion

  • Occurs in species where the cecum is a major site of large intestinal fermentation (Ruminants, rodents, rabbits but not horses)
  • There is strong reverse peristalsis in the proximal colon that propels digesta back into the cecum for more fermentation

The digesta enters via the cecocolonic sphincter

• Large intestinal motility patterns speed digesta passage, including cecal motility in horses
  • There is a movement of fluid/solid digesta with a train of peristalsis from cecal apex to base
  • Mass movement moves from the cecal body into base, then cupola, to base
  • Circular muscle contraction isolates the cupola and forces digesta into the colon, with contraction at the base driving content through the cecocolonic sphincter
• Movement of gas:
  • Fermentation gas cap forms at base of cupula > tension in base is sensed > CNS >1)
  • Contraction of the base forces gas into the cupola
  • Distension of the cupola with gas lowers fluid level
  • Contraction of the base elevates the cecocolonic ostium

The fluid does not cover the ostium of the cecocolonic sphincter This is sensed and CNS opens cecocolonic sphincter to pass gas aborally

• Mass movements of the colon affect all domestic species
• The function is preparation for defecation
  • It attempts to fill the rectum, with modified peristaltic movement of the colon

CNS directed Peristaltic waves traverse the length of distal colon IF THE RECTUM IS NOT FILLED -> NO DEFECATION - It repeats every 2-3 mins for 10-30 mins, if it fills up the rectum, then initiates rectal peristalsis. If not, then disappears for 6 to 12 hours - Mass movements occur a few times a day, initiated by gastrocolic reflex, colonic filling (GVA > CNS > PNS > Mass Movement) or colonic irritation

• Defecation reflex involves complex, voluntary and unvoluntary components
  • The stimulus of afferent path is filling of the rectum, sensed locally and centrally
  • Efferent paths cause activation of local (ENS) and long (parasympathetic) reflexes

Local defecation is important for paraplegic animals

• Action:

• Involuntary

  • Initiation of peristalsis in rectum
  • Relaxation of the internal anal sphincter (smooth muscle)

• Voluntary

  • Initiate Valsalva maneuver (forced expiration against a closed glottis diaphragm that moves caudally to increase abdominal pressure) and contraction of abdominal muscles.
  • Relaxation of the external anal sphincter: > DEFECATION

• External anal sphincter (EAS) is striated muscle

  • The EAS surrounds the internal anal sphincter
  • Composed of skeletal muscle and innervated by the somatic motor nerve (pudendal)

• PS nicotinic action at striated muscle

  • Contraction of external anal sphincter can be overridden centrally, causing bowel incontinence (pain, fear, Gl tests, epilepsy)

Test your knowledge of gastrointestinal motility, including ileus mechanisms, peristalsis, and the ENS. Questions cover segmentation, muscle contraction, neurotransmitters, and regional differences in colonic function. Explore the function of 'shifting pacemaker activity'.