Bowel Neuroanatomy and Physiology

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Bowel Neuroanatomy and Physiology

• Suzanne L. Groah, MD, MSPH

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GI Anatomy
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Digestive System Functions

• Ingestion
• Propulsion
• Mechanical digestion
• Chemical digestion
• Absorption
• Defecation

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Normal GI Function

• Mastication via.
• CN V
• Control of salivation
• Chemoreceptors, salivary nuclei, PNS, CN VII and
  IX
• Deglutition
• Buccal phase (voluntary)
• Pharyngeal phase (involuntary)
• Esophageal phase (involuntary ) via
• Vagus N

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Normal GI Function

• Stomach function(s)
• Reservoir for food
• Mix food with gastric secretions
• Reduce solids to smaller particles
• Contractions controlled by
• Vagus
• Modulated by sympathetics

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Normal GI Function

• Small intestine function
• Digest and absorb nutrients
• Pancreatic amylase digests carbos
• Pancreatic enzymes digest proteins
• Lipase digest fats
• Segmental concentric contractions by.
• Myenteric plexus

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Colon Function

• Function(s)
• Absorb electrolytes and water
• Store fecal material until evacuation
• Normal colonic transit
• 12-30 hours
• Types of movement
• Haustrations
• Circular contractions
• Do not propel contents
• Mass movements
• Propels contents

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Sphincters

• Internal anal sphincter
• Voluntary or involuntary?
• Involuntary
• Type of muscle?
• Smooth muscle
• Tonically contracted

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Sphincters

• External anal sphincter
• Voluntary or involuntary?
• Voluntary
• Type of muscle?
• Striated muscle
• Nerve and segment?
• Pudendal nerve (S2-S4)

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Bowel Neuroanatomy 101

• Neural controls
• Extrinsic (3)
• Sympathetic
• Parasympathetic
• Somatic
• Intrinsic (2)
• Myenteric plexus
• Submucosal plexus
• SNS and PNS modulate the enteric nervous system
  as opposed to directly controlling smooth muscle
  of bowel

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Neuroanatomy Physiology 101

• Autonomic neural pathways
• Parasympathetic
• Upper via Vagus nerve innervates
• Upper segments of GI tract to splenic flexure
• Lower via Pelvic splanchnic nerves (nervi
  erigentes)
• S2-S4 to the descending colon and rectum
• Function
• Stimulates GI secretion, motor activity
• Relaxes sphincters and blood vessels

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Neuroanatomy Physiology 101

• Autonomic neural pathways
• Sympathetic
• Hypogastric nerve
• L1, L2, L3 to the lower colon, rectum, and
  sphincters
• Function
• Inhibition of GI secretion, motor activity
• Contraction of GI sphincters and blood vessels
• Somatic
• Pudendal nerve
• S2-S4
• External anal sphincter and pelvic floor

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Bowel - Autonomic Nervous System

• Parasympathetics
• Increases colonic motility
• Sympathetics
• Promote storage
• Enhance anal tone
• Inhibit colonic contractions
• Bilateral sympathectomy has little clinical
  effect

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Parasympathetic Control

• Neurotransmitter
• Ach
• Near the neurons of myenteric and submucosal
  plexuses
• Nerve(s)
• Vagus
• From esophagus to mid transverse colon
• Pelvic nerve
• Supplies mid-transverse colon to rectum
• Lack of PNS innervation to .
• Small intestine
• Function(s)
• Increase peristalsis, stimulate secretions, relax
  sphincter, increase gut motility

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Bowel Autonomic Nervous System

• Parasympathetic nervous system
• PNS functions
• Increase peristalsis
• Stimulates secretions
• Relaxes sphincters
• Increases gut motility

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Sympathetic Control

• Neurotransmitter
• Norepinephrine
• Location
• Intermediolateral SC (T5-L2)
• Superior and inferior mesenteric nerves (T9-T12)
• Hypogastric (T12-L3)
• Functions
• Decrease peristalsis
• Inhibits secretions
• Contracts sphincters
• Decreases gut motility

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Neuroanatomy Physiology 101

• Intrinsic nervous system
• Submucosal (Meissner) plexus
• Myenteric (Auerbach) plexus
• Regulate segment-to-segment movement of the
  gastrointestinal (GI) tract
• May be considered a 3rd part of the ANS

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Intrinsic Nervous System

• Myenteric plexus (Auerbach)
• Located between the longitudinal and circular
  layers of muscle in the tunica muscularis
• Controls tonic and rhythmic contractions
• Exerts control primarily over digestive tract
  motility
• Submucosal plexus (Meissner)
• Buried in the submucosa
• Senses the environment within the lumen
• Regulates GI blood flow
• Controls epithelial cell function (local
  intestinal secretion and absorption)
• May be sparse or missing in some parts of GI
  tract
• Partially controlled by autonomic nervous system

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Intrinsic Nervous System

• 3 types of neurons in enteric system
• Sensory neurons (5 types)
• Chemoreceptors sensitive to acid, glucose and
  amino acids have been demonstrated which, in
  essence, allows "tasting" of lumenal contents.
  Sensory receptors in muscle respond to stretch
  and tension
• Motor neurons
• Control GI motility and secretion, and possibly
  absorption
• Interneurons
• Largely responsible for integrating information
  from sensory neurons and providing it to motor
  neurons

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Intrinsic Nervous System

• Enteric neurotransmitters
• Acetylcholine
• Excitatory
• Stimulate smooth muscle contraction
• Increase intestinal secretions,
• Release of enteric hormones
• Dilation of blood vessels
• Norepinephrine
• Derived from extrinsic sympathetic neurons
• Inhibitory
• Functions autonomously
• but normal digestive function requires
  communication between the intrinsic system and CNS

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Enteric Endocrine System

• Gastrin
• Secreted from the stomach
• Control of gastric acid secretion
• Cholecystokinin
• Secreted from the small intestine
• Stimulates secretion of pancreatic enzymes and
  bile.
• Secretin
• Secreted from the small intestine
• Stimulates secretion of a bicarbonate-rich fluids
  from the pancreas and liver

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Pancreatic Secretions

• Insulin
• Sodium bicarb
• Neutralizes stomach contents
• Trypsin and chymotrypsin
• Digest proteins
• Amylase
• Digests starches
• Lipase
• Digests fats

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Liver Function

• Liver produces bile
• Emulsify fat
• Stored in gallbladder

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Enteric Endocrine System

• Ex prevention stomach acid from burning the
  epithelium of the small intestine
• Acid-laden ingesta flows into the small intestine
  from the stomach
• Acid in the small intestine stimulates secretion
  of secretin
• Secretin stimulates the pancreas to dump a
  bicarbonate-rich fluid into the lumen of the
  intestine
• The bicarbonate neutralizes acid
• Removes the stimulus for secretion of additional
  secretion

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Peristalsis

• Distinctive pattern of smooth muscle contractions
  that propels foodstuffs distally through the
  esophagus and intestines
• Mediated by.
• Local, intrinsic nervous system
• Ex peristalsis is not affect to any significant
  degree by vagotomy or sympathectomy

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Peristalsis

• Bolus of food ?Mechanical distension and mucosal
  irritation ? stimulates afferent enteric neurons
  ? 2 effects
• Excitatory motor neurons above the bolus
  activated ? contraction of smooth muscle above
  the bolus
• Via Ach, substance P
• Inhibitory motor neurons ? stimulate relaxation
  of smooth muscle below the bolus
• Via nitric oxide, vasoactive intestinal peptide
  and ATP

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GI Reflexes

• Gastrocolic
• Increase in colonic activity after a meal
• Distention of the stomach stimulates evacuation
  of the colon
• Blunted, but still useful after SCI
• Enterogastric
• Distention and irritation of the small intestine
  results in suppression of secretion and motor
  activity in the stomach
• Colocolonic
• Propels stool caudally by proximal muscle
  constriction and distal dilatation
• Mediated by myenteric plexus
• Rectocolic
• Colonic peristalsis due to stimulation of rectum
• Mediated by pelvic nerve

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Normal Defecation

• Rectosigmoid distention stimulates rectorectal
  reflex
• Bowel proximal to bolus
• contracts
• Bowel distal to bolus
• relaxes
• Reflex relaxation of internal anal sphincter
• Rectoanal inhibitory reflex
• Correlates with the urge to go
• Volitional contraction of levator ani

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Normal Defecation

• Volitional control of levator ani
• Opens proximal anal canal
• Relaxes external sphincter and puborectalis
• Allows straighter anorectal passage
• May increase with
• Valsalva
• Increasing intraabdominal pressure (squat)

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Normal Defecation

• Defecation deferred by volitionally contracting
  (2)
• Puborectalis
• External anal sphincter
• Then, internal anal sphincter relaxation reflex
  will fade (within approx 15 sec) and urge will
  resolve until triggered again

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Normal Defecation

• Protective mechanisms
• EAS will tense in response to small colonic
  contractions
• Via spinal cord reflex (conus) and modulated by
  higher centers

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Neurogenic Bowel Dysfunction

• Loss of volitional control of defecation due to
  neurologic dysfunction
• Fecal incontinence
• Difficulty with evacuation

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Impact of Bowel Dysfunction

• Decreases return to home after stroke
• Increases nursing home costs
• Embarrassment and humiliation result in
  vocational and social handicap

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Pathophysiology UMN Bowel

• Bowel dysfunction
• Constipation, reflex defecation
• Transit time (? or ?)
• Increases
• Colonic motility
• GMC reduced
• Anocutaneous, bulbocavernosus reflex
• Present to increased

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Pathophysiology LMN Bowel

• Bowel dysfunction
• Chronic constipation, rectal fecal impaction
• Transit time
• Prolonged
• Anal sphincter pressure
• Reduced resting tone, dilated rectum
• Anocutaneous, bulbocavernosus reflex
• Absent

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Diagnostic Testing

• Colonoscopy
• Manometry
• Measures pressure and volume
• Radiography
• Structural defects
• Colonic transit time via serial radiographs

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Bowel SCI Pathophysiology

• Upper motor neuron lesion
• Increased or decreased gastric motility?
• Decreased
• Shorter or prolonged transit times?
• Prolonged
• Spastic or flaccid anal sphincter?
• Spastic
• Reflexes remain intact or lost?
• Intact

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Bowel SCI Pathophysiology

• Lower motor neuron lesion
• Flaccid or spastic anal sphincter?
• Flaccid
• Voluntary and reflex activity intact or lost?
• Lost

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Bowel - Medications

• Stool softeners
• Example Colace
• Mech Emulsify fat in GI tract and decrease
  reabsorption of water
• Mech Increase water content of stool
• Bulk formers
• Examples Metamucil, Fibercon, Citrucel
• Mech Increase absorption of water

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Bowel - Medications

• Prokinetic agents
• Senna
• Mech Stimulation of Auerbach plexus in colon
• Reglan
• Cholinergic agonist and dopamine antagonist
• Mech Increases gastric motility

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Bowel - Medications

• Contact irritants - Bisacodyl
• Dulcolax
• Vegetable oil base
• Magic Bullet
• Polyethylene glycol base

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Bowel Care Algorithm
Evaluate bowel history and perform physical exam
Assess knowledge, cognition, function, and
performance
Design bowel care program
Reflexic?
Areflexic?
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Bowel Care Algorithm
Reflexic
Areflexic
Chemical/mechanical rectal stimulant
Manual evacuation
Establish consistent, individualized schedule
Monitor elements of personalized bowel program
and evaluate after consistent adherence for 3-5
cycles diet, fluids, activity, assistive
techniques, oral meds, type of rectal
stimulation, positioning, assistive devices
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Bowel Care Algorithm
Effective bowel care?
Yes
No
Reevaluate and modify one element at a time
diet, fluids, activity, frequency, position,
type of rectal stimulant, oral medications
Continue effective bowel program, including
recognize/manage complications, evaluate
for improvements, establish educational program,
perform followup exam