Bowel Neuroanatomy and Physiology

Bowel Neuroanatomy and Bowel Neuroanatomy and PhysiologyPhysiology

GI AnatomyGI Anatomy

Bowel Neuroanatomy 101Bowel Neuroanatomy 101

Neural controlsNeural controls– Extrinsic (3)Extrinsic (3)

1)1) SympatheticSympathetic2)2) ParasympatheticParasympathetic3)3) SomaticSomatic

– Intrinsic (2)Intrinsic (2)1)1) Myenteric plexusMyenteric plexus2)2) Submucosal plexusSubmucosal plexus

– SNS and PNS modulate the enteric nervous system SNS and PNS modulate the enteric nervous system as opposed to directly controlling smooth muscle of as opposed to directly controlling smooth muscle of bowelbowel

Neuroanatomy & Physiology 101Neuroanatomy & Physiology 101

Autonomic neural pathwaysAutonomic neural pathways– ParasympatheticParasympathetic

Upper via Vagus nerve innervates…Upper via Vagus nerve innervates…– Upper segments of GI tract to splenic flexureUpper segments of GI tract to splenic flexure

Lower via Pelvic splanchnic nerves (nervi Lower via Pelvic splanchnic nerves (nervi erigentes)erigentes)

– S2-S4 to the descending colon and rectumS2-S4 to the descending colon and rectum

FunctionFunction– Stimulates GI secretion, motor activityStimulates GI secretion, motor activity– Relaxes sphincters and blood vesselsRelaxes sphincters and blood vessels


Autonomic neural pathwaysAutonomic neural pathways– SympatheticSympathetic

Hypogastric nerveHypogastric nerve– L1, L2, L3 to the lower colon, rectum, and sphinctersL1, L2, L3 to the lower colon, rectum, and sphincters

FunctionFunction– Inhibition of GI secretion, motor activityInhibition of GI secretion, motor activity– Contraction of GI sphincters and blood vesselsContraction of GI sphincters and blood vessels

– SomaticSomaticPudendal nerve Pudendal nerve

– S2-S4S2-S4– External anal sphincter and pelvic floorExternal anal sphincter and pelvic floor

Bowel - Autonomic Nervous Bowel - Autonomic Nervous SystemSystem

ParasympatheticsParasympathetics– Increases colonic motilityIncreases colonic motility

SympatheticsSympathetics– Promote storagePromote storage

Enhance anal toneEnhance anal tone

Inhibit colonic contractionsInhibit colonic contractions

Bilateral sympathectomy has little clinical effectBilateral sympathectomy has little clinical effect

Parasympathetic ControlParasympathetic Control

– NeurotransmitterNeurotransmitterAchAch

Near the neurons of myenteric and submucosal plexusesNear the neurons of myenteric and submucosal plexuses

– Nerve(s)Nerve(s)VagusVagus

– From esophagus to mid transverse colonFrom esophagus to mid transverse colon

Pelvic nervePelvic nerve– Supplies mid-transverse colon to rectumSupplies mid-transverse colon to rectum

– Lack of PNS innervation to ….Lack of PNS innervation to ….Small intestineSmall intestine

– Function(s)Function(s)Increase peristalsis, stimulate secretions, relax sphincter, increase Increase peristalsis, stimulate secretions, relax sphincter, increase gut motilitygut motility

Bowel – Autonomic Nervous Bowel – Autonomic Nervous SystemSystem

Parasympathetic nervous systemParasympathetic nervous system– PNS functionsPNS functions

Increase peristalsisIncrease peristalsis

Stimulates secretionsStimulates secretions

Relaxes sphinctersRelaxes sphincters

Increases gut motilityIncreases gut motility

Sympathetic ControlSympathetic Control

NeurotransmitterNeurotransmitter– NorepinephrineNorepinephrine

LocationLocation– Intermediolateral SC (T5-L2)Intermediolateral SC (T5-L2)– Superior and inferior mesenteric nerves (T9-T12)Superior and inferior mesenteric nerves (T9-T12)– Hypogastric (T12-L3)Hypogastric (T12-L3)

FunctionsFunctions– Decrease peristalsisDecrease peristalsis– Inhibits secretionsInhibits secretions– Contracts sphinctersContracts sphincters– Decreases gut motilityDecreases gut motility


Intrinsic nervous Intrinsic nervous systemsystem– Submucosal (Meissner) Submucosal (Meissner)

plexusplexus– Myenteric (Auerbach) Myenteric (Auerbach)

plexusplexus– Regulate segment-to-Regulate segment-to-

segment movement of segment movement of the gastrointestinal (GI) the gastrointestinal (GI) tracttract

– May be considered a 3May be considered a 3rdrd part of the ANSpart of the ANS

Intrinsic Nervous SystemIntrinsic Nervous System

Myenteric plexus (Auerbach)Myenteric plexus (Auerbach)– Located between the longitudinal and circular layers of muscle in Located between the longitudinal and circular layers of muscle in

the tunica muscularis the tunica muscularis – Controls tonic and rhythmic contractionsControls tonic and rhythmic contractions– Exerts control primarily over Exerts control primarily over digestive tract motilitydigestive tract motility

Submucosal plexus (Meissner)Submucosal plexus (Meissner)– Buried in the submucosaBuried in the submucosa– Senses the environment within the lumenSenses the environment within the lumen– Regulates GI blood flowRegulates GI blood flow– Controls epithelial cell function (local intestinal secretion and Controls epithelial cell function (local intestinal secretion and

absorption)absorption)– May be sparse or missing in some parts of GI tractMay be sparse or missing in some parts of GI tract

Partially controlled by autonomic nervous systemPartially controlled by autonomic nervous system

PeristalsisPeristalsis

Distinctive pattern of Distinctive pattern of smooth muscle smooth muscle contractions that propels contractions that propels foodstuffs distally through foodstuffs distally through the esophagus and the esophagus and intestinesintestinesMediated by….Mediated by….– Local, intrinsic nervous Local, intrinsic nervous

systemsystem– Ex: peristalsis is not affect Ex: peristalsis is not affect

to any significant degree by to any significant degree by vagotomy or vagotomy or sympathectomysympathectomy

PeristalsisPeristalsis

Bolus of food Bolus of food →→Mechanical Mechanical distension and mucosal distension and mucosal irritation irritation → → stimulates afferent stimulates afferent enteric neurons enteric neurons → 2→ 2 effects effects 1.1. Excitatory motor neurons Excitatory motor neurons

above the bolus activated above the bolus activated →→ contraction of smooth contraction of smooth muscle above the bolusmuscle above the bolus

Via Ach, substance PVia Ach, substance P

2.2. Inhibitory motor neurons Inhibitory motor neurons →→ stimulate relaxation of stimulate relaxation of smooth muscle below the smooth muscle below the bolusbolus

Via nitric oxide, vasoactive Via nitric oxide, vasoactive intestinal peptide and ATPintestinal peptide and ATP

GI ReflexesGI Reflexes

GastrocolicGastrocolic– Increase in colonic activity after a mealIncrease in colonic activity after a meal– Distention of the stomach stimulates evacuation of the colonDistention of the stomach stimulates evacuation of the colon– Blunted, but still useful after SCIBlunted, but still useful after SCI

EnterogastricEnterogastric– Distention and irritation of the small intestine results in Distention and irritation of the small intestine results in

suppression of secretion and motor activity in the stomachsuppression of secretion and motor activity in the stomach

ColocolonicColocolonic– Propels stool caudally by proximal muscle constriction and distal Propels stool caudally by proximal muscle constriction and distal

dilatationdilatation– Mediated by myenteric plexusMediated by myenteric plexus

RectocolicRectocolic– Colonic peristalsis due to stimulation of rectumColonic peristalsis due to stimulation of rectum– Mediated by pelvic nerveMediated by pelvic nerve

Normal DefecationNormal Defecation

Rectosigmoid distention stimulates rectorectal Rectosigmoid distention stimulates rectorectal reflexreflex– Bowel proximal to bolus Bowel proximal to bolus

contractscontracts

– Bowel distal to bolus Bowel distal to bolus relaxesrelaxes

Reflex relaxation of internal anal sphincterReflex relaxation of internal anal sphincter– Rectoanal inhibitory reflexRectoanal inhibitory reflex– Correlates with the “urge to go”Correlates with the “urge to go”

Volitional contraction of levator aniVolitional contraction of levator ani


Volitional control of levator aniVolitional control of levator ani– Opens proximal anal canalOpens proximal anal canal– Relaxes external sphincter and puborectalisRelaxes external sphincter and puborectalis– Allows straighter anorectal passageAllows straighter anorectal passage

May increase with May increase with – ValsalvaValsalva– Increasing intraabdominal pressure (squat)Increasing intraabdominal pressure (squat)


Defecation deferred by volitionally Defecation deferred by volitionally contracting (2)…contracting (2)…– PuborectalisPuborectalis– External anal sphincterExternal anal sphincter– Then, internal anal sphincter relaxation reflex Then, internal anal sphincter relaxation reflex

will fade (within approx 15 sec) and urge will will fade (within approx 15 sec) and urge will resolve until triggered againresolve until triggered again


Protective mechanismsProtective mechanisms– EAS will tense in response to small colonic EAS will tense in response to small colonic

contractionscontractionsVia spinal cord reflex (conus) and modulated by Via spinal cord reflex (conus) and modulated by higher centershigher centers

Neurogenic Bowel DysfunctionNeurogenic Bowel Dysfunction

Loss of volitional control of defecation due Loss of volitional control of defecation due to neurologic dysfunctionto neurologic dysfunction– Fecal incontinenceFecal incontinence– Difficulty with evacuationDifficulty with evacuation

Impact of Bowel DysfunctionImpact of Bowel Dysfunction

Decreases return to home after strokeDecreases return to home after stroke

Increases nursing home costsIncreases nursing home costs

Embarrassment and humiliation result in Embarrassment and humiliation result in vocational and social handicapvocational and social handicap

Pathophysiology – UMN BowelPathophysiology – UMN Bowel

Bowel dysfunction =Bowel dysfunction =– Constipation, reflex defecationConstipation, reflex defecation

Transit time (Transit time (↑ or ↓) = ↑ or ↓) = – IncreasesIncreases

Colonic motility =Colonic motility =– GMC reducedGMC reduced

Anocutaneous, bulbocavernosus reflex = Anocutaneous, bulbocavernosus reflex = – Present to increasedPresent to increased

Pathophysiology – LMN BowelPathophysiology – LMN Bowel

Bowel dysfunction = Bowel dysfunction = – Chronic constipation, rectal fecal impactionChronic constipation, rectal fecal impaction

Transit timeTransit time– ProlongedProlonged

Anal sphincter pressureAnal sphincter pressure– Reduced resting tone, dilated rectumReduced resting tone, dilated rectum

Anocutaneous, bulbocavernosus reflexAnocutaneous, bulbocavernosus reflex– AbsentAbsent

Diagnostic TestingDiagnostic Testing

ColonoscopyColonoscopy

ManometryManometry– Measures pressure and volumeMeasures pressure and volume

RadiographyRadiography– Structural defectsStructural defects– Colonic transit time via serial radiographsColonic transit time via serial radiographs

Bowel – SCI PathophysiologyBowel – SCI Pathophysiology

Upper motor neuron lesion Upper motor neuron lesion – Increased or decreased gastric motility?Increased or decreased gastric motility?

DecreasedDecreased

– Shorter or prolonged transit times?Shorter or prolonged transit times?ProlongedProlonged

– Spastic or flaccid anal sphincter?Spastic or flaccid anal sphincter?SpasticSpastic

– Reflexes remain intact or lost?Reflexes remain intact or lost?Intact Intact

Bowel – SCI PathophysiologyBowel – SCI Pathophysiology

Lower motor neuron lesionLower motor neuron lesion– Flaccid or spastic anal sphincter?Flaccid or spastic anal sphincter?

FlaccidFlaccid

– Voluntary and reflex activity intact or lost?Voluntary and reflex activity intact or lost?LostLost

Adapted from: NEUROGENIC BOWELAdapted from: NEUROGENIC BOWEL: GUIDE FOR EFFECTIVE MANAGE: GUIDE FOR EFFECTIVE MANAGEMENT, Nelson et alMENT, Nelson et al

Bowel Care AlgorithmBowel Care Algorithm

Evaluate bowel history and perform physical exam

Assess knowledge, cognition, function, and performance

Design bowel care program

Reflexic? Areflexic?



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]

Reflexic Areflexic

Chemical/mechanical rectal stimulant



Continue effectivebowel program, including

recognize/managecomplications, evaluate forimprovements, establish

educational program, perform followup exam

Yes No

Reevaluate and modify one element at a time [diet, fluids,

activity, frequency, position, type of rectal stimulant, oral

medications]

Effective bowel care?

Bowel Neuroanatomy and Physiology

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