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Physiology of Colon [Ch144 SKF]

Physiology of the Colon and Its Measurement

Overview

  • Main Functions of the Colon:
    • Absorption of water and electrolytes
    • Storage of fecal contents until elimination is convenient
    • Salvaging nutrients via bacterial metabolism of unabsorbed carbohydrates
  • Colonic Transit Time:
    • Normally takes several hours to almost 3 days
    • Regional Differences:
      • Right Colon: Reservoir function (mixing and storage)
      • Left Colon: Conduit function (propulsion)
      • Rectum and Anal Canal: Defecation and continence

Anatomy

Gross Anatomy

  • Length: Approximately 1.5 meters in adults
  • Musculature:
    • Outer Longitudinal Layer:
      • Organized into three thick bands called taeniae coli
      • Taeniae function as suspension cables for efficient contraction
    • Inner Circular Layer:
      • Continuous layer aiding in peristalsis
  • Taeniae Coli:
    • Found in humans, primates, horses, guinea pigs, rabbits
    • At the rectosigmoid junction, taeniae broaden to form a uniform layer
  • Mesentery:
    • Cecum, Ascending, Descending Colon: Narrow mesentery, less mobility
    • Transverse, Sigmoid Colon: Broader mesentery, more mobility

Enteric Nervous System

  • Components:
    • Neurons
    • Interstitial Cells of Cajal (ICC): Pacemaker cells regulating motility
  • ICC Locations:
    • ICCMY: Myenteric plexus between muscle layers
    • ICCSM: Submucosal plexus regulating mucosal absorption
    • ICCIM: Within circular muscle layers
  • Functions of ICC:
    • Generate electrical slow waves
    • Mediate mechanosensitivity
    • Possibly mediate neurotransmission to smooth muscle

Cellular Basis for Motility

  • Electrical Events in Colonic Smooth Muscle:
    • Slow Wave Activity: 2–4 contractions/min, originates near submucosal plexus
    • Membrane Potential Oscillations (MPOs): ~18 contractions/min, originates near myenteric plexus
    • Action Potentials: Superimposed on slow waves and MPOs, leading to contraction
  • Contraction Mechanism:
    • Calcium Influx: Through L-type Ca²⁺ channels during depolarization
    • Smooth Muscle Contraction: Triggered by phosphorylation of myosin light chains
  • Role of Neurotransmitters:
    • Excitatory: Acetylcholine
    • Inhibitory: Nitric oxide, ATP

Extrinsic Nerve Supply to the Colon

  • Parasympathetic Innervation:
    • Vagus Nerve: Proximal colon
    • Pelvic Nerves (S2–S4): Distal colon via pelvic plexus
  • Sympathetic Innervation:
    • Origin: T12–L4 segments via paravertebral ganglia
    • Function:
      • Excitatory to sphincters
      • Inhibitory to nonsphincteric muscle
      • Neurotransmitter: Norepinephrine
  • Modulation of Motility:
    • Sympathetic Nervous System: Tonic inhibition via α₂-adrenergic receptors
    • Drugs:
      • Clonidine (α₂ agonist): Decreases colonic tone
      • Yohimbine (α₂ antagonist): Increases colonic tone

Functions of the Colon

Regional Heterogeneity

  • Right Colon:
    • Acts as a reservoir for mixing and storage
  • Left Colon:
    • Functions as a conduit for propulsion
  • Rectum and Anal Canal:
    • Enable defecation and continence
  • Ileocolonic Sphincter:
    • Regulates transfer of ileal contents
    • Prevents reflux of bacteria into ileum

Colonic Fluid and Electrolyte Transport

  • Absorption Capacities:
    • Fluid: Can absorb up to 5–6 liters/day
    • Sodium and Chloride: Active absorption against electrochemical gradient
  • Mechanisms:
    • Sodium Absorption:
      • Via Na⁺/H⁺ exchange, Na⁺ channels
      • Stimulated by aldosterone, somatostatin
    • Water Absorption:
      • Follows sodium passively
    • Potassium Secretion:
      • Active process; both absorbed and secreted
  • Clinical Significance:
    • Colonic Conservation of Sodium:
      • Vital during dehydration
      • Patients with ileostomies are prone to dehydration

Colonic Metabolism

  • Bacterial Fermentation:
    • Produces Short-Chain Fatty Acids (SCFAs):
      • Acetate, Propionate, Butyrate
    • Source of Energy:
      • SCFAs are absorbed and used by colonocytes and other cells
  • Functions of SCFAs:
    • Enhance Sodium and Water Absorption
    • Regulate Cell Proliferation and Gene Expression
    • Influence Immune Function and Wound Healing

Colonic Microflora

  • Composition:
    • Dominated by Firmicutes, Bacteroidetes, Actinobacteria
    • Approximately 39 trillion bacteria, similar to the number of human cells
  • Impact on Motility:
    • Bacterial Metabolites can affect GI motility
    • Altered Microbiota associated with stool consistency and transit time
  • Clinical Note:
    • Small Intestinal Bacterial Overgrowth (SIBO):
      • Misdiagnosed in IBS patients using breath tests
      • True SIBO is less common than previously thought

Colonic Motility

Assessment of Colonic Motor Function

Colonic Transit Studies

  • Radiopaque Marker Methods:
    • Ingest capsules containing markers over 3 days
    • Abdominal X-rays on days 4 and 7
    • Normal: ≤68 markers remaining
  • Scintigraphic Techniques:
    • Use delayed-release capsules with radiolabels
    • Imaging at specific intervals to track transit
    • Geometric Center Calculation:
      • Higher value indicates more distal transit
  • pH-Pressure Capsule:
    • Ingested capsule measures pH, pressure, temperature
    • pH changes indicate transitions between GI regions

Colonic Motility Studies

  • Manometry:
    • Measures intraluminal pressure changes
    • Can detect motor responses to stimuli
  • Barostat Technique:
    • Uses a compliant balloon to measure colonic tone
    • Detects contractions and relaxations by volume changes

Peristalsis

  • Mechanism:
    • Distention triggers coordinated contractions
    • Orad Contraction and Distal Relaxation facilitate propulsion
  • Types of Contractions:
    • Nonpropagated Contractions: Mixing and segmenting contents
    • Propagated Contractions:
      • Low-Amplitude Propagated Contractions (LAPCs): Less forceful, common
      • High-Amplitude Propagated Contractions (HAPCs): Strong, mass movements
        • Occur ~6 times/day, often after meals or upon waking
  • Colonic Motor Response to Meals:
    • Increase in Tone and Phasic Activity
    • Mediated by Neural and Hormonal Factors
    • Lipids are potent stimuli; amino acids may inhibit response

Defecation

  • Process:
    • Rectal Distention triggers desire to defecate
    • Internal Anal Sphincter: Reflex relaxation
    • External Anal Sphincter and Puborectalis Muscle:
      • Voluntary relaxation
      • Widening of Anorectal Angle facilitates passage
  • Coordination:
    • Requires synchronized abdominal and pelvic floor muscle activity
    • Peristaltic Waves: HAPCs may aid in fecal propulsion

Colonic Sensation

  • Visceral Sensation Pathways:
    • Peripheral Receptors: Mechanoreceptors respond to distention
    • Afferent Fibers: Aδ fibers (fast, sharp pain), C fibers (slow, dull pain)
    • Central Processing: Involves spinal cord and brain regions
  • Assessment:
    • Balloon Distention Tests: Measure thresholds for sensation
    • Factors Influencing Perception:
      • Distention Rate: Rapid distention more perceptible
      • Psychological Stress: Increases perception
      • Relaxation Techniques: Can reduce perception

Perturbations of Colonic Physiology in Disease States

Constipation

  • Types:
    • Slow Transit Constipation: Reduced motility throughout colon
    • Defecatory Disorders (DDs): Impaired evacuation due to pelvic floor dysfunction
  • Assessment:
    • Anorectal Manometry: Measures sphincter pressures
    • Balloon Expulsion Test: Evaluates ability to expel simulated stool
    • Colonic Transit Studies: Determine if transit is delayed
  • Treatment:
    • Dietary Fiber Supplementation
    • Laxatives: For slow transit constipation
    • Biofeedback Therapy: For pelvic floor retraining in DDs

Defecatory Disorders

  • Mechanism:
    • Dyssynergic Defecation: Inadequate relaxation or paradoxical contraction of pelvic floor muscles
  • Symptoms:
    • Excessive straining
    • Sensation of incomplete evacuation
    • Need for digital manipulation
  • Diagnosis:
    • Manometry and Electromyography: Detect paradoxical muscle activity
    • Defecography: Imaging to assess structural and functional abnormalities
  • Treatment:
    • Pelvic Floor Retraining: Biofeedback therapy
    • Behavioral Interventions

Acute Colonic Pseudo-obstruction (Ogilvie Syndrome)

  • Description:
    • Acute dilation of the colon without mechanical obstruction
    • Occurs in seriously ill or postoperative patients
  • Mechanism:
    • Sympathetic Overactivity: Inhibits colonic motility
  • Treatment:
    • Neostigmine: Cholinesterase inhibitor that enhances motility
    • Decompression: Via colonoscopy or tube placement if necessary

Chronic Megacolon

  • Causes:
    • Hirschsprung Disease: Congenital absence of enteric neurons
    • Idiopathic Megacolon: Severe colonic dilation without clear cause
  • Symptoms:
    • Severe constipation
    • Abdominal distention
  • Diagnosis:
    • Imaging Studies: Show colonic dilation
    • Manometry and Biopsy: May reveal neuronal defects
  • Treatment:
    • Surgical Resection: Colectomy with ileorectal anastomosis
    • Medical Management: Laxatives, enemas (often ineffective)

Functional Diarrhea and IBS

  • Irritable Bowel Syndrome (IBS):
    • Symptoms: Abdominal pain, altered bowel habits (diarrhea or constipation)
    • Possible Mechanisms:
      • Altered Motility: Accelerated transit in diarrhea-predominant IBS
      • Visceral Hypersensitivity: Enhanced perception of distention
      • Microflora Alterations: May influence symptoms
      • Psychosocial Factors: Stress and anxiety exacerbate symptoms
  • Treatment:
    • Dietary Modifications: Low FODMAP diet
    • Medications: Antispasmodics, antidiarrheals, antidepressants
    • Psychological Therapies

Other Diarrheal Illnesses

  • Carcinoid Syndrome:
    • Increased Serotonin Production: Accelerates transit
    • Symptoms: Diarrhea, flushing
  • Inflammatory Bowel Diseases:
    • Ulcerative Colitis: Reduced rectal compliance, urgency
  • Post-Ileal Resection Diarrhea:
    • Bile Acid Malabsorption: Leads to secretory diarrhea
    • Treatment: Cholestyramine binds bile acids

Diverticulosis

  • Definition:
    • Formation of mucosal pouches (diverticula) in the colon wall
  • Mechanism:
    • Structural Weakness: Points where vessels penetrate muscle layers
    • Increased Intraluminal Pressure: Due to motility disturbances
    • Reduced Compliance: Thickened muscle layers, elastin deposition
  • Clinical Significance:
    • Symptoms: Often asymptomatic but can cause pain, bleeding, or inflammation (diverticulitis)
    • Risk Factors: Low-fiber diet, aging

Implications of Colonic Physiology for Surgical Practice

  • Preoperative Assessments:
    • Identify Pelvic Floor Dysfunction: Prior to colectomy for constipation
    • Evaluate for Generalized Motility Disorders: To predict surgical outcomes
  • Surgical Considerations:
    • Colectomy with Ileorectostomy: Preferred for intractable constipation with normal anal function
    • Denervation Risks: Left-sided colectomy may affect colonic motility due to nerve damage
  • Fluid and Electrolyte Management:
    • Retention of Colon Segments: Important in short bowel syndrome to enhance absorption
  • Avoiding Unnecessary Surgery:
    • Functional Disorders: Often managed non-surgically with dietary, medical, and behavioral therapies

Note: Understanding the physiology of the colon is essential for diagnosing and managing colonic disorders, planning surgical interventions, and improving patient outcomes.