Defecation and Defecation Reflexes

1. Definition

2. Anatomy Relevant to Defecation

3. Filling of the Rectum - The Trigger

• Occur ~3 times/day (most commonly after meals)
• A large segment of colon contracts as a unit, propelling feces from the transverse colon down into the rectum
• Triggered by gastrocolic reflex and duodenocolic reflex (distension of stomach/duodenum after meals)
• The urge to defecate first appears when rectal pressure rises to about 18 mm Hg (Ganong)

4. Defecation Reflexes

A. Intrinsic (Myenteric) Defecation Reflex

• Stimulus: Feces distend the rectal wall
• Arc: Afferent signals via myenteric plexus → peristaltic waves travel down through the descending colon, sigmoid, and rectum toward the anus → internal anal sphincter relaxes (via inhibitory signals from myenteric plexus)
• Limitation: By itself this reflex is relatively weak and insufficient to cause effective defecation

B. Parasympathetic (Spinal) Defecation Reflex - The Amplifier

• Stimulus: Rectal distension stimulates nerve endings
• Arc:
  1. Afferent signals travel via pelvic nerves → sacral spinal cord (S2-S4)
  2. Efferent parasympathetic signals return via pelvic nerves → descending colon, sigmoid, rectum, anus
• Effect: Greatly intensifies peristaltic waves + relaxes the internal anal sphincter
• This reflex greatly amplifies the intrinsic myenteric reflex, making defecation effective

5. Sequence of Events During Defecation

1. Feces enter rectum via mass movements → rectal wall is distended
2. Intrinsic + parasympathetic defecation reflexes are triggered
3. Peristaltic waves sweep down the colon and rectum
4. Internal anal sphincter relaxes (via myenteric inhibitory signals)
5. If the person voluntarily relaxes the external anal sphincter → defecation occurs
6. Simultaneously:
   • Deep breath is taken
   • Glottis closes (Valsalva)
   • Abdominal muscles contract → raises intra-abdominal pressure
   • Pelvic floor relaxes and descends 1-3 cm
   • Anorectal angle straightens (puborectalis relaxes)
   • These forces push fecal contents out

6. Voluntary Control

• The external anal sphincter is under voluntary control (pudendal nerve, somatic)
• When it is socially inconvenient, a person can voluntarily constrict the external sphincter to postpone defecation
• This causes the rectal wall to accommodate to the increased pressure and the urge temporarily subsides
• If natural reflexes are repeatedly suppressed, constipation results
• When convenient, defecation can be voluntarily initiated by:
  • Taking a deep breath → diaphragm moves down
  • Contracting abdominal muscles → increases intra-abdominal pressure → forces more feces into rectum → triggers the reflex again (though less effective than naturally arising reflexes)

7. Gastrocolic Reflex

• Distension of the stomach by food initiates contractions of the rectum and the desire to defecate
• This is a vagovagal reflex (possibly amplified by gastrin acting on the colon)
• Explains why defecation after meals is common, especially in children

8. Effects of Spinal Cord Transection

9. Hirschsprung Disease (Clinical Correlation)

• Absence of ganglion cells (aganglionosis) in a segment of colon → no myenteric plexus → no intrinsic reflex in that segment → the aganglionic segment remains tonically contracted → megacolon proximal to it
• Treatment: surgical resection of aganglionic segment

10. Summary Table for Quick Revision

Key Points to Score Full Marks

1. Two types of defecation reflexes must be named and distinguished
2. Intrinsic = myenteric plexus; Parasympathetic = sacral spinal cord (S2-S4) via pelvic nerves
3. Intrinsic reflex is weak alone - always needs the spinal reflex to be effective
4. Internal sphincter = smooth, involuntary; External sphincter = skeletal, voluntary (pudendal nerve)
5. Role of Valsalva maneuver components (deep breath + glottis closure + abdominal contraction)
6. Voluntary inhibition/facilitation via the external sphincter
7. Gastrocolic reflex = defecation urge after meals
8. Rectal pressure thresholds: urge at ~18 mm Hg, reflex expulsion at ~55 mm Hg (Ganong)
9. Consequences of spinal cord injury on defecation
10. Clinical relevance: Hirschsprung disease, constipation