Parasympathetic extrinsic reflex: role in defecation mechanism

Area of Science:

• Gastroenterology
• Neuroscience
• Physiology

Background:

• The rectum utilizes both intrinsic (enteric nervous plexus - ENP) and extrinsic (parasympathetic nerves) nervous mechanisms for defecation.
• The rectoanal inhibitory reflex, mediated by the ENP, is considered the primary defecation mechanism.
• The comparative roles of intrinsic versus extrinsic innervation in defecation remain insufficiently explored.

Purpose of the Study:

• To investigate and compare the functional roles of the intrinsic enteric nervous plexus (ENP) and extrinsic parasympathetic innervation in the defecation reflex.
• To determine the relative contributions of intrinsic and extrinsic pathways to rectal expulsion.
• To elucidate the neurogenic control of the defecation act.

Main Methods:

• A balloon expulsion test was performed on anesthetized dogs.
• Rectal and anal pressures were measured during incremental rectal balloon distension.
• The test was repeated after blocking the ENP with adrenoceptor antagonists (phentolamine, propranolol) and after rectal denervation via pelvic ganglionectomy.

Main Results:

• Rectal balloon expulsion occurred at 30-40 ml distension with intact innervation.
• Expulsion volumes increased to 50-60 ml after ENP blockade.
• Expulsion volumes further increased to 80-90 ml after rectal denervation, indicating a weaker intrinsic reflex.

Conclusions:

• The intrinsic defecation reflex mediated by the ENP is functionally weaker than the extrinsic parasympathetic pathway.
• Both intrinsic and extrinsic reflexes are complementary in inducing defecation.
• Understanding these differences is crucial for managing neurogenic rectal dysfunction.