Innervation of the bowel

1. What is the role of the parasympathetic system in the bowel? Where does it originate?
2. What is the role of the sympathetic system in the bowel?
3. What is the role of the somatic nervous system in the bowel?
4. What takes place to allow fecal storage in a neurologically intact person?
5. What occurs during defecation in a neurologically intact person?

Answers:
1. The parasympathetic system increases upper GI motility and enhances colonic motility, via the vagus nerve and the splanchnic nerves (S2-4).
2. Sympathetic system inhibits colonic contractions and relaxes the internal anal sphincter for storage, via the hypogastric nerve.
3. Increases external anal sphincter tone for continence, via the pudendal nerve (S2-4)
4. Internal anal sphincter is sympathetically activated and relaxes, the external anal sphincter tone increases secondary to s.c. reflexes and higher cortical control.
5. Rectosigmoid distention --> reflex internal anal sphincter relaxation --> signal sent to pontine defecation center --> voluntary contraction of levator ani muscle and relaxation of external anal canal, reflexive rectal propulsions.