Question at position 47 What is the parasympathetic nerve supply to the sigmoid colon? L2-L3Greater splanchnic nervesVagus nervePelvic splanchnic nerves

Question

BlackTom AI · Accepted Answer

1) Relevant knowledge points and concepts
Sigmoid colon is part of the hindgut (distal 1/3 of transverse colon, descending colon, sigmoid colon, rectum). Parasympathetic innervation of the gut is divided by embryologic regions: vagus (cranial parasympathetic) supplies foregut and midgut structures (up to the proximal two‑thirds of the transverse colon), while pelvic splanchnic nerves (S2–S4, sacral parasympathetic) supply the hindgut. Sympathetic fibers come from thoracolumbar levels (greater splanchnics for foregut sympathetic, lumbar splanchnics for hindgut sympathetic).

2) Step‑by‑step reasoning
- Identify the organ: sigmoid colon = hindgut derivative.
- Determine parasympathetic source for hindgut: pelvic splanchnic nerves arising from S2–S4.
- Trace the pathway: pelvic splanchnic preganglionic parasympathetic fibers join the inferior hypogastric (pelvic) plexus and reach the sigmoid colon, providing motor and secretomotor parasympathetic input.
- Therefore the parasympathetic nerve supply to the sigmoid colon is the pelvic splanchnic nerves.

3) Calculations / intermediate steps
No numerical calculations are required. Intermediate logical steps are: classify region (hindgut) → recall parasympathetic supply for hindgut (S2–S4) → identify named nerves (pelvic splanchnic).

4) Why the correct answer is correct
Pelvic splanchnic nerves are the sacral parasympathetic outflow (S2–S4) specifically responsible for parasympathetic innervation of hindgut structures including the sigmoid colon. They carry preganglionic parasympathetic fibers that synapse in intramural ganglia of the colon wall to stimulate peristalsis and secretion.

5) Why the other options are incorrect
- L2–L3: these spinal levels refer to lumbar segments associated with sympathetic (lumbar splanchnic) fibers, not parasympathetic supply; they provide sympathetic, not parasympathetic, innervation.
- Greater splanchnic nerves: carry sympathetic fibers from T5–T9 to the foregut, not parasympathetic and not to the hindgut/sigmoid colon.
- Vagus nerve: provides parasympathetic innervation to foregut and midgut (up to proximal two‑thirds of transverse colon) but does not supply the hindgut (sigmoid colon).

6) Conclusion
Because the sigmoid colon is a hindgut derivative supplied parasympathetically by sacral outflow (S2–S4), the correct answer is Pelvic splanchnic nerves, which matches the provided answer.

Question at position 47 What is the parasympathetic nerve supply to the sigmoid colon? L2-L3Greater splanchnic nervesVagus nervePelvic splanchnic nerves单项选择题

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