This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Stretch reflex of the bladder is integrated at 
Select and explain. 
A. Sacral portion, B. Lumber portion, C. Substantia gelatinosa, D. Sympathetic plexus
A: Sacral portion
Answer: A: Sacral portion 
Explanation: The bladder smooth muscle has some inherent contractile activity; however, when its nerve supply is intact, stretch receptors in the bladder wall initiate a reflex contraction that has a lower threshold than the inherent contractile response of the muscle. Fibers in the pelvic nerves are the afferent limb of the voiding reflex, and the parasympathetic fibers to the bladder that constitute the efferent limb also travel in these nerves. The reflex is integrated into the sacral portion of the spinal cord.