What muscle is primarily responsible for bladder contraction and is under parasympathetic control?

The detrusor muscle is primarily responsible for bladder contraction and is indeed under parasympathetic control. This smooth muscle layer forms the wall of the bladder and allows for its expansion and contraction during the storage and release of urine.

When the bladder fills, stretch receptors in the bladder wall signal the autonomic nervous system. When it is time to urinate, the parasympathetic nervous system is activated, releasing acetylcholine, which binds to muscarinic receptors on the detrusor muscle, leading to its contraction. This contraction helps to expel urine from the bladder through the urethra.

In contrast, the internal urethral sphincter is a smooth muscle that is also influenced by the autonomic nervous system but mainly functions to keep the urethra closed during bladder filling. The external urethral sphincter consists of skeletal muscle, which is under voluntary control and enables conscious control over urination. The transverse abdominal muscle plays a role in stabilizing the core and supporting abdominal pressure but does not directly contract the bladder. Hence, the role of the detrusor muscle in facilitating bladder contractions under parasympathetic control makes it the correct response.