Which muscular structure consists of smooth muscle and is under involuntary control?

The internal urethral sphincter is a muscular structure composed of smooth muscle that is under involuntary control. This means that its function is not consciously regulated; rather, it operates automatically in response to physiological signals. This sphincter is located at the junction of the bladder and the urethra, playing a critical role in controlling the flow of urine from the bladder.

Smooth muscle, which makes up the internal urethral sphincter, is characterized by its involuntary nature and is essential for functions like regulating blood vessel diameter and gastrointestinal motility, along with maintaining urinary continence. In contrast, other structures such as the external urethral sphincter are comprised of skeletal muscle and are thus under voluntary control, allowing for conscious regulation over the release of urine. The detrusor muscle, responsible for bladder contraction, also operates involuntarily but is distinct from the sphincter structures in function and nature.

The context here highlights how different muscular structures maintain different types of control and functionality within the urinary system, emphasizing the unique role of the internal urethral sphincter in maintaining urinary retention until voluntary release is appropriate.