What is the role of the internal urethral sphincter?

The internal urethral sphincter plays a fundamental role in the involuntary control of bladder emptying. It is composed of smooth muscle and is under autonomic control, primarily influenced by the sympathetic nervous system. When the bladder fills with urine, the internal urethral sphincter remains contracted to prevent leakage and maintain continence. During the process of micturition, signals from the nervous system promote relaxation of this sphincter, allowing urine to flow from the bladder through the urethra. This involuntary mechanism is crucial for the proper functioning of urinary storage and emptying.

The other options reflect different functions or types of control not associated with the internal urethral sphincter. For instance, voluntary muscle control of urine flow pertains to the external urethral sphincter, which is under conscious control. Sympathetic control of the detrusor muscle refers to a different aspect of bladder function, where sympathetic stimulation inhibits bladder contraction. Lastly, coordination of micturition phases involves both voluntary and involuntary processes but does not specifically describe the role of the internal urethral sphincter itself.