Final answer:
The internal urinary sphincter is a smooth muscle that relaxes to allow urine into the urethra, while the external urinary sphincter, made of striated muscle, provides voluntary control over urination. The bladder also contains the detrusor muscle that contracts during urination after the internal sphincter relaxes.
Step-by-step explanation:
The internal urinary sphincter is composed of smooth muscle located at the juncture of the bladder and urethra. It relaxes as the bladder fills, allowing urine to flow into the urethra. The external urinary sphincter, on the other hand, is made of striated muscle and is controlled by the somatic nervous system, providing voluntary control over the passage of urine out of the body. During urination, the external urethral sphincter must relax to allow urine to leave the body through the external urethral orifice. The bladder also features sensory nerves and stretch receptors that signal the need to urinate. Voluntary control over these sphincters is facilitated by the somatic nervous system, particularly the pudendal nerve.
Additionally, the bladder is made up of the detrusor muscle, which relaxes as the bladder fills and contracts during urination, aided by the relaxation of the internal urethral sphincter.