Final answer:
The detrusor muscle in the wall of the bladder, made of smooth muscle fibers, is responsible for emptying the urinary bladder.
Step-by-step explanation:
The detrusor muscle in the wall of the bladder is responsible for emptying the urinary bladder. It is made of smooth muscle fibers that are controlled by both the autonomic and somatic nervous systems. As the bladder fills, the detrusor muscle automatically relaxes to allow it to hold more urine. When the bladder is about half full, the stretching of the walls triggers the sensation of needing to urinate. When the individual is ready to void, conscious nervous signals cause the detrusor muscle to contract and the internal urethral sphincter to relax and open. As a result, urine is forcefully expelled out of the bladder and into the urethra.