In order for urine to be eliminated, the process of micturition must be initiate

Once the urine reaches the renal pelvis, it is moved into the bladder by wavelike contractions of the smooth muscle cells of the ureter. The bladder is wrapped by smooth muscle cells (detrusor muscle) that converge on the neck of the bladder and form a thickening of the wall that is referred to as the internal urethral sphincter. This sphincter acts as a valve to regulate the flow of urine into the urethra. The urethra passes through the pelvic wall where it is surrounded by a skeletal muscle called the external urethral sphincter. The process of micturition is controlled by the micturition reflex. That reflex is activated by stretching of the bladder as it fills with urine. This spinal reflex stimulates the parasympathetic nervous system, which stimulates contraction of the detrusor muscle. This contraction increases the pressure within the bladder, which will eventually cause the internal urethral sphincter to open. At the same time, the stretch reflex inhibits sympathetic nerve activity, which relaxes the internal urethral sphincter, and somatic nerve activity, which relaxes the external urethral sphincter. The micturition reflex can be inhibited. Signals from the cerebral cortex inhibit 1) the increase in parasympathetic activity to the detrusor muscle and 2) the inhibition of somatic nerve activity to the external urethral sphincter. In addition, one can initiate micturition through voluntary inhibition of the external urethral sphincter. This occurs in concert with the lowering of the pelvic wall. As the pelvic wall is lowered, it causes the internal urethral sphincter to open and stretches the bladder. This, in concert with contraction of the diaphragm and abdominal muscles, can increase the volume of urine voided by an individual.