Micturition (urination) is the process of urine excretion from the urinary bladder. Micturition has two discrete phases: thestorage/continence phase, when urine is stored in the bladder; and the voiding phase, where urine is released through the urethra. These phases require coordinated contraction/relaxation of the bladder and urethral sphincters, which are all under control of the sympathetic, parasympathetic and somatic nervous systems.
Storage of urine requires relaxation of the detrusor muscle and contraction of both the internal and external urethral sphincters. This is controlled by the sympathetic nervous system in the case of the detrusor and internal urethral sphincter, and the somatic nervous system for the external urethral sphincter.
As the bladder fills rugae distend and a constant pressure in the bladder (intra-vesicular pressure) is maintained. This is known as the stress-relaxation phenomenon and is responsible for signalling the need to urinate. The voiding phase of micturition is mainly controlled by the parasympathetic nervous system and also requires voluntary relaxation of the external urethral sphincter to allow passage of urine.