The discharge of urine from the bladder via the urethra.
The bladder accommodates a range of volumes of urine – from 0 ml after the bladder has been emptied, to a maximum of around 300-400ml in normal adults, less in cases of children and adults of below average size.
When urine contained in the bladder causes tension in the walls of the bladder it is perceived in the CNS as a sensation of a full bladder.
Micturition is controlled at cortical, pontine, and spinal micturition centers.
The cortical micturition center decides when we void, for example, at low volumes.
Urine is released from the bladder into the urethra by the actions of muscles innervated by motor neurons acting at neuromusclar junctions.
Urination involves coordination between the central, autonomic, and somatic nervous systems.
Urination may occur as an involuntary reflex in infants, the elderly an in patients with neurological impairment.
The detrusor muscle is the smooth muscle of the bladder wall and, together with the internal urethral sphincter muscle located at the neck of the bladder, is innervated by the sympathetic nerve fibres from the lumbar sections of spinal cord, and also by the parasympathetic nerve fibres from sacral segments 2 – 4 of the spinal cord: these muscles are not under voluntary control.
The external urethral sphincter muscle is under voluntary control, and as such is innervated by the sympathetic nervous system..
Urination is initiated by the micturation reflex, an autonomic spinal cord reflex.
The bladder wall accomodates increasing volumes of urine with little change in the tension of the bladder wall until the volume of urine reaches certain a threshold, typically in the range 300-400 ml, and the micturation reflex is triggered involuntary via the peripheral nervous system.
The micturation reflex can be triggered consciously by the CNS with any level of bladder volume, and is the case most frequently.
Spinal cord reflex urination can be temporarily inhibited by the brain until urination is appropriate.
When the micturation reflex is triggered it is assisted by the CNS relaxing the external urethral sphincter muscle facilitates flow of urine from the body.
The flow of urine is aided by additional reflexes from the urethra that are stimulated by the flow of urine and which also reinforce contraction of the bladder muscle and relaxation of the sphincter muscle.
Completion of the emptying of the bladder is enhanced by contration of the abdominal wall and pelvic floor muscles.
Frequent urination can be due to excessive urine production, small bladder capacity, irritability or incomplete emptying.
Enlargement of the prostate is associated with more frequent urination.
An overactive bladder occursv when a person urinates more than eight times per day.
An overactive bladder can often cause urinary incontinence.
Both urinary frequency and volumes have been shown to have a circadian rhythm.