The bladder stores urine until the accumulated urine stretches the bladder to a significant degree to trigger the reflexes that will result in urination. With the relaxation of the external urethral sphincter, increase in abdominal pressure and contraction of the detrusor muscle, urine will be passed out when the situation is acceptable. This process seems quite simple but is made of up various phases that contribute to both voluntary and involuntary bladder control.
Filling of the Bladder and the Micturition Reflex
The ureters carry urine from the kidney to the bladder. It penetrates the detrusor muscle of the bladder wall, which can also compress the ureters during urination to prevents urine from the bladder emptying into the ureters (vesicoureteral reflux).
As urine collects in the bladder, the pressure rises slightly but once more than 300 milliliters accumulates in the bladder, the pressure rises rapidly. Stretch receptors in the bladder wall, internal urethral orifice and posterior urethra sends signal back to the spinal cord by the way of the sensory fibers of the pelvic nerve. Once the stretching reaches a certain point, it triggers the micturition reflex.
Motor impulses travel through the parsympathetic nerve fibers in the pelvic nerves (mainly from S2 and S3 of the sacral spinal cord) and synapse with short postganglionic nerves in the bladder wall. These nerves in turn innervate the detrusor muscle. The detrusor muscle contracts and relaxes almost immediately causing sudden peaks and dips known as the micturition wave.
As the bladder fills up further, the frequency and intensity of the micturition reflexes increase substantially. Eventually this leads to period of sustained contraction of the detrusor muscle which leads to the typical urgency to urinate. If urine is not passed out at this time, the process subsides for a few minutes to an hour and then starts again.
Bladder Control by the Brain
The micturition reflex can also trigger a second reflex that inhibits the tonic contraction of the voluntary external urethral sphincter through impulses via the pudendal nerve. This occurs once the bladder is distended to a significant degree. However impulses from the brain can inhibit this secondary action of the micturition reflex. This allows a person to find a suitable setting to urinate.
In addition, the brain can also directly act on the external urtheral sphincter to maintain its contracted state and prevent urination despite the effects of the micturition reflex. When the situation is suitable, the higher centers can then relax the external urethral sphincter and stimulate the micturition reflex to commence urination.
Even though a person may voluntary prevent urination despite the urging, eventually the bladder has to be emptied or voluntary control measures will fail. In a suitable setting, a person increases the pressure on the bladder by contracting the abdominal muscles. This forces urine into the bladder neck thereby stretching its wall further. The micturition reflex occurs again and without interruption by the higher centers, the muscles of the external urethral sphincter relax and urine is expelled into the environment.