Emptying of the urinary bladder is referred to as micturition, urination or voiding. It involves the synchronous contraction of the bladder wall and relaxation of the urethral sphincters. These processes are co-ordinated by a combination of autonomic spinal cord reflexes and voluntary control of the external urethral sphincter which is made of striated muscle. Consequently, micturition, like breathing, is a mixture of reflex and voluntary actions. The urinary bladder is progressively filled by inflow of urine from the ureters. At low volumes i.e. less than 200-300ml, the bladder is a relatively compliant organ and filling is accompanied by only modest increases in tension in the bladder wall. Above 300ml, tension increases more markedly and this is detected by stretch receptors in the bladder wall smooth muscle.
It is at this level of filling that a sensation of fullness is felt in the bladder. Impulses from the stretched detrusor muscle are sent via sensory nerves to the pontine micturition centre in the brainstem where the micturition reflex is integrated.In response to these sensory inputs, parasympathetic efferent signals initiate contraction of the detrusor which, since it is a syncitium of smooth muscle cells, contracts. This results in increased pressure and tendency towards expulsion of urine and opening of the bladder neck i.e. internal sphincter. Voluntary control of the bladder arises largely as a result of the external urethral sphincter. This structure is formed where the urethra passes through the urogenital diaphragm located in the pelvic arch, which comprises skeletal muscles of the pelvic floor under voluntary, cortical control.
Contraction of the sphincter prevents the flow of the urine and voluntary relaxation has the reverse effect and initiates the flow of urine during micturition.Further, voluntary control of micturition arises from inhibitory inputs from the cortical and suprapontine centre to the pontine micturition centre which inhibit the micturition reflex until it is socially acceptable to urinate or the sensation of the bladder fullness becomes too intense. Furthermore, sympathetic nerves from the hypogastric plexus inhibit contraction of detrusor and aid the inhibition of micturition by higher cortical centres. During bladder emptying, sensory receptors in the urethra sense the flow of urine and feedback to the micturition center.
This has the effect of enhancing the micturition reflex and increasing the flow of urine. Contraction of the abdominal and the pelvic muscles can also increase the pressure on the bladder wall, thus aiding micturition. The bladder can be voluntary emptied at any time, even when not full, indicating that the higher areas of the brain can initiate the micturition reflex even in the absence of afferent input from stretch receptors in the wall of the bladder. Voluntary control of micturition can be lost due to the damage to inhibitory descending pathways in the spinal cord. Such a condition can result in the urinary incontinence. Furthermore, lesion of the para-sympathetic nerve supply to the bladder results in the incomplete emptying of the bladder leading to micturition, which can further lead to the recurrent urinary tract infections.