Physiology of the urinary bladder and urethra.

Activation of the parasympathetic pathways to the detrusor muscle and inhibition of somatic input to the external urethral sphincter are the essential neuronal events initiating release of urine. The former occurs via a spinobulbospinal pathway, whereas the latter is produced by inhibitory mechanisms in the sacral spinal cord. The sympathetic outflow to the urinary tract promotes urine storage by increasing urethral resistance and depressing detrusor contractions. Sympathetic activity is generated at least in part by a spinal vesicosympathetic reflex pathway. Evidence indicates that the integration of sympathetic and parasympathetic inputs to the bladder can occur at the level of the peripheral autonomic ganglion as well as at levels of the effector organ. The existence of facilitatory and adrenergic inhibitory mechanisms in ganglia and the identification of spontaneously active ganglion cells raise the possibility that vesical ganglia may have a role in modulating or "filtering" the efferent neural input to the bladder.