Physiology of the lower urinary tract

The urinary bladder is a storage vessel for most of the time, when intravesical pressure remains low and the outflow resistance high. During voiding the reverse occurs when intravesical pressure rises and the outflow relaxes. The initiation and control of this change of function is carried out by a complex interplay between sensory mechanisms in the bladder, coordination of responses in the brain and sacral spinal cord, and control over bladder and outflow muscle tone. Measurement of intravesical pressure uses urodynamics, whereby the pressure measured by a rectal catheter (abdominal pressure) is subtracted from that measured in the bladder lumen. The difference (detrusor pressure) represents pressure due to detrusor smooth muscle contraction surrounding the bladder wall. During filling, intravesical pressure rises minimally but the urothelial wall undergoes increasing stretch, releasing activators such as ATP. ATP eventually activates myelinated afferents that convey impulses via the spinal tract to a region in the rostral pons, the peri-aqueductal grey (PAG). This region has connections with the cingulate gyrus and prefrontal cortex that may convey consciousness of sensation and social control over the appropriateness of micturition. When PAG activation is sufficient, a caudal pontine micturition centre is activated that exerts a complex control over the sacral spinal cord: activation of a parasympathetic motor nucleus to excite the detrusor by muscarinic receptors, and inhibition of a somatic (Onuf’s) nucleus to reduce activation of the external sphincter (rhabdosphincter) in the urethra. In this way coordinated voiding against an open outflow tract is possible.