Function




Urine, excreted by the kidneys, collects in the bladder because of drainage from two ureters, before disposal by urination (micturition). Urine leaves the bladder via the urethra, a single muscular tube ending in an opening called the urinary meatus, where it exits the body. Urination involves coordinated muscle changes involving a reflex based in the spine, with higher inputs from the brain. During urination, the detrusor muscle contracts, and the external urinary sphincter and muscles of the perineum relax, allowing urine to pass through the urethra and out of the body.

The urge to pass urine stems from stretch receptors that activate when between 300 - 400 mL urine is held within the bladder. As urine accumulates, the rugae flatten and the wall of the bladder thins as it stretches, allowing the bladder to store larger amounts of urine without a significant rise in internal pressure. Urination is controlled by the pontine micturition center in the brainstem.

Stretch receptors in the bladder signal the parasympathetic nervous system to stimulate the muscarinic receptors in the detrusor to contract the muscle when the bladder is distended. This encourages the bladder to expel urine through the urethra. The main receptor activated is the M3 receptor, although M2 receptors are also involved and whilst outnumbering the M3 receptors they are not so responsive.

The main relaxant pathway is via the adenylyl cyclase cAMP pathway, activated via the β3 adrenergic receptors. The β2 adrenergic receptors are also present in the detrusor and even outnumber β3 receptors, but they do not have as important an effect in relaxing the detrusor smooth muscle.

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