Sphincters

As the urethra leaves the bladder, the smooth muscle in the bladder wall forms spiral longitudinal and circular muscle bundles, which contract to prevent the bladder from emptying prematurely. This is the internal sphincter and is under the autonomic nervous system control.

As the urethra is traversing the pelvic floor diaphragm, a group of skeletal muscle forms the voluntary external sphincter. This sphincter is under voluntary control and thus holds the urine until micturition is convenient.

Micturition is governed by the micturition reflex. When the bladder stretch receptors are activated (urine volume of between 250ml to 400ml of urine), afferent signals are transmitted to the spinal cord. These signals initiate a parasympathetic output to the bladder, causing the smooth muscle to contract and at the same time causing the internal sphincter to relax.

Voluntary control can overide this reflex, allowing emptying at convenient times.