The internal urethral sphincter is controlled by the hypogastric nerve via signals arising in the pontine micturition center, also known as also known as Barrington's nucleus.
Specifically, it is controlled by the hypogastric nerve, predominantly via the alpha-1 receptor. During urination, the preganglionic neurons of this sympathetic pathway are inhibited via signals arising in the pontine micturition center and traveling through the descending reticulospinal tracts, allowing the muscle to relax. The Pontine micturition center (PMC, also known as Barrington's nucleus) is a collection of neuronal cell bodies located in the rostral pons in the brainstem involved in the supraspinal regulation of micturition. 
