The internal urethral sphincter is controlled by the hypogastric nerve via signals arising in the pontine micturition center.
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. 
