The innervation and properties of the urethral striated muscle.

The striated muscle forms an outer sleeve around the urethra and occupies about 80% of the wall. In humans more than 60% of the muscle consists of slow, fatigue resistant type I fibres whereas in male greyhounds only about 6% are type I. Most of the remainder (70% of fibres) are type IIa which are classified as fast but can contract for several hours with little decline in amplitude. Stimulation of pudendal but not pelvic nerves produces contraction of the striated muscle and a rapid increase in urethral pressure. Pelvic nerve branches enter the striated muscle sleeve but pass through it to supply the inner layer of smooth muscle. The striated muscles are well suited to maintain contraction necessary for continence.