The innervation of the external urethral sphincter; an ultrastructural study in male human subjects.

Innervation of the external urethral sphincter (EUS) was studied in male human subjects. In the region of EUS at the distal end of prostatic urethra, a large axon bundle surrounded by perineurium was evident in the intramural connective tissue gap. Because of the presence of dense core vesicles, the small nonmyelinated axon profiles in the bundle were considered to be adrenergic. After ramifications to smooth musculature, the axons were traced to the EUS. In the EUS, axon bundles containing many non-myelinated axons were recognized as a sole autonomic nerve among the striated muscle cells. A single or at most two or three axons were surrounded by a Schwann cell, and some possessed dense core vesicles which suggested an adrenergic function. These autonomic adrenergic nerve ends formed surface junctions with the striated muscle of EUS. The clinical relevance of these data are discussed.