Fiber types in the striated urethral and anal sphincters.

Seven normal human striated urethral and anal sphincters obtained by autopsy were examined using histochemical techniques. In both the urethral sphincter and the subcutaneous (s.c.) and superficial part of the anal sphincter a characteristic pattern with two populations of muscle fibers, abundant connective tissue, and numerous intramuscular nerves are seen. No spindles are observed. The muscle fibers, particularly the predominant type 1 fibers are very small (about 15 micron in diameter). The fiber characteristics of the sphincters indicate that these muscles have a capacity to produce sustained contractions and to react in stress conditions with fast increase in tension.