ABSTRACT
Muscular compression is provided by the circular smooth muscle throughout the urethra, as well as the striated external sphincter and the compressor urethra and urethrovaginal sphincter ( Fig. 61.2 ). The internal sphincter is not a true sphincter from an anatomic viewpoint; i.e., there is no clearly defined circular muscle layer. Further, the inner longitudinal layer of the bladder neck and urethra is actually a continuation of the ureteral muscles that fan out and become the trigone, some of which continue into and terminate in the distal third of the urethra. It is unclear what exact role these muscles play as there are women with an open bladder neck on videourodynamics (VUDS) or ultrasound who are continent. On the other hand, and open vesical neck at rest is considered to be one of the hallmarks of intrinsic sphincter deficiency (ISD) which is discussed in more detail below (4) . The external sphincter is composed of striated circular muscle fibers which are thickest in the middle part of the urethra (5) . It is thought that the striated muscles provide two kinds of sphincteric function. First, slow twitch muscle fibers provide a continuous, modulated tone that keeps the urethra closed. Second, fast twitch fibers respond to voluntary and reflex stimuli that suddenly increase contraction in time of increased abdominal pressure or in voluntary attempts to prevent
incontinence from involuntary detrusor contractions ( 6-8 ). There are, in addition, two extrinsic striated muscles that are thought to provide sphincteric function-the compressor urethrae and the urethrovaginal sphincter ( Fig. 61.2 ) ( 9 , 10 ).
