Evidence-Based: Physical Therapy Pelvic Floor

THE URINARY SPHINCTERIC CLOSURE SYSTEM


(Fig. 3.3 Transverse histologic section of the midurethra of
a 21-year-old woman. On the left structures are visualized
using a sigma-actin smooth muscle stain, which shows the
pubovesical muscle (PVM), the circumferential smooth
muscle (CMU) layer, and the longitudinal smooth muscle
(LMU) layer. On the right, the contralateral side is stained
with Masson’s trichrome to show the arcus tendineus fascia
pelvis (ATFP), the striated urogenital sphincter (SUG), the
levator ani (LA), the anterior vaginal wall (AV), and the
submucosa of the urethra (SM). (From Strohbehn et al
1996, with permission of Lippincott Williams Wilkins,
Baltimore, MD.)



(Fig. 3.4 Lateral view of urethral and pelvic fl oor muscular
anatomy. BC, bulbocavernosus; CU, compressor urethrae;
D, detrusor; LA, levator ani; US, urethral sphincter; UVS,
urethrovaginal sphincter. Puborectalis muscle is removed for
clarity. (© DeLancey 2004.)


Sphincteric closure of the urethra is normally provided
by the urethral striated muscles, the urethral smooth
muscle, and the vascular elements within the submucosa
(Figs 3.2 and 3.3) (Strohbehn et al 1996, Strohbehn
& DeLancey 1997). Each is believed to contribute equally
to resting urethral closure pressure (Rud et al 1980).
Anatomically, the urethra can be divided longitudinally
into percentiles, with the internal urethral meatus
representing point 0 and the external meatus representing
the 100th percentile (Table 3.1). The urethra passes
through the wall of the bladder at the level of the vesical
neck where the detrusor muscle fibres extend below
the internal urethra meatus to as far as the 15th
percentile.

The striated urethral sphincter muscle begins at the
termination of the detrusor fibres and extends to the
64th percentile. It is circular in configuration and
completely surrounds the smooth muscle of the
urethral wall.

Starting at the 54th percentile, the striated muscles of
the urogenital diaphragm, the compressor urethrae, and
the urethrovaginal sphincter can be seen. They are continuous
with the striated urethral sphincter and extend
to the 76th percentile. Their fibre direction is no longer
circular. The fibres of the compressor urethrae pass over
the urethra to insert into the urogenital diaphragm near
the pubic ramus.

The urethrovaginal sphincter surrounds both the
urethra and the vagina (Fig. 3.4). The distal terminus of
the urethra runs adjacent to, but does not connect with,
the bulbocavernosus muscles (DeLancey 1986).

Functionally, the urethral muscles maintain continence
in various ways. The U-shaped loop of the detrusor
smooth muscle surrounds the proximal urethra,
favouring its closure by constricting the lumen.
The striated urethral sphincter is composed mainly
of type 1 (slow twitch) fibres, which are well suited to
maintaining constant tone as well as allowing voluntary
increases in tone to provide additional continence protection (Gosling et al 1981). Distally, the recruitment of the striated muscle of the urethrovaginal sphincter and
the compressor urethrae compress the lumen.
The smooth muscle of the urethra may also play a
role in determining stress continence. The lumen is surrounded
by a prominent vascular plexus that is believed
to contribute to continence by forming a watertight seal
via coaptation of the mucosal surfaces. Surrounding this
plexus is the inner longitudinal smooth muscle layer.
This in turn is surrounded by a circular layer, which
itself lies inside the outer layer of striated muscle.

The smooth muscle layers are present throughout the
upper four-fifths of the urethra. The circular configuration
of the smooth muscle and outer striated muscle
layers suggests that the contraction of these layers has
a role in constricting the lumen. The mechanical role of
the inner longitudinal smooth muscle layer is presently
unresolved. Contraction of this longitudinal layer may
help to open the lumen to initiate micturition rather
than to constrict it.

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