Anatomical components of continence
mechanism in males
Effective urethral closure and
continence at rest and during periods of increased intra-abdominal pressure
depends on several anatomical factors including: well-vascularized urethral
mucosa and submucosa, properly functioning striated and smooth urethral
sphincter, pelvic floor muscles and fasciae .
The male urethra
The male
urethra is 18-20 cm long and extends from the bladder neck to the external
meatus at the end of the penis. It may be considered in two parts (Fig. 1):
Figure (1): Anatomy
of male urethra.
The anterior
urethra
It is about 16 cm long
and surrounded by the corpus spongiosum. It is subdivided into:
The bulbar urethra which is more
proximal, surrounded by the Bulbospongiosus muscles and lie entirely
within the perineum.
The pendulous urethra which is distal and continues to the
tip of the penis .
The posterior
urethra
It is about 4 cm long and
lies in the pelvis proximal to the corpus spongiosum. The posterior urethra is
divided into:
The pre-prostatic part of the urethra
is about 1 cm long, extends from the base of the bladder to the prostate and surrounded
by the proximal (internal) urethral sphincter.
The
prostatic part is the
widest and passes through the prostate.
The membranous
(sphincteric) part is
the shortest and narrowest part. In the deep perineal pouch, it is surrounded
by distal (external) urethral sphincter. The
membranous urethra is called “sphincteric urethra” as it comprises both
striated and smooth muscle components that provide continence at this level .
The urethral mucosa and submucosa
The male
urethral mucous membrane is continuous with the transitional epithelium of the
bladder. The submucous tissue consists of a vascular layer containing
longitudinally arranged collagen fibers and elastin fibers surrounded by a layer
of circular smooth muscle .
The male
urethral sphincter complex (Fig. 2)
The male urethral sphincter complex
is composed of: proximal (smooth) sphincter, distal sphincter (mainly striated) and pelvic floor
(Peri-urethral) muscles .
Figure (2):
Male urethral sphincter complex. PUS (lissosphincter) extends from the
bladder neck through the prostatic urethra above the verumontanum. DUS (rhabdosphincter)
extends from the prostatic urethra below the veru-montanum through the
membranous urethra surrounded by Peri-urethral skeletal muscle (Pelvic
floor) .
The Proximal (internal) (lisso-) urethral sphincter (PUS)
The proximal urethral sphincter
consists of a relatively thick inner longitudinal layer, a thinner outer
circular layer of smooth muscles and a lamina propria layer that is less
developed than detrusor. PUS extends in the form of a cylinder completely
surrounding the urethra from the bladder neck to the perineal membrane with its
main part at the bladder neck and becomes thinner in its further course in the
urethra. The proximal sphincter is innervated by adrenergic autonomic fibers .
The Distal (external) (rhabdo-) urethral sphincter (DUS)
It is 1.5 to 2.4 cm in length and surrounds the membranous “sphincteric” urethra which
extends from the apex of the prostate to the corpus spongiosum of the penis .
The smooth
muscle fibers of DUS are continuous with the PUS and lie internal to the
striated muscle. The striated muscle fibers lie externally and extend from the
base of the bladder and the anterior aspect of the prostate to the full length
of the membranous urethra. The striated muscle form a horseshoe or omega
configuration around the membranous urethra being deficient posteriorly and
bulky anteriorly . The peri-urethral
striated muscles of the pelvic floor lie external to the striated urethral muscle
fibers .
The
innervation of DUS is complex with both autonomic unmyelinated and somatic
myelinated nerves (pudendal nerve and pelvic nerve). The autonomic nerves enter
at the 5 and 7 O’clock positions, while the somatic nerves enter
the striated fibers of the prostatic capsule at the 9 and 3 O’clock
positions. Damage to these nerves lead to loss of sphincteric mechanism .
Role of urethral in continence
mechanism
The urethral
smooth and striated muscles in addition to pelvic floor play the main role in
the anatomical support of continence .
The striated muscle fibers of distal
urinary sphincter is responsible for resting continence because it contains
predominantly type I slow-twitch fibers (see physiology) .
Prostatectomy,
either radical or transurethral, results in a destruction of the proximal
smooth muscle sphincteric mechanism. Continence in post-prostatectomy patients
continues to be maintained through the action of the distal urethral sphincter .
The urethral mucosa and submucosa also
play a role in continence. The flow of blood into the large submucosal venules
can be controlled assisting in forming a water-tight closure of the mucosal
surfaces. So, the urethral mucosa and submucosa function as a filler substance
to effectively close the urethral lumen after the urinary sphincters narrow the
urethral space .
Role of the prostate in continence
In terms of urinary continence the
prostate gland plays an important role. Its enlargement due to benign cause or
prostatic cancer causes voiding difficulties and surgical excision of the
prostate can be complicated by urinary incontinence .
The pelvic floor (Fig 3)
The
pelvic floor lies at the bottom of the abdomino-pelvic cavity and forms a
support for the abdominal and pelvic viscera and plays important role in
continence. The pelvic floor has three layers of support: the endopelvic fascia,
the levator ani muscles and the perineal membrane .
1-
The
endopelvic fascia
The endopelvic fascia (outer stratum
of the pelvic fascia) is a viscero-fascial layer that lies immediately beneath
the peritoneum and connects the viscera to the pelvic sidewalls. It presents an
extension of the transversalis fascia which drapes on the pelvic floor. It can
be considered the first layer of the pelvic floor .
The endopelvic fascia becomes
condensed to form the urethropelvic and puboprostatic ligaments. The Urethropelvic
ligaments are an anterior medial condensation of the endopelvic fascia,
which combines with fibers from the pubococcygeus muscle to span the area from
the anterior aspect of the tendinous arc to the bladder neck and proximal
urethra. The puboprostatic ligament attaches the inferior surface of the pubic
symphysis to the junction of the prostate and the external
sphincter.
2- The levator ani muscles (Fig 3)
The levator ani muscle is the second
layer of pelvic floor and considered the true muscular floor of the pelvis that
provides the main support for the pelvic organs. The layer formed by the muscle
and its fascial layers (superior and inferior) is referred to as the “Pelvic
diaphragm” .
The levator ani can be divided into
three parts: the pubococcygeus, iliococcygeus and ischiococcygeus muscles but
the boundaries between each part cannot be easily distinguished and they
perform many similar physiological functions .
Figure (3):
Pelvic diaphragm muscles and pelvic walls .
a- The Pubococcygeus muscle
Pubococcygeus forms a U-shaped thick
band being deficient at the ventral aspect of the urethra. It arises from the back
of the body of the pubis on either side of the midline and passes back almost
horizontally surronding the urethra and rectum.
Pubococcygeus is often subdivided
into two parts:
The pubourethralis (Puboprostaticus) is the most medial fibers of pubococcygeus
that runs directly lateral to the urethra and its sphincter and inferio-lateral
to the prostate.
The puborectalis is a thick muscular sling that wraps
around the anorectal junction and behind the rectum .
b- The iliococcygeal muscle
It is attached to the inner surface
of the ischial spine. The most posterior fibers are attached to the tip of the
sacrum and coccyx but most join with fibers from the opposite side to form a
raphe which provides a strong attachment for the pelvic floor posteriorly.
c- The ischiococcygeal muscle (also called coccygeus)
It is
the most posterosuperior portion of levator ani and arises as a triangular
musculo-tendinous sheet with its apex attached to the tip of the ischial spine
and base attached to the lateral margins of the coccyx .
3- The perineal membrane (urogenital diaphragm)
This is the third layer
of pelvic floor and it provides some weak support for urethra .
Role of the pelvic floor in
continence mechanism
The levator ani muscle plays an
important role in urinary continence and support of the striated DUS but the exact
anatomical relationship between this muscle and external urethral sphincter
remains incompletely understood .
The levator ani muscle fibers are responsible
for the voluntary active continence during stress conditions such as cough,
abdominal straining or interruption of the urinary stream by contracting
forcefully and rapidly but for a short time because it contains predominately
fast-twitch type IIa fibres .
The pelvic fasciae also play a role
in continence and pelvic viscera support. The puboprostatic ligaments, in conjunction with the
pubourethralis muscle prevent the rotational decent of the proximal urethra .
