Surgical
anatomy and physiology
Surgical anatomy
• Lined by transitional epithelium which covers a connective
tissue known as the lamina propria
• The fibres of the detrusor smooth muscle are intermingled and
not arranged in distinct layers
• When the detrusor hypertrophies from bladder outlet
obstruction, neurological bladder dysfunction or detrusor instability, the
fasciculi of the inner layer, covered by urothelium, stand out to give rise to
the endoscopic and radiological appearance of trabeculation
• Lining the trigone is a separate, thin layer of smooth muscle
to which the epithelium is closely adherent and which extends as a sheath around
the lower ureters and also passes into the proximal urethra
• Around the male bladder neck is the smooth muscle internal
sphincter which fulfils a sexual function, it is innervated by alphaadrenergic
fibres and prevents retrograde ejaculation
• The distal urethral sphincter is a horseshoe-shaped mass of
striated muscle which lies anterior and distal to the prostate or proximal
two-thirds of the female urethra
• The distal sphincter is a somatic, striated muscle, quite
distinct from the pelvic floor and is supplied by S2—S4 fibres via the
pudendal nerve and also by somatic fibres passing directly through the inferior
hypogastric plexus
Fascial and
ligamentous supports of the bladder
Several parts of the surrounding pelvic fascia
are of surgical importance. Posteriorly, there are condensations of the
endopelvic
fascia which are continuous with the lateral ligaments of the rectum; these pass
forward medial to the ureter to join with the fascia surrounding the prostate:
these sheets of fascia need to be divided during radical cystectomy. The
anterior puboprostatic ligaments are
well defined, are condensations of the anterior part of the endopelvic fascia
and are of great surgical importance. Each stretches from the front of the
prostate to the lower part of the periosteum of the pubis. They lie lateral to
the dorsal vein complex of the penis and in their deep parts are closely
adherent to large veins. When they are divided it is important to stay laterally
and very close to the periosteum of the pubis.
The
urachus and obliterated hypogastric arteries, together with the folds of
peritoneum overlying these structures, are called the false
(median and lateral umbilical) ligaments of the bladder. Condensations of
fascia around the blood vessels passing to the bladder are known as the superior
and inferior vascular pedicles.
Arteries
The superior and inferior vesical arteries are
derived from the anterior trunk of the internal iliac artery. Branches from
Veins
The veins form a plexus on the lateral and
inferior surfaces of the bladder; in the male the prostatic plexus is large and
continuous with the vesical plexus, which drains into the internal iliac vein.
Lymphatics
These accompany the veins, and drain into the
lymph nodes along the internal iliac vessels and thence to the obturator and
external iliac chains. Some lymphatics pass to nodes which are situated
posterior to the internal iliac artery lying directly on the sacral fascia.
Physiology (Fig.
65.1)
The nerves concerned in micturition are as
follows.
The
parasympathetic input
This innervation is the most important
component and is derived from the anterior primary divisions of the second,
third and fourth sacral segments (mainly S2 and S3). These fibres pass through
the pelvic splanchnic nerves to the inferior hypogastric plexus, from which they
are distributed to the bladder. The pelvic plexus is easily damaged during
excisions of the rectum, following which disturbances of micturition and sexual
function may occur.
The sympathetic
input
These nerves arise in the 11th thoracic to the
second lumbar segments. These fibres pass via the presacral hypogastric nerve
and the sympathetic chains to the inferior hypogastric plexus, which is situated
lateral to the rectum, and thence to the bladder.
Somatic
innervation
A somatic innervation also passes to the
distal sphincter mechanism via the pudendal nerves and also via fibres which
pass through the inferior hypogastric plexus without synapsing to the distal
sphincter.
The
sympathetic nerves convey afferent painful stimuli following overdistension of
the fundus. Other afferents arise from the mucosa where they respond to touch,
temperature and pain, and also from the muscle of the detrusor and lamina
propria where they convey stretch information. These afferents pass via the
inferior hypogastric plexus to the posterior roots of S2—S4. Efferent fibres
pass via the pelvic parasympathetics. Normal micturition is co-ordinated in
the pons in the midbrain where detrusor contraction is timed with inhibition of
the distal sphincter mechanism. Interruption of this pathway with preservation
of the function of the sacral cord is therefore likely to result in a
contractile detrusor but with a tonically active distal sphincter mechanism
which does not relax during voiding (detrusor-sphincter dyssynergia).