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 the obturator and inferior gluteal arteries (and in the female from the uterine and vaginal arteries) also help to supply the bladder.

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).