(1)
Faculty of Medicine of Montpellier, Montpellier, France
Abstract
The bladder and anal sphincters are responsible for maintaining a safe continence of the pelvic reservoirs during the periods between evacuations (micturition and defecation), which have to be consciously and voluntarily controlled. These functions are dependent on the vegetative nervous system with two polarities: sympathetic and parasympathetic. The sensation of need is functionally important and different, depending on the two reservoirs. For bladder, kidneys produce urine permanently and the need is in relation with the stored volume and the pressure on the detrusor. For rectum, the need for defecation is linked with the beginning of the mandatory contraction of the rectum, which is a mental cortical conditioning explaining the frequent constipation, particularly in women. Two types of sphincters are acting: the smooth one guarantees the major part of the continence and the striated one provides a complementary action in case of urgent need. The two striated sphincters cannot be contracted separately in order, presumably, to avoid a too complex conscious control.
9.1 Introduction
This motor function is devoted to striated cylindrical muscular structures called sphincters, placed around the exhaust ducts of the pelvic reservoirs, marking the metabolic end of renal clearance and digestive tract.
9.2 Technical Problems and Solutions
The pelvic reservoirs occupy the bottom of the pelvic basin. The bladder collects the urinary vector produced continuously by the kidney filters. The rectum collects faeces, with a typical repulsive smell avoiding any temptation to reverse the normal progression of the oral/anal digestive tract.
The evacuation of the excreta being necessarily intermittent and controlled, the first problem is the storage in a deformable, elastic and waterproof reservoir in order to prevent any leakage out of washout periods. The tank must have an entry allowing the filling with a positive pressure against a check valve return. It must avoid reflux in the filling pipe during the phase of active evacuation.
This requires a reduction in volume of the reservoir by a contraction of its walls. Therefore, both reservoirs need a contractile pocket. The bladder is a reservoir formed by a mucosa and a smooth muscle—the detrusor [1–4]—and the rectum is a pocket formed by smooth muscle fibres oriented in two directions, longitudinal and circular (Fig. 9.1).
Fig. 9.1
Dissection of the rectum. (a) Opening of the ampulla showing the different layers: mucosa, deep circular and superficial longitudinal smooth fibre layers: 1. Permanent rectal fold with circular fibres playing the functional role of anti-reflux valve; 2. Longitudinal smooth fibres; 3. Mucosa with folds. (b) External view of a rectal permanent fold: 1. Fibrous tracts maintaining the fold; 2. Deep part of the fold. (c) General view of the rectal pouch: 1. Rectosigmoid junction; 2. Superior part of the rectum with antireflux fold; 3. Inferior part of the faeces container
9.2.1 The Mandatory Continence
The choice must be made between a central controlled striated muscle and a smooth muscle with vegetative autonomic control. This one has a better fatigue resistance and has obviously been chosen. The main problem is in fact the continence between evacuation periods.
There are, at the output channel, three different and complementary devices:
Firstly, a smooth sphincter capable of maintaining permanent contraction providing the bulk of continence.
Secondly, a striated sphincter quickly fatigable after 130 s in women and 60 s in men. But it can intervene to stop the flow of output under voluntary control or maintain continence in case of overpressure in the tank, for example, during coughing or in an imperious urination. For this purpose, inside the striated sphincter, there are neuromuscular spindles capable of inducing a clamping myotatic reflex.
The third feature is based on the optimal use of geometrical resources derived from the shape of the output channels. For the bladder, it refers to the organisation of the vesical trigone showing a superficial muscle layer partly dependent on the outer longitudinal detrusor and a deep inextensible fibro-connective part. For the rectum, the ano-rectal angle of about 95° opened backwards is maintained by the puborectal beam of the levator ani muscle, creating a “back tie” fixed in front of the pelvis (Fig. 9.2). It also sends fibres to the skin of the anal margin, creating the “cutis ani muscle corrugator”, well described by Milligan and Morgan [5].
Fig. 9.2
Puborectal muscle strap on a lateral view of pelvis. 1. Pubic symphysis; 2. Iliac bone; 3. Rectal ampulla; 4. Puborectal muscle; 5. Anorectal angle of 80°
9.2.2 The evacuation process
The sensation of need is essential to control evacuation. It comes from submucosal mechanoreceptors sensitive to distension during filling but especially to the active contraction of the reservoir. It works differently for the bladder and the rectum.
9.2.2.1 Miction
For the bladder, the first sensation appears when the filling reaches between 175 and 200 cc, which can be easily inhibited, except in a spinal cord-injured patient where it can cause leaks. It becomes urgent to urinate for filling at 400 cc, which is accompanied by the beginning of detrusor contraction. Beyond, an imperious urination may arise. The conscious control of evacuation is made by sensory feedback from the mucosa of the urethra [6], the outlet canal, which facilitates the active contraction of the detrusor, but with a possibility of trailing blocking, sometimes imperfectly, by activation of the striated sphincter which inhibits the detrusor (Fig. 9.3).
Fig. 9.3
General view of pelvis and sphincters. (a) MRI sagittal section of the pelvis of a young women: 1. Rectus abdominis muscle; 2. Full bladder; 3. Pubic symphysis; 4. Anus; 5. Vagina; 6. Rectum; 7. Uterus; 8. Sigmoid colon; 9. Sacrum; 10. Spinal muscles. (b) Sagittal section of pelvis in man (right half): 1. Pubic symphysis; 2. Bladder; 3. Bladder neck; 4. Seminal vesicles; 5. Prostate; 6. Veru montanum of prostatic urethra with ejaculator canal; 7. Urethral sphincters; 8. Cavernous body; 9. Testicle; 10. External striated sphincter; 11. Anal canal with anal columns (rich in venous plexus); 12. Internal smooth sphincter (in continuity with the smooth rectal muscles); 13. Rectum; 14. Rectosigmoid junction (anti-reflux valve); 15. Sigmoid colon
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
