A detailed description of complete anatomy and physiology of lower urinary tract is vast and beyond the scope of this book. Nevertheless, a brief mention of the same pertaining to functional understanding of the tract would be relevant here and is described as under.
ANATOMY OF THE LOWER URINARY TRACT
Bladder is divided into two parts—body and base. The base comprises of trigone and bladder neck. The bladder base and posterior urethra along with its muscular surroundings (prostate and sphincteric muscles) are collectively known as ‘the bladder outlet’. In males, the posterior urethra (or the flow-controlling zone) comprises of pre-prostatic, prostatic and membranous parts which then continues as bulbar urethra beyond the pelvic floor musculature. In females, the urethra is short and almost the whole of it is the flow-controlling zone. It is often divided into proximal, mid and distal urethra.
On luminal aspect, the bladder is lined by 6-cell-layered transitional epithelium resting on a basement membrane. The epithelial unit rests on fibromuscular lamina propria, which allows considerable distension. The muscular architecture of bladder is appropriate for emptying of a spherical structure. The smooth muscle is roughly arranged into inner longitudinal, middle circular and outer longitudinal layers. The muscular architecture of the base comprises of a superficial (from lumen) longitudinal layer and a deep circular layer, which is continuous with the detrusor. The bladder neck assists in the maintenance of continence. In men, an anatomically definable circular smooth muscle is present; however, in women, no such muscle can be identified. Nevertheless, urethral pressure profilometry and videourodynamic studies have confirmed the presence of ‘functional’ bladder neck with circumferential compression in both the sexes (Figures 1A and B).
The muscles in relation to urethra are comprised of the following:
- The urethral smooth muscle—in both the sexes, it is arranged in obvious layers, inner longitudinal (thicker) and outer circular (thinner).
- Rhabdosphincter—this is the ‘horse-shoe shaped’ skeletal muscle layer within the wall of urethra and is separate from the ‘pelvic floor periurethral muscles’.Figs 1A and B: Cystometry and urethral pressure profilometry depicting functional bladder neck (BN) in a male with symptomatic benign enlargement of prostate (A) and a female with pelvic floor dysfunction with non-relaxing external sphincter (B). White arrows point towards closed BN before micturition and yellow arrows to open BN during micturition. Green tracing depicts resting urethral pressure profileIn males it is present from bladder base, through the prostate to all along the length of membranous urethra. In females, it extends from bladder neck downwards all along the length of urethra. The configuration of rhabdosphincter is depicted in Figures 2A to C.
- Pelvic floor musculature—the disposition of levator ani complex muscles leaves a visceral hiatus anteriorly through which pelvic viscera, viz urethra and rectum in both the sexes and vagina in females. The part of the muscle-complex bordering these viscera is called ‘pubovisceralis’.
Muscles of the pelvic floor are composed of both slow and fast twitch fibers, whereas the rhabdosphincter fibers are only the former type.
Figs 2A to C: Configuration of rhabdosphincter in females—(A) In the proximal urethra, it surrounds the urethra circumferentially (sphincter urethra); (B) In the mid-urethra, it is horse-shoe-shaped and fibers are inserted into the vaginal wall (compressor urethra); (C) In the distal urethra, it encircles the urethra and vagina (sphincter urethravaginalis)
FUNCTION OF THE LOWER URINARY TRACT
Primary function of the lower urinary tract is the storage of urine and maintenance of continence. To accomplish the storage function, bladder ‘accommodates’ a large volume of urine without increasing the luminal pressure. This requires rearrangement of collagen and elastin fibers, and reorientation of smooth muscles with distension. For the maintenance of continence, all the sphincters remain closed. Urethral pressure profilometry studies have shown maximum closure pressure in the region of external sphincter (rhabdosphincter and levator ani) [Figures 3A and B].
In addition to muscles, the mucosal folds and the cushioning effect of ‘spongy’ submucosa contribute variably to continence. In women, changes in hormonal milieu lead to atrophy of the ‘sponge’ increasing their risk for incontinence.
In addition to the above, other factors contributing to continence in women at times of increased abdominal pressure are as follows:
- Proximal part of the urethra lies within the pelvic cavity and, therefore, during increased abdominal pressure, compression pressure from outside balances the downward pressure from the bladder (Figure 4).
- The Hammock: Pelvic urethral support anteriorly by pubo-urethral ligaments, laterally by urethro-pelvic ligaments and posteriorly by pubocervical fascia (over the firm vaginal base) helps antero-posterior compression of the urethral walls during increased abdominal pressure (DeLancey's hammock hypothesis).
- The guarding reflex: Intact nerve supply to the pelvic floor helps contraction of pelvic floor muscles in advance to direct transmission of abdominal pressure to the urethra.
Voiding function of LUT incorporates relaxation of all the sphincters followed by active detrusor contraction to empty the bladder. In women, opening of bladder neck and maintenance of optimum urethro-vesical angle is important for adequate bladder emptying. Fibro-muscular pelvic support is important in the latter function.
Figs 3A and B: Urethral pressure profilometry in the resting phase showing peak-pressure in the distal part, i.e. in the external sphincter zone (vertical broken line) in male (A) and female (B)
Fig. 4: The role of intrapelvic location of proximal part of female urethra in maintaining continence during rise in abdominal pressure. Downward vesical vector (B) is counterbalanced by transmission of Pabdominal to proximal urethra (inward arrows)
In the presence of deficiency of pelvic-floor support, for example, in the presence of advanced cystocele, bladder emptying may be severely compromised. The interested reader is advised to refer to excellent descriptions of PE Papa Petros.
NEURAL CONTROL OF THE LOWER URINARY TRACT
The LUT is innervated by sensory and motor, somatic (pudendal) as well as autonomic (pelvic and hypogastric) nervous system. The somatic supply is to the EUS and autonomic to bladder and urethral smooth muscles. The pelvic afferent myelinated and unmyelinated neurons respond to mechanical stretch of the bladder. During inflammatory conditions, recruitment of hitherto silent unmyelinated C-fibers in mucosa and muscles leads to ‘hypersensitivity’ and ‘overactivity’.
Storage Phase
During the filling or storage phase, bladder distension reflexively generates the ‘filling response’ in the form of contraction of internal (sympathetic) and external (somatic) sphincter and detrusor inactivity (parasympathetic silence). With continued filling, in the presence of socially appropriate circumstances, heightened afferent discharge from bladder stimulates detrusor contraction (parasympathetic) coupled with sympathetic and somatic inactivity. Pontine micturition center coordinates this switch-over; lesions affecting pontine control (e.g. cervico-dorsal spinal injuries) lead to ‘detrusor-sphincter dyssynergia’ in which detrusor and sphincters contract at the same time leading to voiding dysfunction and pose a threat to the upper tracts.
Reflexes in the Storage Phase
- Bladder to urethra reflex—distension of bladder stimulates central pathways such that sympathetic efferents as well as pudendal efferents are stimulated, resulting in internal and external sphincteric contraction, respectively. It is seen on electromyography as progressively increasing amplitude with filling (Figure 5).
- Urethra to bladder reflex—EUS contraction or contraction of anal sphincter, stimulation from vagina, rectum and perineum inhibit the voiding function through the centrally mediated pathways.
Voiding Phase
There is ‘switch’ of activities which initiate micturition. At the micturition threshold, continued afferent activity from the bladder reverses the activity of the efferent pathway.
- Silence of somatic motor system: relaxation of external sphincter
- Silence of sympathetic system: relaxation of bladder neck sphincter
- Stimulation of parasympathetic system: detrusor contraction and relaxation of bladder neck sphincter (through the release of NO).
Fig. 5: EMG tracing during the filling phase showing progressively increasing EMG activity with progressive filling of bladder (guarding reflex). Sudden relative silence of EMG activity represents sphincteric relaxation with the initiation of the micturitional phase
Reflexes in the Voiding Phase
Urethra to bladder reflex: Urine in the urethra stimulates detrusor contraction; this cascade promotes complete bladder emptying.
The coordination of parasympathetic activity and sympathetic/somatic silence is performed at Pontine Micturition Center (eponymed as Barrington's nucleus). The afferent bladder activity for the ‘switch’ is relayed to the PMC through midbrain's PAG region.
Lesions of PMC or disconnection below the PMC lead to detrusor-sphincter dyssynnergia.
Voluntary control on micturition depends on the connections of frontal cortex, hypothalamus and brainstem.
Lesions of these regions of cortex and hypothalamus lead to the loss of voluntary control on micturition as well as lead to detrusor overactivity.
SUGGESTED READING
- Papa Petros PE. The Female Pelvic Floor: Function, Dysfunction and Management According to the Integral Theory. Second Edition. Springer Medizin Verlag; Heidelberg: 2007.