Urinary Incontinence Relevant Anatomy 

The urethra is approximately 4 cm long in the female. It is imbedded in the connective tissue supporting the anterior vagina.

The urethra is composed of an inner epithelial lining, a spongy submucosa, a middle smooth muscle layer, and outer fibroelastic connective tissue. The spongy submucosa contains rich vascular plexus that is responsible for providing adequate urethral occlusive pressure to create the washer effect, an important female continence mechanism. Urethral smooth muscle and fibroelastic connective tissues circumferentially augment the occlusive pressure generated by the submucosa. (See the image below.)

The urethral epithelium is composed of stratified squamous cells, which variably becomes transitional as the bladder is approached. The epithelium is arranged in longitudinal folds. At the base of the folds are scattered gland openings along the entire urethral length. The epithelium is supported by a loose lamina propria consisting of collagen fibrils and elastic fibers, arranged both circularly and longitudinally. A rich network of blood vessels is in the subepithelial layer.

The smooth muscle of the urethra is arranged longitudinally and obliquely with only a few circular fibers. The nerve supply is cholinergic and alpha-adrenergic. The longitudinal muscles may contribute to shortening and opening of the urethra during voiding. The oblique and circular fibers contribute to urethral closure at rest.

The striated urethral musculature is complex, and the components and their orientation are not agreed upon universally. The voluntary urethral sphincter really is a group of circular and loop-like interrelated muscle fibers, similar to that present in the anorectum. The innermost layer, which is prominent in the proximal two thirds of the urethra, is the sphincter urethrae. More distally, the compressor urethrae and urethrovaginal sphincter are predominant.

These 2 muscles emanate from the anterolateral aspect of the distal half to one third of the urethra and arch over the anterior or ventral surface. These striated muscles function as a unit. Because they are composed primarily of slow-twitch muscle fibers, these muscles serve ideally to maintain urethral tone. The muscles probably do maintain the urethral tone but contribute to voluntary closure and reflex closure of the urethra acutely during times of increased intra-abdominal pressure. The medial-most pubovisceral portion of the levator ani complex also is a major contributor to active bladder neck and urethral closure.

Histologic examination of the striated urethral sphincter indicates that, for the most part, the muscle complex surrounds the urethra in an incomplete fashion. Fibers can be observed to be deficient along the posterior aspect of the urethra. The shape of the muscle complex can be described as resembling a horseshoe or an omega symbol.

Investigations using ultrasonographic imaging of the urethra also have confirmed a paucity of muscle bulk along the posterior urethra.^[1] The urethral meatus empties into the vestibule after the distal-most urethra pierces the perineal membrane. The mucosa of the meatus is continuous with that of the vulva. Support of the urethra and bladder neck is believed to be important in the maintenance of continence during sudden increases in intra-abdominal pressure. The support mechanism is complex and incompletely understood.

The posterior wall of the urethra is embedded in and supported by the endopelvic connective tissue. This sheet of connective tissue consists of collagen, elastin, and a small amount of smooth muscle. The connective tissue envelops the anterior vagina. This supportive tissue has been likened to a sling or a hammock around the urethra and bladder neck.

The endopelvic connective tissue in this area is attached to the perineal membrane ventrally and laterally to the levator ani muscles by way of the arcus tendinous fascia pelvis (ATFP). The ATFP is a condensation of connective tissue, which extends bilaterally from the inferior part of the pubic bone along the junction of the fascia of the obturator internus and levator ani muscle group to an area near the ischial spine. This tissue provides secondary support to the urethra, bladder neck, and bladder base.

Defects in this tissue are believed to result in cystocele and urethral hypermobility. The primary support to this area and the entire pelvic floor is believed to be the levator ani muscles. At rest, the constant tone mediated by slow-twitch fibers constitutes the major supportive mechanism.

Fast-twitch fibers in these muscles cause the sudden stopping of the urinary stream to provide the voluntary guarding reflex. With acute increases in intra-abdominal pressure, forceful contraction of the fast-twitch levator fibers elevates the pelvic floor and tightens intact connective tissue planes, thereby supporting the pelvic viscera.

The anterior distal wall of the urethra is attached to the pubic bone by the pubourethral ligaments (see the image below). These ligaments consist of extensions of the perineal membrane and the caudal and ventral-most portion of the ATFP. The ligaments may limit movement of the anterior wall of the urethra during increases in intra-abdominal pressure but probably exert a lesser degree of support to the posterior wall.

The previously described endopelvic connective tissue, when intact, provides support to the urethra as a whole. With increases in intra-abdominal pressure, some believe that the urethra is compressed shut against this firm support.

Deficiency in the hammocklike support of the endopelvic connective tissue, coupled with relative preservation of the preferentially anterior urethral support of the pubourethral ligaments, may partially explain the complex rotational and descending motion of the bladder neck commonly observed in association with stress incontinence. The pubourethral ligaments may serve to limit downward motion of the anterior urethral wall and provide a pivot point for rotatory motion around the pubic bone. Some theorize that this preferential anterior wall support also may serve to pull the anterior and posterior urethral walls apart during straining, thereby contributing to bladder neck incompetency and stress incontinence.

Unlike male anatomy, in which the bladder neck and the prostate comprise the internal urinary sphincter, the internal sphincter in females is functional rather than anatomic. The bladder neck and proximal urethra constitute the female internal sphincter. The female external sphincter (ie, the rhabdosphincter) has the most prominent effect on the female urethra at the urogenital triangle. Located approximately 1.8 cm distal to the bladder neck, it exerts influence for a distance of approximately 1.5 cm of urethral length (see the images below).

Male urethral anatomy

In men, the urethra averages 20-22 cm in length, and its anatomy is somewhat more complex than the female urethra. The male urethra is divided into anterior and posterior portions. The anterior urethra contains the penile (or pendulous) urethra and the bulbous urethra. The urethra is surrounded by the corpus spongiosum within the penis. The posterior urethra comprises the membranous urethra and the prostatic urethra.

In the prostatic urethra, which is just distal to the bladder neck, the epithelium is transitional. The orifices of the prostatic glands can be found here. On the floor of the prostatic urethra is an elevation called the verumontanum. This area contains a small pocket called the utricle. Distal to the utricle are the orifices of the 2 ejaculatory ducts.

Continuing distally, the membranous urethra is encountered. This short segment traverses the urogenital diaphragm, which contains the external urethral sphincter. Smooth muscle fibers extend into the bladder neck and preprostatic urethra to form the internal sphincter, which preserves continence and prevents retrograde flow of semen into the bladder during ejaculation. Cowper glands are adjacent to the urethra. The epithelial cells become more elongated and appear as stratified columnar cells.

The bulbous urethra derives its name from the bulb of the corpus spongiosum, which it traverses. The ducts of the Cowper glands empty in this location. The epithelium is composed of pseudostratified columnar cells.

The penile urethra passes through the remainder of the corpus cavernosum urethrae and makes up more than half of the total anatomic urethral length. The epithelium comprises pseudostratified columnar cells, except in the fossa navicularis, the slightly widened distal part of the urethra that passes through the glans penis. In this distal-most segment, stratified squamous epithelium is present.

Throughout the penile urethra, the periurethral tissue contains many small mucus-secreting glands called the glands of Littre. These glands are much more numerous along the roof of the urethra, and they empty into small recesses called the lacunae of Morgagni. The bulbous and penile urethra together sometimes are referred to as the cavernous urethra.

The urethra is approximately 4 cm long in the female. It is imbedded in the connective tissue supporting the anterior vagina.

The urethra is composed of an inner epithelial lining, a spongy submucosa, a middle smooth muscle layer, and outer fibroelastic connective tissue. The spongy submucosa contains rich vascular plexus that is responsible for providing adequate urethral occlusive pressure to create the washer effect, an important female continence mechanism. Urethral smooth muscle and fibroelastic connective tissues circumferentially augment the occlusive pressure generated by the submucosa. (See the image below.)

The urethral epithelium is composed of stratified squamous cells, which variably becomes transitional as the bladder is approached. The epithelium is arranged in longitudinal folds. At the base of the folds are scattered gland openings along the entire urethral length. The epithelium is supported by a loose lamina propria consisting of collagen fibrils and elastic fibers, arranged both circularly and longitudinally. A rich network of blood vessels is in the subepithelial layer.

The smooth muscle of the urethra is arranged longitudinally and obliquely with only a few circular fibers. The nerve supply is cholinergic and alpha-adrenergic. The longitudinal muscles may contribute to shortening and opening of the urethra during voiding. The oblique and circular fibers contribute to urethral closure at rest.

The striated urethral musculature is complex, and the components and their orientation are not agreed upon universally. The voluntary urethral sphincter really is a group of circular and loop-like interrelated muscle fibers, similar to that present in the anorectum. The innermost layer, which is prominent in the proximal two thirds of the urethra, is the sphincter urethrae. More distally, the compressor urethrae and urethrovaginal sphincter are predominant.

These 2 muscles emanate from the anterolateral aspect of the distal half to one third of the urethra and arch over the anterior or ventral surface. These striated muscles function as a unit. Because they are composed primarily of slow-twitch muscle fibers, these muscles serve ideally to maintain urethral tone. The muscles probably do maintain the urethral tone but contribute to voluntary closure and reflex closure of the urethra acutely during times of increased intra-abdominal pressure. The medial-most pubovisceral portion of the levator ani complex also is a major contributor to active bladder neck and urethral closure.

Histologic examination of the striated urethral sphincter indicates that, for the most part, the muscle complex surrounds the urethra in an incomplete fashion. Fibers can be observed to be deficient along the posterior aspect of the urethra. The shape of the muscle complex can be described as resembling a horseshoe or an omega symbol.

Investigations using ultrasonographic imaging of the urethra also have confirmed a paucity of muscle bulk along the posterior urethra.^[1] The urethral meatus empties into the vestibule after the distal-most urethra pierces the perineal membrane. The mucosa of the meatus is continuous with that of the vulva. Support of the urethra and bladder neck is believed to be important in the maintenance of continence during sudden increases in intra-abdominal pressure. The support mechanism is complex and incompletely understood.

The posterior wall of the urethra is embedded in and supported by the endopelvic connective tissue. This sheet of connective tissue consists of collagen, elastin, and a small amount of smooth muscle. The connective tissue envelops the anterior vagina. This supportive tissue has been likened to a sling or a hammock around the urethra and bladder neck.

The endopelvic connective tissue in this area is attached to the perineal membrane ventrally and laterally to the levator ani muscles by way of the arcus tendinous fascia pelvis (ATFP). The ATFP is a condensation of connective tissue, which extends bilaterally from the inferior part of the pubic bone along the junction of the fascia of the obturator internus and levator ani muscle group to an area near the ischial spine. This tissue provides secondary support to the urethra, bladder neck, and bladder base.

Defects in this tissue are believed to result in cystocele and urethral hypermobility. The primary support to this area and the entire pelvic floor is believed to be the levator ani muscles. At rest, the constant tone mediated by slow-twitch fibers constitutes the major supportive mechanism.

Fast-twitch fibers in these muscles cause the sudden stopping of the urinary stream to provide the voluntary guarding reflex. With acute increases in intra-abdominal pressure, forceful contraction of the fast-twitch levator fibers elevates the pelvic floor and tightens intact connective tissue planes, thereby supporting the pelvic viscera.

The anterior distal wall of the urethra is attached to the pubic bone by the pubourethral ligaments (see the image below). These ligaments consist of extensions of the perineal membrane and the caudal and ventral-most portion of the ATFP. The ligaments may limit movement of the anterior wall of the urethra during increases in intra-abdominal pressure but probably exert a lesser degree of support to the posterior wall.

The previously described endopelvic connective tissue, when intact, provides support to the urethra as a whole. With increases in intra-abdominal pressure, some believe that the urethra is compressed shut against this firm support.

Deficiency in the hammocklike support of the endopelvic connective tissue, coupled with relative preservation of the preferentially anterior urethral support of the pubourethral ligaments, may partially explain the complex rotational and descending motion of the bladder neck commonly observed in association with stress incontinence. The pubourethral ligaments may serve to limit downward motion of the anterior urethral wall and provide a pivot point for rotatory motion around the pubic bone. Some theorize that this preferential anterior wall support also may serve to pull the anterior and posterior urethral walls apart during straining, thereby contributing to bladder neck incompetency and stress incontinence.

Unlike male anatomy, in which the bladder neck and the prostate comprise the internal urinary sphincter, the internal sphincter in females is functional rather than anatomic. The bladder neck and proximal urethra constitute the female internal sphincter. The female external sphincter (ie, the rhabdosphincter) has the most prominent effect on the female urethra at the urogenital triangle. Located approximately 1.8 cm distal to the bladder neck, it exerts influence for a distance of approximately 1.5 cm of urethral length (see the images below).

Male urethral anatomy

In men, the urethra averages 20-22 cm in length, and its anatomy is somewhat more complex than the female urethra. The male urethra is divided into anterior and posterior portions. The anterior urethra contains the penile (or pendulous) urethra and the bulbous urethra. The urethra is surrounded by the corpus spongiosum within the penis. The posterior urethra comprises the membranous urethra and the prostatic urethra.

In the prostatic urethra, which is just distal to the bladder neck, the epithelium is transitional. The orifices of the prostatic glands can be found here. On the floor of the prostatic urethra is an elevation called the verumontanum. This area contains a small pocket called the utricle. Distal to the utricle are the orifices of the 2 ejaculatory ducts.

Continuing distally, the membranous urethra is encountered. This short segment traverses the urogenital diaphragm, which contains the external urethral sphincter. Smooth muscle fibers extend into the bladder neck and preprostatic urethra to form the internal sphincter, which preserves continence and prevents retrograde flow of semen into the bladder during ejaculation. Cowper glands are adjacent to the urethra. The epithelial cells become more elongated and appear as stratified columnar cells.

The bulbous urethra derives its name from the bulb of the corpus spongiosum, which it traverses. The ducts of the Cowper glands empty in this location. The epithelium is composed of pseudostratified columnar cells.

The penile urethra passes through the remainder of the corpus cavernosum urethrae and makes up more than half of the total anatomic urethral length. The epithelium comprises pseudostratified columnar cells, except in the fossa navicularis, the slightly widened distal part of the urethra that passes through the glans penis. In this distal-most segment, stratified squamous epithelium is present.

Throughout the penile urethra, the periurethral tissue contains many small mucus-secreting glands called the glands of Littre. These glands are much more numerous along the roof of the urethra, and they empty into small recesses called the lacunae of Morgagni. The bulbous and penile urethra together sometimes are referred to as the cavernous urethra.

The bladder wall is made up of muscle fibers extending in all directions. This configuration is well suited to decreasing the bladder size in all dimensions when contracting.

At the bladder neck, the muscular bladder wall is more organized, and 3 relatively distinct layers become apparent. The inner longitudinal layer fuses with the inner longitudinal layer of the urethra. The middle circular layer is most prominent in the proximity of the bladder neck, and it fuses with the deep trigonal muscle. The outer longitudinal layer contributes some anterior fibers to what becomes the pubovesical muscles, terminating on the posterior surface of the pubic bone.

These muscles may be important in bladder neck opening during micturition. Posteriorly, the outer longitudinal fibers interdigitate with deep trigonal fibers and the detrusor muscle. These fibers may aid in bladder neck closure.

The bladder mucosa is transitional epithelium, which is loosely connected to the muscular wall by way of a connective tissue layer called the lamina propria. At the trigone, the epithelium is more densely adherent to the underlying muscle.

The trigone is a triangular structure formed by the internal urethral opening and the orifices of the right and left ureter. The superior border of the trigone is a raised area called the interureteric ridge. Deep to the mucosa are 2 muscular layers. The superficial layer connects to longitudinal urethral musculature. The deep muscle fuses with detrusor and Waldeyer sheath, the fibromuscular covering of the intramural ureter. The intramural ureter enters the bladder wall obliquely. The muscle fibers are longitudinal in orientation at this point. This segment of the ureter is about 1.5 cm in length.

Ventrally, the retropubic space is bounded by the pubic bones and the midline fibrocartilage. The floor and part of the dorsal aspect consist of the bladder, urethra, and endopelvic connective tissue, which extend laterally on both sides to the pelvic side walls at the arcus tendineus fasciae pelvis (ATFP).

The remainder of the dorsal wall consists of the pelvic parietal perineum and the transversalis fascia. Above the arcuate line, the posterior rectus sheath is present. As originally described by Retzius, the potential space extends in a cephalad direction to the level of the umbilicus.^[2]

The pectineal ligament, or Cooper ligament, lies on the superior-dorsal surface of the pubic ramus. A flat triangular extension of Cooper ligament, the lacunar ligament, widens as it travels medially and joins the inguinal ligament at the pubic tubercle. The anterior or ventral aspect of the bladder makes up the floor of the retropubic space. This part of the bladder wall is extraperitoneal in location.

The cephalad wall and part of the posterior wall are covered with peritoneum and can be accessed from within the peritoneal cavity. The inferior aspect of the bladder lies on the anterior vagina, cervix, and lower uterine segment. The tissue between the bladder and the muscular wall of the vagina is the endopelvic connective tissue.

Lateral to the bladder and bladder neck and within the endopelvic connective tissue lies a venous plexus. These prominent veins are a frequent source of bleeding during retropubic urethropexy. The pubovesical ligaments, pubourethral ligaments, and extrinsic muscles of the urethra also lie in the retropubic space.

Intact neuroanatomy and neurophysiology are essential to both the storage and micturition phases of lower urinary tract function. These phases are controlled largely by the peripheral autonomic nervous system, with important modulating information contributed by sensory nerves from the bladder and urethra. Further modification is provided by higher CNS centers, which allow conscious control of lower urinary tract function. Lesions anywhere along these neuroanatomic pathways can contribute to or cause incontinence or voiding dysfunction.

Voluntary control of detrusor activity is thought to arise in the frontal cerebral cortex. This area is in communication with the pontine mesencephalic reticular formation, which serves as the brainstem micturition center. Maturation of these and higher centers are important in the childhood acquisition of the ability to voluntarily suppress micturition. Diseases that involve this area of the brain may cause or contribute to incontinence disorders. Stroke, multiple sclerosis, Parkinson disease, and brain tumors are examples.

Efferent connections beginning in the pons and terminating in the sacral micturition center at the S2 to S4 levels are important to efficient detrusor functioning during micturition. Damage to these tracts (eg, spinal cord injury) results in detrusor areflexia. Neural activity within this system promotes micturition.

A neural loop involving the bladder, sacral micturition center, pontine micturition center, and urethral sphincter mechanism has been described. This pathway allows the coordination of urethral and detrusor function. In other words, coordination of urethral relaxation with detrusor contraction is dependent on this neural pathway being intact.

Dysfunction in this loop may result in detrusor-sphincter dyssynergia. Direct connections between the cerebral cortex and the sacral-pudendal motor neurons are important contributors to voluntary control over the striated urethral-sphincter complex. Severe neuromuscular damage to the striated urethral muscles, along with brain and spinal cord injury, can prevent proper functioning of this system.

Healthy functioning of the lower urinary tract is partly dependent on the interplay of sympathetic (ie, adrenergic) and parasympathetic (ie, cholinergic) input to the bladder and urethra. Bladder filling normally takes place with little or no increase in intravesical pressure. This phenomenon is largely due to the predominance of sympathetic tone during the filling and storage phase. Simplistically, beta-adrenergic receptors predominate in the detrusor muscle.

Stimulation of these receptors promotes bladder relaxation. Alpha fibers also exist, in smaller numbers, in the parasympathetic ganglia supplying the bladder. Stimulation of the alpha fibers results in the inhibition of neural firing at the level of the parasympathetic ganglion, thereby inhibiting bladder contractions. Alpha-adrenergic receptors predominate in the smooth muscle of the bladder neck and urethra. Stimulation results in contraction of the structures.

The sum effect of sympathetic stimulation of the bladder and urethra is the promotion of storage. Parasympathetic or cholinergic stimulation generally is micturition-promoting.

Postganglionic muscarinic fibers to the detrusor muscle promote bladder contractions when stimulated. In addition, stimulation of muscarinic receptors on alpha-adrenergic nerves to the urethra prevents norepinephrine stimulation. The resulting physiologic effect is urethral relaxation. Cholinergic agents, although generally thought of as promoters of detrusor activity, also can stimulate preganglionic sympathetic nicotinic receptors with neural connections to the urethra and bladder neck. Such stimulation promotes contraction of these structures.

Nonadrenergic, noncholinergic nerves with adenosine triphosphate (ATP)-stimulated purinergic receptors have been found in animal models and in the human bladder. These nerves may be very important in bladder contractility. Prostaglandins also may be able to activate these receptors.

Sensory afferent innervation of the bladder originates with stretch and pain receptors in the bladder wall. Stretch receptors, which are responsible for bladder proprioception, are the origin of impulses traveling via the pelvic nerve to the posterior columns ipsilaterally and, eventually, the brain stem micturition center. Connections from the brain stem to the cerebral cortex provide for conscious awareness of bladder distention.

Pain receptors are present in the bladder wall but not as densely as stretch receptors. These receptors are responsible for sensing temperature, touch, and irritative stimuli. The generated impulses travel by way of the hypogastric nerve to synapse in the posterior root ganglia. The impulses cross to the contralateral side before ascending in the spinothalamic tract and the thalamic nuclei, eventually reaching the cerebral cortex.

Afferent impulses from these pain receptors can trigger detrusor contractions via a normally suppressed reflex arc. Under conditions of severe mucosal irritation (eg, urinary tract infection), this reflex may become unmasked. In addition, disorders resulting in the loss of conscious cerebral cortical input may be responsible for the emergence of this reflex.

Somatic efferent innervation to the striated urethral sphincter complex is from the second through the fourth sacral segments. The precise source of these fibers is controversial. Evidence suggests that the sphincter complex is innervated by way of the pelvic nerve rather than the pudendal nerve, as was once thought. The levator ani complex probably has a dual source of innervation from both the pelvic and pudendal nerves.

In women, estrogen receptors can be found in the musculature of the pelvic floor, bladder, bladder neck, and urethra. Estrogen stimulation increases the density of alpha-adrenergic receptors in the urethral smooth muscle. Progesterone may enhance beta-adrenergic activity. Emerging evidence of the presence of gonadotropin receptors in the lower urinary tract also exists.

The pelvic diaphragm lines the floor of the bony pelvis and is composed of 4 sheets of muscles, the pubococcygeus, iliococcygeus, ischiococcygeus, and coccygeus. (See the images below.)

Specialists often refer to the pelvic diaphragm as the levator ani. The levator ani musculature is attached to the inner sides of the bony pelvis by a condensation of pelvic fascia called the arcus tendineus.

The levator ani is the most important component of the pelvic diaphragm because the integrity of the pelvic floor depends upon its function. When the levator ani is damaged, stress urinary incontinence and/or herniation of pelvic organs through the vagina may develop.

Supporting ligaments and fascia

The urethropelvic ligament is a fibrous band of connective tissue that lines the undersurface of the bladder neck and attaches laterally to the arcus tendineus. The urethropelvic ligament provides the major support to the bladder neck and proximal urethra. Laxity of the urethropelvic ligament results in SUI.

The pubocervical fascia is a fibrous sheet of connective tissue that lines the base of the urinary bladder and inserts laterally into the arcus tendineus. An intact pubocervical fascia prevents the herniation of the bladder and the proximal urethra into the vagina. Damage to the pubocervical fascia may cause the bladder to herniate through the vagina, resulting in cystocele formation and SUI.

The cardinal ligaments arise from the arcus tendineus and anchor to the uterine cervix. The cardinal ligaments stabilize and support the uterus, vagina, and bladder. Weakening of the cardinal ligaments may cause a cystocele and uterine descensus (see the image below).

The uterosacral ligaments originate from condensation of the fibrous connective tissue overlying the sacral promontory and insert into the uterine cervix. The uterosacral ligaments stabilize the uterus in the bony pelvis. Weakening of the uterosacral ligaments may cause a prolapsed uterus or vaginal vault prolapse.