Urinary bladder The urinary bladder is a reservoir and varies in size, shape, position and relations, according to its content and the state of neighbouring viscera. When empty, it is entirely in the lesser pelvis but as it distends it expands anterosuperiorly into the abdominal cavity. When empty, has a fundus, neck, apex, a superior and two inferolateral surfaces. The fundus or base is triangular and postero-inferior. In females it is closely related to the anterior vaginal wall; in males it is related to the rectum although separated from it above by the rectovesical pouch and below that by the seminal vesicles and deferent ducts. In a triangular area between the deferent ducts, the bladder and rectum are separated only by rectovesical fascia; the inferior part of this area may be obliterated by approximation of the deferent ducts above the prostate. Although the vesical fundus should be, by definition, the lowest region, the neck is in fact lowest and also the most fixed; it is 3–4 cm behind the lower part of the symphysis pubis. It is pierced by the internal urethral orifice and alters little in position with varying conditions of the bladder and rectum. There is no special constriction of the bladder at its neck. In males the neck rests on, and is in direct continuity with, the base of the prostate; in females it is related to the pelvic fascia which surrounds the upper urethra. The vesical apex in both sexes faces towards the upper part of the symphysis pubis; from it the median umbilical ligament (urachus) ascends behind the anterior abdominal wall to the umbilicus, the peritoneum over it being the median umbilical fold. The triangular superior surface is bounded by lateral borders from the apex to the ureteric entrances and by a posterior border joining them. In males the superior surface is completely covered by peritoneum, extending slightly on to the base and continued posteriorly into the rectovesical pouch, laterally into the paravesical fossae and anteriorly into the median umbilical fold. It is in contact with the sigmoid colon and the terminal coils of the ileum. In females the superior surface is also largely covered by peritoneum but posteriorly this is reflected to the uterus at the level of the internal os (the junction of the uterine body and cervix), forming the vesico-uterine pouch. The posterior part of the superior surface, devoid of peritoneum, is separated from the supravaginal cervix by fibro- areolar tissue. Each inferolateral surface in males is separated anteriorly from the pubis and puboprostatic ligaments by an adipose retropubic pad and posteriorly by fascia from the levator ani and obturator internus. In females the relations are similar, but the puboprostatic are replaced by pubovesical ligaments. The inferolateral surfaces are not covered by peritoneum. As the bladder fills it becomes ovoid. In front it displaces the parietal peritoneum from the suprapubic region of the abdominal wall, so that the inferolateral surfaces become anterior and rest against the abdominal wall without intervening peritoneum for a distance above the symphysis pubis, varying with the degree of distension. At birth, the bladder is relatively higher than in the adult, the internal urethral orifice being level with the upper symphyseal border; the bladder is abdominal rather than pelvic, extending about two-thirds of the distance towards the umbilicus. It progressively descends, reaching the adult position shortly after puberty. Ligaments of the Bladder In both sexes stout bands of fibromuscular tissue extend from the bladder neck to the inferior aspect of the pubic bones. These structures are the pubovesical ligaments and constitute the superior extensions of the pubourethral ligaments in the female or the puboprostatic ligaments in the male. The two pubovesical ligaments lie one on each side of the median plane, leaving a midline hiatus through which pass numerous small veins. In addition to the pubovesical ligaments a number of other so-called ligaments have been described in relation to the base of the urinary bladder. These are formed by condensation of connective tissue around neurovascular structures and as such do not merit the distinction of having specific names assigned to them. The apex of the bladder is joined to the umbilicus by the remains of the urachus (below), which forms the median umbilical ligament.. From the superior surface of the bladder the peritoneum is carried off in a series of folds which are termed the 'false' ligaments of the bladder. Anteriorly there are three folds: the median umbilical fold over the median umbilical ligament and two medial umbilical folds over the obliterated umbilical arteries. The reflexions of the peritoneum from the bladder to the side walls of the pelvis form the lateral false ligaments while the sacrogenital folds constitute the posterior false ligaments. Interior of the Bladder Vesical Mucosa Attached only loosely to subjacent muscle for the most part, it folds when the bladder empties, the folds being effaced as it fills. Over the trigone (13.35), immediately above and behind the internal urethral orifice, it is adherent to the subjacent muscle layer and always smooth. The trigone's anteroinferior angle is formed by the internal urethral orifice, its posterolateral angles by the ureteric orifices. The superior trigonal boundary is a slightly curved interureteric crest, connecting the two ureteric orifices and produced by the continuation into the vesical wall of the ureteric internal longitudinal muscle. (For details of trigonal musculature, consult Uhlenhuth et al 1952; Woodburne 1965, 1968.) Laterally this ridge extends beyond the ureteric openings as ureteric folds, produced by the terminal parts of the ureters running obliquely through the bladder wall. At cystoscopy the interureteric crest appears as a pale band and is a guide to the ureteric orifices in catheterization. Ureteric Orifices Placed at the posterolateral trigonal angles (13.35), they are usually slit-like. In empty bladders they are about 2.5 cm apart and about the same from the internal urethral orifice; in distension these measurements may be doubled. Internal Urethral Orifice Sited at the trigonal apex, the lowest part of the bladder, this is usually somewhat crescentic in section; in adult males, particularly past middle age, immediately behind it is a slight elevation caused by the median prostatic lobe, the uvula of the bladder. Bladder Capacity Mean vesical capacity in male adults varies from 120–320 ml (Thompson 1919); micturition commonly occurs at about 280 ml. Filling to about 500 ml may be tolerated but beyond this pain is caused by tension in the wall, leading to reflex contractions and the urgent desire to micturate. Pain is referred to the cutaneous areas supplied by spinal segments supplying the bladder (T11–L2, S2–4), including the lower anterior abdominal wall, perineum and penis. Bladder Microstructure Histologically the wall of the urinary bladder consists of three layers (13.36): an outer adventitial layer of soft connective tissue which in some regions possesses a serosal covering of peritoneum, a nonstriated muscle coat (the detrusor muscle) and an inner layer of mucous membrane which lines the interior of the bladder. Serous Layer The serous layer is restricted to the superior and, in males, part of the posterior surfaces of the bladder, the rest being devoid of peritoneum. It consists of mesothelium and underlying connective tissue as elsewhere in the peritoneum. Muscular Layer The muscular layer, the detrusor muscle, is composed of relatively large (in diameter) interlacing bundles of nonstriated muscle cells arranged as a complex meshwork. Three ill-defined layers are present and arranged in such a way that longitudinally orientated muscle bundles predominate on the inner and outer aspects of a substantial middle circular layer. Posteriorly some of the outer longitudinal bundles pass over the bladder base and fuse with the capsule of the prostate or with the anterior vaginal wall. Other bundles are carried on to the anterior aspect of the rectum and named the rectovesical muscle. Anteriorly some of the outer longitudinal bundles continue into the pubovesical ligaments and contribute to the muscular component of these structures. As in the muscle coat of the ureter, exchange of fibres between adjacent muscle bundles within the bladder wall frequently occurs so that from a functional viewpoint, the detrusor comprises a single unit of interlacing smooth muscle. An electron-dense basal lamina surrounds each nonstriated myocyte except at certain junctional regions. The most frequently observed type of junction between muscle cells is the region of close approach at which an intercellular separation of 10–20 nm occurs over distances occasionally in excess of 1 mm in length. Junctions of the 'peg and socket' and 'intermediate' types are observed occasionally but gap junctions (nexuses) have not been reported in the detrusor. Since electrotonic spread excitation occurs in the nonstriated muscle of the bladder wall, the regions of close approach may represent the morphological feature which enables this physiological event to take place, although it is more likely that gap junctions exist, albeit few of them. Within muscle bundles, the smooth muscle cells are closely packed together such that the basal lamina of one cell very often becomes confluent with that of its neighbours. Trigone The smooth muscle of this region consists of two distinct layers, sometimes termed the superficial and deep trigonal muscles. The latter is composed of muscle cells which are indistinguishable from the muscle cells of the detrusor. Hence this deep trigonal muscle is simply the postero-inferior portion of the detrusor muscle proper and confusion might be avoided if the term deep trigonal muscle was abandoned in favour of the more accurate definition as trigonal detrusor muscle. The superficial trigonal muscle represents a morphologically distinct component of the trigone which, unlike the detrusor, is composed of relatively small diameter muscle bundles continuous proximally with those of the intramural ureters. The muscle layer comprising the superficial trigone is relatively thin but is generally described as becoming thickened along its superior border to form the interureteric crest. Similar thickenings occur along the lateral edges of the superficial trigone (Bell 1812). In both sexes the superficial trigone muscle becomes continuous with the smooth muscle of the proximal urethra, extending in the male along the urethral crest as far as the openings of the ejaculatory ducts. However, the arrangement of nonstriated muscle in the human trigone has been re-examined recently by Dorschner et al (1994a) who describe the presence of two muscular structures, a musculus interuretericus originating from the muscular systems of both ureters and forming the muscular component of the interureteric ridge, and a musculus sphincter trigonalis or musculus sphincter vesicae, the latter surrounding the internal urethral orifice but not extending into the urethra or its surroundings. This description is at variance with that of Bell (1812). Dorschner et al (1994a) reported that in men the lower part of the sphincter trigonalis is pervaded with prostatic tissue, and suggested that the muscle could have a dual function, assisting in urinary continence and also preventing retrograde ejaculation whilst aiding the release of prostatic secretions. Ureterovesical Junction The distal 1–2 cm of each ureter is surrounded by an incomplete collar of the detrusor nonstriated muscle which forms a sheath (of Waldeyer) separated from the ureteric muscle coat by a connective tissue sleeve. The ureters pierce the posterior aspect of the bladder and run obliquely through its wall for a distance of 1.5–2.0 cm before terminating at the ureteric orifices. This arrangement is believed to assist in the prevention of reflux of urine into the ureter, since the intramural ureters are thought to be occluded during increases in bladder pressure. There is no evidence of a classic ureteral sphincter mechanism in man (Noordzij & Dabhoiwala 1993). The longitudinally oriented muscle bundles of the terminal ureter continue into the bladder wall and at the ureteric orifices become continuous with the superficial trigonal muscle (Tanagho et al 1970). More recently this muscular continuity has been disputed, no evidence of it having been found in the porcine ureterovesical junction (Thomson et al 1994). Bladder Neck The nonstriated muscle of this region is histologically, histochemically and pharmacologically distinct from that which comprises the detrusor muscle proper (Klück 1980). Hence the bladder neck should be considered as a separate functional unit. The arrangement of nonstriated muscle in this region is quite different in males and females and consequently each sex will be described separately. Male In the male bladder neck, the nonstriated muscle cells form a complete circular collar which extends distally to surround the preprostatic portion of the urethra. Because of the location and orientation of its constituent fibres, the terms internal, proximal or preprostatic urethral sphincter are suitable alternatives for this particular component of urinary tract smooth muscle. Distally, the bladder neck muscle merges with and becomes indistinguishable from the musculature in the stroma and capsule of the prostate gland. Female The female bladder neck also consists of morphologically distinct nonstriated muscle, since the large diameter fasciculi characteristic of the detrusor are replaced in the region of the bladder neck by those of small diameter. However, unlike the circularly orientated preprostatic nonstriated muscle, the muscle fasciculi in the female extend obliquely or longitudinally into the urethral wall. The female does not, therefore, possess a smooth muscle sphincter at the bladder neck and it is unlikely that active contraction of this region plays a significant part in the maintenance of female urinary continence. Mucosa The mucosa has a structure similar to that of the ureters and consists of an epithelium (urothelium, transitional epithelium, p. 72) supported by a layer of loose connective tissue, the lamina propria. The latter consists of loose fibro-elastic connective tissue and forms a relatively thick layer, varying in depth from 500 mm in the fundus and inferolateral walls to about 100 mm in the trigone. Small-diameter bundles of nonstriated muscle cells also occur in the subepithelial connective tissue forming an incomplete and rudimentary muscularis mucosae. The soft connective tissue elements immediately beneath the urothelium, particularly in the region of the trigone, are densely packed. At deeper levels they are more loosely arranged, thus allowing the bladder mucosa to form numerous thick folds when the volume of fluid contained within the lumen is small. An extensive network of blood vessels is present throughout the lamina propria and supplies a plexus of thin-walled fenestrated capillaries lying in grooves at the base of the urothelium. Nontrigonal urothelium is often up to six cells in thickness. These cells can be classified according to position and consist of highly differentiated superficial or luminal cells, one or more layers of smaller intermediate cells and a layer of undifferentiated basal cells. The large superficial cells frequently bulge into the bladder lumen and are often binucleate. In contrast the intermediate and basal cells are smaller and each contains a single darkly-staining nucleus. The flattened urothelium of the trigone usually consists of only two or three layers of cells and a similar appearance prevails throughout the bladder when in the distended state. In addition to the basal, intermediate and superficial cells described above, a fourth type of cell occurs in the urothelium of the human bladder neck and trigone. These flask-shaped cells extend throughout the depth of the urothelium and are characterized by the presence of numerous large membrane-bound vesicles each containing a central dense granule. Vesicles of this type are believed to be involved in the storage of amines and it seems likely that these cells belong to the so-called APUD (amine- precursor-uptake and decarboxylation, see p. 1898) series which have a wide distribution throughout the body. Several morphological variations have been described in the mucosa of the bladder which, because of their occurrence in otherwise normal healthy adults, are not considered to represent pathological conditions. One of the commonest epithelial variants found in bladder biopsy samples or at postmortem is the occurrence of so- called Brunn's nests. These consist of proliferations of morphologically normal basal urothelial cells which project into the underlying connective tissue of the lamina propria and are particularly frequent in the trigone. Mucus-secreting glands with single or branched ducts are another frequently observed feature of the bladder mucosa. When present these structures are particularly numerous near the ureteric and internal urethral orifices. Non-keratinizing squamous metaplasia of the vaginal type also frequently occurs in the urinary bladder mucosa, especially over the trigone. This histological appearance, whilst occasionally observed in males and in children, is more common in adult females. Vascular and Lymphatic Supply of the Bladder Arteries The principal arteries of supply to the bladder are the superior and inferior vesical, derived from the anterior trunk of the internal iliac artery (p. 1559). The obturator and inferior gluteal arteries also send small branches to it and in the female additional branches are derived from the uterine and vaginal arteries. Veins These form a complicated plexus on the inferolateral surfaces and pass backwards in the posterior ligaments of the bladder to end in the internal iliac veins (p. 1598). Lymph Vessels These are described on page 1623. Nerve Supply of the Bladder The nerves supplying the bladder form the vesical plexus (see p. 1309) and consist of both sympathetic and parasympathetic components, each of which contains both efferent and afferent fibres. The innervation of the bladder was reviewed in some detail by Burnstock (1990c) and by de Groat in 1993. Efferent Fibres Parasympathetic fibres arise from the second to the fourth sacral segments of the spinal cord (nervi erigentes); the sympathetic fibres are derived from the lower two thoracic and upper two lumbar segments of the spinal cord. In addition to the branches from the vesical plexus, small groups of autonomic neurons occur throughout all regions of the bladder wall. These multipolar intramural neurons are rich in acetyl cholinesterase (AChE) and occur in ganglia consisting of five to 20 nerve cell bodies. Numerous preganglionic autonomic fibres form both axosomatic and axodendritic synapses with the ganglion cells. The majority of these preganglionic nerve terminals correspond morphologically to presumptive cholinergic fibres. Noradrenergic terminals also relay on cell bodies in the pelvic plexus although it is unknown whether similar nerves synapse on intramural bladder ganglia. The urinary bladder (including the trigonal detrusor muscle) is profusely supplied with nerves which form a dense plexus among the detrusor smooth muscle cells. The majority of these nerves contain AChE and occur in abundance throughout the muscle coat of the bladder. Axonal varicosities adjacent to detrusor nonstriated muscle cells possess features which are considered to typify cholinergic nerve terminals and contain clusters of small (50 nm diameter) agranular vesicles together with occasional large (80–160 nm diameter) granulated vesicles and small mitochondria. Terminal regions approach to within 20 nm of the muscle cells' surface and are either partially surrounded by or more often totally denuded of Schwann cell cytoplasm. The human detrusor muscle possesses a sparse supply of sympathetic noradrenergic nerves (Sundin et al 1977). Nerves of this type generally accompany the vascular supply and only rarely extend among the nonstriated myocytes of the urinary bladder. A further component plays a part in the autonomic innervation of the urinary bladder (Ambache & Zar 1970), which has been classified as having a nonadrenergic, noncholinergic nerve mediated effect. A number of other neurotransmitters or neuromodulators have been detected in intramural ganglia, including the peptide somatostatin (see p. 937). The superficial trigonal muscle is associated with few cholinergic (parasympathetic) nerves while those of the noradrenergic (sympathetic) variety occur relatively frequently. These differences support the view that the superficial trigonal muscle should be regarded as 'ureteric' rather than 'vesical' in origin. It should be emphasized that the superficial trigonal muscle forms a very minor part of the total muscle mass of the bladder neck and proximal urethra in either sex and is probably of little significance in the physiological mechanisms which control these regions. In bladder neck of the male, nonstriated muscle is sparsely supplied with cholinergic (parasympathetic) nerves but possesses a rich noradrenergic (sympathetic) innervation (Gosling et al 1977). A similar distribution of autonomic nerves also occurs in the nonstriated muscle of the prostate gland, seminal vesicles and ducti deferentes. From a functional standpoint, sympathetic nerves on stimulation cause contraction of nonstriated muscle in the wall of the genital tract resulting in seminal emission. Concomitant sympathetic stimulation of the proximal urethral muscle causes sphincteric closure of the bladder neck, thereby preventing reflux of ejaculate into the bladder. Although this genital function of the bladder neck of the male is well established it is not known whether the nonstriated muscle of this region plays an active role in maintaining urinary continence. In contrast with this rich sympathetic innervation in the male, the nonstriated muscle of the bladder neck of the female receives relatively few noradrenergic nerves but is richly supplied with presumptive cholinergic fibres. The sparse supply of sympathetic nerves presumably relates to the absence of a functioning 'genital' portion incorporated within the wall of the female urethra. The lamina propria of the fundus and inferolateral walls of the bladder is virtually devoid of autonomic nerve fibres, apart from some noradrenergic and occasional presumptive cholinergic perivascular nerves. However, as the urethral orifice is approached the density of nerves unrelated to blood vessels increases. At the bladder neck and trigone a nerve plexus extends throughout the lamina propria. The constituent nerves are cholinesterase positive and run through the connective tissue unassociated with blood vessels. Some of the larger diameter axons are myelinated and others lie adjacent to the basal urothelial cells. As in the ureter, the subepithelial nerve plexus of the bladder is assumed to subserve a sensory function in the absence of any obvious effector target sites (Gosling & Dixon 1974). Afferent Fibres Vesical nerves are also concerned with pain and awareness of distension. Pain fibres are stimulated by distension or spasm due to a stone, inflammation or malignant disease; they are found in sympathetic and parasympathetic nerves, predominantly the latter. Hence, simple division of the sympathetic paths (e.g. 'presacral neurectomy'), or of the superior hypogastric plexus (p. 1308), does not materially relieve vesical pain. The spinal path for pain is in the anterolateral white columns and considerable relief follows bilateral anterolateral cordotomy. Since nerve fibres mediating awareness of distension are in the posterior columns (fasciculus gracilis), after anterolateral cordotomy the patient still retains awareness of the need to micturate. The nerve endings detecting noxious stimuli are probably of more than one type; a subepithelial plexus of fibres containing dense vesicles, probably afferent endings, has been described. Clinical Anatomy A distended bladder may be ruptured in lower abdominal or pelvic injuries, either extraperitoneally or, if the superior surface is involved, with tearing of the peritoneum and escape of vesical contents into the peritoneal cavity. In progressive chronic obstruction to micturition, for example by prostatic enlargement (p. 1861) or urethral stricture, vesical musculature hypertrophies, its fasciculi increasing in size and interlacing in all directions to produce an enlarged 'trabeculated bladder'. Mucosa between the fascicles forms 'diverticula', which may contain phosphatic concretions. When outflow is thus obstructed, emptying is not complete; some urine remains and may become infected; infection may extend to the ureters and kidneys. Back pressure from a distended bladder may gradually dilatate the ureters, renal pelves and even the renal collecting tubules. Lesions of the fasciculus gracilis (e.g. tabes dorsalis) cause loss of desire to micturate; the distended bladder may empty merely by overflow. Severe spinal cord lesions above its sacral segments, interrupting efferent and afferent tracts involved in normal micturition, may result in 'automatic' emptying. The vesical interior can be examined with a cystoscope, introduced via the urethra after distending the bladder with fluid. A special cystoscope is used to catheterize the ureters, to obtain a direct specimen of urine from either kidney or to inject radio- opaque fluid for retrograde pyelography (13.6). The vesical outline can be similarly demonstrated. The distended bladder may be punctured just above the symphysis pubis without traversing the peritoneum (suprapubic cystostomy). When the bladder contains about 300 ml its anteroinferior surface contacts the anterior abdominal wall directly for about 7.5 cm above the pubis. Surgical access to the bladder is usually by this route. In females, owing to the shorter and more dilatable urethra, small calculi, foreign bodies and growths may be removed through it. Congenital abnormalities of the bladder are described on p. 213. Male Urethra The male urethra (13.32, 38) is from 18–20 cm long, and extends from an internal orifice in the urinary bladder to an external opening, or meatus, at the end of the penis. It may be considered in four regional parts: preprostatic, prostatic, membranous and spongiose, and presents a double curve while the penis is in its ordinary flaccid state (13.32). Except during the passage of fluid along it, the urethral canal is a mere slit; in the prostatic part the slit is transversely arched in transverse section, in the preprostatic and membranous portions it is stellate, in the spongiose portion transverse, while at the external orifice it is sagittal in orientation. Preprostatic Part The preprostatic urethra possesses a stellate lumen and is approximately 1–1.5 cm in length, extending almost vertically from the bladder neck to the superior aspect of the prostate gland. The non-striated muscle bundles surrounding the bladder neck and preprostatic urethra are arranged as a distinct circular collar which becomes continuous distally with the capsule of the prostate gland. The bundles which form this preprostatic or internal sphincter (sphincter vesicae) are separated by connective tissue containing many elastic fibres (Gilpin & Gosling 1983). Unlike the detrusor muscle, the nonstriated muscle surrounding the proximal urethra is almost totally devoid of parasympathetic cholinergic nerves but is richly supplied with sympathetic noradrenergic nerves. Similar nerves also supply the nonstriated muscle of the prostate, ducti deferentes and seminal vesicles and are involved in causing muscle contraction at the time of ejaculation (Learmonth 1931). Contraction of the preprostatic sphincter serves to prevent the retrograde flow of ejaculate through the proximal urethra into the bladder. Prostatic Part The prostatic urethra is approximately 3–4 cm in length and tunnels through the substance of the prostate closer to the anterior than the posterior surface of the gland. It is continuous above with the preprostatic part and emerges from the prostate slightly anterior to its apex (its most inferior point). Throughout most of its length the posterior wall possesses a midline ridge, the urethral crest, which projects into the lumen causing it to appear crescentic in transverse section. On each side of the crest there is a shallow depression, termed the prostatic sinus, the floor of which is perforated by the orifices of the prostatic ducts. About the middle of the length of the urethral crest the colliculus seminalis (verumontanum) forms an elevation on which the slit-like orifice of the prostatic utricle is situated; on both sides of or just within this orifice there are the two small openings of the ejaculatory ducts. The prostatic utricle is a cul-de-sac about 6 mm long, which runs upwards and backwards in the substance of the prostate behind its median lobe. Its walls are composed of fibrous tissue, muscular fibres and mucous membrane; the last presents the openings of numerous small glands. Developed from the paramesonephric ducts or urogenital sinus, it is thought to be homologous with the vagina of the female (p. 205). The prostatic utricle is, therefore, called by some the 'vagina masculina', but the more usual view is that it is a uterine homologue and hence the term 'utricle'. The ejaculatory ducts are described on page 1856. Distally the prostatic urethra possesses an outer layer of circularly disposed skeletal muscle cells which are continuous with a prominent collar of striated skeletal muscle (the external urethral sphincter) within the wall of the membranous urethra. Membranous Part The membranous part is the shortest, least dilatable and, with the exception of the external orifice, the narrowest section of the urethra. It descends with a slight ventral concavity from the prostate to the bulb of the penis (13.32), passing through the perineal membrane about 2.5 cm postero-inferior to the pubic symphysis. The hind part of the bulb of the penis is closely apposed to the inferior aspect of the urogenital diaphragm (perineal membrane) but anteriorly it is slightly separated from the latter, so that the wall of the urethra is related anteriorly neither to the perineal membrane nor the penile bulb. If this part of the anterior wall of the urethra is regarded as the 'membranous' anteriorly the membranous urethra is about 2 cm long, whilst posteriorly it is only 1.2 cm. The wall of the membranous urethra consists of a muscle coat which is separated from the epithelial lining by a narrow layer of fibro-elastic connective tissue. This muscle coat consists of a relatively thin layer of nonstriated muscle bundles continuous proximally with those of the prostatic urethra and a prominent outer layer of circularly orientated skeletal muscle fibres forming the external urethral sphincter. The skeletal muscle fibres which comprise this external sphincter are unusually small in cross-section, with diameters of only 15–20 mm. The fibres are physiologically of the slow twitch type (Gosling et al 1981), unlike the pelvic floor musculature which is a heterogeneous mixture of slow and fast twitch fibres of larger diameter. Moreover, the external sphincter is devoid of muscle spindles and is supplied by the pelvic splanchnic nerves, further distinguishing it from the periurethral levator ani muscle (Donker et al 1976). The slow twitch fibres of the external sphincter are capable of sustained contraction over relatively long periods of time and actively contribute to the tone which closes the urethra and maintains urinary continence. Spongiose Part The spongiose part is contained in the corpus spongiosum penis (p. 1857). It is about 15 cm long and extends from the end of the membranous urethra to the external urethral orifice on the glans penis. Commencing below the perineal membrane, it continues the ventrally concave curve of the membranous urethra to a point anterior to the lowest level of the symphysis pubis. From here, when the penis is flaccid, the urethra curves downwards in the 'free' part of the penis. It is a narrow, transverse, slit when empty, with a diameter of about 6 mm when passing urine; it is dilated at its commencement as the intrabulbar fossa and again within the glans penis, where it becomes the navicular fossa. The enlargement of the intrabulbar fossa affects the floor and side walls but not the roof of the urethra. The bulbo-urethral glands open into the spongiose section of the urethra about 2.5 cm below the perineal membrane (p. 1861). The external urethral orifice is the narrowest part of the urethra: it is a sagittal slit, about 6 mm long, bounded on each side by a small labium. The epithelium of the urethra, except in its most anterior part, presents the orifices of numerous small mucous glands and follicles situated in the submucous tissue and named the urethral glands. Besides these there is a number of small pit-like recesses, or lacunae, of varying sizes; the orifices of these are directed forwards and may intercept the point of a catheter in its passage along the canal. One, larger than the rest, the lacuna magna, is situated on the roof of the navicular fossa. Mucous Membrane of the Male Urethra The epithelium lining the preprostatic urethra and the proximal part of the prostatic urethra is of the typical urothelial type and is in continuity with that lining the bladder; it is also continuous with the ducts of the prostate and bulbo-urethral glands and with the linings of the seminal vesicles, deferent ducts and ejaculatory ducts, a relationship which is important in the spread of urinary tract infections. However, below the openings of the ejaculatory ducts this epithelium changes to a patchily pseudostratified or stratified columnar variety which lines the membranous urethra and the major part of the penile urethra (2.71). Mucus-secreting cells are common throughout this epithelium and frequently occur in small clusters in the penile urethra. The mucuous membrane of the penile urethra shows many recesses which continue into deeper branching tubular mucous glands (of Littré) which are especially numerous on the dorsal aspect. In older men many of the recesses of the urethral mucosa contain concretions similar to those found within the substances of the prostate. Towards the distal end of the penile urethra the epithelium changes once again, becoming stratified squamous in character with well-defined connective tissue papillae. This type of epithelium lines the navicular fossa and becomes keratinized at the external meatus. The epithelial cells lining the navicular fossa are unusual in being glycogen-rich and it has been suggested that they may act as a substrate for an endogenous flora of lactobacteria (Holstein et al 1991). They also lack acid phosphatase activity and lysozyme-like immunoreactivity, both of which are demonstrable in the epithelium lining other parts of the distal male urethra. The epithelium lining the distal urethra in men is thus heterogeneous, a condition which may provide a measure of defence against invasion by pathogenic organisms. Urethral Sphincters Of the two urethral sphincters, the internal sphincter vesicae (p. 1842) controls the vesical neck and the prostatic urethra above the ejaculatory ducts. It is composed of nonstriated muscle and supplied by sympathetic and parasympathetic fibres from the vesical plexus (see above and p. 1309). The external sphincter urethrae (p. 834) surrounds the membranous urethra; it consists of striated muscle and is supplied by the perineal branches of the pudendal nerve (S2, 3 and 4); it is voluntary after early infancy. The existence of an internal sphincter is, however, controversial. Many consider that its nonstriated muscle fibres are multi-directional as in the vesical wall, no true circumferential fibres being identifiable (Woodburne 1961; Angell 1969). However, all agree that a substantial muscular aggregation, with an admixture of elastic and collagenous fibres, exists at the vesical outlet (Vincent 1966). Its significance in micturition is noted below. Female Urethra The female urethra is about 4 cm long and 6 mm in diameter. It begins at the internal urethral orifice of the bladder, approximately opposite the middle of the symphysis pubis, and runs anteroinferiorly behind the symphysis pubis, embedded in the anterior wall of the vagina. It traverses the perineal membrane and ends at the external urethral orifice, an anteroposterior slit with rather prominent margins, which is situated directly anterior to the opening of the vagina and about 2.5 cm behind the glans clitoridis. Except during the passage of urine the anterior and posterior walls of the urethra are in apposition and the epithelium is thrown into longitudinal folds, one of which, on the posterior wall of the canal, is termed the urethral crest. Many small mucous urethral glands and minute pit-like recesses or lacunae open into the urethra. On each side, near the lower end of the urethra, a number of these glands are grouped together and open into a duct, named the para-urethral duct; each duct runs down in the submucous tissue and ends in a small aperture on the lateral margin of the external urethral orifice. Microscopic Structure of the Female Urethra The wall of the female urethra comprises an outer muscle coat and an inner mucous membrane which lines the lumen and is continuous with that of the bladder. The muscle coat consists of an outer sleeve of striated muscle (external urethral sphincter p. 835) together with an inner coat of smooth muscle fibres. The female external urethral sphincter is anatomically separate from the adjacent periurethral striated muscle of the anterior pelvic floor. The constituent fibres of this sphincter are circularly disposed and form a sleeve which is thickest in the middle one-third of the urethra. In this region striated muscle completely surrounds the urethra although the posterior portion lying between the urethra and vagina is relatively thin. The striated muscle extends into the anterior wall of both the proximal and distal thirds of the urethra but is deficient posteriorly in these regions. The myocytes forming the external urethral sphincter are all of the slow twitch variety. As in the male, muscle fibres of the external urethral sphincter are unusually small and have diameters of 15– 20 mm on average. Although the thickness of the external urethral sphincter in the female is less than that of the male, its constituent fibres are able to exert tone upon the urethral lumen over prolonged periods, especially in relation to the middle third of its length. Periurethral striated muscle (pubococcygeus) aids urethral closure during events which require rapid, albeit short-lived, elevation of urethral resistance. The smooth muscle coat extends throughout the length of the urethra and consists of slender muscle bundles, the majority of which are orientated obliquely or longitudinally. A few circularly arranged muscle fibres occur in the outer aspect of the non-striated muscle layer and intermingle with the skeletal muscle fibres forming the inner part of the external urethral sphincter. Proximally the urethral smooth muscle extends as far as the bladder neck where it is replaced by fascicles of detrusor nonstriated muscle. This region in the female is devoid of a well-defined circular non- striated muscle component comparable with the preprostatic sphincter of the male. When traced distally, urethral smooth muscle bundles terminate in the subcutaneous adipose tissue surrounding the external urethral meatus. The smooth muscle of the female urethra is associated with relatively few noradrenergic nerves but receives an extensive presumptive cholinergic parasympathetic nerve supply identical in appearance to that which supplies the detrusor muscle (Ek et al 1977a, b). From a functional viewpoint it seems unlikely that competence of the female bladder neck and proximal urethra is solely the result of nonstriated muscle activity, in the absence of an anatomical sphincter. The innervation and longitudinal orientation of most of the muscle fibres suggest that urethral smooth muscle in the female is active during micturition, serving to shorten and widen the urethral lumen. For further details, see Gilpin and Gosling (1983). The mucous membrane lining the female urethra consists of a stratified epithelium and a supporting layer of loose fibro-elastic connective tissue (the lamina propria). The lamina propria contains an abundance of elastic fibres orientated both longitudinally and circularly around the urethra. Numerous thin- walled veins are another characteristic feature and in the past have been falsely likened to erectile tissue. A fine plexus of acetyl cholinesterase (AChE) positive nerves is present throughout the lamina propria and these fibres are believed to be sensory branches of the pudendal nerves. The proximal part of the urethra is lined by urothelium, identical in appearance to that of the bladder neck. Distally the epithelium changes into a non-keratinizing stratified squamous type which lines the major portion of the female urethra. This epithelium is keratinized at the external urethral meatus and becomes continuous with the skin of the vestibule. Functional Anatomy of the Lower Urinary Tract The urinary bladder performs a dual function, acting at times as a reservoir for fluid accumulating within its lumen and at others as a contractile organ actively expelling its contents into the urethra. In the following account the tissue components and, where appropriate, their neurological control will be considered under the headings continence of urine and micturition. Continence of Urine To achieve urinary continence, the bladder acts as a passive reservoir retaining fluid because the forces acting on the urethra produce an intra-urethral pressure greater than bladder pressure. Several tissue components play a part in generating this urethral resistance and make either an active or passive contribution. Since the nonstriated muscle of the bladder is replaced in the bladder neck region by a different type of nonstriated muscle, the detrusor muscle does not play a part in closing the proximal urethra. In the male, a distinct collar of circularly orientated nonstriated muscle occurs in the bladder neck and preprostatic urethra, continuous distally with the muscular components of the genital tract. This smooth muscle sphincter is supplied by a rich plexus of sympathetic nerve fibres which, on stimulation, cause the sphincter to contract, thereby preventing retrograde flow of semen into the urinary bladder at ejaculation. During seminal emission, the sympathetic nervous system also prevents coincidental contraction of detrusor smooth muscle. This inhibitory effect on bladder contractility is mediated by noradrenergic nerves which synapse in the vesical plexus upon parasympathetic motor neurons. Despite this well-defined genital role, it is not known whether the nonstriated muscle of the bladder neck region and preprostatic urethra plays an active part in the maintenance of continence. Intramural collagen and elastic fibres within the wall of the bladder neck, proximal urethra and prostate generate passive forces which help to close the urethral lumen. However, postoperative incontinence of urine does not usually follow radical surgical excision of the bladder neck, preprostatic urethra and prostate, suggesting that these regions make only a minor contribution to urinary continence. In the female, a nonstriated muscle sphincter cannot be anatomically recognized in the wall of the bladder neck and proximal urethra. Consequently it is even less likely that active smooth muscle contraction can be considered as an important factor in the continence of urine. However, the bladder neck and proximal urethra possess within their walls innumerable elastic fibres which are of particular importance in producing passive occlusion of the urethral lumen (Lapides 1958). Indeed, it has been suggested that the passive elastic resistance offered by the urethral wall is the most important single factor responsible for the closure of the bladder neck and proximal urethra in the continent woman. The anatomy and physiology of urinary continence in women was reviewed in 1990 by DeLancey. In both sexes the urethra contains within its walls the external urethral sphincter, the location of which corresponds anatomically to the zone where maximal urethral closure pressures are normally recorded. This striated muscle sphincter is morphologically adapted to maintain tone over relatively long periods without fatigue and plays an important active role in producing urethral occlusion at rest. It remains to be determined, however, whether the force exerted by the sphincter is maximal at all times between two consecutive acts of micturition or whether additional motor units are recruited during coughing, sneezing, etc. to enhance the occlusive force on the urethra during these events. The external sphincter is innervated by nerve fibres which travel via several routes, not exclusively via the pudendal nerves. The clinical relevance of this arrangement is that pudendal blockade or neurectomy performed in order to reduce urethral resistance will not achieve the desired effect since much of the motor innervation of the striated sphincter remains intact after these procedures. Concerning the role of periurethral muscle in the maintenance of continence, the medial parts of the levator ani muscles in both sexes are related to (but structurally separate from) the urethral wall. These periurethral fibres are innervated by the pudendal nerve and consist of an admixture of large diameter fast and slow twitch fibres. Therefore, unlike the external sphincter, periurethral muscle possesses morphological features which are similar to other 'typical' voluntary muscles. This pelvic floor musculature plays an important part (especially in the female) by providing an additional occlusive force on the urethral wall, particularly during events which are associated with an increase in intra-abdominal pressure. In addition, the muscles provide support for the pelvic viscera. Micturition To enable fluid to flow along the urethra it is necessary for the pressure in the urinary bladder to exceed that within the urethral lumen. Under normal circumstances, in order to initiate micturition, a fall in urethral resistance immediately precedes a rise in pressure within the lumen of the bladder. The fall in urethral resistance may be due, at least in part, to the action of what has been termed the 'musculus dilator urethrae' (Dorschner et al 1994b), a ventral longitudinal muscle system located in the neck of the bladder and in the wall of the urethra. The fasciculi of the upper part of this muscle cross in stages the ventral circumference of the trigonal sphincter muscle, whereas the lower part is closely related to the mucous membrane lining the urethra. Histomorphological evidence suggests that the muscle should be able to oppose the action of the musculus sphincter trigonalis (p. 1840) and the musculus sphincter urethrae (Dorschner & Stolzenburg 1994), causing urethral dilatation. The pressure rise is usually produced by active contraction of detrusor smooth muscle at the onset of micturition. The detrusor muscle coat consists of numerous interlacing bundles forming a complex meshwork of smooth muscle which, on contraction, reduces all dimensions of the bladder. The muscle coat is collectively involved and it is unnecessary to attach special significance to the precise orientation of individual bundles within the wall of the viscus. The preganglionic nerve supply travels in the pelvic splanchnic nerves before synapsing on neurons located within the vesical part of the pelvic plexuses and within the wall of the bladder. These peripheral neurons supply nerve fibres which ramify throughout the thickness of the detrusor smooth muscle coat. The profuse distribution of these motor nerves emphasizes the importance of the autonomic nervous system in initiating and sustaining bladder contracting during micturition. For micturition to occur the pressure differential between the bladder and urethra must overcome the elastic resistance of the bladder neck. Immediately prior to the onset of micturition, the tonus of the external sphincter is reduced by central inhibition of its motor neurons located in the second, third and fourth sacral spinal segments. This inhibition is mediated by descending spinal pathways originating in higher centres of the central nervous system. Concomitantly, other descending pathways activate (either directly or via sacral interneurons) the preganglionic parasympathetic motor outflow to the urinary bladder. This central integration of the nervous control of the bladder and urethra is essential for normal micturition. Clinical Anatomy After urethral rupture, the extravasation of urine may complicate micturition; urine usually extends between the perineal membrane and the membranous layer of the superficial fascia. As both of these are attached firmly to the ischiopubic rami, extravasated fluid cannot pass posteriorly because the two layers are continuous around the superficial transverse perineal muscles. Laterally, the spread of urine is blocked by the pubic and ischial rami; it cannot enter the lesser pelvis through the perineal membrane and, if this remains intact, fluid can make its way only anteriorly into the scrotal and penile loose connective tissue and thence to the anterior abdominal wall. When the lesser pelvis is crushed the urethra may be ruptured between the prostatic and membranous parts; extravasation of urine then occurs into the pelvic extraperitoneal tissue. The lower urinary tract is subject to many congenital anomalies, some amenable to surgical correction.
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