Fig. 2.1
Urinary sphincters in the male and female. BN bladder neck, BU bulbar urethra, EUS external (rhabdo) sphincter, P prostate, PPL puboprostatic ligament, PUL pubourethral ligament, SV seminal vesicle (Reproduced with permission of the authors, Turner-Warwick and Chapple, Functional Reconstruction of the Urinary Tract and Gynaeco-urology)
The bladder neck sphincter (or internal sphincter) is located at the bladder neck and is made up of a concentric ring of smooth muscle. This sphincter is not under voluntary control and remains closed during bladder filling. It is important to note that this is generally less well developed in females compared to males. The bladder neck sphincter is supplied by alpha-adrenergic nerves from the thoracolumbar sympathetic outflow of the spinal cord (T10–L2 spinal segments). In males, the bladder neck sphincter also plays an important reproductive role by remaining closed during ejaculation (to prevent retrograde ejaculation).
The rhabdosphincter (or external sphincter) plays a vital role in urinary continence in both men and women. It is composed of striated muscle fibers and is under voluntary control. In males, it is located between the apex of the prostate and the membranous urethra. In females, the external sphincter is thickest in the middle of the urethra while being horseshoe-shaped both proximally and distally. The muscle fibers are intermixed with connective tissue located on the anterior wall of the vagina. The rhabdosphincter is innervated by nerves originating from the Onuf’s nucleus in the S2–S4 spinal segments (mainly via the pudendal nerve) (Chancellor et al. 1994).
Pelvic Floor and Supporting Structures
In addition to the structures of the lower urinary tract, the pelvic floor musculature and surrounding structures also contribute to urinary continence. The pelvic viscera, levator ani muscles, pelvic ligaments, as well as overlying peritoneum all act in concert to assist the sphincters in maintaining continence. This is especially important in the female where these structures work in conjunction to support and maintain the position of the pelvic organs, bladder neck, and urethra (Fig. 2.2). Weakness of pelvic floor support can lead to pelvic organ prolapse (Fig. 2.3, also see Chap. 4).
Fig. 2.2
Three-dimensional ultrasound with coronal reconstruction of pelvic floor in patient with stress incontinence cured by a midurethral synthetic sling. U urethra, V vagina, R rectum, PF pelvic floor muscle. Note the presence of the sling supporting the urethra
Fig. 2.3
Pelvic organ prolapse (POP). Weakness of the pelvic floor and supporting structures can result in POP (diagram on right) such as cystocele (orange arrow) and rectocele (blue arrow)
The pubourethral ligaments are the fascial support of the midurethra to the inner surface of the inferior pubis (Fig. 2.1). This effectively stabilizes the urethra and anterior vaginal wall (Brubaker et al. 1996). Pubourethral ligament laxity is often attributed as a cause of stress incontinence, whereby there is rotational descent of the urethra and hence distraction with opening of the bladder neck/proximal urethra during times of stress such as coughing (Fig. 2.4).
Fig. 2.4
Urethral hypermobility – transperineal ultrasound image of bladder (B) and urethra (U) at rest (left image) and with straining (right image) demonstrating rotational descent of the urethra with opening of the bladder neck/proximal urethra (arrow) leading to stress incontinence. BN bladder neck, INF inferior/caudal, SUP superior/cranial
The Micturition Cycle and Innervation
Micturition is controlled by neural circuits in the brain and spinal cord that coordinate the activity of the urinary bladder and urethral sphincters. The circuits act as “on/off” switches between two modes – storage and elimination. The control of this is complex and involves the central nervous system and both autonomic and somatic divisions of the peripheral nervous system. At each level of neural control, different pathways and neurotransmitters are involved (Fig. 2.5).
Fig. 2.5
Innervation of the lower urinary tract (see text for explanation)
Innervation of Lower Urinary Tract
When discussing innervation, it is important to distinguish between the types of nerve fibers and the different roles that they play. The peripheral nervous system is divided into the autonomic system which controls visceral function via smooth muscle and the somatic system which allows for voluntary control via striated muscle. The autonomic system is further divided into its sympathetic and parasympathetic pathways.
Afferent (Sensory) Pathways
Sensation of bladder filling and fullness arises from stretch receptors in the detrusor muscle as well as the urothelium and is conveyed via the parasympathetic afferent nerves to the sacral spinal cord. There, they enter ascending spinal pathways to the brain stem (the pontine micturition center) and cerebral cortex.
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