Urethral sphincters

Origin and insertion

The urethral sphincter is made up of a number of muscular components which act together to control micturition and maintain urinary continence.

The internal urethral sphincter is a poorly developed circular bundle of smooth muscle fibers situated around the proximal end of the urethra, at the neck of the bladder. These muscle fibers are continuous with the detrusor vesicae muscle of the bladder.

The external urethral sphincter is located distal to the internal urethral sphincter, in the deep perineal pouch. Its skeletal muscle fibers arise from the ischial rami, inferior pubic rami and adjacent fascia, extending to surround the middle-lower, membranous, part of the urethra. The male and female external urethral sphincters have some differences and intricacies in their anatomy. The male external urethral sphincter, sometimes called the rhabdosphincter muscle, is situated just inferior to the prostate. Its circular muscle fibers surround the intermediate, or membranous, part of the male urethra forming an annular sphincter.

The female external urethral sphincter, also termed the urogenital sphincter, is more complex. It is formed from 3 parts. The innermost part, the female external urethral sphincter proper,  consists of circular muscle fibers located around the proximal two thirds of the female urethra. This part is thicker anterior than posterior, yet forms a true annular sphincter around the urethra. The second part, the compressor urethrae muscle, is a bilateral extension from the innermost part. The fibers arise from the ischiopubic rami on each side and pass anteriorly to the urethra to join with its counterpart. In doing so the extensions form a loop-like band around the urethra, superior to the perineal membrane. The third part is the urethrovaginal sphincter. This is a broad flat band of fibers which pass in a posterior direction to surround both the urethra and vagina as one unit. It also extends to the bulb of vestibule.
Whilst often considered as independent sphincters, cadaveric research has shown the smooth and skeletal muscle components of the urethral sphincter complex to be continuous.
 

Relations

The internal urethral sphincter is located at the junction between the urinary bladder and the urethra. Here, supported alongside the bladder neck by the pubovesical ligaments, it surrounds the intramural (preprostatic) part of the urethra and comes in contact with the base of the prostate (males) or the endopelvic fascia (females). Let's recall that the endopelvic fascia is an umbrella term for the connective tissue that envelops the pelvic organs and attaches them to the lateral walls of the pelvis.

The external urethral sphincter is contained in the deep perineal space (pouch), which is situated between the pelvic diaphragm and the perineal membrane. It spans the urogenital hiatus of the pelvic diaphragm and fills the areas between the pudendal canals, through which the pudendal nerve and internal pudendal artery and veins pass. The external urethral sphincter is closely related to the levator ani, deep transverse perineal and rectourethral muscles. The puboperineal component of the levator ani muscle, forms an open circle around the external sphincter creating a hiatus at the ventral aspect. The external urethral sphincter is also closely associated with the perineal body, the central tendon of perineum, and may adhere to the body along its posterior aspect.

In males, the external sphincter is closely related to the prostate. It sits tightly against the prostate’s apex and is surrounded by a fibrous tissue which is a continuation of the prostate sheath. Semi-circular skeletal muscle fibers at the apex of the prostate blend into those of the external urethral sphincter. The bulbourethral glands are embedded within the sphincter’s muscle fibers.

In females, the striated muscle of the external urethral sphincter is closely related to the puborectalis component of the levator ani muscle, while inferiorly it blends with the smooth muscle of the urethra and vagina.

Innervation

The urethral sphincter complex receives both somatic and autonomic innervation. These supply its voluntary and involuntary components, respectively.

The smooth muscle fibers of the internal urethral sphincter receive both sympathetic and parasympathetic innervation. Sympathetic supply arises from the lower thoracic and upper lumbar (T11 - L2) segments of the spinal cord. These nerves maintain tonic contraction of the internal urethral sphincter, thereby preventing urine outflow from the bladder into the urethra. Conversely, the parasympathetic supply arises from sacral levels S2 - S4 (spinal micturition center). It acts to “inhibit” the internal sphincter muscle, thereby relaxing it and allowing urine to pass from the bladder into the urethra.

The skeletal muscle fibers of the external urethral sphincter receive somatic motor innervation from the perineal branches of the pudendal nerve (S2-4). These nerves stem from neurons found in an area of the spinal cord called Onuf’s nucleus, which in turn receives fibers from the pontine micturition center of the central nervous system. When stimulated, Onuf’s nucleus acts to increase the resting tone in the external urethral sphincter, the skeletal muscle is contracted and urination is stopped or slowed. Central nervous system inhibition of Onuf’s nucleus results in relaxation of the external urethral sphincter. This facilitates voiding.

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Blood supply

The internal urethral sphincter is supplied by the inferior vesical artery (males) or the vaginal artery (females).

The external urethral sphincter receives blood supply from the internal pudendal artery (females) or the artery of bulb of penis (bulbourethral artery), a branch of the internal pudendal artery (males).

All of these arteries stem from the anterior division of the internal iliac artery.

Function

The main action of the urethral sphincter complex is to compress the urethra. This is important as it provides control over urinary continence. The internal sphincter’s smooth muscle resting state is one of contraction or ‘closure’, in which urine is prevented from passing through the internal urethral orifice into the urethra. The external sphincter is also thought to contribute to resting closure, although it’s most important feature is augmenting closure to prevent urination or voluntary opening to allow voiding.

During the voiding phase of micturition, both the internal and external sphincters relax to allow urine to pass through the urethra. When the bladder is full, stretch receptors in its muscular wall stimulate a parasympathetic nervous system response. This results in contraction of the detrusor muscle of the bladder and relaxation of the smooth muscle of the internal urethral sphincter. Thus allowing outflow of urine into the urethra. This process is involuntary, however the flow of urine can be voluntarily stopped by contraction of the external urethral sphincter. At an appropriate time and place, the external urethral sphincter can then be voluntarily relaxed, allowing the passing of urine.

A secondary function of the urethral sphincter complex is to prevent retrograde ejaculation of semen into the bladder. In males, semen and urine share the urethra as a passageway. Sympathetic stimulation during orgasm in males simultaneously causes contraction of the internal urethral sphincter at the neck of the urinary bladder. This serves to prevent semen from traveling proximally along the urethra, through the internal urethral orifice and into the bladder. In women, the 3 part external urethral sphincter also acts to pull the urethra caudally and inferiorly as well as compress the urethra against the anterior wall of the vagina, aiding urinary continence.