The female EUS is more elaborate than the male EUS because it is made of striated muscle and is more properly referred to as a urogenital sphincter [ 4 ]. The female EUS includes 1 the true annular sphincter around the urethra urethral sphincter , 2 a part that passes anterior to the urethra and that attaches to the ischial rami compressor urethral muscle , and 3 a part that encircles both the urethra and the vagina urethrovaginal sphincter.
In females, the urethral sphincter M. It starts from the base of the bladder and is a continuation of the peripheral component of the compressor urethral muscle [ 26 ]. In this sphincter, the ventral side of the urethra is thickest and the dorsal side of the urethra is thin.
There is a dorsal septum into which the fibers attach [ 19 ]. This fibrous septum affects the manner in which muscle fibers constrict the urethra [ 19 ]. Compressor urethral muscles M. These muscle fibers begin as a small tendon attaching to the ischiopubic ramus in the lateral side [ 26 ]. This muscle expands to the anterior surface of the urethra and is a continuation of the corresponding fibers of the opposite side of the body [ 19 ].
It then forms a broad arcing muscle. The role of the compressor urethral muscles is to squeeze the urethra from its ventral part [ 19 ]. This muscle can affect pulling caudally and inferiorly the urethral meatus and assist the urethral elongation as a way of providing continence.
The urethrovaginal sphincter M. This muscle intermingles ventrally with the compressor urethral muscle [ 26 ]. These muscle fibers begin on the ventral side of the urethra to extend dorsally along the lateral wall of the urethra [ 19 ]. These fibers also extend to the vagina to the beginning of the vestibular bulb [ 19 ]. These fibers are continuous with the posterior vagina and correspond to the muscle of the opposite side.
The contraction of these fibers that encircle the vagina and the urethra leads to constriction of both the vagina and the urethra [ 19 ]. The innervation of the urethral sphincter is from both the somatic and the autonomic nervous systems.
Urination is prevented by the voluntary motor innervation of the EUS muscle. The striated sphincter is innervated by the pudendal nerve from the S2 to S4 nerve roots [ 28 , 29 ]. The neurons that innervate the urethral sphincter originate in the cord, in an area termed Onuf's nucleus [ 30 ]. The autonomic nervous system also controls the IUS muscle because it is made of smooth muscle fibers. The sympathetic innervation of the bladder begins at the lower thoracic and upper lumbar spinal cord segments T10 to L2 [ 31 , 32 ].
The preganglionic axons run to sympathetic neurons in the inferior mesenteric ganglion and the ganglia of the pelvic plexus, and the postganglionic fibers from these ganglia pass in the hypogastric and pelvic nerves to the bladder [ 31 ]. The sympathetic activity results in the closing of the IUS [ 18 ]. However, parasympathetic activity causes the bladder to contract and allows the internal sphincter to open [ 18 ]. When the bladder is full, the fully filled bladder increases parasympathetic tone and decreases sympathetic activity, allowing the internal sphincter muscle to relax and the bladder to contract [ 17 ].
Thus, the combination of functional innervations via the somatic pudendal nerve and autonomic innervation manages urination. The urethral sphincter can be used to control micturition. Voiding urine begins with voluntary relaxation of the external sphincter muscle of the bladder.
Parasympathetic impulses induce contractions of the bladder and relaxation of the internal sphincter [ 18 ]. Voluntary control of urination is possible only if neural innervation of the bladder and urethra is intact [ 31 ]. Occasionally, injury and malfunctioning to any of the supplying nerves causes involuntary emptying of the bladder [ 27 ].
This involuntary urination is called incontinence. On the other hand, in males, during ejaculation, the closure of the urethral sphincter prevents mixing between urine and semen and backward flow of semen into the bladder [ 33 ]. UI is the involuntary loss of urine. UI is most commonly caused by urethral sphincter incompetence [ 27 ]. Incontinence can be caused by a weak vagina [ 34 ], pipestem urethra, denervation in neurogenic patients, or estrogen deficiency [ 27 ].
UI has a great influence on quality of life and affects mostly the elderly. There are three main types of UI: urge incontinence, stress incontinence, and mixed incontinence. The notable feature of urge incontinence is frequent micturition accompanied by the urge to void. Stress incontinence is characterized by the loss of urine during coughing, sneezing, or physical activity. Mixed incontinence is a mixed form between urge incontinence and stress incontinence. Stress incontinence is a problem of the closing mechanism of the urinary tract outlet, and the urethral sphincter is most important to this closing mechanism.
In general, incontinence is common in women aged over 50 years, with a peak in incidence around menopause [ 37 ]. UI can negatively affect quality of life.
Therefore, it is very important to understand the main mechanism causing UI. The urethral sphincter and UI are closely related and many theories in that regard exist. However, more intensive research is necessary to understand this relationship and the functional and anatomical mechanism of age-induced UI associated with a problem of the urethral sphincter.
Normal urethral sphincter function depends on the complex interaction of the somatic and autonomic pathways. This bilateral innervation is essential for urethral sphincter functioning. Several neurological conditions can cause UI.
In particular, vaginal distension during childbirth causes direct muscular and supportive tissue damage and neurologic injury. Therefore, further research into the clinical and functional anatomy of the urethral sphincter and the promotion of continence recovery could be a crucial focus to benefit women's health. No potential conflict of interest relevant to this article was reported.
National Center for Biotechnology Information , U. Journal List Int Neurourol J v. Int Neurourol J. Published online Sep Find articles by Junyang Jung. Find articles by Hyo Kwang Ahn. Find articles by Youngbuhm Huh. Author information Article notes Copyright and License information Disclaimer.
Corresponding author. Corresponding author: Junyang Jung. Received Sep 14; Accepted Sep This article has been cited by other articles in PMC. Abstract Continence and micturition involve urethral closure. Keywords: Urethral sphincters, Pudendal nerve, Autonomic nervous system, Urinary incontinence, Urination. EUS The perineal membrane PM is a complex, three-dimensional structure and is an anatomical term for a thick, fibrous, and triangular membrane attached to the bony framework of the pubic arch [ 21 , 22 ].
External Sphincter Muscle of Male Urethra The muscle fibers inferior to the caudal prostate are circular and form the external sphincter of the membranous urethra [ 24 ]. UI UI is the involuntary loss of urine. Footnotes No potential conflict of interest relevant to this article was reported. References 1. Continence mechanism of the isolated female urethra: an anatomical study of the intrapelvic somatic nerves. J Urol. Internal urinary sphincter in maintenance of female continence. Imaging of the posterior pelvic floor.
Eur Radiol. Oelrich TM. The striated urogenital sphincter muscle in the female. Anat Rec. The urethral sphincter muscle in the male. Am J Anat. Urinary incontinence: sphincter functioning from a urological perspective. Hunskarr S. Plymouth: Health Publication; MRI morphology of the levator ani muscle, endopelvic fascia, and urethra in women with stress urinary incontinence.
Risk factors in childbirth causing damage to the pelvic floor innervation. Int J Colorectal Dis. Morales O, Romanus R. Urethrography in the male: the boundaries of the different urethral parts and detail studies of the urethral mucous membrane and its motility.
The length of the male urethra. Int Braz J Urol. Zacharin RF. The suspensory mechanism of the female urethra. J Anat. Phillips C, Monga A. Childbirth and the pelvic floor: "the gynaecological consequences".
Rev Gynaecol Pract. Aanestad O, Flink R. Urinary stress incontinence. A urodynamic and quantitative electromyographic study of the perineal muscles. Acta Obstet Gynecol Scand. A critical analysis of the current knowledge of surgical anatomy related to optimization of cancer control and preservation of continence and erection in candidates for radical prostatectomy.
Eur Urol. The bladder expands as urine flows in from the ureters, but there is a limit to the volume it can contain. At about ml of urine, the detrusor muscle begins to contract and the internal urethral sphincter muscle begins to relax. If this urge is ignored, continence may be threatened. At about ml, detrusor muscle contractions begin to force open the internal urethral sphincter. Unless the external urethral sphincter is powerful enough to prevent it, micturition urination will occur involuntarily.
Smooth muscle stretch initiates the micturition reflex by activating stretch receptors in the bladder wall. This autonomic reflex causes the detrusor muscle to contract and the internal urethral sphincter muscle to relax, allowing urine to flow into the urethra. The stretch receptors also send a message to the thalamus and the cerebral cortex, giving voluntary control over the external urethral sphincter. We usually gain this control of urination between the ages of 2 and 3, as our brains develop.
Download Urinary System Lab Manual. See more from our free eBook library. A description of the urinary bladder from the edition of Gray's Anatomy of the Human Body. Using bladder ultrasound to detect urinary retention in patients from Nursing Times. Urinary System Structures. Give It Up for the Kidneys. The Three Steps of Urine Formation. Common Diseases and Disorders. When you select "Subscribe" you will start receiving our email newsletter.
Use the links at the bottom of any email to manage the type of emails you receive or to unsubscribe. See our privacy policy for additional details. Learn Site. The Two Urethral Sphincters Maintain Urinary Continence The internal urethral sphincter and the external urethral sphincter both provide muscle control for the flow of urine.
Muscles of Micturition: The Detrusor and Urethral Sphincters Micturition, or urination, is the act of emptying the bladder. Get our awesome anatomy emails!
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