The external urinary sphincter is a circular muscle that controls the voluntary release of urine from the bladder, playing a direct part in maintaining urinary continence. This muscle remains in a constant state of contraction to prevent leakage and relaxes only during the intentional act of urination. Its function is distinct from other urinary system muscles, as it is under conscious control.
Anatomy and Location of the External Urinary Sphincter
The external urinary sphincter (EUS) is composed of striated, or skeletal, muscle tissue, which allows for voluntary command over its action. This muscle encircles the urethra, the tube through which urine exits the body. Its precise location differs between the sexes, a distinction with significant clinical implications.
In males, the EUS is situated just inferior to the prostate gland, where it surrounds the membranous part of the urethra. It is a robust, well-defined muscular structure.
In females, the anatomy is more complex; the EUS is located along the middle portion of the urethra, at its distal end. The female sphincter is composed of several parts, including the urethrovaginal sphincter, which encircles both the urethra and the vagina. This structural difference makes the female EUS more susceptible to certain types of injury, particularly during childbirth.
Function and Voluntary Control
The primary function of the external urinary sphincter is to maintain urinary continence by remaining tonically contracted, which seals the urethra shut and prevents urine from leaking out. When a person decides to urinate, a process known as micturition, the brain sends signals that cause the EUS to relax, opening the urethral passage and allowing urine to flow out.
This conscious control contrasts sharply with the function of the internal urinary sphincter, which is made of smooth muscle and operates involuntarily. The EUS works in a coordinated effort with the detrusor muscle, the main muscle of the bladder wall. For urine storage, the detrusor muscle remains relaxed while the EUS is contracted. During urination, this relationship inverts: the detrusor muscle contracts to push urine out, and the EUS relaxes to permit its passage.
The somatic nervous system, which governs voluntary actions, controls the EUS through the pudendal nerve. Signals traveling along this nerve instruct the sphincter to either constrict or relax. This neural pathway allows an individual to consciously regulate urination and delay voiding until an appropriate time.
Common Causes of Sphincter Dysfunction
Weakness or damage to the external urinary sphincter can lead to sphincter dysfunction, most commonly resulting in stress urinary incontinence. This condition involves the involuntary leakage of urine during activities that increase abdominal pressure, such as coughing, sneezing, laughing, or exercising.
In women, pregnancy and childbirth are among the most common causes of EUS dysfunction. The strain on the pelvic floor during vaginal delivery can stretch, weaken, or damage the sphincter muscle and the nerves that control it. For men, a frequent cause of sphincter damage is prostate surgery, such as a radical prostatectomy for cancer treatment. The close proximity of the prostate to the EUS means surgery carries a risk of injuring the sphincter.
Beyond these sex-specific causes, the natural aging process can contribute to a gradual weakening of all muscles, including the pelvic floor and the EUS. Neurological conditions, such as multiple sclerosis, Parkinson’s disease, or stroke, can also disrupt the nerve signals required for proper sphincter control. Spinal cord injuries can sever the connection between the brain and the pudendal nerve, leading to a loss of voluntary command.
Methods for Strengthening the Sphincter
A weakened external urinary sphincter can often be strengthened through Pelvic Floor Muscle Training, also known as Kegel exercises. These exercises isolate and strengthen the muscles of the pelvic floor, including the EUS, to enhance muscle tone and improve the sphincter’s ability to close the urethra effectively.
To perform these exercises correctly, one must first identify the right muscles. A common technique is to imagine trying to stop the flow of urine mid-stream or tightening the muscles used to prevent passing gas. Once identified, the exercise involves contracting these muscles, holding the contraction for a few seconds, and then fully relaxing them. Avoid tensing other muscles, such as the abdomen, thighs, or buttocks, and continue breathing normally throughout the exercise.
Consistency is important for the success of this training. Performing multiple sets of these contractions daily over several weeks or months can lead to significant improvements in muscle strength and a reduction in symptoms like stress urinary incontinence.