The body’s ability to control the release of urine, known as urinary continence, relies on a highly coordinated system centered on the urethral sphincters. These specialized muscles function like a pair of valves, ensuring urine remains stored in the bladder until an appropriate time for voiding. They must maintain a constant seal against the pressure of a filling bladder, a task that requires continuous, yet largely unconscious, muscular effort. Understanding how these two distinct muscles work together is fundamental to recognizing the mechanisms that support normal bladder function and what happens when that control is lost.
Anatomy and Role in the Urinary System
The urethral sphincters are two muscular structures positioned at the base of the urinary bladder and along the proximal section of the urethra, the tube through which urine exits the body. The internal urethral sphincter is situated where the bladder meets the urethra and is composed of smooth muscle tissue. Because it is made of smooth muscle, its function is entirely involuntary, meaning it operates without conscious thought, controlled by the autonomic nervous system.
The external urethral sphincter is located slightly further down the urethra, surrounding the membranous portion. Unlike its involuntary counterpart, this sphincter is made of skeletal muscle, placing it under the direct, conscious control of the somatic nervous system. This muscle also forms part of the larger pelvic floor muscle group, which provides structural support to the pelvic organs. Together, these two sphincters act as a dual-layered barrier to ensure the urethra remains sealed, with the internal sphincter serving as the primary muscle for prohibiting the unintentional release of urine.
The Two Mechanisms of Control
Control over urination is governed by two distinct phases: the storage phase and the voiding phase. During the storage phase, the internal urethral sphincter maintains a constant contraction, which is regulated by the sympathetic nervous system. This involuntary closure prevents urine from entering the urethra while the bladder fills, acting as a high-pressure zone at the bladder neck. This sympathetic signaling also causes the bladder wall’s detrusor muscle to relax, allowing it to expand and accommodate up to 300 to 400 milliliters of urine in adults without significant pressure increase.
As the bladder stretches, sensory nerves send signals to the spinal cord and up to the brain, which consciously perceives the urge to urinate, usually when the bladder holds 150 to 250 milliliters. At this point, the external urethral sphincter, controlled by the somatic nervous system via the pudendal nerve, provides a layer of voluntary control. This allows an individual to consciously contract the skeletal muscle to delay urination, effectively overriding the involuntary signals and guarding against leaks during sudden increases in abdominal pressure, such as coughing or sneezing.
The voiding phase, or micturition, involves a coordinated shift in nervous system activity. The brain releases its inhibitory effect on the pontine micturition center, which initiates the reflex. The parasympathetic nervous system becomes dominant, causing the detrusor muscle in the bladder wall to contract, while simultaneously signaling the involuntary internal sphincter to relax. The voluntary external sphincter then relaxes, allowing urine to flow out through the urethra; this coordination is necessary for complete and unobstructed voiding.
Causes of Sphincter Dysfunction
When the coordinated control of the urethral sphincters fails, it can lead to urinary incontinence, a condition often resulting from damage to the muscles or the nerves that control them. Intrinsic sphincter deficiency occurs when the urethral sphincter loses its ability to seal effectively due to muscle or nerve damage. This deficiency is a common cause of stress incontinence, characterized by urine leakage during activities that suddenly increase abdominal pressure, such as laughing or lifting.
Physical trauma is a frequent contributor to sphincter weakness, particularly in women who have experienced vaginal childbirth, which can damage the pelvic floor muscles and the nerves supplying the external sphincter. In men, prostate surgery, such as a prostatectomy, is a leading cause of stress incontinence because the procedure can directly affect the surrounding sphincter tissue.
Neurological conditions that disrupt the communication pathways between the brain, spinal cord, and sphincters can also cause dysfunction. Diseases like multiple sclerosis, Parkinson’s disease, or spinal cord injuries can impair the nerve signals required for either the internal or external sphincter to function correctly. Age-related muscle weakening, chronic conditions that increase abdominal pressure like obesity, and a lack of estrogen after menopause can all contribute to the progressive loss of sphincter integrity and function.
Strengthening and Support Options
Non-surgical interventions can help maintain or improve urethral sphincter function, particularly targeting the voluntary external sphincter. Pelvic floor muscle exercises, often referred to as Kegels, are a primary method for strengthening the muscles that support the urethra. These exercises involve consciously squeezing and holding the muscles used to stop the flow of urine or prevent passing gas, which directly targets the skeletal muscle of the external sphincter.
Regularly performing these contractions helps to increase the strength and endurance of the pelvic floor, which can improve continence, especially for individuals with mild to moderate stress incontinence. Bladder training is another behavioral technique that supports sphincter control by helping the bladder adapt to holding more urine. This involves gradually increasing the time between scheduled toilet visits, which trains the detrusor muscle and the nervous system to suppress the urge to void.
Lifestyle adjustments, such as managing body weight, can reduce chronic pressure on the pelvic floor and sphincters. Modifying fluid intake, particularly avoiding excessive consumption of bladder irritants like caffeine, can also help in reducing the frequency and urgency of urination. For those who struggle to correctly isolate the muscles, techniques like biofeedback can be used with a healthcare professional to visually or audibly confirm that the correct pelvic floor muscles are being contracted.