There are two main sphincters that control urination, or micturition, working together to ensure urine is stored and released in a coordinated manner. The dual sphincter system maintains urinary continence through both automatic reflexes and conscious, voluntary control. This muscular and neurological arrangement allows for the complex process of holding urine until a socially appropriate time.
The Two Sphincters: Internal and External
The internal and external urethral sphincters regulate the exit of urine from the bladder through the urethra. These two structures differ significantly in composition, location, and nervous control. The internal urethral sphincter is located at the bladder neck and is composed of involuntary smooth muscle, controlled automatically by the nervous system. The external urethral sphincter is positioned further down the urethra within the deep perineal pouch of the pelvic floor. Unlike the internal sphincter, the external sphincter is made of striated, or skeletal, muscle, placing it under the control of the somatic nervous system for voluntary movement.
The Involuntary Mechanism for Urine Storage
The primary goal during the storage phase is to keep the urethra sealed as the bladder fills with urine. Continence is maintained largely by the internal urethral sphincter, which is constantly held in a contracted state to prevent leakage. This closed state is actively maintained by the sympathetic nervous system. Sympathetic nerve signals cause the internal smooth muscle sphincter to contract, increasing resistance to urine outflow. Simultaneously, this system ensures the detrusor muscle, which makes up the bladder wall, remains relaxed so the bladder can expand. This coordinated action is often referred to as the “guarding reflex,” an automatic mechanism that prevents involuntary urination.
The Voluntary Mechanism for Urine Release
When the bladder reaches a certain capacity, stretch receptors send signals to the brain, creating the sensation of needing to urinate. The voluntary decision to void is achieved through the coordinated action of the external urethral sphincter and the detrusor muscle. The external sphincter, composed of skeletal muscle, is the final point of conscious control over the urinary stream. To initiate urination, the brain signals the external sphincter to relax. Simultaneously, the parasympathetic nervous system stimulates the detrusor muscle in the bladder wall to contract forcefully. This contraction increases pressure inside the bladder, pushing urine out past the relaxed internal sphincter and through the relaxed external sphincter.
When Sphincter Control Fails
A loss of proper function in either sphincter or their controlling nerves results in urinary incontinence, the involuntary leakage of urine. Weakening of the external urethral sphincter and pelvic floor muscles can lead to stress incontinence. This leakage occurs when sudden increases in abdominal pressure, such as during a cough or sneeze, overwhelm the sphincter’s ability to remain closed. Failure of the automatic storage mechanism, often due to an overactive detrusor muscle, is linked to urge incontinence. In this condition, the detrusor contracts involuntarily, creating a sudden, intense need to urinate. This forceful contraction can overwhelm the involuntary closing mechanism, causing leakage.