Dominic Stone
The control of urination involves the coordination of the nervous system, the bladder muscle, sphincters, and the pelvic floor. The kidneys produce urine, which flows to the bladder, where it is stored. The bladder is encircled by a muscle called the urinary sphincter, which contracts to close off the urethra and prevent urine from leaving the body. When the bladder is full, messages are sent to the brain via the spinal cord, signalling the urge to urinate. A person can then voluntarily decide to relax the sphincter muscle, allowing urine to flow out. There are two urethral sphincters, the external and internal, which contract to stop or slow urination. The bladder wall is made up of a muscle called the detrusor muscle, which contracts to excrete urine or relaxes to hold it in. Disorders such as urinary incontinence can occur when any component of the urinary system loses normal function.
| Characteristics | Values |
|---|---|
| Muscles that control urination | Bladder sphincter, detrusor muscle, urethral sphincter, muscles in the abdominal wall and floor of the pelvis |
| Function | When contracted, the urethral sphincter narrows the urethra, stopping or slowing urination. When relaxed, it opens up, allowing urine to flow out of the bladder. |
| Disorders | Stress incontinence, urge incontinence, neurogenic bladder dysfunction, urinary retention, overflow incontinence |
| Treatment | Lifestyle changes, medication, surgery, pelvic floor exercises, support devices, pharmacotherapy |
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What You'll Learn
The role of the detrusor muscle
The detrusor muscle, also known as the detrusor urinae muscle, muscularis propria of the urinary bladder, or simply muscularis propria, is a smooth muscle found in the wall of the bladder. It is composed of an interwoven network of multi-directional smooth muscle fibres that form three layers: two longitudinal sheets surrounding a circular middle sheet. The detrusor muscle is innervated by the autonomic nervous system.
During urination, the detrusor muscle contracts to release urine from the bladder. At other times, the muscle remains relaxed to allow the bladder to store urine. The detrusor muscle works in conjunction with the urethral sphincter muscles, which envelop the urethra and control the flow of urine through contraction. When the detrusor muscle contracts, the urethral sphincter muscles relax, opening up and allowing urine to flow out of the bladder through the urethra.
The detrusor muscle fibres arise from the posterior surface of the body of the pubis in both sexes, with additional fibres originating from the prostate in males or the front of the vagina in females. These fibres pass longitudinally up the inferior surface of the bladder, over its apex, and then descend along its fundus to attach to the prostate or vagina. At the sides of the bladder, the fibres intersect obliquely.
The detrusor muscle is responsible for the mechanical power generated during bladder contraction, known as the watts factor (WF). However, issues with the detrusor muscle can cause difficulties in voiding the bladder, leading to uncomfortable urinary retention, particularly in older adults over 60 years of age. In such cases, pharmacological agonists targeting the β3 adrenergic receptors in the bladder may be used to treat overactive bladder symptoms.
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The external urethral sphincter
In both males and females, the internal and external urethral sphincters work together to prevent the release of urine. Specifically, the internal urethral sphincter controls involuntary urine flow from the bladder to the urethra, while the external urethral sphincter controls voluntary urine flow from the bladder to the urethra. When the bladder is full, messages travel from the bladder to the spinal cord and then to the brain, making the person aware of the urge to urinate. A person who has control of urination can then consciously and voluntarily decide whether to release the urine from the bladder or to hold it for a while. When the decision is made to urinate, the external urethral sphincter relaxes, allowing urine to flow out through the urethra, and the bladder wall muscles contract to push the urine out.
The female external urethral sphincter is more complex than the male external urethral sphincter. 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. The female external sphincter is made up of three parts: the sphincter urethrae, the urethrovaginal muscle, and the compressor urethrae. The urethrovaginal muscle fibres wrap around the vagina and urethra, and contraction leads to the constriction of both. The compressor urethrae muscle wraps around the urethra so that when it contracts, it squeezes the urethra against the vagina.
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The internal urethral sphincter
The modern view of the urethral sphincters considers them functionally independent but forming a single continuous layer from the bladder neck to the perineal membrane, constituting a single urethral sphincter complex. The internal sphincter, also called the lissosphincter, arises from the absorption of the Wolffian duct into the bladder and urethra. It is supplied with blood by the inferior vesical artery and drained by the internal iliac vein.
In males, the internal urethral sphincter is an anatomical structure composed of the bladder neck and prostate. In females, the internal sphincter is more of a functional concept than a distinct anatomical entity, with the bladder neck and proximal urethra constituting the internal sphincter. The female external sphincter, or rhabdosphincter, has a more prominent effect on the female urethra.
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Pelvic floor muscles
The pelvic floor muscles play a crucial role in controlling urination. These muscles, along with the nervous system, bladder muscle, and sphincters, work in harmony to ensure the proper functioning of the urinary system. When any component of this system malfunctions, urinary control can be affected, leading to issues such as urinary incontinence.
The pelvic floor muscles are a vital part of the pelvic floor, which provides support for the bladder and other pelvic organs. These muscles can be contracted voluntarily to increase pressure on the bladder and help control urination. In addition, they work in conjunction with the bladder muscle (detrusor muscle) and the urethral sphincters to initiate and regulate urine flow.
The detrusor muscle, located within the walls of the bladder, is composed of smooth muscle fibres that allow the bladder to contract and relax. When the bladder fills with urine, the stretch receptors within the bladder become stimulated, leading to the contraction of the detrusor muscle, signalling the need to urinate. At the same time, the pelvic floor muscles contract to support the bladder and prevent urine leakage.
In the case of urination, the pelvic floor muscles relax, reducing pressure on the bladder. Simultaneously, the detrusor muscle contracts, and the urethral sphincters relax, allowing urine to flow out through the urethra. This coordinated action between the pelvic floor muscles, detrusor muscle, and urethral sphincters ensures controlled urination.
Pelvic floor exercises, such as Kegel exercises, are often recommended to strengthen the pelvic floor muscles and improve urinary control. These exercises are particularly effective in managing stress incontinence, which is the most common type of urinary leakage, affecting up to 88% of patients with incontinence. By strengthening the pelvic floor muscles, individuals can regain better control over their urination and reduce the impact of conditions like incontinence on their daily lives.
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Urinary incontinence
The urinary system comprises several organs that work together to filter, store, and remove waste from the body in the form of urine. When the system functions as expected, individuals can typically reach a bathroom in time to urinate without experiencing any leakage. However, urinary incontinence occurs when these organs malfunction, resulting in a loss of bladder control.
There are various types of urinary incontinence, ranging from mild leakage during certain activities like coughing or laughing, to a complete loss of bladder control. The specific type of incontinence can be diagnosed through a combination of tests, including pelvic exams, digital rectal exams, urine tests, bladder ultrasounds, stress tests, and cystoscopies. These tests help identify any underlying health issues, assess pelvic muscle strength, and determine the patient's ability to empty the bladder completely.
The treatment options for urinary incontinence depend on the specific type and cause of the condition. Medications can help reduce leakage by stabilizing muscle contractions or relaxing muscles to empty the bladder fully. For menopausal women, local hormone treatments, such as vaginal estrogen cream, can be effective in restoring bladder function. In other cases, lifestyle changes or surgery may be recommended.
It is important to consult a healthcare provider for a proper diagnosis and treatment plan. They can help identify the specific type of incontinence and recommend the most suitable treatment option based on the patient's preferences and medical history.
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Frequently asked questions
Urinary incontinence is the loss of urine control, or the inability to hold your urine until you reach a bathroom.
Urinary incontinence can be caused by stress or damage to the sphincters or surrounding muscles. It can also be caused by trauma or disease of the central nervous system, leading to a loss of control of the bladder sphincter muscles. Childbirth and pelvic surgeries are also common causes of urinary incontinence.
The detrusor muscle, located within the walls of the bladder, contracts to excrete urine or relax to hold it in. The urethral sphincter, composed of the internal and external sphincter, also plays a crucial role in regulating the outflow of urine from the bladder.
When the bladder is full, messages are sent along nerves from the bladder to the spinal cord and then to the brain, signalling the urge to urinate. When the decision to urinate is made, the sphincter muscle relaxes, and the bladder wall muscles contract to facilitate urine flow through the urethra.
Yes, treatment options are available for urinary incontinence. These include lifestyle changes, medication, and surgery. Pelvic floor exercises, such as Kegel exercises, are often recommended to control leakage.
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