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The urethral sphincters are two muscles that control the exit of urine from the bladder through the urethra. The two muscles are the external urethral sphincter and the internal urethral sphincter. When these muscles contract, the urethra is sealed shut, preventing the flow of urine. The internal urethral sphincter is composed of smooth muscle and regulates involuntary control of urinary flow, while the external urethral sphincter provides voluntary regulation. The external sphincter is made of skeletal muscle and is, therefore, under the control of the somatic nervous system. Any damage to these muscles can lead to urinary incontinence.
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What You'll Learn
The external urethral sphincter
In contrast, the internal urethral sphincter is made of smooth muscle and is, thus, under involuntary or autonomic control. It is located at the bladder's inferior end and the urethra's proximal end at the junction of the urethra with the urinary bladder. The internal urethral sphincter is a continuation of the detrusor muscle, which lines the bladder wall and allows the bladder to contract to excrete urine or relax to hold it in.
Any damage to either the external or internal urethral sphincters can lead to urinary incontinence, the involuntary loss of bladder control. In women, childbirth, obesity, and age can be risk factors for urinary incontinence, while in men, prostate surgery and radiation therapy can be contributing factors. Pelvic floor exercises, such as Kegels, can help strengthen the pelvic floor muscles and improve bladder control.
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The internal urethral sphincter
The urethral sphincter is a muscular structure that regulates the outflow of urine from the bladder through the urethra. There are two urethral sphincters: the external urethral sphincter and the internal urethral sphincter. The internal urethral sphincter is located at the bladder's inferior end and the urethra's proximal end at the junction of the urethra with the urinary bladder.
In males, the internal urethral sphincter has an additional function of preventing the flow of semen into the bladder during ejaculation. The internal urethral sphincter in males is located inferior to the prostate at the level of the membranous urethra.
Weak pelvic floor muscles, intrinsic sphincter damage, or damage to the surrounding nerves and tissue can cause the urethral sphincter to become incompetent, leading to stress urinary incontinence. Pelvic floor exercises, or Kegels, can be used to strengthen the pelvic floor muscles and treat incontinence issues.
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Pelvic floor muscles
The pelvic floor muscles are a vital part of the human body, supporting essential bodily functions such as urination, defecation, and sexual intercourse. These muscles can weaken over time due to various factors, including injury, childbirth, obesity, and the natural ageing process. Weak pelvic floor muscles can lead to conditions such as incontinence or pelvic organ prolapse.
The pelvic floor consists of layers of muscles and connective tissues that work together to support important organs in the pelvis, including the bladder, bowel (large intestine), and internal reproductive organs. By contracting and relaxing, the pelvic floor muscles help regulate the release of urine from the bladder through the urethra. This process is known as urethral sphincter function, and it is critical for maintaining urinary control.
The urethral sphincter is composed of two sets of muscles: the external urethral sphincter and the internal urethral sphincter. When these muscles contract, they seal the urethra shut, preventing the flow of urine. The external urethral sphincter is under voluntary control, allowing individuals to consciously stop the flow of urine. In contrast, the internal urethral sphincter is involuntary and regulates the flow of urine from the bladder to the urethra without conscious effort.
Pelvic floor exercises, also known as Kegels, are commonly recommended to strengthen the pelvic floor muscles. These exercises involve routinely squeezing and relaxing the pelvic muscles to improve muscle control and flexibility. Biofeedback techniques may also be used during Kegel exercises to help individuals identify and isolate the correct muscles for contraction.
In summary, the pelvic floor muscles play a crucial role in maintaining urinary control and supporting essential bodily functions. Weakness or damage to these muscles can lead to urinary incontinence, emphasising the importance of maintaining their strength and flexibility through exercises such as Kegels.
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Detrusor muscle
The detrusor muscle is a smooth muscle found in the wall of the bladder. It is made up of three layers of muscles: two longitudinal sheets of muscle surrounding a circular middle sheet. The detrusor muscle is innervated by the autonomic nervous system.
The primary function of the detrusor muscle is to contract during urination to push urine out of the bladder and into the urethra. When the bladder is empty, the muscle relaxes to allow the storage of urine. The detrusor muscle is continuous with the internal urethral sphincter, which is also composed of smooth muscle. The ureter passes obliquely through the detrusor muscle to prevent the reflux of urine into the kidney as the bladder fills.
New research in detrusor physiology has added to our understanding of how this muscle contracts in a coordinated fashion. During phasic activity, small regions of the detrusor muscle undergo localized “stretches” that are associated with phasic increases in bladder pressure. During global contractions, most of the bladder smooth muscle is involved.
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Intrinsic sphincter damage
Intrinsic sphincter deficiency (ISD) occurs when there is weakness in the urethral sphincter, causing it to be unable to generate enough resistance to retain urine in the bladder. This can lead to stress urinary incontinence, where the urethral sphincter is unable to close fully, resulting in urine leakage. ISD is commonly associated with multiple incontinence surgical procedures, as well as with hypoestrogenism, aging, or both. In women, childbirth, obesity, and age can also be risk factors for ISD, as they can weaken the pelvic floor muscles that support the urethral sphincter.
The diagnosis and treatment of ISD can be challenging due to the evolving and varied definitions of the condition. Originally, ISD was used to describe a severe form of stress urinary incontinence, but the term has since become more vague and broad in its usage. There is currently no consensus on the treatment of ISD, and various procedures may be considered. However, the recognition of ISD is crucial in evaluating and managing stress urinary incontinence.
To diagnose ISD, a combination of history, clinical examination, and documentation of severe stress urinary incontinence, a fixed urethra, and a low Valsalva leak point pressure is essential. Treatment options for stress urinary incontinence due to ISD may include lifestyle modifications, pelvic floor muscle exercises, and surgical procedures such as tension-free vaginal tape (TVT) or transobturator tape (TOT). However, the effectiveness of these treatments can vary, and there is a risk of failure, especially as the urethral sphincter's ability to generate closing pressure declines.
In summary, intrinsic sphincter damage or deficiency is a condition that affects the urethral sphincter's ability to regulate urine outflow. ISD can lead to stress urinary incontinence due to weakness in the urethral sphincter and insufficient urethral resistance. The diagnosis and treatment of ISD can be complex due to evolving definitions and varying procedures. However, recognizing ISD is crucial for managing stress urinary incontinence effectively.
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Frequently asked questions
The urethral sphincter is a muscular structure that regulates the outflow of urine from the bladder into the urethra.
Yes, there are two urethral sphincters: the external urethral sphincter and the internal urethral sphincter. Both of these muscles work together to control the exit of urine.
Damage to either the external or internal urethral sphincter can lead to urinary incontinence, which is the loss of bladder control. This can range from releasing a small amount of urine when laughing or sneezing to a sudden and strong urge to urinate.
Yes, the detrusor muscle is located within the walls of the bladder and works in conjunction with the urethral sphincters. During urination, the detrusor muscle contracts to push urine out, while the urethral sphincters relax to allow urine to flow out of the body.
Maintaining healthy pelvic floor muscles is important for urethral sphincter function. These muscles can be strengthened through exercises such as Kegels, which involve routinely squeezing and relaxing the pelvic floor muscles.
