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The detrusor muscle is the primary muscle that contracts during urination to push urine out of the bladder and into the urethra. It is located within the walls of the bladder and is composed of smooth muscle fibres that are longitudinal and circular. The pelvic floor muscles also play a crucial role in controlling the bladder and supporting the internal organs. These muscles can be voluntarily contracted to prevent the release of urine.
| Characteristics | Values |
|---|---|
| Muscle that squeezes the bladder | Detrusor muscle |
| Location of detrusor muscle | Within the walls of the bladder |
| Composition of detrusor muscle | Smooth muscle fibres oriented in multiple directions |
| Control of detrusor muscle | Autonomic system |
| Function of detrusor muscle | Contracts during urination to push urine out of the bladder and relaxes to store urine |
| Other muscles involved in bladder control | Pelvic floor muscles, internal urethral sphincter, external urethral sphincter |
| Function of pelvic floor muscles | Support and control of bladder and bowel; stabilization of the core |
| Function of internal urethral sphincter | Controls involuntary urine flow from the bladder to the urethra |
| Function of external urethral sphincter | Controls voluntary urine flow from the bladder to the urethra |
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What You'll Learn
The detrusor muscle and its role in urination
The detrusor muscle is a smooth muscle that forms the walls of the bladder. It is responsible for the contraction of the bladder during urination, allowing urine to be pushed out of the bladder and into the urethra. During urination, the detrusor muscle contracts, while the internal urethral sphincter, which is a continuation of the detrusor muscle, relaxes to allow urine to pass through the urethra. This process is controlled by the autonomic nervous system, specifically the parasympathetic nervous system, which stimulates the detrusor muscle to contract.
The detrusor muscle is composed of smooth muscle fibres that are oriented in multiple directions and arranged in layers. The inner layer of the detrusor muscle is longitudinal, followed by a circular middle layer, and finally another longitudinal layer on the outside. These smooth muscle fibres provide the bladder with the ability to stretch and expand in response to the presence of urine. When the bladder is filling with urine, the detrusor muscle remains relaxed, allowing the bladder to expand and accommodate the increasing volume of urine.
The detrusor muscle plays a crucial role in urination by contracting at the appropriate time to initiate the release of urine. This contraction is triggered by the stimulation of M3 receptors located within the bladder. When the bladder fills with urine, these receptors become stretched and activated, leading to the contraction of the detrusor muscle. This contraction creates the urge to urinate.
Additionally, the detrusor muscle works in conjunction with the urethral sphincters to control the flow of urine. The internal urethral sphincter is a thickening of smooth muscle at the bladder neck, while the external urethral sphincter is a skeletal muscle controlled by the somatic nervous system. When the internal sphincter relaxes and the detrusor muscle contracts, urine is expelled from the bladder through the urethra. The external sphincter controls the voluntary release of urine, ensuring that it only occurs at the appropriate time.
Malfunctions of the detrusor muscle can lead to urinary issues such as urinary retention or incontinence. Overactivity of the detrusor muscle can result in urge incontinence, where urine is released even when the individual is not ready to urinate. On the other hand, if the detrusor muscle is underactive or overstretched, it may be difficult to expel urine, leading to urinary retention.
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Pelvic floor muscles and their impact on bladder control
The pelvic floor muscles are a group of muscles that stretch from the pubic bone to the tailbone (coccyx) at the back. They play a crucial role in supporting the bladder, bowel, and, in women, the uterus. These muscles help to control bladder function and maintain urinary continence.
In both men and women, the pelvic floor muscles wrap around the urethra, vagina (in women), and anus, helping to keep them shut. When the bladder is full, the detrusor muscle contracts, pushing urine out of the bladder and into the urethra. Simultaneously, the pelvic nerve fibres stimulate the internal urethral sphincter to relax, allowing urine to pass out of the body. The urethral sphincters, both internal and external, work together with the detrusor muscle to control the flow of urine.
Weak pelvic floor muscles can lead to a loss of control over the release of urine, a condition known as urinary incontinence. This can be caused by various factors, including childbirth, obesity, age, prostate surgery, and radiation therapy. Pelvic floor muscles can also become too tight, a condition called a hypertonic pelvic floor, which can cause difficulty in emptying the bladder and bowel.
To maintain bladder control, it is essential to keep the pelvic floor muscles strong through specific exercises. These exercises can improve an individual's ability to squeeze and relax the pelvic floor muscles, enhancing their control over bladder function. Over time, with consistent practice, individuals can increase their muscle strength and endurance.
In summary, the pelvic floor muscles play a vital role in supporting the bladder and maintaining urinary continence. Weak or damaged pelvic floor muscles can lead to urinary incontinence, while overly tight pelvic floor muscles can cause difficulties in emptying the bladder. Strengthening and training the pelvic floor muscles can help improve bladder control and overall pelvic health.
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Urethral sphincters and their function in men and women
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. Both men and women have these two sphincters, which work together to prevent the release of urine. When either of these muscles contracts, the urethra is sealed shut.
The external urethral sphincter is a voluntary muscle, controlled by the somatic nervous system. It controls the voluntary flow of urine from the bladder to the urethra. In men, it is situated just inferior to the prostate, surrounding the intermediate, or membranous, part of the urethra. In women, it is located in the deep perineal pouch, at the bladder's distal inferior end. The female external sphincter is more complex than the male one, being 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 wraps around the urethra, so when it contracts, it squeezes the urethra against the vagina.
The internal urethral sphincter is an involuntary muscle, under autonomic control. It controls the involuntary flow of urine from the bladder to the urethra. The internal sphincter is located at the bladder's inferior end and the urethra's proximal end, at the junction of the urethra with the bladder. It is a continuation of the detrusor muscle and is made of smooth muscle fibres.
In men, the internal urethral sphincter has the additional function of preventing the flow of semen into the bladder during ejaculation. Weak pelvic floor muscles, intrinsic sphincter damage, or damage to the surrounding nerves and tissue can make the urethral sphincter incompetent, leading to stress urinary incontinence. In women, childbirth, obesity, and age can all be risk factors, especially by weakening the pelvic floor muscles.
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How the bladder is a hollow, muscular organ
The urinary bladder is a hollow, muscular organ in the abdomen that stores urine. It is derived from the upper segment of the urogenital sinus in the fetus and is connected to the allantois, which eventually becomes the urachus. The bladder is a spherical-shaped organ that can hold 500-700 mL (about two cups) of urine. As the bladder fills with urine, it expands like a balloon, and when it reaches a volume of 200-350 mL, nerves in the bladder signal the brain that it's time to use the restroom.
The bladder wall is primarily composed of the detrusor muscle, which contracts to excrete urine and relaxes to store urine. This muscle is made up of smooth muscle fibres oriented in multiple directions, providing the bladder with the ability to stretch in response to the presence of urine. The detrusor muscle is under the control of the autonomic system, specifically the parasympathetic nervous system, which stimulates the bladder to contract during urination.
In addition to the detrusor muscle, the bladder neck is a narrow group of muscles that connect to the urethra. The urethral sphincters, consisting of the internal and external sphincter muscles, control the exit of urine from the bladder through the urethra. These muscles contract to seal the urethra shut and relax to allow urine to pass through during urination.
The pelvic floor muscles also play a crucial role in bladder control. These muscles support the bladder and help control the release of urine. Weakened pelvic floor muscles can lead to difficulty controlling urination, while overly tight pelvic floor muscles can cause the bladder not to empty properly.
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The effects of weak pelvic floor muscles on bladder function
The detrusor muscle, located within the bladder wall, is responsible for contracting during urination to expel urine from the bladder into the urethra. The urethral sphincters, composed of the internal and external sphincters, control the exit of urine from the bladder. While the internal sphincter functions involuntarily, the external sphincter is under voluntary control.
Weak pelvic floor muscles can have significant effects on bladder function. Pelvic floor muscles span the bottom of the pelvis and provide support to the bladder, bowel, and uterus in women. These muscles help maintain bladder control by wrapping firmly around the urethra to keep it shut, preventing the release of urine. When the pelvic floor muscles are weakened, the bladder and bowel may not empty properly, resulting in a condition called a hypertonic pelvic floor. This can lead to difficulty controlling the release of urine, faeces, or flatus.
In women, childbirth, obesity, and age can contribute to weak pelvic floor muscles. Pregnancy and childbirth are significant risk factors, with women who have had multiple births, assisted births, or perineal tearing being more susceptible to pelvic floor muscle damage. Additionally, reduced oestrogen levels during menopause can cause pelvic floor muscles to weaken. Age-related hormonal changes and the natural ageing process can also lead to muscle weakness over time.
In men, prostate surgery and radiation therapy can damage the pelvic floor muscles and affect bladder control. Being overweight can also increase the risk of urine leakage and place greater stress on the pelvic floor, contributing to weakness.
Weak pelvic floor muscles can lead to stress urinary incontinence, where the urethral sphincter does not close fully due to muscle weakness or damage. This results in the involuntary release of urine during activities such as laughing, coughing, sneezing, or lifting. Urge incontinence can also occur when the bladder contracts uncontrollably, causing a frequent and urgent need to urinate.
To counteract the effects of weak pelvic floor muscles, exercises can be performed to strengthen the muscles and improve bladder control. Pelvic floor exercises can enhance muscle control and coordination, reducing the risk of incontinence and improving bladder function.
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Frequently asked questions
The detrusor muscle is the primary muscle that contracts during urination to squeeze the bladder and push urine out of it and into the urethra.
The pelvic floor muscles, the urethral sphincters, and the bladder neck muscles are also involved in the urination process. The pelvic floor muscles support the bladder and help control the release of urine. The urethral sphincters control the exit of urine from the bladder through the urethra. The internal urethral sphincter is involuntary and prevents the uncontrolled flow of urine, while the external urethral sphincter is voluntary and prevents the flow of urine when you want to hold it in. The bladder neck is a narrow group of muscles that connect to the urethra.
If the muscles that squeeze the bladder are weak, it can lead to urinary incontinence, where urine leaks out unintentionally. This can happen if the pelvic floor muscles are weak, or if there is intrinsic sphincter damage or damage to the surrounding nerves and tissue.
