Maximus Gage
The detrusor muscle is a smooth muscle that makes up the wall of the bladder. When the bladder is empty, the detrusor muscle is relaxed, allowing urine to flow in and fill the bladder. As the bladder fills, the detrusor muscle stretches to accommodate more urine. When the bladder is full, the detrusor muscle contracts, pushing urine out of the bladder and into the urethra. The process of urination is a complex interplay between the detrusor muscle and the internal and external urethral sphincters, which also play a crucial role in controlling the flow of urine.
| Characteristics | Values |
|---|---|
| Name of the muscle | Detrusor muscle, also detrusor urinae muscle, or muscularis propria of the urinary bladder |
| Location | Wall of the bladder |
| Type of muscle | Smooth muscle |
| Number of layers | 3 |
| Innervation | Autonomic nervous system |
| Contraction | Parasympathetic pelvic splanchnic nerves act on postganglionic M3 receptors |
| Relaxation | Sympathetic branches from the inferior hypogastric plexus |
| Issues | In older adults over 60 years, it may cause issues in voiding the bladder, resulting in uncomfortable urinary retention |
| Treatment for overactive bladder | Pharmacological agonists of β3 adrenergic receptors |
| Related muscles | Urethral sphincter muscles envelop the urethra to control the flow of urine |
| Internal urethral sphincter | Involuntary muscle that relaxes to allow urine to pass |
| External urethral sphincter | Voluntary muscle that must be relaxed for urination to occur |
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What You'll Learn
The detrusor muscle
In older adults over 60 years of age, the detrusor muscle may cause issues in voiding the bladder, resulting in uncomfortable urinary retention. A floppy bladder can lose detrusor muscle tone (strength) and not contract for emptying. This type of bladder can be easily overstretched with too much urine, which can damage the bladder wall and increase the risk of infection.
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The internal urethral sphincter
Spasms of the internal urethral sphincter are associated with penile erection. Because the internal urethral sphincter is under involuntary control, it is believed to play a role in paruresis, where a person who perceives themselves to be under observation is unable to urinate. In males, the internal urethral sphincter has an additional function of preventing the flow of semen into the male bladder during ejaculation. Retrograde ejaculation can occur when the internal urethral sphincter fails to contract adequately during ejaculation.
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The external urethral sphincter
The process of urination begins when the bladder is full of urine, and stretch receptors in the bladder wall trigger the micturition reflex. The detrusor muscle that surrounds the bladder contracts, and the internal urethral sphincter relaxes, allowing urine to pass out of the bladder and into the urethra. The external urethral sphincter must also relax for urination to occur, and this is under conscious control.
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The nervous system
Additionally, the locus coeruleus (LC) and the PMC work together, with LC neurons receiving input from the bladder before the PMC. This neuronal activity helps initiate the process of urination by increasing arousal and shifting attention towards the full bladder. The LC sends signals to the PMC, which then communicates with other centers in the central and peripheral nervous systems to coordinate urination.
During urination, the detrusor muscle contracts, and the internal urethral sphincter relaxes, allowing urine to pass from the bladder into the urethra. The detrusor muscle is innervated by the autonomic nervous system, and its contraction is triggered by parasympathetic pelvic splanchnic nerves acting on postganglionic M3 receptors. At other times, the muscle remains relaxed through sympathetic branches, allowing the bladder to fill.
Disruptions in the nervous system's control of urination can lead to conditions such as neurogenic bladder, where neurological issues affect bladder function. This can result in incontinence or difficulty voiding the bladder. Treatment options include lifestyle changes, medications, catheterization, and in some cases, surgery.
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Urinary retention and incontinence
The detrusor muscle, also known as the detrusor urinae muscle, is a smooth muscle found in the wall of the bladder. It contracts during urination to release urine. The urethral sphincter muscles envelop the urethra to control the flow of urine when they contract. The internal sphincter is involuntary, whereas the external sphincter is voluntary.
Urinary retention is the difficulty emptying one's bladder or the inability to completely empty it. This can be due to obstructions in the bladder or urethra, nerve damage, or weak bladder muscles. Urinary retention can lead to urine leakage, usually associated with incontinence, and potentially dangerous backflow to the kidneys, urinary tract infections, bladder damage, and chronic kidney failure. It can also cause bladder stones, which can result in infection, irritation, or pain. Urinary retention is more common in older men, with 1 in 10 men over the age of 70 experiencing some type of retention. Treatment options include draining the bladder using a catheter and addressing the underlying cause.
In contrast, urinary incontinence is the loss of bladder control, making it difficult to hold urine. It can be caused by urinary retention, as an overly full bladder can lead to leakage. Overflow incontinence is a rare type of urinary incontinence characterised by dribbling or leaking urine. It is often due to prostate problems, such as an enlarged prostate or prostate cancer, which can block the urethra. Other causes include an increase in urine volume due to diabetes or medications, and nerve damage. Treatment options for overflow incontinence include bladder training, double voiding, wearing absorbent underwear or pads, and in rare cases, a urinary catheter.
Managing urinary incontinence can be done through Kegel exercises to strengthen the pelvic floor muscles, maintaining a healthy diet and elimination pattern, and scheduling fluid intake to retrain the bladder.
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Frequently asked questions
The bladder expands as urine flows in from the ureters. When the bladder is full, the detrusor muscle contracts and the internal urethral sphincter muscle relaxes, allowing urine to pass out of the bladder into the urethra.
The detrusor muscle is a smooth muscle found in the wall of the bladder. It remains relaxed to allow the bladder to store urine and contracts during urination to release urine.
The internal and external urethral sphincter muscles form a ring around the urethra to keep urine in the bladder. When you are ready to urinate, these muscles relax to allow urine to flow out.
When the bladder is full of urine, stretch receptors in the bladder wall trigger the micturition reflex, creating the "'urge'" to urinate.
The detrusor muscle can be overactive, contracting too much when not needed, or underactive, not contracting when it should. An overactive detrusor muscle can cause urge incontinence, while an underactive muscle can lead to urinary retention.
