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The detrusor muscle, also known as the detrusor urinae muscle, is a smooth muscle found in the wall of the bladder. It is innervated by the autonomic nervous system and plays a crucial role in urination. When the bladder is full, the detrusor muscle contracts, squeezing the bladder and increasing bladder pressure. Simultaneously, the internal and external sphincter muscles, located at the bladder neck and lower urethra, respectively, relax and open. This coordinated action propels urine out of the bladder, through the urethra, and out of the body. The urethral sphincter muscles are also essential in controlling the release of urine, as they can contract to prevent urine leakage and relax to allow urination.
| Characteristics | Values |
|---|---|
| Name of the muscle | Detrusor muscle |
| Location | Wall of the bladder |
| Type of muscle | Smooth muscle |
| Contraction | During urination to release urine |
| Relaxation | To allow the bladder to store urine |
| Innervation | By the autonomic nervous system |
| Issues | In older adults over 60 years, it may cause issues in voiding the bladder, resulting in uncomfortable urinary retention |
| Treatment | Pharmacological agonists of β3 adrenergic receptors are used to treat overactivity |
| Related muscles | Urethral sphincter muscles |
| Function of related muscles | Control the flow of urine when they contract |
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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. Most experimental research pertaining to OAB, urge incontinence, and detrusor overactivity (DO) has relied on increased detrusor contractions on cystometry as a surrogate for this disorder.
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The autonomic nervous system
The detrusor muscle, also known as the detrusor urinae muscle, is the muscle that pushes urine out of the body. It is a smooth muscle found in the wall of the bladder. The detrusor muscle is innervated by the autonomic nervous system. The autonomic nervous system is made up of two divisions, the sympathetic nervous system and the parasympathetic nervous system.
The sympathetic nervous system stimulates the contraction of the internal urethral sphincter, which is a thickening of smooth muscle at the bladder neck, allowing urine to be held in the bladder. The internal sphincter muscle is not under voluntary control. When the bladder is full, messages travel along nerves from the bladder to the spinal cord and then to the brain, signalling the urge to urinate. The detrusor muscle remains relaxed to allow the bladder to store urine.
The parasympathetic nervous system stimulates the contraction of the detrusor muscle, causing the person to urinate. When the decision is made to urinate, the internal sphincter relaxes and opens, while the detrusor muscle contracts and expels the urine out of the body via the urethra. The pelvic floor also moves downward, allowing the bladder to be in the best position to empty.
In older adults over 60 years of age, the detrusor muscle may cause issues in voiding the bladder, resulting in uncomfortable urinary retention. The bladder is connected to the outside by the urethra. The urethra is shorter in females than in males. Nerve and muscle damage can inhibit the functioning of the urethral sphincter, which plays a significant role in releasing urine. Intrinsic sphincter deficiency is a condition where the urethral sphincter cannot close enough to retain urine in the bladder, leading to involuntary urine loss.
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The internal and external sphincters
The urethral sphincter is a muscular structure that regulates the outflow of urine from the bladder into the urethra. There are two urethral sphincters: the external and internal urethral sphincters. When these muscles contract, the urethra narrows, and urination stops or slows. The urethral sphincter is critical for maintaining urinary continence. Urinary incontinence is often associated with the pathology of the urethral sphincter.
The external urethral sphincter, or rhabdosphincter, is primarily made of striated skeletal muscle and is under voluntary control via the deep perineal branch of the pudendal nerve and nicotinic receptors. The internal urethral sphincter is composed of smooth muscle; the autonomic nervous system controls it via the hypogastric nerves and alpha-one receptors. The external sphincter can be controlled, and when it is time to void, this muscle relaxes to allow it to open. Unlike the internal sphincter muscle, which is not under voluntary control, the external sphincter relaxes to allow urine to pass through the urethra.
The internal urethral sphincter is thought to arise from the absorption of the Wolffian duct into the bladder and urethra. The male external urethral sphincter is in the same anatomical area as the membranous urethra, supplied by the bulbourethral artery. Lymphatic drainage from the proximal urethra (prostatic and bulbomembranous sections) drains into the obturator and medial external iliac lymph nodes. The distal male urethra drains into the superficial inguinal nodes. The vaginal artery replaces the inferior vesical artery to supply the bladder, neck, and fundus.
The internal anal sphincter is located inside the rectum, while the external anal sphincter surrounds the outside of the end of the anal canal. The anal sphincter surrounds the anus and controls the release of stool, thereby maintaining continence. The anal sphincter muscles may be damaged if you have trouble holding in stool and/or urine, or if you have pain in your rectum. Several diseases, conditions, and injuries can affect the anal sphincter.
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The urethra
The prostatic urethra is supplied by the inferior vesical artery, a branch of the internal iliac artery that also supplies the lower part of the bladder. The membranous urethra is supplied by the bulbourethral artery, a branch of the internal pudendal artery. The penile urethra is supplied directly by branches of the internal pudendal artery.
The length of the urethra differs between males and females. In males, the urethra is approximately 15-20 cm long and passes through the penis. It has three constrictions: the internal urethral sphincter, the external urethral sphincter, and the external urethral orifice. In females, the urethra is significantly shorter, at about 4 cm in length, and exits the body between the clitoris and the vaginal opening.
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Pelvic surgery and nerve damage
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, which envelop the urethra, also play a role in controlling the flow of urine. These muscles contract to allow the bladder to empty and relax to seal the bladder for storage.
Pelvic surgery can potentially result in nerve damage, specifically to the pudendal nerve. This nerve is responsible for carrying messages from the bladder to the spinal cord and the brain, indicating the urge to urinate. Pudendal neuralgia, or pudendal nerve entrapment syndrome, can be caused by activities that place constant strain on the pelvic area, such as cycling, squatting, horseback riding, or chronic constipation. It can also be a result of complications from surgeries like prostatectomy, childbirth trauma, or pelvic scarring.
The symptoms of pudendal neuralgia include perineal pain, which is exacerbated by sitting and relieved by standing or lying down. It can cause numbness and dysfunction in the genitalia, perineum, rectum, and lower urinary tract. Urinary issues such as urgency, frequency, and painful ejaculation in men may also occur.
To diagnose pudendal neuralgia, healthcare providers will typically perform a physical exam and ask about symptoms and pain locations. They may also perform a vaginal or rectal exam to apply pressure directly to the pudendal nerve. Treatment options include physical therapy, medication, lifestyle changes, and surgery. Pudendal nerve decompression surgery involves removing muscle or tissue away from the pudendal nerve to alleviate pressure and pain.
It is important to note that nerve damage from pelvic surgery may not always be immediate, and that relief from surgery is typically a gradual process. Patients should be educated about avoiding painful stimuli and actively participating in post-surgical physiotherapy and lifestyle modifications.
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Frequently asked questions
The detrusor muscle, also known as the detrusor urinae muscle, is a smooth muscle that makes up the wall of the bladder. During urination, the detrusor muscle contracts to push urine out of the bladder.
The detrusor muscle is a layer of smooth muscle that surrounds the bladder. It is innervated by the autonomic nervous system, which controls its contraction and relaxation. When the bladder is full, the detrusor muscle contracts to push urine out, while the internal and external sphincters relax to open the urethra.
The urethral sphincter is a muscle that surrounds the urethra and controls the flow of urine. During urination, the urethral sphincter relaxes to open the urethra and allow urine to pass through. At other times, it remains contracted to prevent urine leakage.
The control of urination involves the coordination of the nervous system, muscles, and sphincters. Disorders or damage to these structures can interfere with normal urination. For example, intrinsic sphincter deficiency (ISD) is a condition where the urethral sphincter cannot close completely, leading to continuous urine loss. Overactive detrusor muscle can cause urge incontinence, while an underactive detrusor muscle can result in urinary retention.
