It is mostly the detrusor muscle that forms the wall of the bladder, it contracts during urination and relaxes to hold the urine. The muscle is continuous at the inferior end with the internal urethral sphincter. Urination results when the detrusor muscle contracts and the urethral sphincter muscle relaxes. The autonomic nervous system controls the detrusor muscle. Urinary retention or urinary incontinence can result due to detrusor muscle pathology. If there are abnormalities in the muscle and it is left untreated, then the upper urinary tract may deteriorate.
What is detrusor muscle?
The smooth muscle of the urinary bladder is the detrusor muscle and during the process of urination, it facilitates the bladder wall contraction. One component of the bladder wall is the detrusor muscle. Within the bladder cavity the urothelial lining overlies the detrusor muscle. In order for the muscles to act in unison, the muscle fibers of the detrusor muscles are arranged in a swirl pattern in order to enable mass contraction of the bladder. Near the bladder neck this muscle inferiorly continues with the internal urethral sphincter. During the process of micturation, this muscle plays an important role. It possesses stretch receptors that give a sensation of the bladder being full and the efferent parasympathetic impulses enable contraction so that the bladder may empty.
This muscle is also known as detrusor urinae muscle or muscularis propria. In the wall of the bladder this muscle located. The urethral sphincter muscle is related to this muscle that envelops the urethra and when they contract it controls the flow of the urine. It is innervated by the hypogastric sympathetic nerve and the pelvic splanchnic parasympathetic nerve. The sympathetic nerve gives signals for the muscle to relax in order to store urine and the parasympathetic nerve sends signals for the muscle to contract so that the bladder can be emptied. There may be issues in emptying the bladder in older adults above the age of 60 years due to issues with the detrusor muscle which may result in discomfort.
In both the sexes, from the posterior surface of the pubic body, the fibers of this muscle arises. In males, they also arise from the prostate and its capsule. In females, they are attached to the vaginal area. They are arranged obliquely at the sides and then they intersect each other. From the inner layers to the outer layers, the fibers are arranged as longitudinal-circular-longitudinal. Using an EMG, the activity of the detrusor muscle can be recorded.
Contraction during urination is the main function of detrusor muscle so that from the bladder the urine can be pushed out into the urethra. For the urine to be stored in the urinary bladder the detrusor muscle will relax. Micturation or urination is a process when the bladder is emptied. The micturation reflex is triggered by the stretch receptors present in the bladder wall when the bladder is full of urine. Around the bladder the detrusor muscles are present that contract. The urine passes from the bladder into the urethra due to relaxation of the internal urethral sphincter. These are involuntary reactions.
The action of external urethral sphincter is voluntary. For the urine to flow through the urethra to the outside, the external urethral sphincter must voluntarily relax. A study has shown that the electrical activity of the detrusor muscle in relation to abdominal contractions when studied can help to understand the neurogenic disorders.
In a baby, the bladder is found in the upper segment of the urogenital sinus. This is connected with the allantois. The detrusor muscle is derived from the mesoderm and it originates from the neural crest. The superior vesical artery supplies blood to the upper portion of the urinary bladder. The minor blood supply is received by the inferior gluteal and obturator arteries. Through the vesical venous plexus, the venous system is drained out. Then it is further drained by the vesical veins which finally goes to the internal iliac veins. The external iliac lymph nodes drains the lymphatics of the bladder.
Structure of detrusor muscle
This muscle is located within the walls of the bladder. It is further made of two types of smooth muscles and these are longitudinal and circular muscles. Initially, in the inner layers the muscle is longitudinal, then in the middle it becomes circular. And then again in the outer layer becomes longitudinal. This muscle continues with the internal urethral sphincter which is also made of smooth muscles. As the bladder fills, in order to prevent the back reflux of the urine into the kidney, the ureter passes obliquely through the detrusor muscle. The urine may reflux back into the kidney if this ureteral tunnel is not short or oblique. This malfunction would cause renal scarring, infection or renal damage.
Contractility of the pediatric detrusor muscle is comparatively less and its passive stiffness is greater. This is because the ratio of connective tissue to the smooth muscle is more in children. This is also known as bladder compliance, which can lead to damage to the upper urinary tract if this urinary compliance is poor.
Bladder dysfunction may occur due to pelvic surgery. There have been reports of bladder dysfunction due to surgeries such as prostatectomy, radical hysterectomy, perineal resection, and proctocolectomy. In order to avoid damage to any vessels, nerves, urinary system or bladder damage one should take caution during pelvic surgery. In healthy children, the detrusor muscle have been measured by using sonography.
Detrusor muscle dysfunction
There is difficulty in voiding urine due to urinary retention. Also, there is a sensation of incomplete urination, urine is retained, voiding frequently, and overflow incontinence. The main causes of degeneration of detrusor nerve control are stroke and diabetes mellitus. When the bladder is not able to contract, it may cause detrusor areflexia. This is usually neurologic in origin. It may occur due to injury to the spinal cord, herniated disc, infections and fractures. Fibrosis of muscle may occur when chronic overdistention causes damage to the detrusor muscle. The muscle contraction also weakens in this condition. Often this condition is referred to as myogenic bladder. Benign prostatic hypertrophy is one common cause of chronic bladder distension, and this is common in males.
The ability to empty the bladder becomes poor when there are problems with the nerve or muscle. In some women, the detrusor muscle may become overactive due to which contraction becomes poor leading to overflow incontinence. Often urgency incontinence is associated with the over activity of detrusor muscle. In this condition, the urge to urinate is very urgent which can even lead to leakage. In older women, this condition is very common and may be associated with other conditions. The most effective way to treat urgency incontinence is to treat the underlying issue. First, conservative options are used along with modifications in lifestyle, then the person should practice pelvic floor exercises and bladder training. However, if the symptoms do not improve by conservative therapies, then other surgical and pharmacological therapies should be adopted.
Antimuscarinics and beta 3 adrenergics are two medication classes that are commonly used. There are side effects associated with antimuscarinics such as dry mouth, blurred vision, constipation, cognitive issues, and drowsiness. The other two options are sacral nerve stimulation and Botox. Surgery along with augmentation cystoplasty is the other invasive therapies. Another therapy is detrusor myectomy. By these therapies the compliance of the bladder can be improved or the damage to the upper urinary tract due to high bladder pressure can be prevented.
Another reason that may cause issues to the urinary system is bladder cancer. By cystoscopy with biopsy, this can be diagnosed and staged. The removal of the urinary bladder is the standard therapy if the cancer invades the detrusor muscle and the path of the urine is diverted. For diverting the urine, a bowel conduit is created or by harvesting a bowel segment a neobladder is created.
Some conditions associated with detrusor muscle dysfunction include:
- Nocturnal enuresis - is bed wetting. There is no clear cause for this but according to research there may be a genetic link to it. Many studies have found that many incidences of nocturnal enuresis is associated with detrusor instability. In most of the cases of detrusor instability, this condition has been found.
- Urge incontinence - when there is a problem with the detrusor muscle then this condition arises. In this condition, too often the muscle contracts thus the need or urgency to urinate increases. Even though the cause is not clear the detrusor muscle may contract too often because of poor fluid intake, consuming excess alcohol, caffeine, constipation or conditions such as urinary tract infection or certain medications. Various treatment options have been researched to treat urge incontinence.
- Overflow incontinence - it may occur due to various reasons, but one of the reasons is related to detrusor muscle. When complete contraction of the detrusor muscle does not happen then it causes overflow incontinence. It means then when the person goes to the toilet the muscle does not completely contract as a result it stretches even more. Detrusor muscle may not completely contract due to nerve damage caused by surgery of a part of the bowel or some injury to the spinal cord or due to certain medications. Overflow incontinence may also occur due to obstructions. When prostatic obstructions occur, the bladder is usually the first reason.
- Detrusor instability - this is also known as overactive bladder syndrome. In this condition, since the detrusor muscle is not stable, the person may at times experience urgency incontinence. Most of the times the cause is idiopathic. This condition can be quite embarrassing. With age the risk of this condition increases. The risk of detrusor muscle instability increases in conditions such as Parkinson disease, diabetic neuropathy, multiple sclerosis, spinal cord injury or enlarged prostate. Detrusor instability causes symptoms such as nocturia, frequency of urination increases, pain in the abdominal area, and urinary urge incontinence.
Detrusor muscle and urination
The bladder stores urine until the time is appropriate for micturation. In the wall of the bladder, there is the detrusor muscle which is a smooth muscle in the bladder wall. The mucosal layer overlies this muscle, and the uroepithelium lines this. Urination is impacted by detrusor muscle. In two ways, malfunction of the detrusor muscle may occur. It can either become overactive or underactive.
An overactive detrusor muscle will contract too often to squeeze out the urine. Thus the desire to urinate will increase or there is a sense of urgency. The detrusor muscle is said to be overactive when there is an urge to urinate but no urine is lost. If the patient is ready to urinate the urine will be pushed out by the strong contractions of detrusor muscle and this is known as urge incontinence.
The bladder is connected to the urethra so that the urine is pushed out from the body. The urethra is fairly short in females but it is longer in makes. In males, the urethra is divided into three segments. The exit of the urine from the bladder into the urethra is controlled by two sphincters. At the bladder neck is present one internal urethral sphincter and in the pelvic floor is present another external urethral sphincter.
The central nervous system controls the bladder. Two different divisions of the autonomic nervous system control the smooth muscles. The detrusor muscle is stimulated by the parasympathetic nervous system so that it can contract and the person is able to urinate. The contraction of the internal urethral sphincter is stimulated by the sympathetic nervous system so that urine can be held. The contraction of the external urethral sphincter is stimulated by the somatic efferent neurons.