The detrusor is a layer of smooth muscle that constitutes the main component of the urinary bladder wall. Its fundamental purpose is to facilitate the storage and subsequent expulsion of urine. The muscle fibers of the detrusor are interwoven, an arrangement that allows the bladder to expand as it fills with urine.
Function in the Urinary Cycle
The detrusor muscle performs a dual role in the urinary, or micturition, cycle: storage and voiding. During the storage phase, the muscle remains in a relaxed state. This active relaxation allows the bladder wall to stretch and accommodate increasing volumes of urine without a significant rise in internal pressure. This is similar to a balloon slowly filling with air to hold more content.
When it is time to urinate, the cycle enters the voiding phase. The detrusor muscle then contracts forcefully and in a sustained manner. This action increases the pressure inside the bladder, functioning like squeezing a full balloon. This contraction must be coordinated with the relaxation of the urethral sphincters, the muscles that control the urethra’s outlet, allowing urine to be pushed out.
Nervous System Control
The function of the detrusor muscle is governed by the autonomic nervous system, which operates without conscious thought. This system has two main branches, the sympathetic and parasympathetic, which have opposing effects on the bladder muscle. The balance between these two branches dictates whether the bladder stores or expels urine.
During the storage phase, the sympathetic nervous system is dominant, sending signals that promote relaxation of the detrusor muscle and allow the bladder to fill. When urination is appropriate, control shifts to the parasympathetic nervous system. Signals travel through the pelvic splanchnic nerves, causing the detrusor muscle to contract and initiate urination. While these processes are automatic, the brain’s cerebral cortex provides conscious control, allowing a person to decide when to start or delay urination.
Common Detrusor Muscle Conditions
Disruptions in the detrusor muscle’s function can lead to several common urinary conditions. These issues fall into categories of being overactive or underactive. Each condition has a distinct set of symptoms related to how the muscle behaves during the urinary cycle.
Detrusor overactivity, a component of overactive bladder (OAB), is characterized by involuntary contractions of the muscle during the bladder’s filling phase. These unscheduled contractions create a sudden, compelling need to urinate, known as urgency. This can lead to frequent urination during the day and night (nocturia). If the contractions are strong enough to overcome the sphincter muscles, it can result in urge incontinence, the leakage of urine.
Conversely, detrusor underactivity occurs when the muscle’s ability to contract is weakened during urination. This leads to a weak or intermittent urine stream, hesitancy, and a feeling of incomplete bladder emptying. In some cases, it can result in urinary retention. This condition can arise from nerve damage, such as from diabetes or a stroke, or from chronic overstretching of the bladder.
A third condition, detrusor sphincter dyssynergia, involves a lack of coordination between the bladder muscle and the external sphincter. In this state, the detrusor muscle contracts to void, but the external urethral sphincter muscle also contracts instead of relaxing. This conflict obstructs the flow of urine and is associated with neurological conditions like spinal cord injury or multiple sclerosis.
Management and Treatment Options
Treatment for detrusor muscle conditions aims to restore normal bladder function and alleviate symptoms. Management strategies often begin with conservative, non-invasive methods before progressing to other options.
Behavioral therapies are a common starting point. Bladder training involves urinating on a fixed schedule, gradually increasing the time between voids to improve the bladder’s capacity and control. Pelvic floor muscle exercises, also known as Kegels, can help manage detrusor contractions by strengthening the muscles that support the bladder and urethra.
Medications are frequently used, particularly for detrusor overactivity. Anticholinergic drugs work by blocking nerve signals that trigger involuntary bladder contractions, thus relaxing the muscle. Another class of medication, beta-3 agonists, helps the detrusor muscle to relax and increase the bladder’s storage capacity.
For individuals who do not respond to conservative treatments, advanced procedures may be considered. Botulinum toxin (Botox) can be injected directly into the detrusor muscle, which partially paralyzes it to reduce overactivity for several months. Another option is nerve stimulation, or neuromodulation, where a small device sends electrical pulses to the sacral nerves that control the bladder.