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Detrusor Muscle: Function, Anatomy and Disorders

The detrusor is the smooth muscle of the urinary bladder wall that contracts to expel urine during urination. It plays a key role in both bladder storage and controlled emptying.

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Things worth knowing about "Detrusor"

The detrusor is the smooth muscle of the urinary bladder wall that contracts to expel urine during urination. It plays a key role in both bladder storage and controlled emptying.

What Is the Detrusor?

The detrusor muscle (Latin: Musculus detrusor vesicae) is the smooth muscle that makes up the wall of the urinary bladder. It consists of interlocking layers of smooth muscle cells arranged in longitudinal, circular, and spiral patterns, forming a coordinated network that enables efficient bladder function. The detrusor is responsible for both urine storage and controlled voiding.

Anatomy and Structure

The bladder wall is composed of several layers, with the detrusor forming the thickest and most functionally important layer. It is organized into three main layers:

Inner longitudinal layer: runs parallel to the bladder axis
Middle circular layer: encircles the bladder in a ring-like pattern
Outer longitudinal layer: runs longitudinally again on the outside

At the bladder outlet, the detrusor fibers thicken to form the internal urethral sphincter, which is under involuntary (autonomic) control and contributes to urinary continence.

Function of the Detrusor

The detrusor serves two key functions during the micturition cycle (the process of urination):

Storage phase: As the bladder fills with urine, the detrusor remains relaxed and compliant, keeping intravesical pressure low and allowing for a large storage capacity.
Voiding phase: In response to nerve signals, the detrusor contracts, increasing bladder pressure and propelling urine through the urethra.

Control of the detrusor is regulated by the autonomic nervous system. Parasympathetic nerve fibers (via the pelvic nerve) stimulate detrusor contraction, while sympathetic fibers promote relaxation during the filling phase.

Common Conditions Affecting the Detrusor

Detrusor Overactivity (Overactive Bladder)

Detrusor overactivity occurs when the muscle contracts involuntarily and prematurely. This leads to symptoms of the overactive bladder (OAB), including urgency, urge incontinence, and nocturia (waking at night to urinate). Causes may be neurogenic (e.g., following a stroke or in multiple sclerosis) or idiopathic (no identifiable cause).

Detrusor-Sphincter Dyssynergia

In this condition, the detrusor and sphincter contract simultaneously rather than in a coordinated manner. This results in impaired bladder emptying, elevated post-void residual urine, and an increased risk of urinary tract infections. It is commonly associated with spinal cord injuries or neurological diseases.

Detrusor Underactivity

Detrusor underactivity is characterized by reduced or absent detrusor contractile strength, leading to incomplete bladder emptying, urinary retention, and recurrent urinary tract infections. It is frequently observed in elderly patients and those with diabetic neuropathy.

Diagnosis

Several diagnostic tools are used to evaluate detrusor function:

Urodynamic studies: Comprehensive assessment of bladder pressure, capacity, and detrusor behavior during filling and voiding
Cystometry: A component of urodynamics that specifically evaluates the filling phase of the bladder cycle
Uroflowmetry: Measures the urine flow rate to identify voiding dysfunction
Post-void residual measurement: Ultrasound-based assessment of urine remaining after voiding

Treatment

Treatment depends on the underlying condition and may include:

Anticholinergics / Antimuscarinics (e.g., oxybutynin, solifenacin): Reduce involuntary detrusor contractions in overactive bladder
Beta-3 adrenergic agonists (e.g., mirabegron): Relax the detrusor during the storage phase
Botulinum toxin injections: Injected directly into the detrusor for refractory overactivity
Sacral neuromodulation: Electrical stimulation of sacral nerves to restore coordinated bladder function
Bladder training and pelvic floor physiotherapy: Conservative approaches to improve continence and bladder control

References

Abrams P. et al. - The standardisation of terminology of lower urinary tract function: report from the Standardisation Sub-committee of the International Continence Society. Neurourology and Urodynamics, 2002.
Wein AJ, Kavoussi LR et al. - Campbell-Walsh Urology. 11th Edition. Elsevier, 2016.
Andersson KE, Arner A. - Urinary bladder contraction and relaxation: physiology and pathophysiology. Physiological Reviews, 2004.

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