Detrusor Muscle

1. Overview

The detrusor muscle is the smooth muscle layer of the urinary bladder wall responsible for contracting during micturition to expel urine. It is essential for both bladder filling and emptying phases, playing a central role in urinary continence and voiding. The muscle’s tone and contraction are regulated by autonomic innervation and modulated by various reflex pathways and hormones.

2. Location

The detrusor muscle is located in the wall of the urinary bladder, sandwiched between the submucosa and the outer serosa/adventitia. It surrounds the entire bladder, including the dome, lateral walls, and posterior wall, and continues down into the bladder neck, contributing to the internal urethral sphincter, particularly in males.

3. Structure

Histologically, the detrusor muscle is composed of irregularly arranged bundles of smooth muscle fibers organized into three indistinct layers:

• Inner longitudinal layer

• Middle circular layer (thicker and better defined, particularly around the bladder neck)

• Outer longitudinal layer

Between these layers, connective tissue, blood vessels, and autonomic nerve fibers are present. The muscle fibers contract as a unit due to gap junctions facilitating coordinated contractions.

4. Function

The detrusor muscle serves two primary opposing functions, depending on the phase of the bladder cycle:

• During filling: It remains relaxed to allow bladder expansion and accommodate urine volume without a significant rise in pressure (compliance).

• During voiding: It contracts forcefully to expel urine from the bladder into the urethra, overcoming outlet resistance.

This dual function is tightly regulated by the autonomic nervous system and higher brain centers involved in micturition control.

5. Physiological Role(s)

The detrusor muscle plays several critical physiological roles:

• Urinary continence: Maintains relaxation during storage phase under sympathetic control (via β3-adrenergic receptors).

• Micturition: Contracts under parasympathetic stimulation (via muscarinic M3 receptors) to initiate urination.

• Stretch-sensing: Contains mechanoreceptors that sense bladder fullness and trigger the micturition reflex.

• Compliance modulation: Adjusts wall tension to accommodate varying volumes of urine without triggering urgency.

6. Clinical Significance

Detrusor Overactivity (Overactive Bladder)

A condition where the detrusor contracts involuntarily during the bladder filling phase, leading to:

• Urgency

• Frequency

• Nocturia

• Urge incontinence

Common in aging and neurogenic conditions; managed with anticholinergics, β3 agonists, or bladder training.

Detrusor Underactivity (Atonic Bladder)

Reduced or absent detrusor contractions during voiding, resulting in:

• Urinary retention

• Increased residual volume

• Overflow incontinence

Causes include diabetic autonomic neuropathy, spinal cord injury, or long-term bladder outlet obstruction.

Neurogenic Bladder

Detrusor muscle dysfunction due to spinal cord injury or neurological disease can result in:

• Detrusor-sphincter dyssynergia: Poor coordination between detrusor contraction and sphincter relaxation.

• Detrusor hyperreflexia: Involuntary contractions with reduced capacity.

Managed with catheterization, medications, or neuromodulation.

Benign Prostatic Hyperplasia (BPH)

In males, bladder outlet obstruction due to BPH can lead to secondary detrusor hypertrophy and later detrusor fatigue or failure.

Pharmacologic Targets

The detrusor is a major target for:

• Anticholinergics: Reduce involuntary contractions in overactive bladder (e.g., oxybutynin).

• β3-adrenergic agonists: Promote relaxation during storage (e.g., mirabegron).

• Botulinum toxin injections: Used in refractory detrusor overactivity.

Surgical Implications

During bladder surgeries (e.g., augmentation cystoplasty or tumor excision), care must be taken to preserve detrusor integrity to maintain normal bladder function.