Membranous Urethra

1. Overview

The membranous urethra is the shortest and narrowest portion of the male urethra. It extends from the apex of the prostate to the bulb of the penis and passes through the urogenital diaphragm, where it is surrounded by the external urethral sphincter. It serves as a conduit for urine (and semen in males) and is a key anatomical and functional region in maintaining urinary continence.

2. Location

The membranous urethra is located entirely within the deep perineal pouch, between the prostatic urethra superiorly and the spongy (penile) urethra inferiorly. It traverses:

• The urogenital diaphragm (a muscular and fascial structure)

• Just posterior to the pubic symphysis

• Anterior to the anal canal and rectum

This segment is only present in males; the female urethra does not have a distinct membranous portion.

3. Structure

The membranous urethra measures approximately 1–2 cm in length and is characterized by:

• Lumen: Narrowest and least distensible part of the male urethra.

• Epithelium: Transitional epithelium (urothelium) proximally, transitioning to pseudostratified columnar distally.

• Surrounding muscle: Enveloped by the external urethral sphincter (skeletal muscle), providing voluntary control.

• Supportive tissue: Lies within the deep perineal pouch, surrounded by fascia and connective tissue.

4. Function

The membranous urethra has several functional roles:

• Urine conduction: Transports urine from the bladder through the prostatic urethra to the penile urethra.

• Voluntary urinary control: Works with the external urethral sphincter to regulate micturition.

• Barrier function: Maintains continence during increases in intra-abdominal pressure.

5. Physiological Role(s)

While the membranous urethra does not directly engage in urine formation or modification, it plays a critical role in urinary physiology:

• Continence control: Contains the external urethral sphincter, innervated by the pudendal nerve (S2–S4), which allows voluntary retention of urine.

• Coordination with bladder activity: Must relax during micturition, synchronized with detrusor muscle contraction.

• Structural transition: Provides a mechanical link between the rigid prostatic urethra and the flexible penile urethra.

6. Clinical Significance

Urethral Injury

The membranous urethra is particularly vulnerable to injury in pelvic trauma (e.g., motor vehicle accidents with pelvic fractures). Features include:

• Disruption between prostatic and membranous urethra

• Urinary retention or leakage into the perineum

• Blood at the urethral meatus

Diagnosis may involve retrograde urethrogram, and treatment could require catheterization, realignment, or surgical repair.

Incontinence After Prostatectomy

The membranous urethra’s external sphincter may be damaged during radical prostatectomy, leading to stress urinary incontinence. Preservation of this region is crucial for postoperative continence.Urethral Stricture

Scarring and narrowing (stricture) of the membranous urethra may occur after trauma, instrumentation, or infection. Symptoms include:

• Weak urinary stream

• Straining to void

• Incomplete bladder emptying

Treatment includes dilation, urethrotomy, or urethroplasty.

Catheterization Challenges

Due to its narrow lumen and angle, the membranous urethra may present resistance during catheter insertion. Gentle technique is essential to avoid trauma or false passage.

Electromyography and Urodynamics

Studies of the external urethral sphincter function focus on the membranous urethra, especially in patients with voiding dysfunction or neurogenic bladder. These tests help assess voluntary sphincter activity and coordination with detrusor contraction.

Imaging

Visualized on imaging techniques like:

• Retrograde urethrography (RUG) – Gold standard for assessing trauma or strictures.

• MRI pelvis: Used for preoperative planning in prostate cancer and continence-preserving surgery.