Membranous Urethra

The membranous urethra is a short portion of the male urethra situated between the prostatic and spongy urethral segments. It begins at the apex of the prostate, passes through the region of the external urethral sphincter, and continues to the proximal spongy urethra at the bulb of the penis.

This segment is anatomically and clinically important because it is closely surrounded by the external urethral sphincter, which provides voluntary control of urination. Its relatively fixed position within the deep perineal region also makes it relevant in pelvic trauma and urethral injury.

Position Within the Male Urethra

The male urethra extends from the internal urethral orifice of the urinary bladder to the external urethral orifice at the tip of the glans penis. The membranous urethra lies between the prostatic urethra proximally and the spongy urethra distally.

Segments of the Male Urethra

1. Preprostatic or intramural urethra
2. Prostatic urethra
3. Membranous urethra
4. Spongy urethra

Extent and Length

The membranous urethra begins immediately distal to the apex of the prostate and extends through the deep perineal region to the point where the urethra enters the proximal corpus spongiosum within the bulb of the penis. It is relatively short, measuring approximately 1 to 2 cm in the adult, although its precise length varies between individuals.

Course

From the apex of the prostate, the membranous urethra passes inferiorly and slightly anteriorly through the region of the external urethral sphincter. It traverses the deep perineal region and perineal membrane before entering the bulb of the penis, where it becomes continuous with the spongy urethra.

Relationship to the Prostate

The membranous urethra begins immediately below the apex of the prostate. The prostatic urethra passes through the prostate and becomes continuous with the membranous segment as it exits the gland.

Relationship to the Spongy Urethra

Distally, the membranous urethra approaches and enters the proximal corpus spongiosum. From this point onward, the urethral channel is termed the spongy urethra and continues through the penis.

Deep Perineal Region

The membranous urethra passes through the deep perineal region between the prostate and bulb of the penis. It is closely associated with skeletal muscle fibers of the external urethral sphincter and the connective tissues supporting the pelvic outlet.

Perineal Membrane

The urethra pierces the perineal membrane as it passes from the pelvic region toward the bulb of the penis. This relationship contributes to the relative fixation of this urethral segment.

External Urethral Sphincter

The most important muscular relationship of the membranous urethra is the external urethral sphincter. This predominantly striated skeletal muscle surrounds the urethra in the region immediately inferior to the prostate and is a major component of voluntary urinary continence.

Function of the External Urethral Sphincter

Contraction of the external urethral sphincter compresses the urethral lumen and increases resistance to urine flow. Relaxation of the sphincter is required during normal voluntary micturition.

Somatic Control

Voluntary control of the external urethral sphincter is mediated through somatic motor pathways associated primarily with the pudendal nerve. These fibers arise from sacral spinal cord levels, principally S2 to S4.

Relationship to the Pelvic Floor

The membranous urethra occupies the transition between the pelvic urinary tract and the perineal urethra. Its relationships with the pelvic floor, external sphincter complex, and perineal membrane contribute to stabilization of the urethral outlet and urinary continence.