External Urethral Sphincter

The external urethral sphincter is a striated skeletal muscle that surrounds and compresses the urethra in the deep perineal region. It is an important component of the urinary continence mechanism and provides voluntary control over the passage of urine through the urethra.

Unlike the smooth muscle associated with the bladder neck and proximal urethra, the external urethral sphincter is composed predominantly of skeletal muscle. Its activity can therefore be consciously modified, although it also participates in reflex mechanisms that maintain continence during bladder filling.

Location

The external urethral sphincter lies in the deep perineal region and surrounds the urethra as it passes through the pelvic outlet toward the perineum.

Its precise anatomical arrangement differs between males and females because of differences in the urethra and surrounding reproductive structures.

General Muscular Arrangement

The external urethral sphincter consists of striated muscle fibers arranged around the urethra. Contraction of these fibers compresses the urethral lumen and increases resistance to urinary flow.

The muscle functions as part of a larger continence system that also includes the bladder neck, urethral smooth muscle, pelvic floor, connective tissues, and neural control mechanisms.

External Urethral Sphincter in Males

In males, the external urethral sphincter is closely related to the membranous urethra and the apex of the prostate. Its fibers surround the urethra immediately distal to the prostate and contribute substantially to voluntary urinary continence.

Relationship to the Membranous Urethra

The membranous urethra passes through the region occupied by the external urethral sphincter. The close relationship between these structures explains why the membranous urethra is functionally important in voluntary control of urination.

Relationship to the Prostate

In males, the external urethral sphincter lies immediately inferior to the apex of the prostate. Some sphincteric fibers extend toward and are closely related to the distal portion of the prostate.

This anatomical relationship is particularly important during surgical procedures involving the prostate.

Relationship to the Perineal Membrane

The external urethral sphincter is closely associated with the perineal membrane and structures of the deep perineal region. The urethra passes through this region before entering the corpus spongiosum in males.

External Urethral Sphincter in Females

In females, the external urethral sphincter complex surrounds the urethra and is integrated with additional striated muscle components associated with the urethra and vagina.

The female sphincter complex is therefore more anatomically differentiated than a simple circular ring of muscle.

Female External Urethral Sphincter Complex

The female external urethral sphincter complex is commonly described as including several muscular components:

• Sphincter urethrae
• Compressor urethrae
• Urethrovaginal sphincter

Sphincter Urethrae

The sphincter urethrae consists of striated muscle fibers surrounding the urethra. Its contraction compresses the urethral lumen and contributes to urinary continence.

Compressor Urethrae

The compressor urethrae is a component of the female external sphincter complex. Its fibers pass around the urethral region and contribute to compression of the urethra.

Urethrovaginal Sphincter

The urethrovaginal sphincter surrounds both the urethra and vagina. Its contraction contributes to urethral closure and integrates the continence mechanism with surrounding female perineal anatomy.

Comparison Between Males and Females

Feature	Male	Female
Primary relationship	Membranous urethra and prostatic apex	Urethra and anterior vaginal region
Muscle type	Striated skeletal muscle	Striated skeletal muscle
Major function	Voluntary urethral closure	Voluntary urethral closure
Distinctive arrangement	Closely surrounds the membranous urethral region	Includes sphincter urethrae, compressor urethrae, and urethrovaginal sphincter components

Skeletal Muscle Composition

The external urethral sphincter is composed predominantly of striated skeletal muscle fibers. This distinguishes it from the smooth muscle of the bladder wall and proximal urethral outlet.

Its skeletal muscle composition allows rapid contraction and voluntary modulation of urethral resistance.

Somatic Innervation

The external urethral sphincter receives somatic motor innervation associated primarily with the pudendal nerve.

The relevant motor neurons originate from sacral spinal cord levels, principally S2 to S4.

Pudendal Nerve

The pudendal nerve arises from the sacral plexus and supplies several skeletal muscles of the perineum. Motor pathways associated with this nerve allow voluntary contraction of the external urethral sphincter.

Onuf's Nucleus

Motor neurons involved in sphincter control are located in a specialized region of the sacral spinal cord commonly referred to as Onuf's nucleus. These neurons contribute to somatic innervation of the external urethral and anal sphincters.

Sacral Spinal Cord

Sacral spinal cord segments S2 to S4 play a central role in lower urinary tract control. They contain neural pathways involved in parasympathetic bladder function as well as somatic pathways controlling the external urethral sphincter.

Function in Urinary Continence

The primary function of the external urethral sphincter is to increase urethral outlet resistance and help prevent involuntary urine leakage.

During bladder filling, tonic sphincter activity helps maintain closure of the urethra.

Function During Bladder Filling

As the bladder fills, the detrusor muscle generally remains relaxed while urethral outlet resistance remains elevated. The external urethral sphincter contributes to this resistance through sustained skeletal muscle activity.

Guarding Reflex

Increasing bladder filling can produce reflex increases in external urethral sphincter activity. This response, often described as part of the guarding reflex, helps maintain continence as bladder pressure changes.

Response to Increased Abdominal Pressure

Activities such as coughing, sneezing, lifting, or straining can rapidly increase intra-abdominal pressure. Coordinated activation of the urethral sphincter and pelvic floor helps oppose the resulting increase in bladder pressure and maintain continence.

Voluntary Urinary Control

Because the external urethral sphincter is composed of skeletal muscle, an individual can voluntarily increase its contraction to delay urination even when the bladder is full.

This voluntary control distinguishes the external sphincter from predominantly autonomically controlled smooth muscle mechanisms of the urinary tract.

Function During Micturition

Normal micturition requires coordinated relaxation of the external urethral sphincter while the detrusor muscle of the bladder contracts.

Relaxation reduces urethral resistance and permits urine to flow through the urethra.

Coordination with the Detrusor

Efficient bladder emptying depends on coordination between the bladder and urethral outlet. Detrusor contraction without appropriate sphincter relaxation creates resistance to urinary flow.

Micturition Sequence

1. The urinary bladder gradually fills.
2. Stretch receptors in the bladder wall respond to increasing volume.
3. Sensory information reaches the sacral spinal cord and higher nervous system centers.
4. During storage, the external urethral sphincter remains active.
5. When voiding is appropriate, central pathways facilitate the micturition response.
6. External urethral sphincter activity decreases.
7. Urethral outlet resistance falls.
8. The detrusor contracts.
9. Urine passes through the urethra.

Higher Neural Control

Voluntary control of micturition depends on communication between the cerebral cortex, brainstem micturition centers, spinal cord, bladder, and urethral sphincters.

This organization allows urination to be delayed until an appropriate time and place.

Pontine Micturition Center

The pontine micturition center participates in coordinating bladder contraction with relaxation of the urethral outlet during voiding.

Descending pathways help suppress somatic sphincter activity while promoting the coordinated autonomic responses required for bladder emptying.

Relationship to the Pelvic Floor

The external urethral sphincter functions together with muscles of the pelvic floor. These structures provide support for pelvic organs and contribute to mechanisms that resist urinary leakage.

Relationship to Levator Ani

The levator ani muscles are anatomically distinct from the external urethral sphincter but contribute to pelvic support and continence. Coordinated activity of the pelvic floor can reinforce urethral closure during increases in abdominal pressure.

Internal and External Urethral Sphincters

The terms internal and external urethral sphincter describe different components of urethral outlet control.

The internal sphincter refers primarily to smooth muscle mechanisms at the bladder neck and proximal urethra, whereas the external sphincter consists predominantly of striated skeletal muscle.

Comparison with the Internal Urethral Sphincter

Feature	Internal Urethral Sphincter	External Urethral Sphincter
Muscle type	Smooth muscle	Striated skeletal muscle
Location	Bladder neck and proximal urethra	Deep perineal urethral region
Control	Predominantly autonomic	Somatic and voluntary
Major neural pathway	Autonomic pelvic pathways	Pudendal-associated somatic pathways
Major role	Bladder outlet regulation	Voluntary continence

Blood Supply

The external urethral sphincter receives blood from small arterial branches supplying the urethral and deep perineal regions.

These vessels arise from branches of the internal pudendal artery and neighboring pelvic arterial systems.

Venous Drainage

Venous blood drains through accompanying perineal and pelvic veins. These vessels communicate with pudendal and pelvic venous plexuses before ultimately draining toward the internal iliac venous system.

Clinical Importance of the External Urethral Sphincter

The external urethral sphincter is clinically important because damage to the muscle or its neural supply can compromise voluntary urinary continence.

Injury can occur through neurological disease, pelvic trauma, childbirth, or surgical procedures involving structures adjacent to the urethra.

External Sphincter and Prostate Surgery

In males, the close relationship between the external urethral sphincter and the apex of the prostate is especially important during prostate surgery.

Preservation of functional sphincter tissue and its innervation contributes to postoperative urinary continence.

Radical Prostatectomy

During radical prostatectomy, the prostate is separated from the urethra near its apex. The remaining urethral sphincter mechanism is therefore an important anatomical consideration during apical dissection and reconstruction.

Damage to sphincteric tissue or supporting structures can contribute to postoperative urinary incontinence.

Urinary Incontinence

Weakness or dysfunction of the urethral sphincter mechanism can reduce urethral closure pressure and contribute to urinary incontinence.

The clinical pattern depends on the underlying anatomical, neurological, and functional abnormalities.

Stress Urinary Incontinence

Stress urinary incontinence occurs when increases in abdominal and bladder pressure exceed the ability of the urethral closure mechanism to maintain continence.

Pelvic support, urethral position, connective tissues, and sphincter function can all contribute to this mechanism.

Neurological Injury

Neurological disorders affecting the brain, spinal cord, sacral nerve roots, or peripheral nerves can disrupt normal control of the external urethral sphincter.

The resulting dysfunction depends on the location and severity of the neurological lesion.

Detrusor-Sphincter Dyssynergia

Detrusor-sphincter dyssynergia refers to inappropriate contraction or failure of relaxation of the striated urethral sphincter during detrusor contraction.

This loss of coordination can obstruct urinary flow and may occur with certain neurological lesions affecting pathways between the brainstem and sacral spinal cord.

Consequences of Sphincter Dyssynergia

Failure of the external sphincter to relax appropriately during bladder contraction can increase outlet resistance and impair bladder emptying.

Depending on severity and duration, this can contribute to elevated bladder pressures, residual urine, urinary tract complications, and changes in upper urinary tract function.

Pelvic Floor Training

Exercises designed to strengthen and improve voluntary control of pelvic floor musculature can support urinary continence in selected patients.

Training may improve coordination of muscles that contribute to urethral closure and pelvic support.

Urodynamic Assessment

External urethral sphincter function can be evaluated as part of urodynamic testing. Measurements of bladder pressure, urinary flow, urethral function, and sphincter muscle activity can help characterize abnormalities of storage and voiding.

Electromyography

Electromyography can be used to assess electrical activity associated with the striated urethral sphincter and pelvic floor during bladder filling and voiding.

Appropriate reduction in sphincter activity is expected during coordinated micturition.

Urinary Catheterization

During male urethral catheterization, the catheter passes through the region of the external urethral sphincter after traversing the spongy urethra.

Reflex or voluntary sphincter contraction can increase resistance to catheter passage. Force should not be used to overcome unexpected resistance.

External Sphincter During Continence and Voiding

Phase	External Urethral Sphincter Activity	Effect
Bladder filling	Active	Maintains urethral closure
Voluntary urine holding	Increased activity	Raises urethral resistance
Coughing or straining	Reflexly reinforced	Helps prevent leakage
Normal micturition	Relaxes	Reduces outlet resistance

Key Anatomical Features

• The external urethral sphincter is composed predominantly of striated skeletal muscle.
• It lies in the deep perineal region around the urethra.
• In males, it is closely related to the membranous urethra and apex of the prostate.
• In females, the external sphincter complex includes sphincter urethrae, compressor urethrae, and urethrovaginal sphincter components.
• Its principal somatic motor supply is associated with the pudendal nerve.
• The relevant spinal cord levels are primarily S2 to S4.
• It maintains tonic activity during bladder filling.
• Voluntary contraction allows urination to be delayed.
• It relaxes during normal micturition.
• Its activity must be coordinated with detrusor contraction.
• Reflex activation helps maintain continence when abdominal pressure increases.
• Damage to the sphincter or its neural pathways can contribute to urinary incontinence.
• Failure of appropriate relaxation can contribute to obstructed voiding.
• Its close relationship with the prostate makes it important during prostate surgery.

Clinical Importance

The external urethral sphincter is a major component of the urinary continence mechanism because it provides somatic and voluntary control over urethral closure. During bladder filling, tonic sphincter activity helps retain urine. During normal micturition, the sphincter relaxes in coordination with detrusor contraction so that urine can pass through the urethra with minimal outlet resistance.

Its anatomy is particularly important in prostate surgery, pelvic floor dysfunction, urinary incontinence, neurological bladder disorders, and urodynamic evaluation. Damage to the muscle itself, its supporting structures, or the neural pathways controlling it can interfere with continence or coordinated voiding.

Understanding the external urethral sphincter provides the foundation for studying urinary continence, micturition, pudendal nerve function, pelvic floor anatomy, membranous urethra, prostate surgery, stress urinary incontinence, neurogenic bladder, and detrusor-sphincter dyssynergia.