Internal Urethral Sphincter

1. Overview

The internal urethral sphincter is a ring of involuntary smooth muscle located at the junction of the urinary bladder and urethra. It plays a critical role in the storage phase of micturition by maintaining continence and preventing urine leakage. In males, it also serves an additional function by preventing retrograde ejaculation. This sphincter is regulated by the autonomic nervous system, particularly sympathetic fibers.

2. Location

• In males: The internal urethral sphincter encircles the bladder neck and the proximal prostatic urethra, just superior to the prostate gland. It lies just anterior to the seminal vesicles and inferior to the ureteric orifices.

• In females: It is located around the bladder neck, at the internal urethral orifice, and is less well-defined than in males due to the absence of a prostate and shorter urethra.

It surrounds the internal urethral orifice and marks the anatomical and functional transition between the bladder and urethra.

3. Structure

The internal urethral sphincter is composed of smooth muscle fibers, arranged in a circular pattern. Key structural characteristics include:

• Muscle type: Involuntary smooth muscle, unlike the striated external urethral sphincter.

• Autonomic innervation: Primarily sympathetic nerves (hypogastric plexus, T11–L2) that keep it contracted during urine storage. Parasympathetic input (S2–S4) promotes relaxation during voiding.

• Mucosal lining: Covered by transitional epithelium (urothelium), continuous with the bladder and proximal urethra.

The muscle is thicker in males, contributing more prominently to continence and ejaculatory function.

4. Function

The internal urethral sphincter primarily functions as a valve mechanism that prevents urine leakage during bladder filling. Its specific functions include:

• Maintaining continence: Remains contracted during the storage phase of the bladder cycle to prevent involuntary leakage.

• Coordinating micturition: Relaxes in synchrony with detrusor muscle contraction during voluntary urination.

• Preventing retrograde ejaculation (in males): Contracts tightly during ejaculation to prevent semen from entering the bladder.

5. Physiological Role(s)

The internal urethral sphincter contributes to urinary physiology in several key ways:

• Storage phase control: Sympathetic activation keeps the sphincter contracted, allowing the bladder to fill without leaking.

• Voiding reflex: Parasympathetic input during the micturition reflex causes detrusor contraction and sphincter relaxation, allowing urine to pass.

• Barrier function: Helps protect against infections by preventing ascending pathogens during storage.

• Male reproductive function: Prevents mixing of urine and semen during ejaculation.

6. Clinical Significance

Urinary Incontinence

Weakness or dysfunction of the internal urethral sphincter may lead to urge incontinence or overflow incontinence, especially in elderly individuals or after pelvic surgery. This is more prominent in males after prostate surgery where the internal sphincter may be damaged.

Bladder Outlet Obstruction

Inappropriate contraction or hypertrophy of the internal sphincter may contribute to functional obstruction, leading to symptoms like:

• Hesitancy

• Straining

• Incomplete emptying

Retrograde Ejaculation (Males)

Damage to the sympathetic innervation (e.g., post-prostatectomy, spinal injury) can impair closure of the internal sphincter, allowing semen to flow backward into the bladder during ejaculation. This condition may cause:

• Dry orgasm

• Infertility

• Hematuria post-ejaculation

Neurogenic Bladder

In patients with spinal cord injury or neurological diseases (e.g., multiple sclerosis), loss of autonomic control over the internal sphincter can result in detrusor-sphincter dyssynergia, causing:

• High intravesical pressure

• Vesicoureteral reflux

• Upper urinary tract damage

Pharmacologic Target

The internal urethral sphincter is influenced by drugs affecting smooth muscle tone:

• Alpha-1 blockers (e.g., tamsulosin): Relax the sphincter in conditions like benign prostatic hyperplasia (BPH) to improve urine flow.

• Anticholinergics: Reduce detrusor overactivity but may impair sphincter relaxation, requiring careful use.

Imaging and Endoscopy

The internal sphincter can be indirectly assessed through:

• Video urodynamic studies – Assess coordination between detrusor and sphincter.

• Cystoscopy – Visualizes the internal urethral orifice and evaluates sphincteric tone.