The urethral sphincter is a critical component of the urinary system, responsible for maintaining continence and regulating the flow of urine from the bladder to the external environment. It is a muscular structure composed of two distinct parts: the internal urethral sphincter and the external urethral sphincter.[2] These sphincters work together to ensure voluntary and involuntary control over urination.
The internal urethral sphincter, made of smooth muscle, operates under involuntary control, while the external urethral sphincter, composed of skeletal muscle, allows for conscious control over the release of urine. Proper functioning of these sphincters is vital for maintaining urinary health and preventing incontinence.
Anatomy
The urethral sphincter is anatomically divided into two main components: the internal urethral sphincter and the external urethral sphincter.[3] These components differ in structure, location, and control mechanisms.
Internal Urethral Sphincter
- Location:
- Structure:
- Composed of smooth muscle fibers arranged in a circular pattern.
- Encircled by connective tissue, providing additional support.[5]
- Control:
- Operates under involuntary control via the autonomic nervous system, specifically sympathetic innervation.
- Gender Differences:
- In males, it also functions as a barrier during ejaculation to prevent retrograde flow of semen into the bladder.
- In females, it serves primarily for urinary continence.
External Urethral Sphincter
- Location:
- In males, it surrounds the membranous urethra, which is the shortest and narrowest part of the urethra.
- In females, it surrounds the mid-urethra and is closely associated with the pelvic floor muscles.
- Structure:
- Composed of skeletal muscle fibers, allowing for voluntary control.
- Supported by connective tissues and ligaments in the pelvic floor.[7]
- Control:
- Regulated by the somatic nervous system via the pudendal nerve.
- Allows conscious control over urine release.
Gender Differences
- The male urethral sphincter system is longer and more complex due to its dual role in urinary and reproductive functions.
- The female urethral sphincter system is shorter and simpler but works in conjunction with the pelvic floor muscles to maintain continence.
The anatomical distinction between the internal and external urethral sphincters highlights their complementary roles in urinary regulation and continence, emphasizing their importance in both involuntary and voluntary control mechanisms.[8]
Location
The urethral sphincter is located along the urethra, with its position and surrounding structures differing between males and females. It consists of two components: the internal urethral sphincter and the external urethral sphincter, each with specific locations and anatomical relationships.
Internal Urethral Sphincter
- In Males:
- In Females:
- Situated at the junction of the bladder and the proximal urethra.
- Its position is less well-defined compared to males due to the absence of a prostate gland.
External Urethral Sphincter
- In Males:
- Surrounds the membranous urethra, located in the urogenital diaphragm.
- Lies distal to the prostate gland and proximal to the spongy (penile) urethra.
- In Females:
- Encircles the middle portion of the urethra.
- Located within the deep perineal pouch, closely associated with the pelvic floor muscles.
Gender Differences in Location
- Males: The urethral sphincter system is positioned to accommodate the dual urinary and reproductive roles of the urethra, with the internal sphincter also functioning to prevent retrograde flow of semen during ejaculation.[1]
- Females: The shorter urethra places the sphincter system closer to the bladder neck and external opening, with additional support from the pelvic floor muscles to maintain continence.
Structure and Composition
Internal Urethral Sphincter
- Composition:
- Made of smooth muscle fibers, which are involuntary in nature.
- Circularly arranged muscle bundles surround the internal urethral orifice to form a tight seal.
- Connective tissue surrounds the smooth muscle, providing additional structural support.
- Histological Features:
- Smooth muscle cells are spindle-shaped with centrally located nuclei.
- Dense innervation by autonomic nerves supports its involuntary action.
External Urethral Sphincter
- Composition:
- Consists of striated skeletal muscle, enabling voluntary control of urination.
- Muscle fibers are organized in a circular or semicircular arrangement around the urethra.
- Reinforced by connective tissues of the pelvic floor and perineum.
- Histological Features:
- Skeletal muscle fibers are multinucleated and striated, indicating their voluntary function.
- Dense innervation by the somatic nervous system through the pudendal nerve.
Supporting Structures
- Connective Tissue:
- Surrounds the sphincters to provide mechanical strength and maintain their shape.
- Includes collagen and elastic fibers, which allow flexibility and durability.
- Blood Supply:
- Rich vascularization supports muscle function and tissue health.
- Arteries and veins supply oxygen and remove metabolic waste from the surrounding muscle tissue.
- Associated Ligaments:
- In females, the urethral sphincter is supported by the pubourethral ligaments.
- In males, the puboprostatic ligaments contribute to its structural stability.
Gender-Specific Differences
- Males:
- The internal sphincter has a more robust structure to function as a barrier during ejaculation, preventing retrograde semen flow into the bladder.[4]
- The external sphincter surrounds the membranous urethra and is positioned deeper in the pelvic cavity.
- Females:
- The external sphincter is less complex but reinforced by adjacent pelvic floor muscles, including the compressor urethrae and urethrovaginal sphincter.
Function
The urethral sphincter plays a critical role in regulating the flow of urine from the bladder to the outside of the body, maintaining urinary continence, and, in males, assisting in reproductive functions. Its dual components, the internal and external sphincters, work together to ensure precise control.
Internal Urethral Sphincter
- Involuntary Control:
- Operates under the autonomic nervous system, primarily controlled by sympathetic and parasympathetic signals.
- Sympathetic stimulation keeps the sphincter contracted to maintain continence.
- Parasympathetic stimulation relaxes the sphincter during urination.
- Barrier Function:
- In males, it acts as a functional barrier during ejaculation, preventing the retrograde flow of semen into the bladder.
- In females, it ensures urinary continence by maintaining a tight seal at the bladder neck.
External Urethral Sphincter
- Voluntary Control:
- Operates under the somatic nervous system and is controlled by the pudendal nerve.
- Allows conscious regulation of urine release during urination and prevents involuntary leakage.
- Fine-Tuned Control:
- Provides additional support to the internal sphincter, especially during increased intra-abdominal pressure, such as in coughing, sneezing, or heavy lifting.
Coordination During Micturition
- Storage Phase:
- Both sphincters remain contracted to keep the urethra closed and maintain continence.
- The internal sphincter prevents involuntary urine leakage, while the external sphincter offers voluntary control.
- Voiding Phase:
- The internal sphincter relaxes under parasympathetic control, allowing urine to pass into the urethra.
- The external sphincter relaxes voluntarily to permit urine flow.
Gender-Specific Functional Roles
- Males:
- The internal sphincter plays a dual role in preventing urine leakage and controlling semen flow during ejaculation.
- The external sphincter ensures precise control over the release of both urine and semen.
- Females:
- The external sphincter works in close coordination with the pelvic floor muscles to maintain continence, especially given the shorter urethra.
Clinical Significance
The urethral sphincter plays a critical role in maintaining urinary continence, and its dysfunction or damage can lead to various clinical conditions that significantly impact quality of life.
Common Clinical Conditions include:
- Urinary Incontinence: Weakness or damage to the external sphincter can result in stress incontinence (leakage during activities like coughing or lifting) or urge incontinence (involuntary leakage due to an overactive bladder).[6]
- Neurogenic Bladder: Conditions such as spinal cord injuries, multiple sclerosis, or stroke can disrupt the neural control of the urethral sphincter, leading to retention or incontinence.
- Urethral Strictures: Scarring or narrowing of the urethra may impair sphincter function and lead to difficulty in urination or incomplete emptying of the bladder.
- Post-Surgical Complications: Procedures like prostatectomy in males or pelvic surgeries in females can damage the urethral sphincter, causing incontinence or sphincter dysfunction.
- Overactive Bladder: Dysfunctional coordination between the sphincters and the bladder muscle can lead to frequent and urgent urination.