The sphincter urethrae is an essential component of the human urinary system, responsible for controlling the release of urine from the bladder through the urethra. Its function is crucial for maintaining continence and ensuring that urination occurs only when it is socially and physiologically appropriate. Found in both males and females, this muscle plays a key role in the voluntary control of urination, which involves a complex interaction between neural signals, muscular coordination, and internal pressure within the urinary tract.
Overview of the Sphincter Urethrae
Definition and Function
The sphincter urethrae is a circular band of skeletal muscle that surrounds the urethra and allows voluntary control over urination. It is part of the external urethral sphincter mechanism, as opposed to the internal sphincter which is composed of smooth muscle. Its primary function is to close the urethra, preventing the passage of urine until the individual consciously decides to void the bladder.
Location and Structure
The location of the sphincter urethrae differs slightly between sexes due to anatomical differences:
- In males: The sphincter urethrae surrounds the membranous part of the urethra and is located just below the prostate gland, within the deep perineal pouch.
- In females: It encircles the urethra more diffusely and is part of a broader structure known as the urethrovaginal sphincter, which also supports the vagina.
Components and Associated Muscles
External vs Internal Sphincters
- External urethral sphincter: This is the sphincter urethrae muscle, under voluntary control, composed of striated skeletal muscle fibers.
- Internal urethral sphincter: Located at the bladder neck, this smooth muscle is under involuntary control and plays a role in preventing urine leakage and retrograde ejaculation in males.
Pelvic Floor Muscles
The sphincter urethrae works in concert with other muscles of the pelvic floor, including the levator ani and the coccygeus, to maintain continence. These muscles provide support to pelvic organs and assist in resisting increases in intra-abdominal pressure during actions like coughing, lifting, or sneezing.
Innervation and Control
Somatic Nervous System
The sphincter urethrae is innervated by the pudendal nerve, which arises from the sacral spinal cord segments S2S4. Because this muscle is made of skeletal muscle fibers, its action is consciously controlled, distinguishing it from the autonomically-controlled internal sphincter.
Neural Coordination
Urination, or micturition, is governed by a combination of voluntary and involuntary mechanisms. The brain’s pontine micturition center and the spinal cord coordinate the signals that either keep the sphincter contracted or allow it to relax during voiding. Damage to this coordination such as in spinal cord injuries or neurological diseases can lead to urinary incontinence or retention.
Physiological Role in Urination
Storage Phase
During the storage phase of bladder filling, the detrusor muscle (muscle wall of the bladder) is relaxed, while the internal and external urethral sphincters remain contracted. The sphincter urethrae holds urine within the bladder by sealing off the urethra.
Voiding Phase
When a person decides to urinate, the brain sends signals that cause the detrusor muscle to contract and the sphincter urethrae to relax. This coordination allows urine to flow through the urethra and out of the body. Involuntary urination occurs when this mechanism is disrupted.
Clinical Relevance
Urinary Incontinence
Incontinence refers to the inability to control urination. Weakness or dysfunction in the sphincter urethrae can lead to stress urinary incontinence, where urine leaks during activities that increase abdominal pressure. This condition is more common in women, especially after childbirth or menopause.
Urinary Retention
Conversely, excessive tightness or spasticity of the external sphincter may prevent normal urination, resulting in urinary retention. This can be seen in individuals with neurological diseases such as multiple sclerosis or spinal cord injuries.
Pelvic Floor Rehabilitation
Pelvic floor muscle training, including exercises like Kegels, can strengthen the sphincter urethrae and surrounding musculature. This non-invasive therapy is widely used to treat mild to moderate cases of urinary incontinence.
Surgical Considerations
In cases of prostate surgery, particularly radical prostatectomy, the sphincter urethrae may be at risk of damage, leading to postoperative incontinence. Surgical techniques often aim to preserve this muscle to maintain continence.
Development and Histology
Embryological Origin
The sphincter urethrae develops from the mesodermal tissues during fetal development, forming as part of the urogenital diaphragm. It begins to function as the nervous system matures, allowing infants to gradually gain voluntary control over urination as they age.
Tissue Characteristics
Histologically, the sphincter urethrae is composed of striated skeletal muscle fibers arranged in a circular pattern around the urethra. These fibers are capable of sustained contraction, which is necessary for continuous continence throughout the day.
Differences Between Males and Females
Male Anatomy
- Encircles the membranous urethra
- Located within the deep perineal pouch
- Closely related to the prostate gland
Female Anatomy
- Surrounds a shorter urethra
- Integrated into a broader muscular structure with the vaginal wall
- May be more prone to weakening due to childbirth or hormonal changes
The sphincter urethrae is a vital muscle that allows for voluntary control of urination. Through its role in the external urethral sphincter mechanism, it helps maintain urinary continence and prevents involuntary leakage. Innervated by the pudendal nerve and composed of skeletal muscle, it operates under conscious control and responds to various physiological and neurological cues. Its proper function is essential for daily life, and dysfunction can lead to significant medical and quality-of-life issues. Understanding the anatomy, physiology, and clinical importance of the sphincter urethrae provides valuable insight into the broader workings of the urinary system and pelvic floor health.