Among the many muscles that play an essential role in the human body’s urinary system, the external sphincter urethrae muscle is particularly important for maintaining voluntary control over urination. This small but powerful muscle acts as a valve around the urethra, allowing individuals to delay urination until an appropriate time. Both men and women have this muscle, and while its structure and position slightly differ between sexes, its overall function remains the same ensuring continence and regulating the release of urine.
Location and Structure of the External Sphincter Urethrae Muscle
General Anatomy
The external sphincter urethrae is a skeletal muscle that surrounds the membranous part of the urethra. Unlike smooth muscles that operate involuntarily, this muscle is under voluntary control, meaning a person can consciously contract or relax it.
In Males
In males, the external sphincter urethrae encircles the membranous urethra, located just below the prostate gland. It is part of the deep perineal pouch and contributes to the formation of the urogenital diaphragm. This muscle is more tube-like in structure, forming a circular band around the urethra that tightly closes it when contracted.
In Females
In females, the external sphincter urethrae surrounds the middle portion of the urethra. However, the female version is not a complete circular band like in males. Instead, it is more of a U-shaped or semicircular structure and is closely associated with additional muscular components such as the compressor urethrae and urethrovaginal sphincter. These associated muscles aid in increasing the strength of voluntary closure.
Function of the External Sphincter Urethrae
Voluntary Control of Urination
The primary function of the external sphincter urethrae muscle is to maintain urinary continence by closing the urethra. It contracts to prevent the outflow of urine from the bladder, especially in situations where urination would be inappropriate, such as during social activities or sleep. This muscle provides a conscious level of control over bladder function.
Coordination with the Internal Urethral Sphincter
The external sphincter works in coordination with the internal urethral sphincter, which is composed of smooth muscle and located at the junction of the bladder and urethra. While the internal sphincter offers involuntary control, the external sphincter adds a layer of voluntary regulation, forming a dual mechanism that ensures urine is only released at the right time.
Role During Micturition
During the process of micturition, or urination, the external sphincter must relax to allow the passage of urine. The brain sends a signal through the spinal cord to this muscle, initiating relaxation. This relaxation, combined with bladder contraction, enables the flow of urine. After urination, the external sphincter contracts again to maintain continence.
Nerve Supply
Pudendal Nerve
The external sphincter urethrae is innervated by the pudendal nerve, which originates from the sacral spinal cord segments S2 to S4. This somatic nerve provides voluntary control, making it possible for individuals to consciously hold or release urine depending on the social context or physical situation.
Neurological Control and Reflexes
The brain, spinal cord, and peripheral nerves must coordinate effectively for proper control of this muscle. Damage to the spinal cord or pudendal nerve can lead to incontinence, as the muscle may lose its ability to contract when needed. Neurological diseases such as multiple sclerosis or spinal injuries can affect this control mechanism significantly.
Clinical Significance
Urinary Incontinence
If the external sphincter urethrae becomes weak or damaged, urinary incontinence may occur. This condition is more common in older adults and women who have experienced childbirth. In males, surgical procedures such as prostatectomy can sometimes damage the muscle or its nerve supply, leading to incontinence.
Pelvic Floor Dysfunction
Since the external sphincter is part of the broader pelvic floor muscle group, dysfunction in this region can lead to both urinary and fecal incontinence. Strengthening exercises like Kegels are often recommended to enhance the function of the external sphincter and surrounding pelvic floor muscles.
Stress Incontinence
In cases of stress incontinence, actions like coughing, sneezing, or laughing put pressure on the bladder and may lead to leakage if the external sphincter is weak. Strengthening this muscle can significantly reduce or eliminate stress incontinence symptoms.
Exercises and Strengthening
Pelvic Floor Exercises
Kegel exercises are commonly prescribed to improve control over the external sphincter urethrae. These exercises involve consciously contracting and relaxing the pelvic floor muscles, which include the external sphincter. Regular practice can strengthen the muscle and enhance continence.
Biofeedback Therapy
In some cases, biofeedback is used to help individuals locate and train the correct muscles. This therapy provides visual or auditory signals when the muscle is properly contracted, helping users better control and strengthen the sphincter.
Development and Anatomy Across Life Stages
Fetal Development
The external sphincter urethrae begins to form during fetal development, and its muscle fibers become more distinct as the fetus matures. The full development of nerve control over this muscle does not occur until after birth, which is why infants do not have voluntary control over urination.
Childhood and Toilet Training
During early childhood, as the nervous system matures, children gain the ability to control the external sphincter urethrae. This development is essential for successful toilet training, typically achieved between the ages of 2 and 4 years.
Aging and Muscle Tone
As people age, muscle tone in the pelvic floor including the external sphincter can weaken. Hormonal changes, decreased activity, and muscle atrophy can contribute to reduced control, which is why urinary incontinence becomes more common among elderly individuals.
Summary of Key Points
- The external sphincter urethrae muscle is a voluntary skeletal muscle surrounding the urethra.
- Its primary function is to maintain urinary continence by constricting the urethra.
- It is controlled by the pudendal nerve (S2S4), allowing conscious regulation.
- This muscle works in coordination with the internal urethral sphincter for full bladder control.
- Pelvic floor exercises can improve the strength and function of the external sphincter.
- Dysfunction or injury can lead to urinary incontinence, often requiring therapeutic or surgical intervention.
The external sphincter urethrae muscle plays a critical role in the urinary system, offering the ability to consciously control when urine is released. Whether in daily life or during medical recovery, maintaining the health and strength of this muscle is essential for continence and quality of life. Understanding its anatomy, function, and clinical relevance provides valuable insight into how the human body regulates one of its most fundamental functions urination.