The external urethral sphincter muscle, also known in Latin asmusculus sphincter urethrae externus, plays a vital role in urinary continence. It is a skeletal muscle that encircles the urethra and allows voluntary control over the release of urine. Both males and females have this muscle, although there are some anatomical differences between the sexes. Understanding the structure, function, and importance of this muscle is essential in fields such as anatomy, urology, and physical therapy.
Anatomical Overview
Location and Structure
The external urethral sphincter is part of the urogenital diaphragm and is located in the deep perineal pouch. It surrounds the membranous portion of the urethra in males and the mid-portion of the urethra in females. Unlike the internal urethral sphincter, which is made of smooth muscle and functions involuntarily, the external sphincter is composed of striated skeletal muscle, allowing it to be consciously controlled.
Gender Differences
- In males: The muscle forms a circular ring around the membranous urethra. It is situated inferior to the prostate gland and superior to the bulb of the penis.
- In females: The muscle is less circular and more of a band-like structure that wraps around the urethra. It is closely associated with the vaginal wall and the compressor urethrae muscle.
Innervation and Blood Supply
Nerve Supply
The external urethral sphincter is innervated by thepudendal nerve, specifically its perineal branch. This nerve arises from the sacral spinal cord segments S2S4 and provides the motor input necessary for voluntary contraction of the sphincter. Because of its somatic innervation, individuals can learn to consciously control this muscle, for example, during toilet training.
Blood Supply
Arterial blood reaches the external urethral sphincter primarily through branches of theinternal pudendal artery, ensuring sufficient oxygen and nutrients to support its function. Venous drainage mirrors the arterial supply and feeds into the internal pudendal vein.
Function of the External Urethral Sphincter
Maintaining Continence
The most important role of the external urethral sphincter is to maintain urinary continence. When contracted, the muscle compresses the urethra and prevents the passage of urine. During activities that increase intra-abdominal pressure such as sneezing, coughing, or lifting the muscle reflexively contracts to maintain continence.
Voluntary Urination Control
This muscle also enables voluntary urination. When a person decides to urinate, the external urethral sphincter relaxes, allowing urine to flow through the urethra. This relaxation is coordinated with bladder contraction and is part of a complex reflex arc involving the central and peripheral nervous systems.
Development and Control
Early Life
Control over the external urethral sphincter develops during infancy and early childhood. This process is part of the maturation of the central nervous system and the pelvic floor musculature. Children gradually learn to consciously contract and relax this muscle, which is a key milestone in toilet training.
Neurological Regulation
The muscle’s function is regulated by both thepontine micturition centerin the brainstem and higher cortical centers. Voluntary suppression or initiation of urination involves signaling from these brain regions to the spinal cord and then to the pudendal nerve. Damage to these areas can result in loss of control over the muscle and lead to incontinence.
Clinical Relevance
Urinary Incontinence
Weakness or damage to the external urethral sphincter can result in stress urinary incontinence. This is especially common in women after childbirth or menopause. In men, incontinence can occur after prostate surgery, such as a radical prostatectomy, where the sphincter may be injured.
Pelvic Floor Dysfunction
The muscle is part of the larger pelvic floor group. Dysfunction in this area may involve not only the external sphincter but also surrounding muscles, nerves, and connective tissue. Symptoms may include leakage, urgency, or pain.
Neurogenic Bladder Disorders
Diseases such as multiple sclerosis, spinal cord injury, or Parkinson’s disease can impair the neural control of the external urethral sphincter. Patients may experience either retention of urine due to failure to relax the muscle or incontinence due to involuntary relaxation.
Strengthening and Rehabilitation
Pelvic Floor Exercises
Also known asKegel exercises, these are commonly prescribed to strengthen the external urethral sphincter and other pelvic floor muscles. Regular performance of these exercises can significantly improve urinary control and reduce the risk of incontinence.
Biofeedback and Physical Therapy
Biofeedback devices and physical therapy can help patients learn to engage the muscle properly. These tools provide visual or auditory signals to guide correct muscle contraction and relaxation.
Electrical Stimulation
In some cases, low-level electrical impulses are used to stimulate the muscle and enhance neuromuscular control. This method is often used when voluntary control is weak or compromised.
Surgical Considerations
Artificial Urinary Sphincter
For patients with severe incontinence due to sphincter damage, implantation of an artificial urinary sphincter is a surgical option. This device mimics the function of the natural muscle and allows the patient to control urination via a manual pump placed in the scrotum or labia.
Urethral Slings
In women, sling procedures are often performed to support the urethra and external sphincter, especially after childbirth or menopause. These slings reposition the urethra to improve closure pressure and support sphincter function.
Themusculus sphincter urethrae externus, or external urethral sphincter, is a critical structure in maintaining urinary continence. As a voluntary muscle, it allows individuals to control when they urinate, contributing to both physical health and quality of life. Its anatomical position, innervation, and function make it central to urological health in both males and females. Understanding how it works, what can go wrong, and how to strengthen or treat it when needed is vital for healthcare providers and patients alike. From childhood development to age-related decline, the external urethral sphincter remains an essential part of human anatomy and continence management.