M. Sphincter Urethrae Internum Nedir

The muscle known asm. sphincter urethrae internumplays a critical role in the urinary system by helping regulate the flow of urine from the bladder to the urethra. Although it operates involuntarily, this smooth muscle is essential for continence and urinary control. Understanding what the sphincter urethrae internum is, where it is located, and how it functions gives us valuable insight into the complex mechanisms of the lower urinary tract, especially in relation to bladder control and urological health.

Definition and Overview

The termm. sphincter urethrae internumrefers to theinternal urethral sphincter. This is a ring-like structure composed of smooth muscle fibers that surrounds the internal opening of the urethra at the neck of the bladder. Unlike the external urethral sphincter, which is made of skeletal muscle and under voluntary control, the internal sphincter isinvoluntary, meaning its actions are regulated by the autonomic nervous system without conscious input.

Primary Function

The main function of the internal urethral sphincter is to maintain urinary continence by keeping the urethra closed when urine is stored in the bladder. It also plays a role in preventing retrograde ejaculation in males by closing off the bladder during the emission phase of ejaculation.

Location and Structure

The m. sphincter urethrae internum is located at the junction between the bladder and the urethra. It is more developed in males than in females due to anatomical differences in the urinary tract.

Position in the Body

• In males: Found at the bladder neck, just above the prostate gland, and surrounds the beginning of the prostatic urethra.
• In females: Located at the internal urethral orifice, although it is less prominent compared to males.

Composition

This sphincter is made entirely ofsmooth muscle tissue. Its fibers are arranged circularly around the urethral opening, allowing it to contract and relax as needed. It is innervated by sympathetic and parasympathetic fibers of the autonomic nervous system.

Role in Urinary Function

The internal urethral sphincter is a key player in the regulation of urine flow from the bladder. It works in conjunction with other parts of the urinary system, including the detrusor muscle (which contracts to expel urine) and the external urethral sphincter (which provides voluntary control).

Urine Storage Phase

• The detrusor muscle remains relaxed.
• The internal urethral sphincter stays contracted to keep the urethra closed.
• This prevents urine from leaking into the urethra during bladder filling.

Urination (Micturition) Phase

• Parasympathetic stimulation causes the detrusor muscle to contract.
• The internal sphincter relaxes to open the urethra.
• Simultaneously, the external sphincter relaxes (under voluntary control) to allow the flow of urine.

Neurological Control

The function of m. sphincter urethrae internum is tightly regulated by theautonomic nervous system. This includes both sympathetic and parasympathetic pathways that coordinate bladder filling and emptying.

Sympathetic Innervation

During urine storage, sympathetic fibers from the hypogastric nerve stimulate the internal sphincter to stay contracted. This prevents accidental urine leakage and helps maintain continence.

Parasympathetic Innervation

When the bladder is full, parasympathetic nerves (primarily via the pelvic splanchnic nerves) signal the detrusor muscle to contract and the internal sphincter to relax. This process is part of the micturition reflex, allowing urination to occur.

Clinical Relevance

Any dysfunction in the internal urethral sphincter can lead to urinary problems. Understanding its role helps in diagnosing and treating several urological and neurological conditions.

Urinary Incontinence

Incontinence, or involuntary urine leakage, may result if the internal sphincter fails to remain closed during bladder filling. This can be due to aging, childbirth, nerve damage, or certain diseases such as multiple sclerosis or spinal cord injury.

Neurogenic Bladder

Damage to the nerves controlling the internal sphincter may cause problems with urine retention or release. Conditions such as spinal cord injuries, Parkinson’s disease, or diabetes can impact sphincter control.

Prostate Surgery Complications

In men, prostate surgery (like prostatectomy) can sometimes affect the function of the internal sphincter, resulting in temporary or permanent incontinence. Surgeons take special care to preserve the sphincter during such procedures.

Retrograde Ejaculation

In males, failure of the internal sphincter to close during ejaculation may result in semen entering the bladder instead of exiting through the urethra. This is called retrograde ejaculation and may be caused by nerve damage, certain medications, or surgical procedures.

Diagnostic and Therapeutic Approaches

Medical evaluation of internal sphincter function typically involves imaging and functional tests. Treatment may include medications, exercises, or surgery depending on the cause and severity of dysfunction.

Diagnostic Tools

• Urodynamic testing: Measures bladder pressure and sphincter activity during filling and emptying.
• Ultrasound or MRI: Helps visualize the bladder and sphincter anatomy.
• Cystoscopy: Allows direct inspection of the bladder neck and urethra.

Therapies and Interventions

• Pelvic floor exercises: Help strengthen surrounding muscles to support sphincter function.
• Medications: Anticholinergics or alpha-blockers may be used to regulate sphincter tone.
• Surgical procedures: In some cases, sphincter repair or artificial sphincter implantation is required.

Differences Between Internal and External Urethral Sphincters

It is important to distinguish between the internal and external sphincters, as both serve different roles in urinary control.

Internal Sphincter (M. Sphincter Urethrae Internum)

• Involuntary control
• Made of smooth muscle
• Located at the bladder neck
• Regulated by the autonomic nervous system

External Sphincter (M. Sphincter Urethrae Externus)

• Voluntary control
• Made of skeletal muscle
• Surrounds the membranous urethra
• Regulated by the somatic nervous system (pudendal nerve)

Them. sphincter urethrae internumis a vital smooth muscle responsible for involuntary control of urine flow from the bladder. Situated at the junction between the bladder and urethra, this internal sphincter ensures that urine remains in the bladder until it is time to void. By coordinating with the detrusor muscle and the external sphincter, it maintains urinary continence and contributes to normal urination. Understanding its anatomy, function, and related disorders is essential in managing urological health and addressing bladder dysfunctions effectively.