How the External Urethral Sphincter Works in Males

The external urethral sphincter in men is a ring-like band of skeletal muscle that wraps around the membranous urethra, just below the prostate, and its primary job is keeping you dry. Unlike the internal sphincter at the bladder neck, which operates on autopilot through smooth muscle, the external sphincter is the one you can voluntarily squeeze to stop the flow of urine. It also plays a role in ejaculation and becomes a critical structure during prostate surgery, where even small amounts of damage can tip the balance toward incontinence.

Where It Sits and What It Actually Looks Like

Textbook diagrams often show the external urethral sphincter as a neat circular ring sitting below the prostate, sometimes labeled as part of an “urogenital diaphragm.” The real anatomy is less tidy. Detailed cadaver studies have shown that in adult men, the muscle does not form a complete circle around the urethra. Instead, the striated muscle fibers are concentrated along the front and sides of the membranous urethra, forming an omega-shaped loop that thins out or disappears entirely behind the urethra, where the fibers insert into the perineal body. The notion of a strong circular band completely encircling the urethra has not held up under careful dissection.¹

The fibers themselves are small. They run about 25 to 30 percent smaller than fibers in surrounding muscles, and they sit embedded in connective tissue that makes the whole structure surprisingly hard to see with the naked eye.² The fibers extend not just at the level of the membranous urethra but also travel upward along the front and sides of the prostate toward the bladder base. This vertical spread means the sphincter is less like a doorstop and more like a muscular sleeve that hugs a sizable stretch of the lower urinary tract.

Supporting the sphincter from behind are additional muscular slings. Two muscles, the puboperinealis and rectourethralis, run from the pubic bone posteriorly to flank the urethra and converge behind it. Together they act as a hammock, helping hold the urethra in position and contributing to continence.³

How It Stays “On” All Day

You do not consciously think about keeping your sphincter contracted, yet it stays shut around the clock. The explanation lies in muscle fiber type. The external urethral sphincter is composed almost entirely of type I (slow-twitch) fibers, with a mean diameter of roughly 17 micrometers and no muscle spindles. Slow-twitch fibers are built for endurance rather than power: they resist fatigue and maintain a low-level contraction, or “tone,” for hours without tiring out.⁴ This is the same fiber type that dominates in postural muscles like those running along the spine. You can also squeeze the sphincter harder on demand, for instance when you feel a sudden urge, but its default state is a steady background contraction that keeps the urethra sealed.

The Nerves That Run It

The external sphincter is under voluntary (somatic) control, meaning the signals come from nerves you can consciously influence rather than from the autonomic system that governs things like heart rate. The bulk of its nerve supply comes from sacral spinal roots S2 and S3, traveling through both the pudendal nerve and branches from the pelvic plexus. Dissection studies have shown that stimulating the S3 nerve root alone accounts for roughly 70 percent of the sphincter’s closure pressure, with the remaining 30 percent coming from S2 and S4.⁵

The wiring gets more complex closer to the prostate. The sphincter also receives small nerve branches that split off from the dorsal nerve of the penis after it separates from the pudendal nerve. These branches sit very close to the tip of the prostate, typically within a few millimeters of the apex. In half of cadavers studied, these branches appeared on only one side or consisted of just a single twig per side.⁶ Their proximity to the prostate makes them vulnerable during surgical dissection, which is one reason even nerve-sparing techniques cannot always guarantee full continence afterward.

Additional pathways exist through the pelvic splanchnic nerves. In most men, the lowest branch of the pelvic splanchnic nerve (typically from S4) forms a shared trunk with the nerve to the levator ani, and connections between the pudendal nerve and these pelvic-plexus branches have been observed.⁷ This redundancy in nerve supply is actually a good thing: it means the sphincter can sometimes retain partial function even if one pathway is damaged.

What Happens When You Urinate

Urination requires a carefully coordinated sequence. The external urethral sphincter relaxes first, but that alone is not enough to start the stream. Voiding does not begin until the bladder generates enough pressure to equal or exceed the pressure at the bladder neck.⁸ So the process is a handoff: the brain tells the sphincter to stand down, the detrusor (bladder wall muscle) contracts, and once the pressure gradient favors outflow, urine passes. After the bladder empties, the sphincter contracts again and background tone resumes.

When this coordination breaks down, the result is a condition called detrusor-sphincter dyssynergia. Instead of relaxing while the bladder contracts, the sphincter contracts at the same time, working against the detrusor. This is most commonly seen in people with spinal cord injuries or neurological conditions like multiple sclerosis, because the brain’s coordinating signals can no longer reach the lower spinal centers properly. Diagnosis typically involves urodynamic studies combined with electromyography to catch the mistimed contraction.⁹ Measuring sphincter pressure alone has proven unreliable for detecting this problem: one study found that external urethral sphincter pressure measurement picked up only about 15 percent of cases that EMG and imaging caught.¹⁰

Treating Detrusor-Sphincter Dyssynergia

For men with spinal cord injuries whose sphincter fights the bladder during voiding, botulinum toxin injections into the sphincter have become a standard treatment. The toxin temporarily weakens the sphincter’s involuntary clenching, allowing the bladder to empty more completely. In a study of 24 spinal cord injury patients, the treatment significantly improved dyssynergia in 21 of them, with most also seeing a drop in the amount of urine left in the bladder after voiding.¹¹

Results are not universal, though. Poor outcomes are significantly more likely when bladder neck dyssynergia is also present or when the detrusor itself is not generating a proper contraction. The severity of the dyssynergia before treatment also matters: the worse the residual volumes going in, the worse they tend to remain after injection.¹²

The Sphincter’s Role in Ejaculation

The external urethral sphincter does more than manage urine. During ejaculation, the bladder neck (internal sphincter) closes to prevent semen from traveling backward into the bladder, while the external sphincter relaxes to allow semen to exit. When this coordination fails, the consequences extend beyond urinary problems. In rat models, denervation of the external urethral sphincter produced both urinary incontinence and retrograde ejaculation, where semen flows back into the bladder instead of being expelled.¹³

Dynamic ultrasound imaging of men with complete retrograde ejaculation has identified distinct functional patterns. In spinal cord injury patients, the problem involves a timing mismatch: the bladder neck closes too late and the external sphincter fails to relax, leading to post-ejaculatory reflux.¹⁴ This dual function is one reason that procedures affecting the sphincter can have reproductive as well as urinary consequences.

Why Prostate Surgery Puts the Sphincter at Risk

Radical prostatectomy, the surgical removal of the prostate for cancer, is one of the most common situations where the external urethral sphincter faces direct threat. The prostate sits immediately above the sphincter, and dissecting it away inevitably brings instruments close to the sphincter’s muscle fibers and the tiny nerve branches feeding them. Postoperative urinary incontinence is one of the most feared side effects for patients.

Surgical techniques that prioritize preserving the full functional length of the sphincter have shown clear benefits for early recovery. One study comparing a full-length preservation technique against standard dissection found that about half of the patients in the preservation group were continent within a week of catheter removal, compared to roughly a third using the standard approach. By one year, both groups converged at around 95 percent continence, but those early weeks matter enormously to quality of life.¹⁵

The length of the membranous urethra before surgery also predicts how quickly continence returns. A meta-analysis pooling nearly 7,000 patients found that each additional millimeter of preoperative membranous urethral length increased the odds of regaining continence at three, six, and twelve months after surgery. Every extra millimeter was associated with about 9 percent greater odds of recovery overall.¹⁶ MRI measurement of this length before surgery is now recommended as a planning tool, because men with shorter membranous urethras may need more aggressive sphincter-sparing techniques or should be counseled to expect a longer road to dryness.

Preoperative testing of sphincter function may also help predict who will struggle. A study combining urethral pressure measurements with needle electromyography of the sphincter found that certain pressure and wave-amplitude patterns could predict post-prostatectomy incontinence with roughly 88 percent sensitivity.¹⁷ A sphincter that is already weak before the prostate comes out has less margin for any surgical insult.

Pelvic Floor Training and the Sphincter

Pelvic floor muscle training is the most accessible intervention for strengthening the external urethral sphincter, both before and after prostate surgery. A randomized controlled trial comparing a pre-surgery training program to no training found that the trained group leaked significantly less after catheter removal and returned to continence faster. Their pelvic floor function measures remained consistently better at every follow-up time point, and they reported less bother from urinary symptoms.¹⁸

MRI evidence suggests the sphincter itself can physically grow in response to training. In older patients with stress urinary incontinence, 12 weeks of structured pelvic floor exercises produced visible hypertrophy of the external urethral sphincter on imaging, indicating that at least some of the age-related decline in sphincter function can be reversed.¹⁹ This is encouraging because many men assume that once the sphincter weakens, exercise cannot rebuild it, but the slow-twitch fibers that dominate the sphincter respond to sustained low-level training just as postural muscles do elsewhere in the body.

When Training Is Not Enough: Artificial Sphincters

For men with severe stress urinary incontinence that does not respond to conservative measures, the artificial urinary sphincter remains the gold standard surgical option. The device consists of an inflatable cuff placed around the urethra, a pressure-regulating balloon, and a pump implanted in the scrotum. The cuff keeps the urethra compressed until the patient squeezes the pump to deflate it, allowing urination.

Recent trial data show strong results at one year: 94 percent of participants experienced at least a 50 percent reduction in urine leakage, and 60 percent reported using no pads at all. Quality-of-life scores and daily incontinence episodes also improved substantially after device activation.²⁰ Patient satisfaction is generally high, with up to 90 percent reporting satisfaction and nearly all saying they would recommend the device to someone in the same situation.²¹

The catch is durability. Long-term nationwide data show that the device survival rate drops from about 76 percent at two years to roughly 57 percent at ten years, with urethral erosion and infection being the main reasons for removal. About one in four patients will need revision surgery at some point.²² This does not mean the device fails catastrophically for most people: among those whose device was still in place at last follow-up, about 81 percent described their urinary condition as causing no more than minor discomfort, and 99 percent would recommend it. But anyone considering the procedure should understand that it is not a one-time fix. There is a realistic chance of needing a second operation years down the road.

How the Sphincter Develops Before Birth

The external urethral sphincter begins to take shape surprisingly early in fetal life. By about ten weeks of gestation, both male and female fetuses already show an omega-shaped sphincter around the urethra. At nine weeks, the tissue is still undifferentiated connective-tissue cells, but even at that stage the tissue is denser at the front of the urethra and looser behind it, foreshadowing the adult anatomy where striated fibers concentrate anteriorly and thin out posteriorly.²³ In fetal specimens, striated muscle fibers do form a more complete ring around the urethra, including the dorsal (back) side. It is only after birth, during growth into adulthood, that the dorsal fibers largely disappear, leaving the omega shape seen in adults.²⁴

Pharmacological Approaches and Regenerative Research

Drug-based treatments targeting the external sphincter are still mostly experimental, but a few avenues have attracted serious attention. Nitric oxide donors, a class of drugs that includes medications already used for heart conditions, have been shown to reduce the resting pressure and contractile strength of the external sphincter when given sublingually. The effect lasted at least an hour without accelerating muscle fatigue, raising the possibility that these drugs could one day help men whose sphincter refuses to relax properly during voiding.²⁵

On the regenerative medicine front, animal studies have demonstrated that stem cells injected around a damaged sphincter can differentiate into muscle-like cells and measurably improve sphincter function. In one rat model of stress urinary incontinence, bone marrow stem cells restored the damaged external sphincter and significantly raised the pressure required to cause leakage.²⁶ Translating this to humans remains a work in progress, with early-phase clinical investigations exploring both stem cell and exosome-based approaches for post-prostatectomy incontinence.²⁷ Neither pharmacological relaxants nor regenerative injections are standard care yet, but the sphincter’s relatively simple structure and accessible location make it a plausible target for both approaches.

The Rhesus Monkey Comparison

Researchers who study the sphincter in animal models have a practical problem: how closely does any given animal’s urethral anatomy match a human’s? The rhesus monkey has been considered one of the closest matches, and histological comparison confirms that both species share a sphincter muscle with distinct smooth and striated components, independent of the pelvic floor musculature. Neither species has a true urogenital diaphragm, despite what older textbooks claim. One difference, though, is that the bladder-neck sphincter present in human men does not exist in rhesus monkeys, which limits how well monkey models can replicate conditions affecting the bladder outlet as a whole.²⁸