Sphincteric incontinence is the involuntary loss of urine due to malfunction of the urinary sphincter. It’s like a leaky faucet.
The urinary sphincter is a specialized structure which lies in the wall of the urethra whose function in both sexes is to maintain urinary control.
How does the (Urinary) Sphincter work?
Normally, the urinary sphincter is kept closed in large part due to contraction of the smooth (involuntary) & striated (voluntary) muscle inside and around the urethral wall. During urination these muscles relax and open, allowing urine to pass through. Of course, if the muscles have become damaged or weakened, they do not stay tightly closed and incontinence can occur. The sphincter is actually composed of a number of different elements, not just muscle. Other components of the urethral wall including collagen & elastin provide the architectural framework for the sphincter, like the frame of a house. In addition, the inner lining of the urethra (mucosa) secretes a mucus layer that acts like an additional seal, similar to a rubber gasket on a garden hose. In women, this whole arrangement is rather simple.
What are the Internal and External Sphincters?
The junction between the urethra and the bladder is called the bladder neck or internal sphincter. Its wall is composed mostly of smooth muscle, which is arranged in both a circular and longitudinal pattern around the urethra. The smooth muscle of the urethra is called the internal sphincter.
There is striated muscle in the wall of the urethra and there is striated muscle outside the urethra called the periurethral striated muscle. Together, these constitute the external urethral sphincter, which is much less important than the internal sphincter. If the internal sphincter has been badly damaged, incontinence occurs even if the external sphincter works normally. The main function of the external sphincter is to allow you to suddenly interrupt the stream if you are in the middle of urinating or to hold back once you get a strong urge. If your internal sphincter has been damaged and you have severe incontinence, the chances are that you are still able to momentarily prevent incontinence by contracting your external sphincter. However, since it is a voluntary (striated) muscle, it fatigues easily (after only 10 – 15 seconds or so) and once that happens the urinary loss continues.
In women, because the sphincter is naturally much weaker than in men, incontinence is much more common. It occurs, in part, as a consequence of repeated stretching and damage to the nerves and muscles of the sphincter which occur during pregnancy and childbirth and in part to the effects of gravity which tend to make the vaginal muscles sag and weaken. Incontinence is so common in women that many consider it a normal part of aging.
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