The role of the pelvic floor in urination

May 26, 2025
10:30 am

The bladder functions in two phases: voiding and resting. We could also call them the "on" phase (micturition) and the "off" phase (rest). During the off phase, the detrusor muscle of the bladder wall is relaxed, so that the bladder can fill with urine without difficulty. At the same time, the internal sphincter (located in the bladder neck) and the external sphincter (part of the pelvic floor) are contracted. It is important to know that the functioning of the internal sphincter does not depend on our voluntary control, but on the autonomic nervous system, and in particular the sympathetic nervous system (the opposite of the parasympathetic). Thus, we cannot simply ask our internal sphincter to contract more or less. This mechanism will depend on certain reflexes, and will be affected by certain pathologies that affect the autonomic nervous system (central or spinal neurological diseases, stress) or that stimulate the production of more muscle cells at this level, as in the case of prostatic hypertrophy. Men with an enlarged prostate (hypertrophy), a very common pathology from the age of 50-60, often have an internal sphincter that is also hypertrophic. In addition to the difficulty in emptying the bladder due to the obstruction caused by the enlarged prostate at the level of the urethra, there is also hyperactivity of this muscle. This is why the treatment for this problem is often alpha-blocker drugs, which block the alpha-adrenergic receptors of the bladder neck, belonging to the sympathetic nervous system, and relax the muscle.

But back to the pelvic floor, unlike the involuntarily controlled internal sphincter, this set of muscles, and especially the external sphincter of the bladder, can be voluntarily controlled by our brain, via the pudendal nerve. When we feel the urge to urinate and cannot find a toilet, we can voluntarily contract this muscle to prevent urine leakage, even though our bladder is pushing to empty. The same happens if we exert ourselves, cough, laugh, etc.

The external sphincter does not function in an "on" and "off" mode like the bladder, but actually has three positions. Under normal conditions, it is in a state of mild tonic contraction. This allows it to prevent urine leakage when the bladder is filling and the pressure inside the abdomen is not very high. But, if the pressure is increased by any straining, the external sphincter can contract much more to prevent incontinence at that time. On the other hand, when we decide to empty our bladder, the sphincter has to relax completely. It should also be noted that, although it is a voluntarily controlled muscle, its control becomes automatic in childhood. Thus, when we are going to sneeze, we do not have to think "I have to contract my sphincter", but this act is carried out automatically. In the same way, if we want to urinate, our sphincter should be able to relax without having to actively think about it. The problem is that many people do not acquire these reflexes correctly, which leads to many urinary problems. This condition is known as "uncoordinated voiding" or "dysfuctional voiding". It should not be confused with dyssynergia (detrusor-sphincter dyssynergia), which is a pathology of neurological origin, where the external sphincter is constantly contracted, not because of a poorly acquired automatism, but because of an underlying nervous system problem (Parkinson's disease, spinal cord injury, disc herniation, diabetes with peripheral neuropathy, etc.). Although the result is the same, as the bladder has to empty when the "door is closed", the treatment is very different. I am not going to develop the subject of dyssynergia, as these are very complex pathologies that are not the subject of this article. I will focus on explaining voiding dysfunction of non-neurological origin, which is very common and is one of the main causes of urinary tract infections in children and adults. If you want to learn more about it, don't miss the next article.

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