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I wonder about this. What's the biology of the transition from wetting the bed at night to holding urine at night? Is there a chemical change with how the bladder muscles contract?

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    $\begingroup$ When I first read the question, I thought the question deals with the difference between bladder activity during the day and during the night. A second read two days later makes me think you are actually asking about maturation of the bladder, or in other words, what is the process by which children "learn" to control the bladder during the night when they are off diapers. It seems the answer you accepted deals with the former interpretation, but your phrasing points to the latter. Can you please clarify your intention? $\endgroup$ – Don_S Feb 16 '17 at 8:58
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I'm not sure how much the bladder has a role in this. Mostly, the reduced frequency of urination during the night is the result of less urine production by the kidneys. This is in response to a hormone called ADH, antidiretic hormone (it has other names as well). ADH acts on the distal convoluted tubules of the nephrons in the kidney to fine-tune the reabsorbtion of water in the tubule bringing it back into the circulatory system. This hormone is released in reponse to a few factors including blood volume. But it's release is also cyclic over a 24 hour cycle. https://www.ncbi.nlm.nih.gov/pubmed/202988

It can happen that this daily cycle of ADH release intensity gets out of adjustment and you urinate more frequently at night, perhaps more than in the day. This has happened to me. Also, according to my GP, it is a frequent cause of bed wetting in children.

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The bladder is controlled by both the autonomic and somatic nervous systems. This means that part of the bladder activity is voluntary and another part is involuntary - more precisely, the detrusor (which is the smooth muscle in the bladder wall) is controlled involuntarily, while the external sphincter (the muscle around the bladder neck) is controlled voluntarily (see picture, this is the female bladder).Bladder anatomy

I found all kinds of references that describe the maturation process of the bladder during infancy. This paper, from Pheonix Children's Hospital, is about voiding disorders in children. It notes in the first paragraph three conditions that make up the bladder's maturation process (the rest of the paper discusses voiding disorders):

  1. The bladder capacity must increase.
  2. The child must develop voluntary control over the sphincter muscle in order to start and stop voiding.
  3. the child must develop voluntary control over the infantile voiding reflex which allows the child to stop involuntary bladder contractions.

This article discusses the topic of bladder maturation in greater detail (see the first section titled: MATURATION OF BLADDER CONTINENCE). From the article: "As bladder urine increases, a critical volume is reached (functional bladder capacity), which initiates a spontaneous reflex contraction that is mediated by the sacrospinal ganglia. As the bladder contraction ensues, the increased bladder pressure overcomes the detrusor muscles at the bladder neck and spontaneous emptying occurs. As the infant grows and matures, control centres in the pons and medulla initiate inhibitory signals that slowly dampen this reflex over a period of years ... Nighttime continence is achieved in association with the abolition of spontaneous bladder contractions, mediated by the pons and medulla; a nocturnal bladder volume less than the nocturnal functional bladder capacity (NFBC), which is ADH-dependent; and the achievement of prolonged rapid eye movement (REM) sleep and attainment of sleep arousal."

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