system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Why does the urge to urinate or deficate come and go, instead of remaining constant until It's relieved?
gpt
Nerves really can only detect changes in signals.  If you've been drinking at night sit outside and stare at one particular light as strongly as you can. You'll notice your peripheral vision fade to black and you'll experience temporary blindness.


Also if you're laying in bed with your arm across your body, lay as perfectly still as possible and eventually you'll loose sensation of your arm.   

All sensory nerves work in this way.   When fecal matter moves into your rectum, your pressure nerves signal that your bowels are filling/stretching, and you get the urge to empty them.  If the volume stays the same you will not feel as strongly.    However getting up/ jumping up and down, will reactivate the nerves.