This "Scishow" video is about internal and external anal sphincters, its length is just 3:51. The speaker, Hank Green, says:
The muscle that keeps things from leaking out of your body, your anal sphincter, is always clenching ...
... when smooth muscle contracts, it can stay contracted for a long time without using much ATP, so it won’t run out of fuel.
it comes with a slower contraction speed ...
The internal layer contracts involuntarily, but it relaxes when it gets the “gotta go” signal from your body.
The external sphincter is technically skeletal muscle, so it can contract voluntarily, ...
I think, these statements also can be said about internal and external urethral sphincters.
it is under the control of the autonomic nervous system
Voiding urine begins with voluntary relaxation of the external urethral sphincter.
In this Reddit askscience page NeuroBill says:
... A neuron either spikes, or does not. This causes it's target cells to be excited to a small, graded degree. If enough of these small graded inputs summate, then the cell receiving these inputs will spike.
So, involuntary sphincter should get neuron spikes for every its reclench action.
Main question is:
Where from does human male internal urethral sphincter get the spikes? Ie what mechanism provides regular spikes for it?
How frequently does i. u. s. get spikes?
How decision to pee makes that mechanism to stop sending spikes? (After writing this question i come to conclusion that the spikes frequency must be more than 1 per several seconds).
Answers may be generalised up to all involuntary sphincters, if you easily can. You can answer additionally just about human female internal urethral sphincter, human internal anal sphincter.
If you do not know about urethral sphincter, but just about anal sphincter, or do not know about male sphincter, but just about female, i would like to read about it, because it can be, potentially, for my point of view, (level of knowledge), similar to male internal urethral sphincter.