hyoscyamine posted this great pic below

Here is my summary of the picture/video that was posted:

There are 3 pathways involved in peeing:

1. Pelvic n. (aka pelvic splanchnic n.) sends parasympathetic fibers to deltrusor to contract --> squeeze bladder and pee.

2. Hypogastric n. sends sympathetic fibers to the deltrusor to relax, as well as the internal sphincter to contract --> hold back pee

3. Pudendal n. sends somatic fibers (under conscious control) to the external sphincter to contract --> hold back pee

In this question, the patient's bladder is filling up so much that it's forced to overflow. That means there is a problem with scenario 1 - damage to pelvic n. so that he can't squeeze his bladder even when it's super full.

The sacral micturition center is 1 of 3 components of micturition reflex (other 2 are from pontine reticular formation and cerebral cortex). Sacral mict center = S2-S4 spinal cord level traveling from ventral white matter in the Pelvic nerves, responsible for bladder contraction via the cholinergic nerves (think it acts on the M3 receptor in the bladder detrusor?). If you lose these nerves (i.e. in pelvic fracture), will result in overflow incontinence.

pelvic nerve--> increases voiding urine, m3 receptor, parasympathetic hypogastric nerve--> increases urinary retention, sympathetic, beta 3 and alpha1 receptors. pudendal nerve--> not part of micturition, its a somatic component and has a nicotinic receptor.

surprisingly good explanation and picture found on this site. something along the lines of micturination reflex (aka bladder is full and you piss reflex) is gone so the afferent fibers carrying the signal that the bladder is full aren't working. those fibers are carried in the pelvic nerves

ttps://www.toppr.com/guides/biology/excretory-products/micturition/