hyoscyamine posted this great pic below
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.
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