Cerebral cortex MS plaques can impair upper motor neurons that inhibit the micturition reflex.  This can lead to increased involuntary (ie, spastic) contractions and overactivity of the detrusor muscle, resulting in a small, contracted bladder resulting in urge incontinence These bladder spasms do not allow adequate time between the sensation to void and the loss of urine, which can be worsened in patients with gait abnormalities that limit their ability to quickly reach a bathroom

Demyelination of the sacral nerve roots can lead to impairment of the lower motor neuron input to the bladder, resulting in detrusor underactivity_>overflow incontinence. these patients would have a distended bladder and a large (>150 mL) postvoid residual urine volume.  In addition, multiple sclerosis affects the CNS, and sacral spinal roots are a part of the peripheral nervous system.