4-Aminopyridine 4-aminopyridine Animals Axons Brain Injuries, Traumatic C57BL mouse Demyelination Electrophysiology Female Humans Kv1.2 Male Mice Mice, Inbred C57BL Multiple Sclerosis Traumatic brain injury White matter animal axon controlled study fampridine female human male mouse multiple sclerosis pathology randomized controlled trial traumatic brain injury