Frequency distribution of myelinated axons according to their diameters. Diameters of all myelinated axons were measured in the region containing the corticospinal tract at the lumbar segments of 2- and 6-month-old NFL-/- (N = 3 animals per age group) and control animals (N = 3 animals per age group). Data is presented based on diameters of all axons. Note the marked shift in distribution of axonal diameters towards smaller fibres. (*p < 0.05, **p < 0.01).