MAPT and Alzheimer disease: Importantly, these functional neural ratios (neuronal and non-neuronal cells) kept below the “dysfunctional cellular ratio thresholds” would not be simply determined by the passive accumulation of a single or multiple brain pathologies (e.g. accumulation of extracellular insoluble β-amyloid in the cerebral cortex or intracellular formation of hyperphosphorylated-tau neurofibrillary tangles as in AD) but also on how certain “predisposing genes” in a specific CNS/brain/person have mutually interacted with a specific group of environmental factors throughout his/her life.