MV wrote the “Site-Specific Onset and Spread of Neurodegeneration in ALS” section. MJ wrote the “Role of Glial Cells in Driving Disease Progression” section. SS wrote the “Aging” section. AR conceived and prepared the figures, and wrote the “Introduction,” and “Anatomy of the Motor System,” “Genetic Mutations and Risk Factors in ALS,” and “Intrinsic Factors Specific to MN Subpopulations” sections. JA conceived the article, wrote the “Conclusion” section, contributed text in numerous sections, and edited the manuscript throughout for content and style consistency.