In SMA, reduced levels of SMN protein trigger abnormal function and apoptosis of α-motor neurons in the spinal cord and in brainstem regions, which in turn progressively reduces the number of muscle fibers and decreases strength in trunk muscles including respiratory-related muscles, limb muscles, and swallowing-associated muscles (regulates by the medulla oblongata region). This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.