1 Because alpha motor neurons are the only way to stimulate extrafusal muscle fibers, the loss of these neurons causes a decrease in muscle tone. 2  Hyporeflexia. 3  The myotatic (stretch) reflex is weak or absent with lower motor neuron disorders, because the alpha motor neurons that cause muscle contraction are damaged. When alpha motor neurons die, the muscle fibers that they innervate become deprived of necessary trophic factors and eventually the muscle itself atrophies. 2  Weakness. 3  Because of the damage to alpha motor neurons and the atrophy of muscles, weakness is profound in lower motor neuron disorders.