Motor neuron disease refers to a diverse group of disorders where only upper motor neurons, lower motor neurons, or both are affected. Upper motor neurons comprise the corticospinal tract and the corticobulbar tract, controlling the anterior horn cells and cranial nerve motor nuclei (collectively the lower motor neurons), respectively. "Bulb" refers to the lower brain stem, which is the location of the cranial nerve motor nuclei that innervate muscles of the jaws, face, palate, pharynx, larynx, and tongue. The severity and prognosis varies widely amongst the degenerative motor neuron diseases discussed here. In other instances, motor neurons can also be destroyed by rare immune-mediated disorders or viruses like polio. Spinal muscular atrophy is the term for a group of disorders involving just anterior horn cells. Many are hereditary in nature. The clinical findings consist of the lower motor neuron signs of weakness, atrophy, fasciculations, and loss of reflexes. Werdnig- Hoffman disease is an infantile onset spinal muscular atrophy with fatal outcome due to respiratory weakness. Less common types of spinal muscular atrophy become symptomatic in childhood or adult life, creating nonfatal disabilities. Benign focal manifests in adults as a slowly progressive atrophy of one limb or restricted amyotrophy segments of limbs, with a normal life span.