Motor neuron disease comprises a group of severe disorders of the nervous system characterized by progressive degeneration of motor neurons (neurons are the basic nerve cells that combine to form nerves). Motor neurons control the behavior of muscles. Motor neuron diseases may affect the upper motor neurons, nerves that lead from the brain to the medulla (a part of the brain stem) or to the spinal cord, or the lower motor neurons, nerves that lead from the spinal cord to the muscles of the body, or both. Spasms and exaggerated reflexes indicate damage to the upper motor neurons. A progressive wasting (atrophy) and weakness of muscles that have lost their nerve supply indicate damage to the lower motor neurons.