The disorder causes muscle weakness and muscle shrinking (atrophy) throughout the body. Both the upper motor neurons and the lower motor neurons die, making them stop sending messages to the muscles. Unable to function, the muscles gradually weaken and waste away (atrophy). Eventually, the ability of the brain to start and control voluntary movement is lost. However, even patients in later stages of the disease may still have the same intelligence, memory, and personality they had before it started.