Amyotrophic lateral sclerosis (ALS), sometimes called Lou Gehrig's disease or classical motor neuron disease, is a rapidly progressive, invariably fatal neurological disease that attacks the nerve cells (neurons) responsible for controlling voluntary muscles. In ALS, both the upper motor neurons and the lower motor neurons degenerate or die, ceasing to send messages to muscles. Unable to function, the muscles gradually weaken, waste away, and twitch. Eventually the ability of the brain to start and control voluntary movement is lost. Symptoms are usually first noticed in the arms and hands, legs, or swallowing muscles. Muscle weakness and atrophy occur on both sides of the body. Individuals with ALS lose their strength and the ability to move their arms and legs, and to hold the body upright. When muscles in the diaphragm and chest wall fail to function properly, individuals lose the ability to breathe without ventilatory support. The disease does not affect a person's ability to see, smell, taste, hear, or recognize touch. Although the disease does not usually impair a person's mind or personality, several recent studies suggest that some people with ALS may develop cognitive problems involving word fluency, decision-making, and memory. The cause of ALS is not known, and scientists do not yet know why ALS strikes some people and not others.