A type of disease in which the nerve cells in the brain that act on muscle cells break down and stop working. This affects basic activities such as speaking, walking, breathing, and swallowing. Symptoms include muscle weakness, wasting, twitching, trouble swallowing, and slowly becoming paralyzed. Motor neuron diseases are sometimes inherited. Amyotrophic lateral sclerosis (ALS or Lou Gehrig’s disease) is the most common type of motor neuron disease.