Motor neuron disease, also known as amyotrophic lateral sclerosis (ALS) or Lou Gehrig's disease, is a progressive disease that attacks motor neurons, components of the nervous system that connect the brain with the skeletal muscles. Skeletal muscles are the muscles involved with voluntary movement, like walking and talking. When your brain sends a signal telling a part of your body to move, the motor neurons transmit the command to the skeletal muscles. The muscles respond by contracting.

In ALS, the motor neurons deteriorate and eventually die, and though a person's brain is fully functioning and alert, the command to move never reaches the muscle. The patient may want to reach for a glass of water, for example, but is not able to do it because the lines of communication from the brain to the arm and hand muscles have been destroyed. The muscles eventually waste away from disuse, and a person in the late stages of Lou Gehrig's disease is completely paralyzed. Currently, there is no cure for ALS, although drug therapy can slow progression of the disease.