Amyotrophic lateral sclerosis, also known as ALS, is a progressive disease of the nervous system. Nerve cells, called neurons, are located in the spinal cord and brain and normally give signals to muscles. ALS causes these neurons to degenerate, or break down. So, when neurons stop sending signals to muscles, the muscles atrophy, which means they weaken and shrink. This may progress to paralysis. Spasticity is increased muscle tone that can make it hard to coordinate movement, and it may also develop as a result of ALS.

What is going on in the body?

The muscle weakness in ALS can affect the muscles of the limbs or of the tongue, mouth, and throat. The nerves that connect the brain and spinal cord, called motor neurons, can also degenerate.

What are the causes and risks of the disease?

Cases that run in families are often due to a defect on one of the chromosomes, the structures that hold genetic information. Otherwise, the cause of ALS is not known.