ALS vs MS

http://www.cnsonline.org/www/archive/ms/ms-04.html

"sclerosis," which literally means hardening (as a result of increased connective tissue or glia).

Multiple sclerosis is a disease of myelin, not primarily of nerve cells. This myelin surrounds the axons, or the long process of the nerve cell.

The principle characteristic in the pathology of amyotrophic lateral sclerosis(ALS) is loss of motor nerve cells in the anterior horns of the spinal cord and in the motor nuclei of the brain stem.

  Thus, it is not primary demyelination, as it is in multiple sclerosis, that is the primary destructive effect in ALS.