ALS damages the motor neurons (nerve cells that control voluntary muscle movement) in the brain and spinal cord. Messages from the brain’s motor neurons are called the upper motor neurons, and are transmitted to the lower motor neurons in the spinal cord, and from there to specific muscles.
As the neurons degenerate, they no longer send the impulses necessary for movement to the muscle fibers. When the muscles lose their connections to the motor neurons, they waste away, which is known as atrophy. Eventually the brain can no longer start and control voluntary muscle movement.
As the disease progresses, problems with moving, swallowing, and speaking develop, along with loss of the ability to use arms or legs and walk or stand. In the later stages of the disease, patients may become totally paralyzed. The ability to see, smell, taste, and hear is not affected.