Amyotrophic lateral sclerosis (ALS) - commonly known as Lou Gehrig's disease - is a neuromuscular disorder. It's caused when the motor neurons that send electric "messages" from the brain to the spinal cord or from the spinal cord to the muscles are slowly being destroyed. These neurons are responsible for muscle movement.
When the messages don't get through from the spinal cord, the muscles eventually lose strength and waste away. This process is called atrophy or amyotrophy. When signals from the brain to the spinal cord are blocked, the muscles become stiff and slow. This is called spasticity.
Up to 5 people out of every 100,000 get ALS. It's more common in men than women, with symptoms usually appearing between the ages of 50 and 75. About 10% of people with ALS have a family history of the disease. Life expectancy averages 2 to 5 years after diagnosis.
What causes ALS isn't known. Aside from a genetic predisposition in some cases, we don't really understand why the motor neurons are destroyed. One theory is that environmental factors such as heavy metal exposure can damage the nerve cells. Viral infections are yet another factor that may play a role in the development of ALS.
Another theory involves a chemical called glutamate, which is responsible for transmitting many of the messages between nerve cells in the brain. When glutamate isn't removed from the spaces between the nerve cells (called synapses), it builds up and causes an overflow of calcium into motor neurons. This eventually leads to destruction of nerve cells.
Motor neurons might also be damaged by free radicals. These molecules wreak havoc in cells and damage genetic material. In some people who have familial ALS, the gene for a particular enzyme that breaks down free radicals (superoxide dismutase) is abnormal. Free radicals might also be linked to environmental factors or excessive levels of glutamate.