The causes of ALS for most cases (90-95% of the cases) is unknown. However, in some cases, 5-10%, the cause is a gene inherited from a parent. In such familial ALS, the culprit is a mutated gene inherited from a parent. More than a dozen such mutated genes have been discovered to cause ALS. One of the major causes of familial ALS is a defect in chromosome 21 (out of the 23 chromosomes that humans have), which codes for superoxide dismutase (an enzyme). It is believed to be the cause of 20% of familial ALS cases.
However, the cause of the symptoms of ALS, such as muscle weakness, difficulty in speaking, swallowing or breathing, and muscle stiffness, is known. These symptoms are caused due to neuron degeneration, specifically the degeneration of the motor neurons. The upper motor neurons present within the brain degenerate causing spasticity (muscle tightness). The lower motor neurons in the brainstem and spine degenerate to cause muscle atrophy (shrinking of muscles), muscle weakness and twitching. These groups of neurons are responsible for the control of voluntary muscles throughout the body. They transfer messages to the peripheral nervous system which gives the messages to muscles. When the motor neurons degenerate, a person loses the ability the control voluntary muscles. Since ALS is a progressive disease, a person loses this ability gradually over a period of time, generally three to four years. In the later stages of ALS, a person may lose the ability to breathe since the muscles involved in the respiratory system weaken to the point at which they cannot move air in and out of the lungs. |