Normally, these motor neurones relay signals, which they receive from the brain, to the muscle cells. When these neurones fail to function, the muscles deteriorate.
Intelligence is normal.
From the description just given it can be seen why the condition is called Spinal Muscular Atrophy - the muscles atrophy because of a problem in the nerve cells in the spine. The major difference between these types are the age of onset and the severity of the condition. It usually becomes evident in the first six months of life. The child is unable to roll or sit unsupported, and the severe muscle weakness eventually causes feeding and breathing problems. These children usually do not live beyond about 18 months of age.