(SMA) SMA is a genetic disease affecting the part of the nervous system that controls voluntary muscle movement. SMA involves the loss of nerve cells called motor neurons in the spinal cord and is classified as a motor neuron disease. There is a wide variability in the age of onset, symptoms and rate of progression. Typically the age at which SMA symptoms begin correlates with the degree to which motor function is affected. The muscles closer to the center of the body are usually more affected in SMA than are the muscles farther from the center.