SMA is a disease resulting in worsening muscle weakness due to progressive loss of motor nerve cells (neurons) in the spinal cord (and brainstem).
The motor neurons in the spinal cord are also known as anterior horn cells, these cells relay messages  for muscle movement from the nerve cells in our brains, to the muscles in our limbs, chest, throat, face and mouth.
SMA is not a disease of the brain, the muscles or the sensory nerves (the nerves that relay feelings back to our brains). The muscles do however eventually weaken and get smaller (atrophy) because they do not receive stimulation from motor neurons. SMA types I and II are due to abnormalities of the SMNI gene (the Survival Motor Neuron I gene). If the SMNI gene has reduced or no function, then the motor neurons in the spinal cord and brainstem do not survive, and gradually die off. Children with SMA type 1 do not usually live beyond two years of age.
SMA is inherited in an autosomal recessive pattern, meaning that if two carriers have children; there is a 25% chance (one in four) of having an affected child in each pregnancy.