SMA is a group of hereditary diseases that progressively
destroys motor neurons—nerve cells in the brain stem and spinal
cord that control essential skeletal muscle such as
speaking, walking, breathing, and swallowing, leading to
muscle weakness and atrophy. Motor neurons control movement in the arms,
legs, chest, face, throat, and tongue.
When there are disruptions in
the signals between motor neurons and muscles, the muscles gradually
weaken, begin wasting away and develop twitching (called fasciculations).
Learn more about SMA at The National Institute of Neurological Disorders and Stroke website