Motor Neuron disorders (MNDs) are a group of diseases that severely affect the function of motor neurons. Motor neurons are cells located in the brain and spinal cord that primarily control voluntary movement through their axonal connections to muscles. Most motor neuron diseases are characterized by a rapidly progressing degeneration resulting in severe disability and even death.

Motor neurons are essential for voluntary muscle functions such as walking, chewing, speaking, swallowing, and breathing. Upper motor neurons, also known as corticospinal neurons, originate in the central nervous system and descend through the spinal cord to activate lower motor neurons. The lower motor neurons innervate muscle fibers and regulate contraction of the muscle. MNDs may start with a localized degeneration, only effecting specific neurons and therefore dysfunction may be limited to one muscle group. Over time, other neurons may eventually degenerate leading to broader effects.

Motor neuron diseases affect both children and adults. Most MNDs found in children are inherited disorders caused by a pathogenic mutation in a known gene, the most common of which is Spinal Muscular Atrophy, an autosomal recessive disease caused by the inheritance of two dysfunctional copies of a gene. The cause of many adult MNDs is not known with some forms being inherited and others developing sporadically, with no known family history of the disease.