The motor neuron diseases (MNDs) are a group of progressive
neurological disorders that destroy motor neurons, the cells
that control essential voluntary muscle activity such as
speaking, walking, breathing, and swallowing. Normally, messages
from nerve cells in the brain (called upper motor neurons) are
transmitted to nerve cells in the brain stem and spinal cord
(called lower motor neurons) and from them to particular
muscles. Upper motor neurons direct the lower motor neurons to
produce movements such as walking or chewing. Lower motor
neurons control movement in the arms, legs, chest, face, throat,
and tongue. Spinal motor neurons are also called anterior horn
cells. Upper motor neurons are also called corticospinal
neurons. When there are disruptions in the signals between the
lowest motor neurons and the muscle, the muscles do not work
properly; the muscles gradually weaken and may begin wasting
away and develop uncontrollable twitching (called
fasciculations). When there are disruptions in the signals