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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.

When there are disruptions in these signals, the muscles do not work properly; the result can be gradual weakening, wasting away, and uncontrollable twitching (called fasciculations). When upper motor neurons are affected, the manifestations include spasticity or stiffness of limb muscles and over-activity of tendon reflexes such as knee and ankle jerks. Eventually, the ability to control voluntary movement can be lost. MNDs may be inherited or acquired.