Medicine. Motor neuron diseases are a group of diseases characterized by a selective degeneration of the motor neurons of the spinal cord, brainstem, or motor cortex. The most common type is amyotrophic lateral sclerosis (ALS) in which both types of motor neurons are involved. Less common types of motor neuron diseases affect either the first or the second motor neuron.
Sometimes the term 'motor neuron disease' is used as synonym for amyotrophic lateral sclerosis.