Motor neuron disease: A group of related diseases of the nervous system that are characterized by steadily progressive deterioration of the motor neurons in the brain, brainstem, and spinal cord. Abbreviated MND. Motor neurons are the nerve cells along which the brain sends instructions, in the form of electrical impulses, to the muscles. The degeneration of motor neurons leads to weakness and wasting of muscles. MND usually first affects the arms or legs. Then shoulders and other muscles may be affected. Weakness and wasting in the muscles of the face and throat may cause problems with speech, chewing, and swallowing. MND does not affect touch, taste, sight, smell, or hearing, nor does it directly affect bladder, bowel, or sexual function. In the vast majority of cases, the intellect remains unchanged. Subtypes of MND are distinguished by the major site of degeneration of the motor neurons'for example, amyotrophic lateral sclerosis (Lou Gehrig's disease), progressive spinal muscular atrophy, progressive bulbar palsy, and primary lateral sclerosis. The prognosis is dependent upon the type of MND.