A term used to define a heterogeneous group of neurodegenerative diseases, some of them hereditary, that exclusively or predominantly affect the motor neurons of the cerebral cortex and/or the motor nuclei of the brainstem and/or the anterior horn of the spinal cord. Currently there is no cure but there are treatments that can alleviate the symptoms.

Motor neurons are found in the brain and spinal cord and are responsible for transmitting signals from the brain to the muscles of the body to control movement. If the emission of these signals is interrupted, it causes progressive paralysis. Symptoms can be very varied: spasms, muscle stiffness, speech, swallowing and breathing difficulties. Symptoms worsen as the muscles become weaker.

The most common form is amyotrophic lateral sclerosis (ALS).