Motor neurons can be classified as Upper Motor Neurons, originating from the brain and Lower Motor Neurons, originating from the brain stem and spinal cord. Also known as motoneurons, their primary function is to synapse muscle fibers and glands to convey signals from the brain. Motor neurons can be further divided according to the type of muscle they innervate. Somatic motor neurons directly innervate skeletal muscle. Brachial motor neurons innervate brachial muscles in the face and neck, whereas visceral motor neurons synapse with visceral muscle in the arteries and heart.
Studying the differentiation of embryonic stem cells into motor neurons is particularly important for research focused on Motor Neuron Diseases (MNDs). These progressive neurological disorders are characterized by the degeneration of neurons that are responsible for controlling muscle activity. The most studied of these conditions is Amyotrophic Lateral Sclerosis (ALS, also known as Lou Gehrig's disease), in which patients suffer from loss of voluntarily muscle control and movement. Established markers of differentiated motor neurons include transcription factors Islet-1, Islet-2, and Olig2.