Degradation of motor neurons, conceptual computer illustration. Motor neuron diseases are a group of neurodegenerative disorders including amyotrophic lateral sclerosis, progressive bulbar palsy and other. Amyotrophic lateral sclerosis, also referred to as Lou Gehrig's disease is the most common of the motor neuron diseases. Nerve degeneration and restricted production of the neurotransmitter dopamine results in deterioration of muscle tissue. This debilitating disease is characterized by rapidly progressive weakness, muscle atrophy and twitching, muscle spasticity, difficulty speaking (dysarthria), difficulty swallowing (dysphagia), and difficulty breathing (dyspnea).