Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Motor Neuron Diseases ?
    ### Response:
    The motor neuron diseases (MNDs) are a group of progressive neurological disorders that destroy cells that control essential muscle activity such as speaking, walking, breathing, and swallowing. Normally, messages from nerve cells in the brain (called upper motor neurons) are transmitted to nerve cells in the brain stem and spinal cord (called lower motor neurons) and from them to particular muscles.
    