Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) spastic paraplegia type 31 ?
    ### Response:
    Spastic paraplegia type 31 is one of a group of genetic disorders known as hereditary spastic paraplegias. These disorders are characterized by progressive muscle stiffness (spasticity) and the development of paralysis of the lower limbs (paraplegia) caused by degeneration of nerve cells (neurons) that trigger muscle movement.
    