Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Hereditary sensory and autonomic neuropathy ?
    ### Response:
    Hereditary sensory autonomic neuropathy (HSAN) is a group of rare peripheral neuropathies where neurons and/or axons are affected. The major feature of these conditions is the loss of large myelinated and unmyelinated fibers. Myelin is an insulating layer, or sheath that forms around nerves, made up of protein and fatty substances, that allows electrical impulses to transmit along the nerve cells.
    