18. Which one of the following statement is correct? (A) Changes in excitability occur in second-order neurons when repetitive or prolonged high-intensity input is received from primary afferent C-fibers. (B) A characteristic of spinal nociceptive systems is that they are under tonic descending inhibition such that a common effect of injury to spinal pathways is a release from this inhibition. (C) The midbrain periaqueductal grey (PAG) and the rostral ventral medulla (RVM), in particular the medullary nucleus raphe magnus (NRM) figured prominently in the original analyses of descending modulation of pain. (D) Stimulation of both somatosensory cortex (SSC) and motor cortex (MC) have been used in the clinical treatment of neuropathic pain, central poststroke pain, and phantom limb pain. (E) All of the above are correct.