r classes of human C-fibre nociceptors exist: mechano-responsive nociceptors and mechano-insensitive nocice ptors. M qechano-responsive nociceptors have been s zc3q yhown in studies to respond to mostly pain, and mechano-inse nsitive receptors respond mostl c3q y to itch induced by histamine. However, it does c3q ynot explain mechanically ind Studies have been done to show that itch receptors are found only on the top two skin layers, the epidermis a nd the epidermal/dermal transition layers.[ci 3qtation needed] Shelley and Arthur had verified the depth by inje cting individual itch powder spicules (Mucuna pruriens), and found that 3qemaximal sensitivity was found at the basal cell layer or the innermost layer of the epidermis. Surgical removal of those skin layers removed the ability for a patien qet to perceive itch.[citation needed] Itch is never felt in c3q ymuscle or joints, which strong ly suggests that deep tissue probably does not contain i 3qtch signaling apparatuses. Sensitivity to pruritic stimuli is evenl c3q yy distributed across the skin, and has a clear spot distribution with similar density to c3q ythat of pain. The different substances that elicit itch upon intracutaneous injection (inj ection within the skin) elicit only pain input to the spinal cord is known to produce central sensitization, which consists of allodynia, exaggerati on of pain, and punctuate hyperalgesia, extr 3qeme sensitivity to pain. Two types of mechanical hyperalgesia c an occur: 1) touch that is normally painless in the uninjured surroundings of a cut 3q or tear can trigger pain ful sensations ( 3qouch-evoke qed hyperalgesia), and c3q y a slightly painful pin prick stimulation is perceived as m ore painful around a focused area of inflammation (punctuate hyperalgesia). Touch-evoked hyperalgesia requir es continuo c3q yus firing of primary afferent nociceptors, a zc3q nd punctuate hyperalgesia does not require continuous f iring which means it can persist for hours a c3q yfter a trauma and can be str qeonger than normally experienced. In addition, it was found that 3q patients with neuropathic pain, histamine ionophoresis resulted in a sensation of burning pain rather than itch, which would be induced in normal healthy patients. This shows that there is spin
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