Wikify: date=April 2008

Itch (Latin: pruritus) is an unpleasant sensation that evokes the desire or reflex to scratch. Itch has many similarities to pain and both are unpleasant sensory experiences but their behavioral response patterns are different. Pain creates a reflex withdrawal while itch leads to a scratch reflex. Unmyelinated nerve fibers for itch and pain both originate in the skin, however information for them is conveyed centrally in two distinct systems that both use the same peripheral nerve bundle and spinothalamic tract.

Historically, the sensations of itch and pain have not been considered to be independent of each other until recently where it was found that itch has several features in common with pain but exhibits notable differences. The physiological mechanisms of itch are currently poorly understood and this is mainly due to the lack of animal models of itch. Pruritic stimuli mostly create the same reactions as noxious stimuli in experimental animals, but humans are capable of discerning the distinct features of itch and pain. Therefore human studies have provided most of the information regarding the processing of pruritic stimuli.

Mechanism

Itch can originate in the peripheral nervous system (dermal or neuropathic) or in the central nervous system (neuropathic, neurogenic, or psychogenic).

Dermal/pruritoceptive

Itch originating in the skin is considered pruritoceptive and can be induced by a variety of stimuli, including mechanical, chemical, thermal, and electrical stimulation. The primary afferent neurons responsible for histamine induced itch are unmyelinated C-fibers. In human C-fiber nociceptors, two major classes exist: mechano-responsive nociceptors and mechano-insensitive nociceptors. Mechano-responsive nociceptors have been shown in studies to respond to mostly pain and mechano-insensitive receptors respond mostly to itch induced by histamine. However it does not explain mechanically induced itch or when itch is produced without a flare reaction which involves no histamine. Therefore it is possible that pruritoceptive nerve fibers have different classes of fibers, which is unclear in current research.

Studies have been done to show that itch receptors are only found on the top two skin layers, the epidermis and the epidermal/dermal transition layers.Fact: date=August 2008 Shelley and Arthur had verified the depth by injecting individual itch powder spicules (Mucuna pruriens) and found that maximal 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 patient to perceive itch.Fact: date=August 2008 Itch is never felt in muscle, joints, or inner organs, which show that deep tissue does not contain itch signaling apparatuses.