An itch (Latin: pruritus) is a sensation felt on an area of skin that makes a person or animal want to scratch it.

Mechanism

The central chemical in itch is histamine, a molecule released by mast cells in the skin. Histamine is the chemical that causes the itch and reddening when bitten by insects. It binds to local nerve endings on specific receptors.

Itch from cutaneous (skin-related) stimuli, such as movement of small hairs on the body, is transmitted along the same pathway as pain. Itch caused by histamine is transmitted to the brain by a different neural pathway, described in 2001 by Andrew et al. As with pain, histamine-induced itch travels via the spinothalamic tract, but in fibres specific for itch.

Causes

The feeling of itchiness can be caused by movement of hairs, or release of a chemical (histamine) from cells under the skin. Itchiness is regarded as protective, as it helps creatures remove parasites that land on them.

Other causes for itching can be:

• Skin conditions (such as psoriasis, eczema and many others). Most are of inflammatory nature.
• Jaundice (bilirubin is a skin irritant at high concentrations)
• Medication:
  • Allergy
  • Photodermatitis - (sun)light reacts with chemicals in the skin, leading to the formation of irritant metabolites
  • Directly (e.g. morphine)

References

• Andrew D, Craig AD (2001). Spinothalamic lamina I neurons selectively sensitive to histamine: a central neural pathway for itch. Nature Neuroscience Jan;4(1):9-10.