What is itching?
The first definition of itching was proposed in 1660 by someone called Samuel Haffenreffer as an unpleasant desire to itch. Approximately 280 million people globally have problems with itching. But why do we itch?
Strangely enough itching and pain both share the same common pathways, although you may think of them as very different sensations.
“itching and pain both share the same common pathways”
Much like you can catch a yawn, you can also catch an itch. It is known as a contagious itch and sometimes even just a discussion about itching can give people the need to itch. So, don’t be surprised if you are reaching for the back scratcher. In one study, itching was induced by visual stimuli. One hypothesis is that the human mirror neurons exists in which we imitate certain motor actions when we view others performing the same action. You copycats.
Apart from contagious itching, itching can be caused by many things ranging from skin conditions, internal diseases to pregnancy and nerve conditions. However, there is sometimes itching without any identifiable cause called essential itch.
The stimulation of free nerve endings, usually at the junction of the dermis and epidermis of the skin, will evoke a desire to scratch. However, there is more going on below the surface that just free nerve endings.
In the human skin there are C fibres, which are identical to those associated with pain but are functionally distinct and will only covey a charming itch sensation. An itch originating in the skin can be induced by a variety of stimuli, e.g. mechanical, chemical or thermal. There are two classes of C fibres: mechano-responsive nociceptors and mechano-insensitive nociceptors. The mechano-responsive nociceptors have been shown to respond mostly to pain while the mechanics-insensitive receptors respond most to itch induced by histamine (present when an itch is caused by an allergy). However, this does not explain mechanically induced itch or when an itch is produced without a flare reaction. These remain unexplained.
“C fibres … are identical to those associated with pain but are functionally distinct and will only convey a charming itch sensation”
When stimulated superficially on the skin the C fibres carry signals along the nerve to the spinal cord to the brain. In the brain they are processed generating a scratching response. The scratching interferes with the sensation of itching arising from pruritus by stimulating various pain and touch receptors in the same area. Unfortunately, this will only offer brief freedom. Scratching may also turn on nerves that stimulate pleasure systems in the brain. Sensitivity to stimuli of itching is distributed evenly across the skin and itch receptors are found only on the top two skin layers.
Despite over a century of research, there is no generally accepted treatment for itching. However some topical anesthetic and other drugs will offer some temporary salvation.
Gwen Ward
Sources used by Gwen in researching this article:
http://www.clevelandclinicmeded.com/medicalpubs/diseasemanagement/dermatology/pruritus-itch/
https://vbn.aau.dk/ws/portalfiles/portal/219083590/4442_9.pdf
https://medical-.dictionary.thefreedictionary.com/essential+pruritus
https://www.nature.com/articles/nrn1950
https://www.scientificamerican.com/article/experts-why-we-itch-and-scratch/
Featured image courtesy of John K Thorne via Flickr. No changes were made to this image. Image use license here.
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