Genetic differences in more than 70 genes have been associated with increased itchiness.  Calcium and serotonin levels may be involved in increased itch or arthritis pain signals being sent/perceived.  See the excerpts below:
Summary: Too much or too little calcium and magnesium can affect pain, itching, and mood. The minerals are both electrically active, and provide energy for ion channels which control the transport of messenger chemicals like serotonin across cell membranes – such as nerve cell membranes.
“For neurons to become excited, you need a receptor to communicate with an ion channel,” said Dr. Bautista. “We tried a variety of experiments and found that HTR7 communicates with the TRPA1 ion channel. Both receptors seem to be working together to mediate chronic itch.” “The researchers found more than 70 genes whose expression was higher in the more itch-sensitive mice. Of these, the gene for the HTR7 receptor was the most closely linked to itch. In fact, the HTR7 gene was twice as active in the itchiest mice compared to the least sensitive mice. ” 
- An Itch You Just Can’t Scratch; NIH-funded study identifies proteins that may cause chronic itch (Oct. 27, 2015) http://www.ninds.nih.gov/news_and_events/news_articles/pressrelease_chronic_itch_10272015.htm
Adam Horvath, et al., Transient receptor potential ankyrin 1 (TRPA1) receptor is involved in chronic arthritis: in vivo study using TRPA1-deficient mice, Arthritis Res Ther. 2016; 18: 6. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4718022/
Excerpt: “TRPA1 is also directly stimulated by intracellular calcium  and a broad range of noxious endogenous oxidative products, such as 4-hydroxy-2-nonenal, hydrogen peroxide, hypochloride, hydrogen sulphide, 15-delta prostaglandin J2 [25–28]. Furthermore, there are several exogenous irritants like mustard oil (allyl isothiocyanate: AITC) , cinnamaldehyde [30, 31], allicin [32, 33] and formalin  that are known to be potent agonists of TRPA1. Inflammatory mediators, such as bradykinin and serotonin, can sensitize this receptor and increase the responsiveness of the nerve endings [19, 35]. These findings suggest that TRPA1 may be involved in the development and maintenance of arthritic pain, but the precise mechanisms are still unknown.”
Rs6295: The “Single” and “Self-Transcendent” Gene (5-HT1A Receptor) https://selfhacked.com/2015/07/23/rs6295-the-single-and-self-transcendent-gene-5-ht1a-receptor/ “Magnesium and Calcium increase the binding of serotonin to the 5HT1A receptors in the cortex (purkinje cells). (R)“
#Scratching That #Itch Really Does Make It Worse #serotonin #5HT1A @beeing_sophie http://www.medicaldaily.com/scratching-itchy-skin-causes-brain-release-hormone-serotonin-intensifies-itchy-sensation-308458#.V2vk1pKuFes.twitter …
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