I have attached a few of the research articles on that subject. But primarily the center on Cymbalta's interaction with spinal 5-HT2A or α2-noradrenergic receptors. The 5HT2A receptors are regulated by serotonin and the α2-noradrenergic receptors are regulated by noradrenaline. Because Cymbalta helps control both serotonin and noradrenaline it is logical that it would markedly relieve pain systemically. The problem is once the Cymbalta is removed from the spinal cord and other nerves the body has to relearn how to control its own serotonin and noradrenaline. Until then the nerves are susceptible to many other neuroreceptors.
https://www.ncbi.nlm...les/PMC3171868/
https://www.ncbi.nlm...pubmed/25154730
https://www.ncbi.nlm...pubmed/24686187