TAC1 and fibromyalgia: Chronic stimulation of C fibers, a phenomenon which is observed in numerous cases of fibromyalgia, may lead to the apoptotic loss of inhibitory opioid and GABAergic interneurons; however, this phenomenon is accompanied by a continuous release of glutamate and substance P onto the second-order neurons, alongside diminished central levels of opioid peptides and serotonin [139,140].