NGF and irritable bowel syndrome: IBS=Irritable Bowel Syndrome; FMS=Fibromyalgia syndrome; FGIDs=Functional gastrointestinal disorders; EGC=Enteric glial cells; MC=Mast cells; NGF=Nerve growth factor; IBS-D=Irritable bowel syndrome with Diarrhea; mRNA=Messenger RNA; TLR=Toll-like receptors; VIP=Vasoactive intestinal polypeptide