The sciatic nerve is the largest, thickest, and longest nerve in the human body.  It originates from spinal roots L4 through S3, located in the lower back.  This terminology simply describes the specific location of the vertebrae from which the nerves exit the spinal cord, and merge together to form the large sciatic nerve.  The sciatic nerve therefore begins in the lower back, and runs down through the buttocks and all the way down the leg.  It supplies almost the entire skin and musculature of the lower limb, including the buttocks, thigh, leg, and foot.  For this reason irritation of the sciatic nerve in the low back or buttocks often causes symptoms that are felt all the way down the leg or foot.