The sciatic nerve is the largest nerve in your body. It travels from your lower back, through your hip area and buttocks and down the back of each leg . The term sciatica refers to pain that radiates along the path of this nerve. Sciatica is the compression of the Sciatic nerve in your lower back. The most common cause of this nerve compression is a bulging or herniated lumbar disc. Piriformis syndrome is another common cause of sciatica. The piriformis is a muscle that lies directly over the sciatic nerve. If this muscle becomes tight or if you have a spasm in this muscle, it puts pressure directly on the sciatic nerve.