The sciatic nerve forms from the merger of the fourth and fifth lumbar nerves with the first, second, and third sacral nerves.... Click to read more below. The sciatic nerve is the largest and longest spinal nerve in the human body.Extending from the lumbar and sacral plexuses in the lower back, the sciatic nerve runs through the buttocks and into the thighs. It delivers nerve signals to and from the muscles and skin of the thighs, lower legs and feet.he sciatic nerve forms from the merger of the fourth and fifth lumbar nerves with the first, second, and third sacral nerves.... Click to read more below. The sciatic nerve is the largest and longest spinal nerve in the human body.