What Exactly is the Sciatic Nerve? The sciatic nerve is the longest and thickest nerve in the human body. It begins in your lower back, runs through your hips and buttocks, and branches down each leg. It is the "main highway" for signals between your brain and your lower limbs, controlling your ability to walk, run, and feel sensations. What is Sciatica? (And What It Isn’t) "Sciatica" is not a diagnosis in itself; it is a description of symptoms. It occurs when the nerve root