The sciatic nerve runs from your lower back, through your buttock, down the back of your thigh, and all the way to your heel. It is the largest nerve in the human body, measuring roughly 6 to 7 millimeters thick at its widest point, about the diameter of a pencil. Because it covers so much territory, problems anywhere along its path can produce pain, numbness, or weakness in surprisingly distant parts of the leg.
Where It Starts: The Lower Spine
The sciatic nerve forms from five nerve roots that branch off the spinal cord in your lower back and upper tailbone area, specifically from the levels labeled L4, L5, S1, S2, and S3. These individual roots merge together inside the pelvis to create a single large nerve trunk. This is why a herniated disc or bone spur in the lower spine can trigger sciatic pain even though the nerve itself runs through the leg. The compression happens at the source, where the roots are still close to the vertebrae.
Through the Buttock and Past the Piriformis
Once formed, the sciatic nerve exits the pelvis through a bony opening called the greater sciatic foramen. It passes just below a small, deep muscle in the buttock called the piriformis. In about 87% of people, the nerve travels beneath the piriformis as a single trunk. But in roughly 12 to 20% of the population, the anatomy is different: one branch of the nerve passes directly through the piriformis while the other passes below it. This variation matters because it can make certain people more vulnerable to nerve irritation in the buttock, a condition sometimes called piriformis syndrome.
The area deep in the buttock where the nerve sits is a common compression zone. Tight muscles, prolonged sitting on hard surfaces, inflammation, or even a blood collection in the area can press on the nerve here. This deep buttock region is also the spot where you might feel a firm, cord-like structure if you press deeply into the center of the glute.
Down the Back of the Thigh
From the buttock, the sciatic nerve descends straight down the back of the thigh. It travels deep to the large hamstring muscle (the long head of the biceps femoris), sitting on top of the adductor magnus, a broad muscle along the inner thigh. Along this route, it supplies the hamstring muscles that bend your knee and help extend your hip. This is the stretch of nerve responsible for the classic “shooting pain down the back of the leg” that people associate with sciatica.
The nerve sits deep enough in the thigh that you can’t feel it through the skin the way you might bump your “funny bone” in the elbow. But because it runs through a corridor of muscle, anything that causes significant swelling or tightness in the posterior thigh can potentially irritate it.
Where It Splits: Behind the Knee
Just before reaching the back of the knee, in the hollow space called the popliteal fossa, the sciatic nerve divides into its two terminal branches. In over 70% of people, this split happens at or near the knee. The two branches are:
- The tibial nerve, which continues down the back of the calf and into the sole of the foot. It controls the calf muscles that point your toes and provides sensation to the bottom of your foot.
- The common peroneal (fibular) nerve, which wraps around the outside of the knee and travels down the front and outer side of the lower leg. It controls the muscles that lift your foot and toes upward and provides sensation to the top of the foot and outer shin.
Together, these two branches mean the sciatic nerve is ultimately responsible for nearly all movement and feeling in the leg below the knee. This is why severe sciatic nerve damage can cause foot drop (difficulty lifting the front of the foot) or widespread numbness from the knee down.
What the Sciatic Nerve Controls
The sciatic nerve handles both motor function (muscle movement) and sensory function (feeling) for a large portion of the lower body. In the thigh, it powers the hamstrings. Below the knee, through its two branches, it controls essentially every muscle in the calf, shin, and foot. On the sensory side, it provides feeling to most of the leg below the knee, including the sole, the top of the foot, and the outer calf.
The only major part of the lower leg the sciatic nerve does not supply is a strip along the inner calf and inner foot, which is handled by a separate nerve. Everything else, from the ability to push off while walking to the sensation of stepping on something sharp, runs through the sciatic nerve or one of its branches.
Common Spots Where Problems Occur
Sciatic nerve pain can originate from several distinct locations along its path, and identifying the right one matters for treatment. The most common source is the lumbar spine itself, where a bulging disc or narrowed spinal canal presses on the nerve roots before they even merge into the sciatic nerve. This is the classic cause of sciatica.
The second major trouble spot is the deep buttock area. Compression here, sometimes grouped under the term “deep gluteal syndrome,” can come from a tight or spasming piriformis muscle, friction from fibrous bands of tissue, or narrowing of the space between the hip bones. People with the anatomical variant where the nerve passes through the piriformis may be especially prone to irritation in this zone.
Less commonly, the nerve can be compressed in the thigh by trauma, tumors, or prolonged pressure (such as sitting on a hard edge for an extended period). And at the knee, the common peroneal branch is vulnerable where it wraps around the bony bump on the outer side of the knee, which is why crossing your legs for a long time can cause temporary tingling or foot numbness.
Knowing the nerve’s exact route helps explain why sciatica symptoms can feel so different from person to person. Pain centered in the buttock, a burning sensation down the back of the thigh, numbness in the foot, or weakness when trying to lift your toes can all trace back to the same nerve, just compressed or irritated at different points along its long path.