The sciatic nerve is the largest single nerve in your body. It runs from your lower back, through your buttock, and down the back of each leg to your foot. When people talk about “sciatica,” they’re referring to pain caused by irritation or compression of this nerve, but the nerve itself plays a critical role in everyday movement and sensation in your lower body.
Where the Sciatic Nerve Starts and Where It Goes
The sciatic nerve forms from five nerve roots that branch off the bottom of your spinal cord, specifically from the fourth lumbar vertebra down to the third sacral vertebra (L4 through S3). These roots merge together inside the pelvis, creating a single thick nerve trunk that exits through an opening in the pelvis called the greater sciatic foramen. From there, it passes beneath a deep buttock muscle called the piriformis and descends between the hip joint and the sit bone.
The nerve then travels straight down the back of your thigh. Near the back of your knee, it splits into two branches: the tibial nerve and the common peroneal (fibular) nerve. The most common splitting point is roughly at the junction of the middle and lower thirds of the thigh, near the top of the hollow behind the knee. These two branches continue down through the lower leg and into the foot, which is why sciatic nerve problems can cause symptoms anywhere from your lower back to your toes.
What the Sciatic Nerve Does
The sciatic nerve carries two types of signals. Motor fibers send instructions from your brain to the muscles in your leg, controlling movements like bending your knee, pointing your foot, and curling your toes. Sensory fibers carry information back to your brain about touch, temperature, and pain from the skin of your lower leg and foot. Without a functioning sciatic nerve, you’d lose both the ability to move most of your leg below the knee and the ability to feel much of it.
In the thigh, the nerve also supplies the hamstring muscles, which are responsible for bending the knee and helping extend the hip. After the nerve splits near the knee, its tibial branch controls the calf muscles and sole of the foot, while its peroneal branch controls the muscles that lift your foot and toes upward and provides sensation to the top of the foot and outer shin.
Anatomical Variations Around the Piriformis
In most people, the sciatic nerve passes below the piriformis muscle as a single trunk. But in roughly 13% of the population, the nerve takes a different route. It may pass through the piriformis muscle itself, or one of its two components may travel above the muscle while the other goes below. These variations matter because they can make some people more prone to nerve compression in the buttock, a condition sometimes called piriformis syndrome.
What Causes Sciatic Nerve Pain
Sciatica happens when something compresses or irritates the nerve roots that form the sciatic nerve. The most common cause is a herniated disc in the lumbar spine. Spinal discs have a soft center surrounded by a tougher outer ring, and when part of the soft center pushes through a crack in that outer ring, it can press directly on a nearby nerve root. Bone spurs, which are bony overgrowths on the vertebrae, are another frequent cause. Less commonly, a tumor or narrowing of the spinal canal (spinal stenosis) can put pressure on the nerve.
Other causes include piriformis syndrome (where the piriformis muscle tightens or spasms around the nerve), direct trauma to the buttock or thigh, and prolonged sitting or compression. Pregnancy can also trigger sciatica because the expanding uterus and shifting posture place extra pressure on the lower spine and pelvis.
What Sciatica Feels Like
The hallmark of sciatica is pain that radiates from the lower back or buttock down the back of one leg. It rarely affects both legs at the same time. The pain can range from a dull ache to a sharp, burning sensation or even an electric shock-like jolt. It often worsens with prolonged sitting, standing up, coughing, or sneezing.
Beyond pain, you might experience numbness, tingling, or a “pins and needles” feeling along the path of the nerve. Some people notice muscle weakness in the affected leg, making it harder to lift the foot (a condition called foot drop) or push off while walking. The specific location of your symptoms depends on which nerve root is compressed. Pressure on the nerve root near the fifth lumbar vertebra, for example, tends to cause pain and weakness in the outer calf and top of the foot, while pressure near the first sacral root typically affects the back of the calf and the sole.
How Sciatica Is Diagnosed
A physical exam is usually the first step. One of the most common bedside tests is the straight leg raise: you lie on your back while a clinician slowly lifts your affected leg. If this reproduces your radiating leg pain between 30 and 70 degrees of elevation, it suggests nerve root irritation. This test picks up most cases of disc-related sciatica, with a sensitivity of 72% to 97%, though it can also flag problems that aren’t actually sciatica (its specificity ranges from just 11% to 66%). A variation called the crossed straight leg raise, performed on the opposite leg, is less sensitive (23% to 42%) but much more specific (85% to 100%), meaning a positive result on that side is a strong signal of true nerve root compression.
If symptoms are severe, persistent, or accompanied by progressive weakness, imaging comes next. An MRI is the preferred tool because it shows both the soft tissues (discs, nerves) and the bony structures of the spine in detail. CT scans or X-rays may be used in specific situations but provide less information about the nerve itself.
How Sciatica Is Treated
Most sciatica improves with time and non-surgical care. The initial approach typically includes staying as active as you reasonably can (prolonged bed rest tends to make things worse), applying heat or ice to the affected area, and using over-the-counter pain relief. Physical therapy focused on core strengthening, gentle stretching, and posture correction is one of the most effective conservative treatments. Many people see significant improvement within six to twelve weeks.
When pain is more stubborn, steroid injections near the affected nerve root can reduce inflammation and provide relief lasting weeks to months. These are generally used as a bridge, buying time for the underlying problem to heal. Surgery becomes an option when conservative treatment fails after several months, or when there are red-flag symptoms like progressive leg weakness, loss of bladder or bowel control, or severe and unrelenting pain. The most common surgical procedure removes the portion of the herniated disc pressing on the nerve, and most patients experience rapid pain relief afterward.
Reducing Your Risk
You can’t control every risk factor for sciatica, but several habits help protect the nerve. Regular exercise that strengthens your core and back muscles supports the spine and reduces the chance of disc problems. When sitting for long periods, take breaks to stand and move every 30 to 60 minutes. Proper lifting technique matters too: bend at the knees and hips rather than rounding your lower back, and keep heavy objects close to your body. Maintaining a healthy weight reduces the mechanical load on your lumbar spine, which is where most sciatic nerve compression originates.