The sciatic nerve is the largest and thickest nerve in your body, roughly 2 centimeters wide, running from your lower back all the way down each leg. When this nerve gets irritated or compressed, the resulting pain is called sciatica, a condition that affects 10% to 40% of people at some point in their lives. Understanding the nerve itself helps make sense of why sciatica pain can show up in so many different places.
Where the Sciatic Nerve Starts and Travels
The sciatic nerve forms from five nerve roots branching off the lower spinal cord, specifically from the fourth lumbar vertebra down to the third sacral vertebra (L4 through S3). These roots merge together in the pelvis, creating a single thick nerve bundle that exits through an opening called the greater sciatic foramen, passing just beneath a deep hip muscle called the piriformis.
From there, the nerve travels down through the back of the buttock and into the rear of the thigh. Just above the back of the knee, it splits into two branches. One branch, the tibial nerve, continues down the back of the lower leg and into the sole of the foot. The other, the common peroneal nerve, wraps around to supply the outer and front portions of the lower leg and the top of the foot. Together, these branches are responsible for nearly all movement and sensation below the knee.
What the Sciatic Nerve Does
This nerve carries two types of signals. Motor signals travel from the brain down through the nerve to control muscles in the back of the thigh, the entire lower leg, and the foot. These are the muscles that let you bend your knee, point and flex your foot, and push off the ground when walking. Sensory signals travel in the opposite direction, carrying information about touch, temperature, and pain from the skin of the leg and foot back up to the brain.
Because the nerve covers such a large territory, damage or irritation at any point along its path can produce symptoms in places that seem far from the actual problem. A pinched nerve root in the lower back, for example, can cause numbness in your toes.
What Causes Sciatica
The most common cause is a herniated disc in the lumbar spine. The soft, gel-like center of a spinal disc (the nucleus pulposus) pushes outward and presses against one of the sciatic nerve roots. But the story is more than just physical pressure. Research has shown that the disc material itself releases inflammatory chemicals, particularly a substance called TNF-alpha, that irritate nerve tissue even without direct compression. Animal studies confirm that disc material can cause nerve damage through chemical inflammation alone. This explains why some people with small herniations have severe pain while others with large ones feel relatively little.
Other common causes include spinal stenosis, where the bony canal around the spinal cord narrows with age and squeezes the nerve roots, and spondylolisthesis, where one vertebra slips forward over another. Piriformis syndrome is a less common but notable cause: the piriformis muscle in the buttock tightens or spasms and compresses the sciatic nerve where it passes underneath. In about 1% to 3% of people, the nerve actually splits and passes both above and below the piriformis, making it more vulnerable to compression from that muscle.
How Sciatica Feels
The hallmark of sciatica is pain that radiates from the lower back or buttock down the back of one leg. It typically affects only one side. The pain can range from a dull ache to a sharp, burning sensation or even an electric shock-like jolt, especially with sudden movements like coughing or sneezing.
Beyond pain, you may notice tingling, numbness, or a “pins and needles” sensation along the path of the nerve. Some people experience weakness in the affected leg, making it harder to lift the foot or push off while walking. The specific location of your symptoms often corresponds to which nerve root is involved. Compression of the L5 root, for instance, tends to cause pain and numbness along the outer calf and top of the foot, while S1 involvement typically affects the back of the calf and the outer edge of the foot.
Symptoms often worsen with prolonged sitting, standing up from a chair, or bending forward. Many people find that walking or lying down with a pillow under the knees provides some relief.
How Sciatica Is Diagnosed
Diagnosis usually starts with a physical exam. One of the most common tests is the straight leg raise: you lie flat on your back while a clinician lifts your affected leg. If this reproduces your radiating leg pain, it suggests nerve root irritation. This test is quite sensitive, catching up to 91% of disc herniations in younger adults, but it produces a lot of false positives because many other conditions can cause a positive result. Its accuracy also drops significantly with age, falling to about 33% sensitivity in people over 60.
Because no single physical test is definitive, imaging is often used when symptoms are severe or persistent. An MRI can reveal disc herniations, spinal stenosis, and other structural problems pressing on the nerve. Most clinicians hold off on imaging unless symptoms haven’t improved after several weeks, since many cases resolve on their own.
Recovery Timeline
The majority of sciatica cases improve without surgery. About three out of four people see significant improvement within a few weeks. Initial treatment typically involves staying active (bed rest can actually slow recovery), using over-the-counter pain relief, and applying heat or ice. Physical therapy focused on core strengthening and gentle stretching often helps reduce pressure on the nerve.
For people who aren’t improving after six weeks, more targeted options come into play, including steroid injections near the affected nerve root to reduce inflammation. Surgery, usually a procedure to remove the portion of disc pressing on the nerve, is considered when pain remains severe and disabling. Surgery tends to speed up recovery, but the long-term picture is telling: by six to twelve months, people who have surgery generally end up in a similar place as those who let the body heal on its own.
When Sciatica Signals Something Serious
Rarely, the nerve roots at the very base of the spine can become severely compressed, a condition called cauda equina syndrome. This is a medical emergency. The warning signs include sudden numbness in the groin or inner thighs (sometimes called saddle numbness), loss of bladder or bowel control, and rapidly worsening weakness in both legs. People with progressive weakness in both legs are nearly 15 times more likely to have this condition compared to those with typical one-sided sciatica. If you develop any of these symptoms, get to an emergency room immediately, as delayed treatment can result in permanent nerve damage.