The sciatic nerve is the largest and longest nerve in your body, running from your lower spine all the way down each leg. When this nerve gets compressed or irritated, the resulting pain is called sciatica, a condition that affects 10% to 40% of people at some point in their lives. Understanding what the sciatic nerve does, where it runs, and what happens when something goes wrong with it helps make sense of a type of pain that can range from a mild ache to a debilitating jolt down your leg.
Where the Sciatic Nerve Runs
The sciatic nerve forms from a bundle of nerve roots that branch off the lower spinal cord, specifically from the fourth lumbar vertebra down to the third sacral vertebra (the five lowest segments of your spine). These individual nerve roots weave together into a single thick nerve, roughly the diameter of a finger, that exits the pelvis through an opening called the greater sciatic foramen. It passes just beneath a deep buttock muscle called the piriformis.
From there, the nerve travels down the back of each thigh. Just above the knee, it splits into two branches: one that serves the front and outer part of the lower leg, and one that serves the back of the calf and the sole of the foot. Together, these branches control nearly all the muscles below the knee and provide sensation to most of the lower leg and foot. The sciatic nerve also powers the hamstring muscles in the back of the thigh. This is why problems with the nerve can produce symptoms anywhere from the low back to the toes.
What the Nerve Actually Does
The sciatic nerve carries two types of signals. Motor signals travel from the brain down through the nerve to tell muscles when and how hard to contract. This is what allows you to push off while walking, point your toes, or lift your foot to clear a step. Sensory signals travel the opposite direction, carrying information about touch, temperature, and pain from your leg back up to the brain. When the nerve is working normally, you never notice it. When it’s compressed or inflamed, both types of signals can get disrupted, producing pain, numbness, tingling, or weakness.
Common Causes of Sciatic Nerve Pain
The most frequent cause of sciatica is a herniated disc in the lower spine. The discs between your vertebrae act as cushions, with a soft gel-like center surrounded by a tougher outer ring. Over time, the outer ring can weaken as it loses water content and its internal structure changes. If the soft center pushes through, it can press directly against a nearby nerve root. This creates two problems at once: physical pressure on the nerve and a flood of inflammatory chemicals from the damaged disc material. Both the compression and the inflammation cause nerve root pain.
Other common causes include spinal stenosis (narrowing of the spinal canal that squeezes the nerves), bone spurs from arthritis, and degenerative disc disease. A less recognized cause is piriformis syndrome, where the piriformis muscle in the buttock tightens or spasms and traps the sciatic nerve as it passes underneath. Piriformis syndrome produces pain and numbness radiating from the buttock down to the foot, closely mimicking a spinal problem, which can make it tricky to diagnose.
Peak incidence of sciatica occurs in people in their 40s, likely because disc degeneration has progressed enough by that age to cause problems, while the spine hasn’t yet stiffened enough to limit disc movement.
What Sciatica Feels Like
Sciatica pain typically starts in the lower back or buttock and radiates down the back of the leg, often below the knee. People describe it differently: a sharp, burning sensation, an electric shock, or a deep ache. It usually affects only one side. Sitting for long periods, coughing, or sneezing can make it worse because these actions increase pressure on the spinal discs.
Some people also experience numbness or tingling in the leg or foot, or a feeling of weakness, like the leg might give out. In milder cases, the pain is annoying but manageable. In severe cases, the pain can make it difficult to stand, walk, or sleep. The specific location of your symptoms often corresponds to which nerve root is affected. Compression higher up tends to cause pain in the thigh and outer calf, while compression lower in the spine is more likely to affect the back of the calf and foot.
How Sciatica Is Diagnosed
A physical examination is usually the first step. One of the most well-known tests is the straight leg raise, where you lie flat on your back while a clinician slowly lifts your affected leg with the knee kept straight. The test is considered positive if it reproduces your radiating leg pain (not just back tightness) when your leg is raised between 30 and 70 degrees from horizontal. This test is quite sensitive, catching 72% to 97% of true disc herniations. Pain that only appears in the lower back, or only in the back of the thigh, or that occurs outside the 30-to-70-degree window points toward something other than nerve root compression.
A variation called the crossed straight leg raise, where the unaffected leg is lifted instead, is less sensitive (23% to 42%) but much more specific (85% to 100%). If lifting your good leg reproduces pain in your bad leg, there’s a very high chance a disc herniation is involved. Imaging like MRI is typically reserved for cases that don’t improve with conservative treatment or when there are signs of a more serious problem.
Treatment and Recovery
Most people with sciatica improve within several weeks to a few months without surgery. The first-line approach combines staying active (bed rest actually slows recovery), gentle movement, and over-the-counter pain relief. Targeted exercises are one of the most effective tools. A physical therapist can assess which movements help and which aggravate your specific situation, then build a routine around that.
Exercises that strengthen the muscles supporting your spine and hips are particularly useful. Glute bridges, for example, strengthen the buttocks and core while keeping the lower back in a neutral position: you lie on your back with knees bent, then lift your hips toward the ceiling, holding for 5 to 30 seconds. Nerve gliding exercises, which gently move the sciatic nerve through its surrounding tissues, can also reduce irritation. Doing a set of 8 to 10 repetitions at least twice a week is a reasonable starting point, and daily practice is fine if it feels good.
For pain that doesn’t respond to these measures, steroid injections near the affected nerve root can reduce inflammation and provide temporary relief. Surgery, most commonly a microdiscectomy to remove the portion of disc pressing on the nerve, is considered when pain is severe, persistent, or accompanied by progressive weakness.
Red Flags That Require Immediate Attention
Rarely, pressure on the nerves at the very base of the spine can cause a condition called cauda equina syndrome. Warning signs include sudden difficulty urinating or loss of bladder control (urinary retention is the most common symptom), numbness in the groin or inner thighs, sexual dysfunction, or rapidly worsening weakness in both legs. This is a surgical emergency because permanent nerve damage can result if pressure isn’t relieved quickly. These symptoms are distinct from typical sciatica, which, while painful, doesn’t affect bladder or bowel function.