The sciatic nerve affects nearly everything below the knee and much of the back of the thigh. It controls the muscles that bend your knee, point and flex your foot, and curl your toes. It also carries sensation from the outer leg, the top and bottom of the foot, and parts of the calf. As the largest nerve in the human body, it serves as the primary communication line between your spinal cord and your lower leg.
Where the Sciatic Nerve Runs
The sciatic nerve forms from five spinal nerve roots in the lower back and sacrum (L4, L5, S1, S2, and S3). These roots merge into a single thick nerve bundle that exits the pelvis through an opening called the greater sciatic foramen, passing just below the piriformis muscle deep in the buttock. From there it travels down the back of the thigh, running beneath the hamstring muscles.
At the back of the knee, the nerve splits into its two terminal branches: the tibial nerve and the common peroneal (fibular) nerve. These two branches are actually distinct nerves bundled together inside the same sheath for the entire journey down the thigh. In roughly 12% of people, they separate earlier, splitting as they leave the pelvis rather than at the knee. This anatomical variation explains why the same “sciatic” problem can produce very different symptoms from person to person.
Muscles It Controls
Before it splits, the sciatic nerve powers the major muscles of the back of the thigh: the biceps femoris (both heads), the semitendinosus, the semimembranosus, and part of the adductor magnus. These are your hamstrings, the muscles responsible for bending the knee and helping extend the hip.
Once the nerve divides at the knee, each branch takes over a different set of muscles in the lower leg and foot:
- Tibial nerve: Controls the calf muscles (gastrocnemius and soleus) and the deep muscles of the back of the lower leg. These handle pointing your foot downward (plantarflexion), curling your toes, and stabilizing your ankle when you push off the ground during walking.
- Common peroneal nerve: Controls the muscles along the front and outer side of the shin. These handle pulling your foot upward (dorsiflexion), extending your toes, and turning your foot outward. The peroneal nerve is especially important for clearing your foot off the ground with each step.
Together, these branches give the sciatic nerve control over virtually every movement of the foot and ankle, plus knee bending. The only major lower-limb muscles it doesn’t supply are the quadriceps at the front of the thigh and the inner thigh muscles, which are served by different nerves.
Sensation It Provides
The sciatic nerve’s sensory territory covers a large portion of the lower leg and foot. Through the common peroneal branch, it provides feeling to the outer lower shin, most of the top of the foot, and the small web of skin between the big toe and second toe. Through the tibial branch and its extensions, it covers the sole of the foot, the heel, and the outer edge of the foot along the Achilles tendon. It also sends sensory branches to the knee and ankle joints themselves, which is how your brain monitors the position and stress on those joints even when you’re not consciously thinking about them.
The inner calf and the front of the shin are not part of the sciatic nerve’s territory. If you have numbness there, a different nerve is likely involved.
What Happens When It’s Compressed
Because the sciatic nerve is so long and passes through tight spaces, it’s vulnerable to compression at several points. The resulting condition, sciatica, produces symptoms that follow the nerve’s path. Common symptoms include a burning or electric shock feeling that shoots down one leg, numbness, and a pins-and-needles tingling. The pain often worsens when you cough, sneeze, bend forward, or lift your legs while lying on your back.
The most frequent cause is a herniated disc in the lower spine pressing on one of the nerve roots that form the sciatic nerve. Another common culprit is piriformis syndrome, where the piriformis muscle in the buttock swells, spasms, or develops scar tissue and squeezes the nerve as it passes underneath. Spinal stenosis (narrowing of the spinal canal) and bone spurs can also compress the nerve roots.
The specific symptoms depend on which fibers within the nerve are affected. Compression of the peroneal division tends to cause difficulty lifting the foot, a condition called foot drop that makes walking feel like your foot is slapping the ground. Compression of the tibial division affects your ability to push off with your foot or flex your toes. Many people experience a mix of both, along with pain radiating from the lower back or buttock down through the calf.
How Doctors Test the Nerve
The most common bedside test is the straight leg raise: you lie flat on your back while a clinician lifts your affected leg with the knee straight. If this reproduces your shooting leg pain, it suggests compression of a lower lumbar nerve root. Studies put this test’s accuracy at about 81%, with a sensitivity of 77% (meaning it correctly identifies most people who have nerve root compression) and a specificity of 81% (meaning it correctly rules out most people who don’t). It’s a useful first step but not definitive on its own, which is why imaging like an MRI is often used to confirm the exact location and cause of compression.
When Sciatica Signals Something Serious
Most sciatica improves with time and conservative treatment. But in rare cases, severe nerve compression at the base of the spine can affect a bundle of nerves called the cauda equina, creating a surgical emergency. The warning signs that distinguish this from ordinary sciatica include sudden loss of bladder or bowel control, inability to urinate, numbness spreading across the inner thighs and buttocks, and rapidly worsening weakness in both legs. This situation, called cauda equina syndrome, typically requires surgery within 24 to 48 hours to prevent permanent damage. Progressive muscle weakness or foot drop that develops over days or weeks also warrants urgent evaluation, even without bladder symptoms.