Yes, a person has two sciatic nerves, one in the left leg and a mirror image nerve in the right leg, as the human body is bilaterally symmetrical. This nerve is the longest and widest single nerve in the entire body, often reaching the diameter of a finger near its origin point in the lower back. Its massive size and central pathway highlight its importance for movement and sensation in the lower limbs. The presence of this nerve on both sides is fundamental to the body’s ability to control both legs independently for standing, walking, and running.
The Origin and Path of the Sciatic Nerves
The sciatic nerve is not a single strand emerging from the spine but rather a complex cable formed from a collection of nerve roots. These fibers originate from the lower spinal cord, specifically the nerve roots that exit the vertebrae from the fourth lumbar segment (L4) through the third sacral segment (S3). These individual roots converge to form the single, large sciatic nerve within the pelvis, creating what is known as the sacral plexus.
From the pelvis, the nerve exits the greater sciatic foramen, a large opening in the hip bone, typically passing just beneath the piriformis muscle. It then runs deep through the buttock and continues its long descent down the back of the thigh, traveling deep to the hamstring muscles.
Just above the back of the knee, in the area called the popliteal fossa, the nerve branches into its two major terminal components. These branches are the tibial nerve, which runs down the posterior leg, and the common fibular (or common peroneal) nerve, which supplies the anterior and lateral parts of the lower leg. This extensive pathway explains why an issue with the sciatic nerve can cause symptoms anywhere from the lower back down to the foot.
Motor and Sensory Roles
The sciatic nerve is classified as a mixed nerve because it contains both motor fibers that control muscle movement and sensory fibers that transmit feeling. Its motor function is responsible for powerful movements in the lower limb, particularly in the thigh. It directly controls the hamstring muscles, which are necessary for flexing the knee and extending the hip.
Through its branches, the sciatic nerve also controls all the muscles in the lower leg and foot. The tibial nerve branch enables plantar flexion, the action of pointing the toes and standing on the balls of the feet. The common fibular nerve branch controls dorsiflexion, which is the action of lifting the foot upwards at the ankle.
The sensory role of the nerve provides feeling to large areas of the lower body. While it has no direct sensory role for the skin of the thigh, its terminal branches supply sensation to the skin of the lower leg and the entire foot. This sensory input allows a person to perceive touch, temperature, and pain in those regions.
Why Sciatica Pain is Often One-Sided
The condition commonly referred to as sciatica is a set of symptoms caused by the irritation or compression of one of the two sciatic nerves or their contributing nerve roots. Although a person has a sciatic nerve on each side, the pain almost always presents unilaterally, meaning it affects only one leg at a time. This is because the underlying mechanical cause of the irritation is typically localized to one side of the spinal column.
The most frequent cause of this one-sided compression is a herniated or bulging intervertebral disc in the lower spine. A disc bulge on the right side, for instance, will press only on the nerve roots destined for the right sciatic nerve, leaving the left nerve unaffected. Other common causes include spinal stenosis, which is a narrowing of the spinal canal, or piriformis syndrome, where the piriformis muscle in the buttock spasms and compresses the nerve passing beneath it.
When the nerve is compressed, the resulting symptoms of sharp, shooting pain, numbness, or tingling travel along the exact path of the affected nerve. This means the sensation can start in the lower back or buttock and radiate down the back of the thigh and into the lower leg and foot. The localized nature of a single disc herniation or muscle spasm is the primary reason why the discomfort typically follows the nerve pathway on just one side of the body.