Anatomy of Sciatica

The sciatic nerve is the longest and largest nerve in your body. Five sets of paired nerve roots combine to create it, and it’s about the diameter of a finger.

The sciatic nerve starts in your low back, which is called your lumbar spine. The nerve roots are at the L4 and L5 vertebrae (the ‘L’ means lumbar, and the numbers indicate the level of the vertebra-where it is in your back). The sciatic nerve also travels through your pelvic region (sacrum).

In most people, the sciatic nerve runs under the piriformis muscle, which moves your thigh side to side. From there, the sciatic nerve descends through the buttocks and the back of the thighs.

Behind your knee, smaller nerves branch out from the sciatic nerve and travel down to your feet.

Your sciatic nerve is part of a complex structure: your body’s nervous system. That system is responsible for transmitting pain and sensation to other parts of your body. So, when something presses on a nerve, you’ll feel it, and it won’t feel good.

With sciatica, something in your low back-a herniated disc, for example-compresses the sciatic nerve, which then transmits pain down your legs.