Sciatica is a symptom of a problem with the sciatic nerve, a large nerve that runs from the lower back down the back of each leg. It controls muscles in the back of your knee and lower leg and provides feeling to the back of your thigh, part of your lower leg and the sole of your foot. When you have sciatica, you have pain, weakness, numbness or tingling. It can start in the lower back and extend down your leg to your calf, foot, or even your toes. It's usually on only one side of your body.