The sciatic nerve is the largest single nerve fiber in the human body, and it is often also referred to as the ischiadic or ischiatic nerve.   This long, tubular nerve can be as thick as three quarters of an inch in diameter in an adult human.

The sciatic nerve is, in a way, the continuation of the spinal chord down each leg.  Near the spinal chord’s lower extremity, in the lumbar region of the spine, the last major spinal nerves -  the fourth and fifth lumbar, and the first, second, and third sacral – extend to form a sciatic nerve on either side of the spine.  One sciatic nerve then travels down along the left leg, and the other travels down along the right leg.  This generates the paths for sensory and motor perceptions by these sciatic nerves, through the spinal chord, to the brain.  Commands from the brain can then, logically, reach the lower limbs through the same channels.

Each sciatic nerve is subdivided into articular branches and muscular branches, which allows the nerve to perform different functions.   The articular branches control articulations, particularly those of the hip joint.  The muscular branches control the muscles in the whole trajectory down each limb to the knee, and all the way to the feet, thanks to the branching of the sciatic nerve into the tibial nerve and the common peroneal nerve, which happens behind the knee.