The sciatic nerve is the one of the largest nerves in the Human body, reaching approximately two centimeters (2 cm) in width. This nerve runs through the pelvis bone and branches off into other nerves near the knee. Almost all of the skin between these two points have nerves connected to the sciatic. The severing of this nerve causes paralysis from the point of the severing.
In 2365, Commander Riker was infected with a bacterio-virus which invaded his nervous system, starting from his sciatic nerve. The contagion was neutralized only with unconventional neural therapy by Dr. Pulaski and Counselor Troi. (TNG: "Shades of Gray")