The "sciatic nerve" (pronounced "sigh + attic") is the longest nerve in the human body. It provides sensory communication between the brain and most of the buttocks and legs. This sensory communication goes in both directions, and includes pain messages that travel from the legs to the brain, as well as information on how to move the leg that travels from the brain to the muscles in the leg. Five separate nerves from the upper body combine to form the sciatic nerve. These nerves, known as S1, S2, S3, L4 and L5, come together in the upper part of the buttocks. From there, small nerve branches emerge and provide sensory communication to the hip joint. The main strand extends down the back of the leg to the knee, where it splits again into separate strands. The "tibial" strand continues down the back the leg and "innervates," or provides sensory perception and communication for, the heel and sole of the foot. The "peroneal" strand travels around the outside of the knee, down the side of the leg, and innervates the front part of the lower leg and the upper part of the foot. The largest segment of the nerve is the size of a person's thumb. The most common health problem associated with the sciatic nerve is known as "sciatica." Sciatica occurs when the S, L, or main sciatic nerve branches are compressed or pinched. Symptoms for sciatica include pain, numbness, tingling, weak muscles or movement difficulties in the lower back, buttocks, legs or feet. Spinal disc herniation is one common cause of sciatica. It may result from twisting and bending motions, which place unusual stress on the soft material inside the vertebrae. These movements may cause the soft material to rupture, spilling fluids into the narrow opening that allows the nerves to pass through the vertebrae, thus causing inflammation that compresses the nerve. Another common cause of sciatica is "piriformis syndrome." In most people, the sciatic nerve is below the piriformis muscle, a muscle in the buttocks region. However, in some people, the nerve passes through that muscle. When that muscle is compressed, the sciatic nerve is also compressed. Piriformis compression can be caused by something as simple as a large wallet in a hip pocket.