The sciatic nerve is the longest nerve in the body. It is responsible for relaying sensations and movement impulses between the foot/leg/hip and the brain.
The sciatic nerve exits the spinal column from a grouping of nerves in the low back area. This grouping is called a plexus. From the plexus, it begins its journey down the back of the hip to the back of the thigh and then to the foot, branching into smaller nerves as it goes.
The sciatic nerve is significant because when it is stretched or irritated in some way, it can cause mild to intense leg pain, a condition known as sciatica.
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Wheeless' Book of Orthopaedics. Retrieved January 10, 2007, from Duke Orthopaedics Web site: http://www.wheelessonline.com/ortho/greater_sciatic_foramen