Your spine comprises 24 bone blocks called vertebrae, stacked on top of each other, resting on the pelvis and topped by the skull. Between each vertebra are spongy but tough cushions called discs, which act as shock absorbers and give the spine its flexibility.
Strong elastic ligaments hold all the vertebrae and discs firmly together in a column. Muscles are attached to the vertebrae by rubbery connections called tendons. The contractions and expansions of the complex layers of back muscles produce the movements of your back and your upper body.
The spine also provides a protective case for the spinal cord, which runs from the base of the brain into a hollow canal down the vertebrae. This is the main communications cable between the brain and the rest of the body, and nerves from the spinal cord branch out and leave the spine through spaces between the vertebrae.
Doctors divide the back into the five regions, shown in the diagram on the right. Back pain most often affects the lumbar region, right at the bottom of the flexible part of the back, just above the pelvis. This part of the back bears the weight of the upper body, plus any weight the person is carrying. It also twists and bends more than the upper part of the back.
This article was last medically reviewed by Dr Rob Hicks in September 2007. First published in March 1999.
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