How the Spine Works

spine
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The complete spinal column viewed from the side.

While most back pain is muscular in nature, there are a host of problems that can stem from your backbones. For instance, a slipped disk or herniated disk can be quite painful. Here is an examination of the skeletal structure of the back.

Spine Basics

In simple terms, your spine is nothing more than a group of bones in a line up the center of your back. The individual bones are called vertebrae. There are 33 bones in all -- 24 vertebrae, the sacrum (which is actually 5 vertebrae fused together), and the tailbone, or coccyx (4 vertebrae fused together). The individual vertebrae are stacked on top of each other from about the level of your belly button up to your head.

When healthy, the bones of the spine are strong and dense. As is the case with all bones, the vertebrae get their strength primarily from two important minerals -- calcium and phosphate. When there is a shortage or loss of these minerals because of an inactive lifestyle (weight-bearing activity actually helps to keep bones strong) or diseases such as osteoporosis, the bones lose their structure and strength. Unless they are weakened in this way or are damaged in an accident or fall, bones do not typically break or wear out.

The spine has four regions. Starting with the neck and progressing down to the pelvis, the regions are the cervical region, the thoracic region, the lumbar region, and the sacral region. All of these regions work together to provide support and stability for much of the weight of the upper body. Each bony level has a passage that forms a tunnel for the spinal cord, thus protecting the spinal cord as it extends downward from the brain.

But the spine is more than just a protective tube. While the spine provides protection and support, its design also allows you to bend, twist, rotate, and otherwise move your upper body in every direction. The bones themselves do not actually bend or twist; the flexibility of the spine comes from structures between the bones called facet joints and intervertebral disks.

Intervertebral Disks

The infamous disks -- technically known as intervertebral disks -- are situated between the bones of the spine, creating a space for nerves branching out from the spinal cord to other areas of the body. The disks are made of tough rings of fibrous elastic material called cartilage; if you sliced a disk horizontally, it would look something like an onion cut through its middle. At the center of the rings of fibro-elastic tissue is a thick fluid with the consistency of very cold molasses.

View of an intevetebral disk showing the pulpy, liquid cneter.
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View of an intevetebral disk showing the pulpy, liquid center.

Together, the rings and the jellylike center of the disk act as a shock absorber, much like the shock absorbers on a car. When healthy, they take up much of the shock that walking, running, jumping, and even sitting can place on your spine. Every time you bend, extend, or twist, there is a change of pressure in the fluid-filled area of the disk. In moderate amounts, this change of pressure is actually good for the disks. In the long term, however, excessive forward bending movements -- with the back rounded and the legs straight -- can damage the rings that hold the fluid in place. Unfortunately, this bending motion is one that many people use repeatedly throughout a typical day to reach or lift objects. Even slumped sitting may expose the disks to possible injury.

This damage to a disk starts at the rings in the center of the disk closest to the fluid, and then progresses toward the outermost rings. The condition has often been called a slipped disk. However, the disk does not actually slip out from between the bones. Rather, the fluid begins to break through the rings. When this condition has advanced to the point where there are only a few rings left holding the fluid inside the disk, the condition is known as a bulging disk. The rings can push into the spinal cord or the nerves exiting the cord. If all the rings tear, the disk has ruptured, or herniated.

The three stages of disk herniation.
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The three stages of disk herniation. First, a healthy disk with the fluid center in place (left). Second, a bulging disk in which the fluid has begun to break through the fibrous layers (center). Third, the fully herniated, or ruptured, disk where the fluid has broken out of all the layers and leaks into the surrounding tissue (right).


Some people experience a condition called degenerative disk disease. This condition usually progresses over many years. In this process, one or more disks dry out, losing their ability to absorb the loads and shock placed on them with everyday activities. Age also has an effect on the disks in the spine. At age 20, the disks are made up of about 70 percent water. With increasing age, the disks naturally lose their water content.

If a disk wears out, dries out, bulges, or tears, it loses height. This forces the bones closer together, so the facet joints end up having to take much more of the shock as you move. The facet joints, in turn, can wear out prematurely. The loss of height also narrows the opening between the bones through which nerves exit the spinal cord. This narrowing can pinch a nerve, often causing pain.

All of the conditions that affect the disk can, in the later stages, be extremely painful and debilitating. They can interrupt normal work, play, and even sexual function. Taking steps to protect the disks can pay great dividends in the long run. However, most of the time when people "throw out" their backs, it is due to muscle tears or spasms. Learn all about the back muscles in our next section.

This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.