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How does the spine work?

The spine is our body’s central support structure. It keeps us upright and connects different parts of the skeleton to each other, such as the head, chest, pelvis, shoulders, arms and legs. Although the spine is made up of a chain of bones, it is flexible due to elastic ligaments and spinal discs.

The length of someone’s spine depends on their height. The average length is 71 cm in men and 61 cm in women. Your spine has many functions: it carries the weight of your head, torso and arms, and allows your body to move in every direction. Some sections of the spine are more flexible than others. The neck is the most flexible section. The spine also encases and protects the spinal cord, which is an important part of the nervous system.

Viewed from the side, there are four slight natural curves in a healthy adult spine: the cervical (neck) and lumbar (lower back) sections of the spine curve inward, and the thoracic (upper back) and sacral (bottom of the spine) sections curve outward. These curves make the spine more stable: they help you keep your balance when you are in an upright position, act like shock absorbers when you walk, and protect the individual bones in the spine (the vertebrae) from fractures.

Adults normally have 26 vertebrae:

7 cervical vertebrae
12 thoracic vertebrae
5 lumbar vertebrae
1 sacrum (5 fused sacral vertebrae)
1 tailbone (4 fused coccygeal vertebrae)

The lower down the vertebrae are in the spine, the more weight they have to carry, and the bigger they are.

There are 23 intervertebral (spinal) discs in the human spine. These discs are found between the vertebrae. But there are no discs between the skull and the first vertebra, or between the first and second vertebrae. Each disc is made up of two parts: a ring of fibrocartilage on the outside and a soft center, which is a bit like a fluid-filled cushion. The spinal discs make it possible for the spine to move sideways and rotate, and they also act as “shock absorbers”.

When we put pressure on our spine, the spinal discs release fluid and become thinner (“compressed”); when the pressure is relieved they absorb fluid again and become thicker (“decompressed”). Because we usually put more pressure on our spines during the day and relieve the pressure at night, we are a bit taller in the morning than we are in the evening: adult men and women are about 1.5 to 2 cm shorter at the end of the day due to spinal disc compression. As we age, our spinal discs become thinner, the vertebrae become compressed and the spine curves more. That is why we are usually a few centimeters smaller in old age than we are as younger adults.

The spinal cord runs down the center of the spine. Each individual vertebral arch has a slight recess (indentation) at the top and bottom edge. At the point where two vertebrae come together they form two gaps – one on the left and one on the right side of the spine – through which spinal nerves exit the spinal canal. Because the nerve cords (so-called spinal nerves) branch off as they go down the spine, the spinal cord gets thinner towards the bottom of the spine, and the spinal canal becomes narrower too.

The spinal nerves carry electrical signals from the brain to muscles of the skeleton and internal organs via the spinal cord. Similarly, they carry sensory information like touch, pressure, cold, warmth, pain and other sensations from the skin, muscles, joints and internal organs to the brain via the spinal cord. Together, the spinal cord, spinal nerves and brain make up the central nervous system.

Author: German Institute for Quality and Efficiency in Health Care (IQWiG)