How does the spine work?

The individual bones in the spine are called vertebrae. Adults typically have 33 vertebrae. They are divided into five areas (from top to bottom):

• Cervical spine (7 vertebrae)
• Thoracic spine (12 vertebrae)
• Lumbar spine (5 vertebrae)
• Sacrum (5 fused vertebrae)
• Tailbone, or coccyx (4 fused vertebrae)

Doctors have numbered the vertebrae in each region from top to bottom together with one or two letters standing for each part: Cervical vertebrae are given the letter "C," thoracic vertebrae the letters "Th," and lumbar vertebrae the letter "L," for example. Each region is then numbered from top to bottom, e.g. with C1 to C7 for the cervical vertebrae and so on.

Some regions of the spine are more flexible than others. The cervical spine is the most flexible. It allows us to turn and tilt our head in nearly every direction. The sacrum and tailbone can't be moved, and are made up of only bone in adults. Children still have separate vertebrae making up the sacrum and tailbone, which then gradually grow together.

The lower down the vertebrae are in the spine, the more weight they have to carry. For that reason, the lumbar vertebrae are larger and more stable than vertebrae near the top of the spinal column, for instance. Because of this greater load, normal wear and tear commonly develops here as we age.

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 a little. This normal curvature is known as lordosis – cervical lordosis in the neck area, and lumbar lordosis in the lower back.
• The thoracic (upper back) and sacral (bottom of the spine) sections naturally curve outward a little. An excessive forward-rounding of the spine is known as kyphosis – thoracic kyphosis in the upper back, and sacral kyphosis in the pelvic area.

The natural S-shaped curvature of the spine makes it stable: It helps you keep your balance when you're in an upright position, acts like a shock absorber when you walk, and protects the individual bones in the spine (the vertebrae) from fractures. This curvature develops after birth once more strain is put on the spine as babies start to lift their heads up, crawl and (finally) walk.

In certain diseases, such as Scheuermann's disease (Scheuermann's kyphosis) and ankylosing spondylitis, the curvature of the spine is too strong. This can lead to a poor posture and other problems.

The spine looks straight when viewed from the back. Curvature to the left or right may be a sign of scoliosis, either in children or in old age.

Aside from the first two cervical vertebrae at the top of the spine and the sacrum and tailbone at the bottom end of the spine, all vertebrae have a vertebral body that faces inwards relative to the body and a spinous process that faces outwards. Spinous processes of the vertebrae are visible in some thin people, especially when they bend forwards.

The other parts of a verebra include:

• The vertebral arch: It connects the vertebral body to the spinous process, and surrounds an opening called the vertebral foramen. The column formed by the verebral formens stacked in a line is the vertebral canal, which contains the spinal cord.
• The articular processes: Together with the articular process from the vertebra above, it forms a facet joint.
• The articular facets: These surfaces are covered with a thin layer of cartilage to allow smooth movement.
• The transverse processes: They are found on the two sides of the vertebra, and are attached to muscles and ligaments.

There are a total of 23 elastic spinal discs spread out between the vertebrae – except for between the skull and the first cervical vertebra, and between the first and second cervical vertebrae. Each spinal disc consists of a soft gel-like core (nucleus pulposus) that is surrounded by a tough, multi-layered case made of fibrous cartilage (annulus fibrosus).

Spinal discs act as shock absorbers between the vertebrae in our spine. They also act as shock-absorbers when we run or jump, for instance. They keep the spine flexible so that we can lean over and rotate our upper body. Together with the muscles in the upper body, the spinal discs also help to keep the spine stable in order to prevent the vertebrae from slipping.

The spinal discs act a bit like sponges: When we put pressure on our spine, the spinal discs release fluid and become thinner (“compress”); when the pressure is relieved, they absorb fluid again and become thicker (“decompress”). Because we usually put more pressure on our spine during the day and relieve the pressure at night, we are around 1.5 to 2 centimeters shorter (about 0.6 to 0.8 inches shorter) by the end of the day.

Signs of wear and tear may start appearing after many years. As we get older, the spinal discs become narrower, the vertebrae move closer to each other, and the spine becomes more curved. Because of this, older people are usually a few centimeters shorter than they used to be when they were younger.

Wear and tear on the vertebrae and spinal discs (degeneration of the spine) is also known as spondylosis. Many people have degenerative changes in their spine but they don't always have back pain or other symptoms. So when people have these symptoms, it usually isn’t absolutely clear whether they are caused by degeneration of the spine.

The bones of the spine protect the spinal cord, which runs along the spinal canal. The vertebral arches are slightly indented at the top and bottom of each side. At the point where two vertebrae come together, they form two gaps where the indented parts meet – one on the left and one on the right side of the spine. The spinal nerves connect to one sciatic nerve on the left and on the right in the pelvis. The sciatic nerves run down the back of the legs.

The spinal nerves carry electrical signals along the spinal cord, from the brain to muscles of the skeleton and internal organs. They also carry sensory information along the spinal cord in the other direction – from the skin, muscles, joints and internal organs to the brain. This sensory information includes things like touch, pressure, cold, warmth and pain. Together, the spinal cord and the brain make up the central nervous system.

The spinal cord ends at the top portion of the lumbar spine. From that point, only a bundle of nerves called the cauda equina continues downward.