How does the spine work?

Adults normally have 33 spine bones called vertebrae. They include (from top to bottom):

• 7 cervical vertebrae (in the neck area)
• 12 thoracic vertebrae (in the upper back area)
• 5 lumbar vertebrae (in the lower back area)
• 5 sacral vertebrae (in the pelvic area – fused together to form the sacrum)
• 4 coccygeal vertebrae (fused together to form the tailbone)

The lower down the vertebrae are in the spine, the more weight they have to carry. For that reason, the lower movable vertebrae are larger and more stable than the top vertebrae.

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 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 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. 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 bad posture and other problems.

The front part of each of the vertebrae is called the vertebral body. There are a total of 23 elastic spinal discs spread out between the vertebrae – except for between the skull and the first cervical (neck) vertebra, and between the first and second cervical vertebrae. The sacrum and the tailbone can't be moved, and are made up only of bone.

Each spinal disc consists of a soft gel-like core that is surrounded by a tough, multi-layered case made of fibrous cartilage. They keep the spine flexible so that we can lean over and rotate our upper body. They also act as shock-absorbers when we run or jump, for instance.

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.

Apart from the first two vertebrae in the neck, as well as the sacrum and tailbone, all of our vertebrae have a vertebral body at the front and a "spinous process" at the back. Both parts are connected by the vertebral arch, which forms a space in the middle called the vertebral foramen. These spaces align to form the spinal canal, which surrounds the spinal cord.

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 leave the spinal canal through these gaps. Because the spinal nerves branch off as they go down the spine, the spinal cord thins out at the lower end of the spine.

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.