The human body

How does the spine work?

The spine is our body’s central support structure. It keeps us upright and connects the different parts of our skeleton to each other: our 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 disks.

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 most flexible part is the cervical spine (neck area). The bones that make up the spine also protect the spinal cord, which runs through the spinal canal.

 Illustration: Areas and curvature of the spine, location of spinal disks and spinal nerve

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. This S-shaped curvature makes the spine stable: It helps you keep your balance when you are in an upright position, acts like a shock absorber when you walk, and protects the individual bones in the spine (the vertebrae) from fractures.

Adults normally have 33 vertebrae, from top to bottom:

  • 7 cervical vertebrae
  • 12 thoracic vertebrae
  • 5 lumbar vertebrae
  • 5 sacral vertebrae (fused together to form the sacrum)
  • 4 coccygeal vertebrae (fused together to form the tailbone, also known as the coccyx)

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.

Vertebrae and spinal disks

The front part of each of the vertebrae is called the vertebral body. There are 23 elastic spinal disks between the vertebrae – except for between the skull and the first cervical 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.

The disks have a solid, multi-layered casing of cartilage fiber and a gel-like core. They keep the spine flexible so that we can lean over and rotate our upper body. They also absorb shocks that are transferred to the spine when we run or jump, for instance.

When we put pressure on our spine, the spinal disks release fluid and become thinner ("compress"); when the pressure is relieved, they absorb fluid 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 by the end of the day. Over many years our spine starts to wear, meaning that as we age our spinal disks become thinner, the vertebrae become compressed and the spine curves more. That is why we usually "shrink" a little (a few centimeters) as we age.

Except for the first two vertebrae in the neck, as well as the sacrum and tailbone, all of our vertebrae are made up of a vertebral body at the front, and a "spinous process," which points backwards. Both parts are connected by the vertebral arch, which forms a cavity in the middle – the spinal foramen. These cavities align to form the spinal canal, which surrounds the spinal cord.

Illustration: Bones and joints of the thoracic spine: Side view with separated parts (left) and top view (right)Bones and joints of the thoracic spine: Side view with separated parts (left) and top view (right)

Spinal cord and nerves

The vertebral arches have a slight recess at the top and bottom of each edge. At the point where two vertebrae come together they form two gaps where the recesses meet – one on the left and one on the right side of the spine. The spinal nerves exit 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 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 and the brain make up the central nervous system.