Anatomy of the spine

The spine is a chain of 33 stacked bones called vertebrae. The spine has 24 movable vertebrae. Between them are intervertebral discs. In addition, there are 9 vertebrae that have fused together. These last vertebrae are only briefly mentioned. Vertebrae are connected to each other by muscles and ligaments.

Types of vertebrae

Vertebrae are abbreviated with a letter and a number. From top to bottom, the vertebrae gradually increase in size.

There are 3 types of movable vertebrae:

• 7 neck vertebrae or cervical vertebrae. These are C1 through C7. The skull rests on the 1st cervical vertebra (atlas). Below it is the 2nd cervical vertebra, the axis. These two vertebrae have an atypical shape. From the third cervical vertebra onward, they increasingly resemble the other movable vertebrae of the spine.
• 12 thoracic vertebrae or thoracic vertebrae. These are T1 through T12. The ribs that form the rib cage are attached to the thoracic vertebrae. These ribs limit the mobility of the thoracic spine and protect internal organs such as the lungs and heart.
• 5 lumbar vertebrae or lumbar vertebrae. These are L1 through L5. These vertebrae are located in the lower back and are the largest of all. They have to be, because they bear the most weight.

Additionally, there are 9 vertebrae that fused together during human development (evolution). There are no intervertebral discs between these vertebrae and they cannot move relative to each other:

• 5 sacral vertebrae or sacral vertebrae. These are S1 through S5. The sacrum connects the two halves of the pelvis and the spine. The sacrum is triangular in shape.
• 4 coccygeal vertebrae or coccygeal vertebrae. There is no abbreviation for these. Usually there are 4 coccygeal vertebrae, but sometimes there are 3 or 5. The coccygeal vertebrae are a remnant of a tail. Because humans no longer have a tail, this part has lost its function.

Structure of a vertebra

With the exception of the top 2 cervical vertebrae, the vertebrae look more or less the same, though they are each slightly different in size and shape.

A vertebra roughly consists of 2 parts. The vertebral body at the front and behind it a vertebral arch. The vertebral body bears most of the weight and makes contact with the intervertebral disc. On the vertebral arch are 2 lateral projections (transverse processes) and 1 (spinous process) posteriorly. These projections serve as attachment points for ribs, muscles and ligaments.

Two consecutive vertebrae contact each other at various points. They therefore form multiple joints together. At the vertebral body this is via an intervertebral disc. The joints on the vertebral arches have a cartilaginous surface and are surrounded by a joint capsule for stability.

Intervertebral discs

There are a total of 23 intervertebral discs. These have a shock-absorbing function and increase the mobility of the spine. The intervertebral discs are made of strong concentric rings and are firmly attached to the vertebral bodies. In the center is a soft core (nucleus). In a herniated disc, this soft core bulges out and presses on the nerves.

The spinal cord and the nerves

Between the vertebral body and the vertebral arch lies the spinal canal. This is where the spinal cord runs. The spinal cord is the extension of the brainstem and together with the brain it belongs to the central nervous system. At the level of the first lumbar vertebra, the spinal cord ends. From that point, ever longer nerves run down the spinal canal. This resembles a horse's tail and is called the cauda equina.

The nerves that leave the spinal canal control the muscles and allow us to experience stimuli such as touch, heat and cold, and pain. On both sides of the spine, one nerve exits between 2 consecutive vertebrae.

The muscles of the spine

The muscles ensure that the spine can move. There are very long muscles that run the entire length of the back. But there are also short muscles that move only the neck or lower back. There are even muscles that go from one vertebra to the other. These are therefore very short and mainly give the spine stability.

The joint capsules and ligaments of the spine

For the joint capsules and ligaments, roughly the same applies as for the muscles. There are very long, but also very short ligaments. They provide stability to the joints, keeping the vertebrae nicely aligned. They limit movements when they threaten to become too large.

Movements of the spine

Because the spine consists of so many different vertebrae and joints, it is very mobile. The spine can bend forward, extend backward, bend sideways and rotate on its axis. Combinations of these movements are also possible.

Posture of the spine

Seen from the front, the spine is straight. The vertebrae are neatly stacked on top of each other. If this is not the case, then there is a scoliosis. But from the side it looks very different and several curves are visible. That is not for nothing. Precisely because of those curves, the spine is mobile and able to absorb the large forces that the spine has to endure daily.
There are two types of curves:

• The lordosis. This is a forward curve as in the neck and lower back.
• The kyphosis. This is a curve that protrudes backward, as is the case in the thoracic spine.