Anatomy of the spine

The spine is a chain of 33 stacked bones called vertebrae. It includes 24 movable vertebrae separated by intervertebral discs, along with 9 fused vertebrae at the base. 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 cervical vertebrae. These are C1 to C7. The skull rests on the first cervical vertebra (atlas). Below it is the second cervical vertebra, known as the axis. These two vertebrae have an irregular shape. From the third cervical vertebra onward, they increasingly resemble the other movable vertebrae of the spine.
• 12 thoracic vertebrae. These are T1 to T12. The ribs, which 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. These are L1 to L5. These vertebrae are located in the lower back and are the largest of all, as they bear the most weight.

Additionally, there are 9 vertebrae that have 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. These are S1 to S5. The sacrum connects the two halves of the pelvis and the spine. The sacrum is triangular in shape.
• 4 coccygeal vertebrae. There is no abbreviation for these. Usually, there are 4 coccygeal vertebrae, but sometimes there are 3 or 5. These vertebrae are remnants of a tail. Since humans no longer have a tail, this part has lost its function.

Structure of a vertebra

With the exception of the top two cervical vertebrae, all vertebrae look more or less the same, though they vary slightly in size and shape.

A vertebra roughly consists of two parts: the vertebral body at the front and the vertebral arch at the back. The vertebral body bears most of the weight and connects with the intervertebral disc. The vertebral arch has two lateral projections (transverse processes) and one posterior projection (spinous process). These projections serve as attachment points for ribs, muscles, and ligaments.

Two consecutive vertebrae touch each other at multiple points, forming several joints. The vertebral bodies are connected by an intervertebral disc, while 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 rings that are tightly attached to the vertebral bodies. In the center is a soft core (nucleus). In a herniated disc, this soft core bulges out and presses against the nerves.

Spinal cord and nerves

Between the vertebral body and the vertebral arch lies the spinal canal, through which the spinal cord runs. The spinal cord is an extension of the brainstem and, together with the brain, forms the central nervous system. The spinal cord ends at the level of the first lumbar vertebra. From that point, increasingly longer nerves run downward in the spinal canal, resembling a horse's tail, known as the cauda equina.

The nerves that exit the spinal canal control the muscles and enable us to experience sensations such as touch, temperature, and pain. On both sides of the spine, one nerve exits between two consecutive vertebrae.

Muscles of the spine

The muscles enable the spine to move. There are very long muscles that run the entire length of the back, as well as shorter muscles that move only the neck or lower back. There are even muscles that extend from one vertebra to another, providing stability to the spine.

Joint capsules and ligaments of the spine

The joint capsules and ligaments function similarly to the muscles. There are very long and very short ligaments. They provide stability to the joints, keeping the vertebrae properly aligned and limiting excessive movements.

Movements of the spine

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

Posture of the spine

When viewed from the front, the spine is straight, with the vertebrae neatly stacked on top of each other. If this is not the case, it is referred to as scoliosis. However, when viewed from the side, it looks very different, with several curves visible. These curves are not accidental. They make the spine mobile and capable of absorbing the large forces that the spine endures daily.

There are two types of curves:

• Lordosis. This is a forward curve, as seen in the neck and lower back.
• Kyphosis. This is a backward curve, as seen in the thoracic spine.