What structures make up the spine?
The spine is a fascinating and important part of our anatomy. Its structure contributes to our upright posture. Functionally, it also houses our spinal cord, a critical part of our central nervous system, which relays all kinds of messages from the brain to the rest of the body.
Sections and curves of the spine
The spine can be divided into sections, each of which has a natural curve, which combine to create its strength and stability. The cervical spine is the section that we know as the neck. This section has a concave curve, which is a curve that goes in towards the body. The thoracic spine is in the area of the mid-back. It has a convex curve, which is a curve that goes out away from the body.
The lumbar spine is in the area of the lower back, posterior to the thoracic spine and anterior to the sacrum. It has a concave curve. Finally we have the sacrum, which is the lowest part of the back. It continues on to connect to the coccyx. The sacrum and coccyx together have a convex curve.
Bones in the spine
The bones that make up the spine are called vertebrae. There are seven cervical vertebrae that make up the neck section. There are twelve vertebrae in the thoracic section. There are five vertebrae in the lumbar section. The spinal bones could also include the sacrum, which is usually composed of five fused vertebrae, and the coccyx (the tailbone), which is usually composed of four fused vertebrae.
Joints in the spine
Where each of the vertebra meets the next vertebra there are three joints, one on the anterior side and two on the posterior side. The movement at each of these joints adds up to the total available movement that we see in the spine. The joints where each of the vertebra meets the next on the anterior side occur at the vertebral bodies. These anterior intervertebral joints are cartilaginous joints and this is where you find the vertebral discs.
The joints where each of the vertebra meets the next vertebra posteriorly, are called the posterior intervertebral joints or facet joints. These are synovial gliding joints which allow the vertebrae to articulate with one another to create small amounts of movement. When we add up each of these small movements, we get the large amount of movement that we see from the spine as a whole.
Muscles and ligaments around the spine
There are several layers of muscles that attach along the posterior vertebrae along most of the spine. At the deepest layer, we find the rotatores which attach from the transverse processes below to the spinous processes above. Each of the small rotatores muscles crosses one or two vertebrae. The rotatores are found only in the thoracic area.
The next most superficial layer are the multifidi, which attach primarily from the transverse processes below to the spinous processes above, although the origins of each section of multifidus vary slightly in the lower lumbar area and cervical area. Each section of the multifidus crosses two to four vertebrae.
Most superficially, we have the erector spinae group of muscles, which includes iliocostalis, longissimus, and spinalis. Each of those can also be broken down further into sections. When combined, the three sections of each of the iliocostalis, longissimus, and spinalis muscles span the vertebrae from the lumbar spine all the way to the cervical spine.
In each area, we find additional muscles that have attachments to the vertebrae there.
In the cervical area we also have:
- Levator scapulae
- Upper trapezius
There are several muscles that span both the cervical and thoracic areas:
- Splenius capitis
- Splenius cervicis
- Semispinalis capitis
- Semispinalis cervicis
In the thoracic area we have:
- Rhomboids major
- Middle and lower sections of trapezius
In the lumbar area we have:
- Quadratus lumborum
- Psoas major
There are also several thick ligaments that help maintain the integrity of the spine. The anterior longitudinal ligament runs along the anterior surface of the vertebral bodies and discs. The posterior longitudinal ligament runs along the posterior surface of the vertebrae. The ligamentum flavum connects the posterior bony wall of adjacent vertebrae. The supraspinous ligament runs along the spinous processes of the vertebrae and becomes the ligamentum nuchae in the cervical area. Finally, the interspinous ligament also connects the spinous processes along the whole spine.
Movements of the spine
Movements of the spine generally include:
- Lateral flexion or side-bending
There is more or less movement of each of these types in different sections of the spine. The amount of movement in a particular direction is affected by the shape of the vertebrae in that section and how they fit together.
In the cervical spine, we can see lots of movement in all of these directions. In the thoracic spine, due to the shape of the vertebrae, there is more rotation available than in the lumbar spine and less movement available in the direction of flexion and extension. In the lumbar spine, due to a slightly different shape of the vertebrae, there is more movement in the flexion and extension direction than we find in the thoracic spine, but less rotation is available.
If you’d like to learn more about some important muscles that move the spine, check out our article on the erector spinae muscles.