The spine, also called the back bone, plays a vital role in stability, smooth movement and protection of the delicate spinal cord. It is made up of bony segments called vertebrae with fibrous tissue called intervertebral discs between them. The vertebrae and discs form the spinal column from the head to the pelvis, giving symmetry and support to the body.
A single vertebra is made up of two parts, the front portion is called the body and the back portion is referred to as the vertebral or neural arch. The body is cylindrical in shape, strong and stable. Two strong pedicle bones join the vertebral arch to the body of the vertebrae.
The laminae of the vertebra can be described as a pair of flat arched bones that form a component of the vertebral arch. The transverse processes spread out from the side of the pedicles, like wings, and help to anchor the surrounding muscle to the vertebral arch. The spinous process forms a steeple at the apex of the laminae, and is the part of our spine that is felt directly under the skin.
Spinal canal is formed by the placement of single vertebral foramina one on top of the other to form a canal. The purpose of the canal is to create a bony casing from the head to the lower back through which the spinal cord passes.
The spine can be divided into 4 parts: cervical, thoracic, lumbar and sacral region. The cervical spine comprises the first 7 vertebrae of the spinal column, which form the neck.
The cervical spine is highly mobile compared to other regions of the spine such as thoracic or lumbar spine. In contrast to other parts of the spine, the cervical spine has transverse foramina in each vertebra through which the vertebral arteries supply blood to the brain.
Based on the structural diversity in the cervical spine, it can be divided into two parts: upper and lower cervical spine.
Upper cervical spine
The upper cervical spine comprises of the atlas (C1) and axis (C2), which are different from rest of the cervical vertebrae. The atlas vertebra articulates with the occiput superiorly at the atlanto-occipital joint and with the axis inferiorly at the atlantoaxial joint. Generally, the atlantoaxial joint is accountable for half of the cervical rotational movements; while the atlanto-occipital joint is responsible for half of the flexion and extension movements of the neck.
In contrast to other vertebrae, the atlas is ring-shaped without a body. The odontoid process, or dens of the axis, represents the fused remnants of the body of the atlas. The transverse ligament firmly opposes the odontoid process to the posterior portion of the anterior arch of atlas and provides stability to the atlantoaxial joint.
The atlas comprises of a thick anterior arch, a thin posterior arch, two prominent lateral masses, and two transverse processes. The transverse process surrounds the transverse foramen, through which the vertebral artery passes. A zygapophyseal joint is present on the superior and inferior aspect of the lateral mass. The superior articular facets articulate with the occipital condyles and are kidney-shaped, concave, projecting upward and inward, while the inferior articular facets are comparatively flat projecting downward and inward and articulate with the superior facets of the axis.
The axis possesses a large vertebral body with the odontoid process or dens. The axis also has heavy pedicles, laminae, and transverse processes, which help in the attachment of muscles.
Lower cervical spine
The remaining five cervical vertebrae, C3-C7, form the lower cervical spine. They are like each other but distinct from C1 and C2. Each of these cervical vertebrae has a body with a concave superior surface and convex inferior surface. The superior surfaces of the bodies have raised processes called uncinate processes that articulate with the inferior lateral portion of the vertebral body present above, called echancrure or anvil.
The C3-C6 vertebrae have bifid spinous processes, meaning they are split in two parts, whereas C7 has a non-bifid, rounded spinous process.
The intervertebral discs are flat and round, present between the lumbar vertebrae and act as shock absorbers when you walk or run. There is a soft, gelatinous material in the center (nucleus pulposus) which is encased in strong elastic tissue forming a ring around it called annulus fibrosus.
Facet joints are synovial joints which give the spine it's flexibility by sliding on the articular processes of the vertebra below. The joint capsules are quite loose in the cervical region, compared to other parts of the spine, to facilitate smooth movement.