Anatomy of the Spine
The spine, also called the backbone, 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 neck to the pelvis, giving symmetry and support to the body.
Parts of a Vertebra
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.
The 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.
Cervical Spine Anatomy
The spine can be divided into 4 parts: cervical, thoracic, lumbar and sacral region. The cervical spine comprises of the first 7 vertebrae, which form the neck.
The cervical spine is highly mobile compared to the 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: the 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. It also has heavy pedicles, laminae and transverse processes that help in the attachment of muscles.
Lower Cervical Spine
The remaining five cervical vertebrae, C3-C7, form the lower cervical spine. They are similar to 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 into two parts, while 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) that is encased in strong elastic tissue, forming a ring around it called annulus fibrosus.
Facet joints are synovial joints that give the spine it's flexibility by sliding on the articular processes of the vertebra below. Compared to other parts of the spine, the joint capsules are quite loose in the cervical region to facilitate smooth movement.
The thoracic spine is the central part of the spine, also called the dorsal spine, which runs from the base of the neck to the bottom of your rib cage. The thoracic spine provides the flexibility that holds the body upright and protects the organs of the chest.
The spine is made up of 24 spinal bones, called vertebrae, of which, the thoracic region of the spine is made up of 12 vertebrae (T1-T12). The vertebrae are aligned on top of one another to form the spinal cord, which gives your body its posture. The different parts of the thoracic spine include bone and joints, nerves, connective tissues, muscles, and spinal segment.
Each vertebra is made up of a round bony structure called the vertebral body. The protective bony ring attaches to each vertebral body and surrounds the spinal cord to form the spinal canal. The bony ring is formed when two pedicle bones join two lamina bones that connect to the back of the vertebral body directly. These lamina bones form the outer rim of the bony ring. When vertebrae arrange one on top of the other, the bony ring forms a hollow tube that surrounds the spinal cord and nerves and provides protection to the nervous tissue.
A bony knob-like structure projects out at a point where the two laminae join at the back of the spine. These projections are called spinous processes, and the projections at the side of the bony ring are called transverse processes.
Joints of the Vertebrae
Between each vertebra, there are small bony knobs at the back of the spine that connect the two vertebrae together called facet joints. Between each pair of vertebra, two facet joints are present, one on either side of the spine. The alignment of the two facet joints allows the back and forth movement of the spine.
The facet joints are covered by a soft tissue called the articular cartilage, which allows the smooth movement of the bones.
Nerves of the Thoracic Spine
On each side, the left and right side of the vertebra, is a small tunnel called neural foramen. The two nerves that leave each vertebra pass through this neural foramen. These spinal nerves group together to form a main nerve that passes to the organs and limbs. These nerves control the muscles and organs of the chest and abdomen. An intervertebral disc is present in front of this opening which is made up of connective tissue. The discs of the thoracic region are smaller compared to the cervical and lumbar spine.
Soft Tissue of the Thoracic Spine
Connective tissue holds the cells of the body together and ligaments attach one bone to another. Anterior longitudinal ligament runs down to the vertebral body and the posterior longitudinal ligament attaches on the back of the vertebral body. A long elastic band called ligamentum flavum connects the lamina bones.
The muscles in the thoracic spine are arranged as layers. Strap-shaped spine muscle, called erector spinae make up the middle layer of the muscle. The deepest layer of muscles attaches along the back of the spine bones and connects to the vertebrae. These muscles connect one rib to the other.
The spinal segment includes two vertebrae separated by an intervertebral disc, nerves that leave the spinal column at each vertebra, and small facet joints of the spinal column.