The vertebral column (spine or backbone) is a curved structure composed of bony vertebrae that are interconnected by a series of cartilaginous intervertebral discs. The vertebral column can be divided into the cervical, thoracic and lumbar vertebrae, the sacrum and coccyx. No two vertebrae are identical, however, each can fall under the category of typical or atypical vertebrae, depending on their structural composition.
The first cervical vertebra, also known as the atlas, bears very little resemblance to any other vertebra, and is thus categorized as an atypical vertebra. The unusual name for this bone is derived from a deity of Greek mythology, Atlas. In a similar manner to how the Greek god Atlas bore the weight of the world on his shoulders, C1 or the atlas of the vertebral column supports the globe-like cranium.
This article will discuss the anatomy and function of the atlas.
|Location||First vertebra of the cervical region, between the base of skull and axis (C2)|
|Components||Anterior arch, posterior arch, lateral mass, transverse process, vertebral canal|
|Anterior arch||Facet for dens, anterior tubercle of atlas|
|Posterior arch||Groove and canal for vertebral artery, posterior tubercle of atlas|
|Lateral mass||Superior articular surface, inferior articular surface, transverse ligament tubercle|
|Transverse process||Anterior root, posterior root, foramen transversarium, apex of transverse process|
|Joints||Atlantooccipital joint, median atlantoaxial joint (x1), lateral atlantoaxial joint (x2)|
Location and structure
The atlas is the first vertebra of the vertebral column, located in the most superior portion of the cervical region of the vertebral column. The atlas articulates superiorly with the occipital condyles found on each side of the foramen magnum of the skull. Inferiorly, it articulates with the second cervical vertebra (C2), which is also known as the axis.
The atlas is a ring shaped bone that does not contain a vertebral body or a spinous process and is therefore classified as an atypical vertebra. Instead of a vertebral body, the atlas is made up of an anterior and posterior arch, which are joined by a lateral mass on each side. Each lateral mass bears articular facets superiorly and inferiorly and projects a transverse process laterally. The anterior and posterior arch and the two lateral masses form a ring-like anatomical space centrally, called the vertebral canal, that provides passage for the spinal cord.
Anterior arch of atlas
The anterior arch of the atlas forms the anterior portion of the atlantal ring. Located on the anterior aspect of the arch is a bony roughening known as the anterior tubercle, which provides attachment to the anterior longitudinal ligament. The superior and inferior surfaces of the anterior arch provide an attachment point for the anterior atlanto-occipital membrane and lateral parts of the anterior longitudinal ligament.
On the posterior surface of the anterior arch is a circular articular facet which articulates with the dens of axis (C2) to form the median atlanto-axial joint. This is a synovial pivot joint which facilitates rotational movements of the head on the neck such as when you shake your head no, or turn your head from the left to the right.
Posterior arch of atlas
The posterior arch of the atlas forms the posterior three-fifths of the circumference of the atlantal ring and is homologous with the pedicles and laminae of typical vertebrae. The superior surface of the posterior arch bears a groove which is simply known as the groove for the vertebral artery. As its name suggests, this groove transmits the vertebral artery, vertebral venous plexus as well as the suboccipital (first spinal) nerve. Variably, this groove may progress and convert into a canal, forming the canal for the vertebral artery.
The borders of the posterior arch act as important attachment points for the surrounding ligamentous structures. The superior border gives attachment to the atlanto-occipital membrane, while the highest pair of ligamenta flava attach to the inferior border.
At the apex of the posterior arch is a bony roughening known as the posterior tubercle. The posterior tubercle is homologous to the spinous process of typical vertebrae, and as such functions as an attachment point for the ligamentum nuchae.
Located between the anterior and posterior arch of the atlas are two ovoid lateral masses. The lateral masses function in place of a vertebral body. They are the thickest and strongest portion of the atlas and support the weight of the cranium. On the superior and inferior surfaces of each lateral mass are articular facets.
- The kidney-shaped superior articular facets articulate with the occipital condyles of the cranium to form the atlantooccipital joint. The atlantooccipital joint facilitates flexion and extension and slight lateral flexion of the head on the neck. This joint is utilised when nodding the head
- The circular inferior articular facets articulate with the superior articular facets of the axis (C2) to form the lateral atlantoaxial joints. The paired lateral atlantoaxial joints work together with the median atlantoaxial joint to produce rotational movements of the head.
Located on the medial surface of each lateral mass is a tubercle known as the transverse ligament tubercle. As its name suggests, this tubercle provides the attachment point for the transverse ligament, which stabilizes the odontoid process of the axis (C2). The transverse ligament divides the vertebral canal (centre of ring) into an anterior and posterior compartment. The anterior third of the canal is occupied by the dens axis, while the posterior compartment is occupied by the spinal cord and its coverings.
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Projecting inferolaterally from each lateral mass of the atlas is the transverse process. The transverse processes of the atlas are the second longest of all the cervical vertebrae and function as strong levers for the muscles that aid in rotating the head. The broad apex of the transverse process is homologous with the posterior tubercle of a typical cervical vertebrae.
Each transverse process is formed by anterior and posterior roots, which extend laterally and merge, creating a small hole between them, known as the foramen transversarium. The transverse foramina of the cervical vertebrae allows for the passage of the vertebral artery and vein which ascend and descend to supply and drain the regions of the brain, respectively.
Feeling confident in your knowledge of the vertebral column? Check out our quiz on the cervical spine to put your knowledge to the test!
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