The vertebral column (spine or backbone) is a curved structure composed of bony vertebrae, 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 axis, also known as the epistropheus, is the second cervical vertebra (C2) that has some similarities to a typical cervical vertebra but is categorized as an atypical vertebra because of its unique features. Its most characteristic feature is the prominent superior projection known as the dens axis, or odontoid process. The dens axis plays an important function for the movement of the head, acting as a stable pivot around which the atlas and head rotate.
This article will discuss the anatomy and function of the axis.
|Location||Second vertebra of the cervical spine, found between the atlas (C1) and C3|
|Components||Vertebral body, dens axis, pedicle (x2), transverse process (x2), lamina (x2), spinous process|
|Body||Medial longitudinal ridge, superior articular surface, inferior articular surface, transverse process|
|Dens axis||Apex of dens axis, neck of dens axis, anterior articular facet, posterior articular facet|
|Pedicle||Groove for the vertebral artery, inferior intervertebral notch|
|Transverse process||Transverse foramen, apex of transverse process|
|Joints||Median atlantoaxial joint (x1), lateral atlantoaxial joint (x2)|
- Location and structure
- Anterior components
- Posterior components
Location and structure
The axis is the second vertebra of the vertebral column, located in the superior portion of the cervical region of the spine. It articulates superiorly with the atlas, and inferiorly with the third cervical vertebra.
The axis can be divided into anterior and posterior components. Anteriorly, it is formed by a vertebral body, dens axis, two pedicles, and two transverse processes. Posteriorly, it contains two thick laminae and a large, often bifid, spinous process.
These anterior and posterior components together form a circumference called the vertebral canal, that provides passage for the spinal cord.
The body of the axis is small compared to the vertebral bodies in the rest of the spinal column. It is a composite structure, made up of the partly fused center of the atlas and the axis, and a rudimentary intervertebral disc between them.
The body of the axis is elongated downwards in its anterior portion, thus it overlaps the upper and anterior part of the third cervical vertebra. On its anterior aspect, the body of the axis presents a medial longitudinal ridge, which separates two lateral depressions that serve as attachment points for the longus colli muscle.
The anterior border of the body of the axis provides attachment to the anterior longitudinal ligament, while the posterior longitudinal ligament and tectorial membrane attach posteriorly, to the lower border of the body of axis.
The superior surface of the body gives the impression of projecting an upward tooth-like process called the dens axis (odontoid process). However, this structure actually represents the body of the atlas that separated during early fetal life and fused with the body of the axis.
Lateral to the dens axis on each side are the superior articular facets, which slope from the superior aspect of the body and extend over the pedicles. On the inferior aspect of the body are the inferior articular facets.
- The superior articular facets articulate with the inferior articular facets of the atlas to form the lateral atlantoaxial joints. The superior articular facets permit the forward gliding of one lateral mass while the other glides backwards, allowing the atlas to rotate around the dens, and thus allowing rotational movements of the head.
- The inferior articular facets start at the inferior aspect of the body and extend over the pedicles of the axis. These facets articulate with the superior articular facets of the third cervical vertebra (C3).
The dens axis (literally ‘tooth of the axis’), also called the odontoid process or the peg, is the separated body of the atlas that fused with the body of the axis during early life. It exhibits a small constriction called the neck, on the place where it fuses with the body of the axis. The posterior surface of the dens axis is smooth and contains a posterior articular facet traversed by the transverse ligament of the atlas, which keeps the dens axis secured in position.
The anterior surface of the dens axis contains an oval anterior articular facet which articulates with the facet of the anterior arch of the atlas 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 to say ‘no’, or turn your head from the left to the right.
Lateral to the articular facet of the dens on each side is a rough impression that serves as an attachment point for the alar ligaments, which connect the dens to the tubercles on the medial sides of the occipital condyles. The apex of the dens axis is slightly pointed and provides attachment to the apical ligament, which connects the dens axis to the anterior margins of the foramen magnum.
The pedicles of axis are broad and strong, and project laterally on each side from the body of the axis to the transverse processes. On their superior surface, each pedicle contain a part of the superior articular facet which slopes from the body of the axis. The anterolateral surface of the pedicle of axis contains a deep groove for the vertebral artery, which transmits the vertebral artery, vertebral venous plexus as well as the suboccipital (first spinal) nerve.
The inferior surface of each pedicle contains a deep inferior intervertebral notch, which bears the large root sheath of spinal nerve C3.
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Projecting laterally and inferiorly from each side of the junction of the pedicle and lamina is the transverse process.
The transverse processes are small and pointed, and contain no anterior tubercles seen on the typical cervical vertebrae. The rounded apex of the transverse process is homologous with the posterior tubercle of a typical cervical vertebrae.
Each transverse process is perforated by a foramen transversarium (transverse foramen), which is directed obliquely superiorly and laterally. The transverse foramina of the cervical vertebrae allow for the passage of the vertebral artery and vertebral vein.
The laminae of the axis are thick and strong and project backwards and medially from the transverse process, joining together in the midline to form the spinous process. The laminae of the axis provide attachment to the ligamenta flava.
The spinous process forms from the merger of the two laminae of axis. It is large, and usually contains a broad base and a bifid tip. The spinous process projects almost 1 cm further posteriorly than the posterior tubercle of atlas and covers the spinous process of the third cervical vertebra (C3).
The apical notch of the spinous process of the axis serves as an attachment point for the ligamentum nuchae.
To test and consolidate your knowledge about the axis and other bones of the cervical spine, take the quiz below!
Axis: want to learn more about it?
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