Full Title: The Atlantoaxial Joint-Turning Heads
Author: Christine Wynne
Date of Publication: January 1, 2017
Research Paper Text:
The atlantoaxial joint is a synovial pivot joint consisting of two cervical vertebral arches, the C1 Atlas and the C2 Axis. The Atlas, the widest of the cervical vertebra, derives its name from the Greek deity in mythology who supported the heavens. Appropriately named as it is the atlas vertebra which supports the skull (containing wisdom gained from the heavens). Axis is a Latin term meaning “axle” or epistropheus. An axle is defined as being a straight line about which a body or geometric object rotates. The axis, the longest cervical vertebra, forms the pivot point upon which the atlas rotates. The ventral arch of the atlas and the body of the axis face into a synovial cavity forming the atlantoaxial joint.
The atlantoaxial joint begins the spinal column in the neck and allows for rotational movement of the head about a longitudinal axis (eg. shaking of the head). C1, an irregular bone, is composed of two lateral masses joined by dorsal and ventral arches. The wings of the atlas are shelflike transverse processes of this vertebra projecting from the body of the atlas which allow the spinal column to articulate with the base of the skull by providing a resting place for the occipital condyles. The dens of the axis, C2, is a projection that articulates with the caudal end of the atlas. It is held down by the transverse ligament of the atlas which provides stabilization of the atlantoaxial joint, distinguishing it from other joints between vertebrae, as there are no discs present.
There are several major ligaments involved in the movement of the atlantoaxial joint. The transverse ligament of the atlas, as mentioned, is a thick, strong band arching across the ring of the atlas which retains the dens in contact with the anterior arch. Its function is to divide the atlas into two unequal parts and serve for transmissions of the spinal cord. The alar ligaments are two strong rounded cords that attach the skull at the medial aspect of the occipital condyles to the axis on either side of the dens. These ligaments are taut in flexion, limit rotation and side flexion and also serve to stabilize the atlas and axis especially in rotation. The nuchal ligament, another strong ligament, originating from attachments along the dorsal extremities of the spinous process of the thoracic vertebrae, extends from the end of the spinous process of T1 cranially to insert on the caudal area of the spinous process of C2. The apical ligament spans between C2 and the anterior margin of the foramen magnum where the brainstem exits the skull and becomes the spinal cord.
Where vertebrae come together, an “opening” or invertebral foramen is formed on each side of the spinal column. The spinal nerve travels through these foramina in the cervical vertebrae and branches off into the dorsal branch and then to medial and lateral branches. The dorsal branch innervates the epaxial muscles which are extensors of the vertebral column. It also innervates the dorsal cutaneous muscles. The ventral branch of the spinal nerve, the largest branch, innervates the hypaxial muscles, the flexors of the neck. The ventral branch of C2 also innervates the lateral and ventral cutaneous nerves, as well as the great auricular nerve serving the sensory function of the external ear canal. The C1 nerve is small and does not contribute to cutaneous innervation.
- Transverse processes (Wings on the atlas and axis)
- Dens of the axis
- Transverse foramen
- Lateral vertebral foramen
- Vertebral foramen
- Articular processes
Running through the foramina of the transverse process of the cervical vertebrae six through one (the atlas) are branches of the vertebral artery which are the major blood suppliers of the cervical vertebral canal and the spinal cord. The ventral spinal artery constitutes the major blood source for the ventral surface of the spinal cord. The dorsal surface of the atlantoaxial joint is supplied by the dorsal spinal arteries formed by the dorsal branches of the ventral radicular arteries. Blood is circulated from the atlantoaxial joint via the ventral spinal vein and the dorsal spinal vein.
Veterinary Gross Anatomy: General Anatomy and Carnivore Anatomy Lecture Notes by Thomas F. Fletcher, DVM, PhD and Christina E. Clarkson, DVM, PhD
Canine Accupoint Energetics and Landmark Anatomy, Amy Snow & Nancy Zidonis
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