The atlas is the first cervical vertebra that supports the skull. It forms an articulated connection with the parietal bone. Fractures in the area of the atlas ring can destroy the medulla oblongata, causing death.
What is the atlas?
The cervical spine of humans and most other mammals consists of a total of seven vertebrae. Compared to the cervical spine, the lumbar and thoracic spines are much less capable of movement. The cervical spine owes its ability to move primarily to the so-called atlas. This is the first cervical vertebra that carries the entire head and a particularly rotatable vertebral joint. The Atlas takes its name from the Greek Titan Atlas, who according to legend had to shoulder the burden of heaven. The atlas is thus the part of the cervical spine closest to the skull and meets with the second cervical vertebra, the axis, in a functional unit called the nicker. The nicker allows the head to bend in an anterior direction, thus enabling the person to nod. In clinical practice, the abbreviation C1 stands for the atlas. In their anatomy, the atlas and axis are distinctly different from the rest of the vertebrae in the spine.
Anatomy and structure
The atlas appears as a ring-like shape. The cervical vertebra lost its vertebral body during evolution. Laterally and ventrally, the atlas bears bony thickenings called massae laterales that terminate in semicircular arches of bone. The bony arches are called atlas arches and correspond to the arcus anterior and posterior atlantis. The spinous process of the atlas is not pronounced, but is replaced by an elevation of the dorsal side on the arcus posterior, also called the tuberculum posterius. A tuberculum anterius also carries the ventral side of the arcus anterior. On the sides of the massae laterales are transverse processes, which are considered remnants of the processus costales and bear foramina transversaria. On the massae laterales lie articular surfaces which, together with the occipital bone, correspond to the atlantooccipital joint. With the axis, the atlas articular surfaces form the atlantoaxial joint. In the atlas, the foramen transversarium is traversed by the so-called vertebral artery, which then passes through the foramen magnum and enters the skull. The vertebral foramen is divided in two by the ligamentum transversum atlantis. Ventral to the ligamentum transversum atlantis is the dens axis and dorsal to it is the spinal cord.
Function and Tasks
The atlas takes its name from the Greek titan Atlas. This alone suggests the main function of the first cervical vertebra. As with the Greek titan, strength is required of Atlas above all else. The atlas is the cervical vertebra in direct proximity to the skull. It is thus supposed to stabilize the transition between the cervical spine and the head and must also bear the full weight of the skull. However, the first cervical vertebra is only relevant for stabilizing the head and spine. The head must be mobile. From an evolutionary biological point of view, the human ability to rotate the head has contributed to the survival of the human species. As soon as a human individual heard a sound and suspected danger behind it, it turned its head in the direction of the supposed danger. This process could take place within milliseconds due to the articulated connection between the spinal column and the cranial bone, allowing direct fixation and identification of danger through the eyes. The atlas is essentially involved in the articulated connection between the cervical spine and the cranial bone. Accordingly, it gives the head the mobility and ability to rotate in the cervical spine. Without this articulated connection, everyday movements such as nodding would be completely unfeasible. The joint of the atlas is quite different from other intervertebral joints of the spine. Basically, the human spine is most mobile in the area of the cervical spine. On the other hand, the atlas in its entirety also prevents dislocations and twisting, as it is stabilized by strong ligaments.
Diseases
Some pathological changes of the atlas are particularly well known. These include various developmental disorders and the resulting deformation of the first cervical vertebra. During embryonic development, disorders may occur in the development of the atlas that cause proportional fusion at the sclerotomes of the upper four somites.In this way, the os occipitale can fuse with the atlas completely or incompletely. This disorder is also referred to as atlas assimilation. Apart from congenital maldevelopments of the first cervical vertebra, the atlas can be affected by malpositions like all other vertebrae. The spinal cord runs directly through the atlas, so misalignment of the vertebra can be associated with central nervous system disorders and often causes a rigid spine. In addition, when the atlas is misaligned, the circulation of blood and cerebrospinal fluid is often impeded, adding further deficits. Besides these phenomena, the most severe symptomatology related to the atlas is the fracture of the neck. This is a fracture of the dens axis, which lies with spinal cord in the atlas ring. In most cases, the ligamentum transversum atlantis and apicis dentis (ligaments) are torn off. The dens axis thus has substantial freedom of movement, often injuring the medulla oblongata. This injures the respiratory center of the body and death occurs in seconds. This injury occurs when people hang themselves. Less well known is the Jefferson fracture, which is a special form of atlas fracture and causes the ring of the atlas to explode entirely.
