The spine is made up of 24 vertebrae and 110 joints. There are 3 sections: 7 cervical, 12 dorsal or thoracic and 5 lumbar vertebrae.
The cervical, or neck section, is the most mobile. In the thoracic section each vertebrae has a rib attached to it on each side. Below the lumbar section is the diamond-shaped sacrum which locks like a keystone into the pelvis. The joints between the sides of the sacrum and the rest of the pelvis are called the sacroiliac joints. This can be where you feel the first symptoms of AS.
|Cervical (7)||Dorsal / Thoracic (12)||Lumbar & Sacrum (5)|
Click on the headings below to read more about the different sections of the spine.
The spine is composed of 7 cervical vertebrae, forming the neck, 12 thoracic vertebrae, which form the upper back, and 5 lumbar vertebrae, which form the lower back.
The sacrum is also part of the spinal column. This is a bone made of 5 fused vertebrae which anchors the spine to the pelvic girdle, and the tailbone (coccyx), along with a semi-flexible series of 4 or fewer vertebrae which help to protect the lower alimentary tract. Between each vertebra is an intervertebral disk made of cartilage, which acts as a shock absorber, to cushion the vertebral column from trauma.
The spine is held together by a series of ligaments, including the intertransverse ligaments which run the length of the spine, attached to the transverse processes of each vertebra. The spinal cord, which serves as the primary nerve pathway to and from the brain, proceeds down a canal in the centre of the spinal column.
The Upper Vertebral Column
The cervical vertebrae are the first (upper) 7 in the vertebral column. The first cervical vertebra is the atlas, so called because it directly bears the weight of the skull. The second cervical vertebra is called the axis, because it admits the rotation of the skull by allowing the atlas to pivot upon it. The other 5 cervical vertebrae have no names, but are called by their number (for example the third cervical vertebra).
Each of the cervical vertebra features a body (anterior, or frontal, portion) and an arch (posterior, or rear, portion). The body of each vertebra in the column bears the weight of the vertebrae above it (and the skull), while the arch serves to create a canal-like area along the spine to house and protect the spinal cord.
Every cervical vertebra has an opening (foramen) in each of its transverse processes (lateral protrusions). The arch of the vertebra features a small knob or prominence, called an anterior tubercle. The anterior tubercles on the sixth cervical vertebra are particularly large and are known as the carotid tubercles.
The atlas is the first of the 7 cervical vertebrae, and is called such because it bears the direct weight of the skull, just as the mythical Greek hero Atlas bore the world on his shoulders. The atlas vertebra meets with the occipital condyles which flank the foramen magnum in the basilar part of the occipital bone of the skull. This junction forms the atlanto-occipital joint, and is responsible for the primary articulation between the spine and the skull. It is the only vertebra in the spine which has no vertebral body.
The atlas vertebra in turn rests upon the axis vertebra, which is the second of the cervical vertebra in the spine, with the articulation between these two vertebra occurring at lateral articular surfaces and an unique juncture between a concave facet (on the atlas) and an upward-protruding structure on the axis called a dens.
The axis is the second of the 7 cervical vertebrae, and is called such because it allows axial (rotational) movement of the skull. The axis lies directly beneath the atlas vertebra, their junction occurring at lateral articular surfaces and an unique juncture between a concave facet (on the atlas) and an upward-protruding dens (on the axis). This articulation is regulated by the alar ligament, which attaches to both atlas and axis.
The Middle Vertebral Column
The thoracic vertebrae are the middle 12 in the vertebral column. Most of the thoracic vertebrae feature costal facets on the body (meaning relating to the ribs) and transverse processes of the vertebra.
The body of each thoracic vertebra in the spinal column bears the weight of the vertebrae above it (and the skull), while the arch serves to create a canal-like area along the spine to house and protect the spinal cord.
Most vertebrae exhibit pronounced lateral protrusions (or processes), one on each side of the vertebra. These transverse processes serve as the attachment sites for ligaments (intertransverse ligaments) and muscles, which control the bending and twisting of the vertebral column.
The base of each transverse process in most vertebrae is just off of the main body of the vertebra, located instead at the pedicle. The pedicle is part of the ring-like structure of a vertebra, which also includes the body and lamina of a vertebra, forming the vertebral foramen which protects the spinal cord.
Cartilage disks are located between the vertebrae, and serve to cushion the spinal column from shock. Each disk features an inner, pulpy centre, called the nucleus pulposus, and a fibrous outer ring, called the annulus fibrosus, which is visible in a lateral view of the spine.
These intervertebral disks are easily torn or dislocated when the vertebra column is subjected to stresses. Typical stresses might include lifting a heavy load in the wrong way or twisting the back sharply.
A "slipped" disk is only one of many causes of back pain. Others include arthritis, spinal meningitis, and inflammation of a tendon or muscle. Such back problems can cause extreme pain, which may be increased by changes in the weather or poor diet. This can make lifting, walking, and sitting an excruciating ordeal. Since so many muscles place stress on the spine when they operate, simple activities such as going to the bathroom, coughing, laughing, and even breathing may be intolerable with such a condition.
The Lower Vertebral Column
The lumbar vertebrae are the 5 vertebrae which are below the thoracic vertebrae and above the fused vertebrae of the sacrum. The lumbar vertebrae feature no facets on the body or transverse processes (as the thoracic vertebrae have) and the bodies of the lumbar vertebrae are much larger than those of the cervical or thoracic vertebrae. The vertebral foramen is usually triangular, while the spinous process points backward and is rectangular or hatchet-shaped.
The transverse processes of the lumbar vertebrae (which also represent their rib elements) lack the foramina which characterise the cervical vertebrae. The large body of each lumbar vertebra bears the weight of the vertebrae above it (and the skull), while the arch serves to create a canal-like area along the spine to house and protect the spinal cord.
The tailbone (coccyx) is made up of between 3 and 5 rudimentary vertebrae. Often, the first of these coccygeal vertebrae is separate, while the rest are fused together. The articulation between the coccygeal vertebrae and the sacrum allow some flexibility in the coccyx, which is particularly beneficial in taking the stresses of sitting and falling.
The coccyx is extremely susceptible to shock fracture, as might be induced from a fall. Furthermore, since a number of nerve pathways pass near this area, damage to the coccyx threatens damage to the nerves of the lower body. The juncture of the first coccygeal vertebra with the sacrum occurs at the lower facet of the sacrum.