Your spine, also called your backbone or vertebral column, is composed of 33 bones, called vertebrae, which provide your body with support and protect your spinal cord from injury. The vertebrae can be divided into five groups: cervical, thoracic, lumbar, sacrum, and the coccyx, each of which has unique features suited to their functions. There are 7 cervical vertebrae, numbered C1-C7, 12 thoracic vertebrae, numbered T1-T12, 5 lumber vertebrae, numbered L1-L5, 5 sacral vertebrae, numbered S1-S5, and the coccyx. The upper 24 vertebrae are articulating, separated by intervertebral disc s. The sacral vertebrae and the bones of the coccyx are fused.
The cervical vertebrae - which are the vertebrae of your neck - support your head, which weighs around 10-13 pounds. C1 and C2 are specialized vertebrae that allow for the greatest range of motion of all the vertebrae. C1 is called atlas, after the man from Greek mythology who held up the earth. It is ring-shaped and attaches directly to your skull. This vertebra allows you to nod your head. C2, called “axis”, serves as an axis about which C1 pivots, allowing you to shake your head when saying “no”. C1 is able to pivot on this vertebra thanks to a specialized process called the dens, or odontoid process.
The thoracic vertebrae - which are the vertebrae of your mid to upper back - hold the rib cage and protect your heart and lungs. They have a limited range of motion. The lumbar vertebrae - which are the vertebrae of your lower back, bear the weight of your upper body and are larger in order to support the stress, especially when you lift something heavy. The sacral vertebrae connect your spine to your hip bones. These vertebrae are fused together. Together with your hip bones, they form the pelvic girdle. The coccyx, or tailbone, is made up of four fused bones, which provide attachment points for ligaments and muscles of the pelvic floor.
Now let’s cover the parts of a vertebra. A vertebra has 3 parts - the body, vertebral arch, and the processes. The body bears weight, the vertebral arch houses the spinal cord, and the processes allow for muscle attachment. The vertebral arch is made of two supporting pedicles and two laminae. The space inside, which houses the spinal cord, is called the vertebral foramen. Under each pedicle, spinal nerves exit and pass through the intervertebral foramen.
7 bony processes arise from each vertebral arch to form facet joints and processes for muscle attachment. Facet joints allow for back motion. Each vertebra has two pairs of facet joints - 1 pair connects to the vertebra above (superior facets), and 1 pair that connects to the vertebra below. There is also a pair of transverse process and a spinous process.
Between the vertebrae are the intervertebral discs, which provide cushioning and prevent the bones from rubbing together. Note that there is no intervertebral disc between C1 and C2. Discs are composed of am annulus, which is made up of several layers, or laminae, of fibrocartilage - crisscrossing fibrous bands that attach between the bodies of the vertebrae above and below, and a gel-filled center called the nucleus. The nucleus distributes the pressure evenly within each disc during compression.
The nucleus is full of proteoglycans - large molecules with sugar subunits that are very hydrophilic. This structure swells with water, unless it experiences constraints from the surrounding tissues. Hence, the nucleus absorbs fluid at night or when you’re lying down and then this fluid is pushed out during the day when you are upright and pressure is applied to the intervertebral discs. We shorten as we age because the discs lose the ability to reabsorb fluid during rest, flatten, and get more brittle.
3D models from:
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