The Regions of the Spine
The spine is an intricate set of bones, muscles, nerves and discs. It is divided into five regions: cervical (neck bones); thoracic (in the chest); lumbar (low back); sacral (attached to the pelvis); and, coccygeal (the tail bone). Each region has a number of vertebral bones. There are usually seven cervical vertebral bones, twelve thoracic bones, and five lumbar vertebral bones. The sacrum is a single, large, fused bone. The coccyx is made of one or two small bones.
Occasionally, an individual may have one extra level, or one bone less than normal. Although it is normal to have five lumbar bones, it is not unusual to see four or six lumbar levels.
The spine bones are often referenced with letters and numbers. C, T, L, and S refer to the cervical, thoracic, lumbar and sacral regions respectively. Within each region, the vertebral bones are numbered from the top down. C1 refers to the top cervical bone. T3 would indicate the third thoracic level from the top. L5 would be the lowest lumbar level. S1 indicates the first sacral level.
The Bones and Discs
Each of the vertebral bones is separated from its neighbor by a disc. The discs are in the anterior (front part) of the spine. Together, the discs allow movement and cushion shocks. Posterior to (behind) each of the discs, and between every pair of vertebral bones, is a foramen or hole.
One spinal nerve root exits through from each of these holes. The nerves go to the arms, chest and legs. The posterior parts of the vertebral bones are connected by small joints called facet joints. Behind the facet joints and along the midline are spinous processes. The spinous processes are bumps which can easily be felt along the back of the neck, thoracic spine and low back.
Each Vertebral Bone
Each of the vertebral bones has an anterior (front) part and a posterior (back) part. The anterior portion is called the vertebral body. The body provides the surfaces against which the discs rest. Two pedicles, or struts, project posteriorly from the body and support an arch called the vertebral lamina. The arch over the body of the vertebra forms a canal through which the spinal cord and nerves pass. The arch also supports the small facet joints which connect the backs of the vertebral bones.
Each Intervertebral Disc
The discs consist of two parts. The inner area is called the nucleus, and the outer area is called the annulus. The disc is like a jelly donut.
The nucleus, or inner core of the disc, consists of a gelatinous material. The annulus, or outer ring, is the strongest portion of the disc. It keeps the jelly-like center from leaking out, supports the weight of the spine, and prevents excessive motion. The annulus is built from layers of fibers, much like a tire.
Normally, the disc remains firmly contained between the vertebral bones. The disc can, however, rupture. Ruptures occur when the fibrous covering of the annulus is torn.
The Motion Segment
Every two vertebral bones are separated from one another by a disc. The two bones, and the disc together, make one motion segment. There is only a small amount of movement at any one motion segment. The strong fibers of the disc annulus, and the posterior facet joints, prevent excessive movement. The spine is able to bend and extend because there are many motion segments which act together.
When there is damage to a disc, to one of the facet joints, or to a portion of the vertebral bone, there can be too much movement at the motion segment. Excessive movement can be painful. Fusion surgeries, where the bones are welded together, are sometimes recommended when there is excessive movement.
The Muscular Spine
The back is supported by strong ligaments and by even more powerful muscles. The muscles are arranged in much the same way as the guy wires in antennas. The strength of an antenna is determined by the strength of the guy wires. The strength of the back is determined by the strength of the muscles.
See the comparison pictures to the left. The muscles are arranged exactly as are the guy wires of a large antenna.
The psoas muscles are perhaps the most important of the muscles. Other paraspinal muscles posteriorly and the abdominal muscles anteriorly also contribute substantially.1
The Spinal Cord and Nerves
The spinal cord begins at the base of the brain and runs down the spine to the low back. It is protected within the spinal canal, a bony arch formed by each of the vertebral bones. Between every two vertebral bones, two nerves exit the spinal canal. One is on the left and another is on the right. The nerves exit right behind the disc and directly in front of the facet joint.
If there is a ruptured disc, it will pinch the nerve. Similarly, a damaged facet joint can also press on a nerve.
Each nerve goes to a specific area of the body. The nerves from the neck go to the arms. The nerves from the thoracic spine go to the chest wall and abdomen. The nerves from the lumbar spine go down the legs. Each nerve serves sensation in a particular area and controls specific muscles. Some of the nerves have reflexes that can be tested. Some of the nerves also regulate blood flow to the skin and tissues, and the functions of the internal organs.
When a nerve is pinched, the pain radiates along the path of the damaged nerve. If one has a pinched nerve in the neck, one generally gets pain in the arm. If a low back nerve is damaged, there is usually leg pain. If a nerve is badly pinched, some of the muscles may become weak, and there may be abnormal reflexes with changes in skin color or temperature.
1 Spinal anatomy picture copyright Primal Pictures, reproduced with permission.