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The individual bones of the spine are called vertebrae. As a general rule, the spine or vertebral column consists of 33 vertebrae. The upper 24 vertebrae are distinct and movable. The top seven vertebrae form the cervical spine (neck), the twelve rib bearing vertebrae form the thoracic spine, and the next five vertebrae form the lumbar spine (low back). The nine remaining vertebrae join together to form two bone structures - the sacrum, and the coccyx or tailbone.

The first and second cervical vertebrae are shaped different from all others which allow them to carry the weight of the head while also permitting rotation of the head on the upper neck.

All remaining movable vertebrae are composed of two distinct bone areas - the vertebral body and the vertebral arch.

The front part of the vertebra is the vertebral body. This is a strong cylinder of bone designed to withstand compression forces as the spine bears our weight. The flat surfaces on the top and bottom are called the vertebral endplates.

Did You Know? The vertebral endplates have a cartilaginous surface that intimately interacts with the intervertebral discs. Essential nutrients and oxygen diffuses through the vertebral end plates to the intervertebral discs.

The front part of the vertebra is the vertebral body. The back part of the vertebra is called the vertebral arch. The vertebral arch attaches to the rear of the vertebral body with two thick posts of bone called pedicles. The back sections of the vertebral arch are called the laminae. An extension of bone called the spinous process projects from very back of the vertebral arch. The spinous processes can be felt as the small, hard lumps just under the skin that running down the center of the back. An extension of bone projects from each side of the vertebral arch called the transverse process. The transverse processes and spinous process serve as muscular attachments, and act as short levers through which muscles work to position and move the vertebrae. In the thoracic spine, the transverse processes also serve as attachment points for the ribs.

The large opening in the center of each vertebra is called the vertebral canal. The vertebrae are arranged so that the vertebral canal becomes a continuous bone tunnel that extends from the base of the skull to the tailbone. The vertebral canal contains the spinal cord and spinal nerves roots, and their supporting blood vessels. By surrounding them with bone, the vertebral canal protects theses important structures.

Did You Know? The overall dimensions of the vertebral canal are determined by the length of the pedicles and the shape of neural arch. These dimensions vary between individuals. Some people have vertebral canals that are unusually narrow, with very little extra space available for the passing spinal canal and nerve roots. This condition is called congenital spinal stenosis.

When examining the sides of each vertebra, a shallow notch is noted above, and a deep notch is noted below each pedicle. When the vertebrae are placed together, these notches form openings on each side of the spine, called neural foramina. It is through the neural foramina that the spinal nerves leave the vertebral canal and travel into the body.

From the top and bottom of the vertebral arch are boney projections, two on the right side and two on the left side. These are called the superior (above) and inferior (below) articular processes.

These superior articular processes connect with corresponding inferior articular processes from the vertebrae above to form facet joints (also called zygapophyseal joints). These joints are similar to knuckles in the hand in that they have cartilage surfaces and joint capsules that produce synovial fluid - a joint lubricant.

Facet joints allow movements between vertebrae, while also acting as mechanical stops to limit the amount of movements that can occur. Facet joints also keep the vertebrae in their correct positions, preventing forward slippage of the upper vertebra on the vertebra below. This is accomplished because of the back facing superior articular process blocks the front facing inferior articular process of the vertebra from moving forward. This is an important function since forward slippage results in a narrowed vertebral canal with potential damage to the nervous system structures contained within it.

The small areas of bone between the upward and downward bony projections that form the facet joints on each side of the vertebral arch are called the pars interarticularis (translated: bridge between the joints).

Did You Know? This area of bone is under high stress as it is situated between the superior articular process that is being pushed forward, and the inferior articular process that is being pushed backwards.

Between the flat endplates of neighboring vertebral bodies are soft-tissue structures called intervertebral discs. The spine has a total of 23 discs, and their height account for about one quarter of the overall length of the spine.

The discs perform three essential functions.

First, the discs join the vertebrae together. The is accomplished by the outer wall of the disc called the anulus fibrosis (also spelled annulus fibrosis), which consists of between 25 - 40 bands of very strong fibers that connect to the end plates above and below, attaching the vertebral bodies together.

Did You Know?The fibers of consecutive rings of the anulus run obliquely between the vertebrae, and alternate their slant to the right and left. This allows the anulus to have enough give to allow movements between the vertebrae, while also producing great strength to resists the forces placed on it.

The second function of discs is to allow movement between the vertebrae so that the spine is flexible. This is accomplished by the disc having slight elastic properties. Indeed, the center of the disc called the nucleus fibrosis has a gel-like consistency that allows it to mold itself to the changing shape of the space between the vertebral endplates during movement.

Did You Know? The nucleus pulposus consist of a complex matrix of molecules that attract and trap water, along with a fine network of fibers that bind the nucleus pulposus together and attach it to the bordering vertebrae and the surrounding annulus fibrosis. Overall, water makes up 80% of the nucleus pulposus. This water content of the discs fluctuates slightly on a daily basis. When lying down, chemical forces attract water into the discs. With upright postures, compression of the disks forces some water out. This results in a slight fluctuation of our height during the day. The water content of disc generally decreases with normal aging, and is often reduced to around 70% during our later decades. This contributes to the loss of height noted by many middle and older age people.

The third function of the disc is to absorb the compression forces on the spine, and distribute these forces evenly between vertebrae. The spine is continuously compressed by combinations of 1) the weight of the body while upright, 2) the weight of items that we may hold and carry, 3) the forces produced by the contraction of the spinal and abdominal muscles that support and move the spine and 4) the impact forces that are absorbed by the spine as we move around throughout our day, such as when the feet strike the ground with walking. These compression forces are substantial, often totaling two or more times the weight of the body.

The disc structure is designed to withstand and distribute these compression forces. In general, compression forces on the discs pressurize the gel-like nucleus pulposus. The rings of the anulus fibrosis contain that pressure, become very tense in the process.

Did You Know? The bands of the anulus fibrosis act to limit the ability of the water filled nucleus pulposus to deform, gives the disc a hydraulic characteristic. This characteristic helps the nucleus pulposus to distribute it pressure throughout the structures which surround it - the vertebral end plates and the anulus fibrosis. This prevents the concentration of forces on any particular section of the anulus and vertebrae, particularly towards the edge which the vertebral column is bent. Although the anulus fibrosis is also compress, these forces are much less than the tension (hoop stress) that is produced as the anulus fibrosis resists the pressures generated in the nucleus pulposus. These forces have been calculated to reach three or more time the load on the spine.

The rings of the healthy anulus fibrosis are extremely strong, and can withstand huge compression forces. Indeed, the anulus fibrosis of a healthy disc is so strong that when the healthy spine is placed under excessive compression, the bone of vertebral bodies will collapse and break before the anulus will fail.

Did You Know? The collapse of the otherwise healthy vertebrae body under severe compression is called a burst fracture. These are seen after severe trauma such as falls from modest heights and motor vehicle (cycle) accidents. Schmorl's nodes are a common type of failure of the endplate of the vertebral body. These occur when a small amount of the nucleus pulposus is forced through the endplate and pushes into the vertebral body. Schmorl's nodes rarely produce symptoms.

Ligaments are fibrous tissues that connect two or more bones. In the spine, ligaments tie the vertebrae together and control the movements between our vertebrae. Thick ligaments called the anterior and posterior longitudinal ligaments run the length of the spine and assist the discs in connecting the vertebral bodies.

An important ligament called the ligamentum flavum binds together the laminae of adjacent vertebrae and enclosing the back of the vertebral column. The thickness of the ligamentum flavum determines, in part, the dimension of the spinal canal.

Did You Know? The ligamenta flavum is very elastic. With bending forward and backward the distance between the laminae of adjacent vertebrae changes dramatically. The elastic properties of the ligamenta flavum allow it to change its length during these motions. Without this elastic ability, the ligament would fold in during these movements, and potentially diminish the dimensions of the spinal canal.

Our spines are very mobile, and can bend forward, backwards, sideways, and can rotate. The major muscles of the spine, called the erector spinae, are involved with extension of the vertebral column, and are most active in controlling forward bending, and restoring the spine to the upright position. Of interest, during full forward bending, the paraspinal mucles are inactive and we are supported mostly by our ligaments. This has been termed the "relaxation response". Smaller and deep muscles of the spine rotate and bend the spine to the side. In the low back region, bending forward, rotating and tilting to the sides are mostly due to contraction of the large flank and abdominal wall muscles.

Did You Know? Some evidence suggest that the small muscles are most involved with adjusting the position of neighboring vertebrae than with actual bending of the vertebral column.

The central nervous system is comprised of the brain and the spinal cord. The spinal cord is the part of the central nervous system that extends outside of the skull and into the vertebral canal, ending behind the first lumbar vertebrae. The vertebral canal surrounds the spinal cord with bone, offering protection similar to the skull protecting the brain. Like the brain, the spinal cord consists of both nerve cells (neurons) and bundles of nerve fibers through which the nerve cells communicate. In essence, the spinal cord connects the brain with the body, thus allowing sensory information (sensations) to reach the brain, and movement commands from the brain to reach the muscles. Of importance, the spinal cord is not just a conduit to transmit these nerve messages along its fibers, the nerve cells of spinal cord also act to refine these messages in ways that affects our sensations and controls our movements.

The spinal cord is surrounded by a membrane of specialized tissue called the dura mata. Between the spinal cord and dura mata is a space filled with a cushioning liquid called cerebral spinal fluid. The remaining space in the spinal canal is occupied by a thin layer of fat (epidural fat). This space is called the epidural space.

Pairs of spinal nerve roots leave the spinal cord at the level of every vertebra, passing though the neural foramina to enter the body. When examining the sides of each vertebra, a shallow notch is noted above, and a deep notch is noted below each pedicle. When the vertebrae are placed together, these notches form openings on each side of the spine, called neural foramina (single-foramen). Nerve roots consist of hundreds of distinct nerve fibers. Many of the nerve fibers are coated with a thin layer of myelin, a complex molecule, made and maintained by specialized Schwann cells. Myelin is essential for correct function of the nerves. Similar to insulation on a wire, myelin protects the nerve fibers and also speeds up the conduction of electrical impulses along the nerve fibers. 

Nerve roots combine and mix with each other to form peripheral nerves, which connect the trunk, arms or legs to the central nervous system. In general, nerve roots have patterns in terms of the area of the body (areas of skin and muscles) to which they connect, but slight variations are noted between different people.