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Spinal Anatomy

The spinal column is one of the most vital parts of the human body. Modern osteopathy and chiropractic both agree that the health of the spine is one of the dominant factors affecting overall health; and rightly so. Your spine is the keystone that holds your body together and is involved in almost every movement you make. The spine's complex arrangement of bones, ligaments, nerves, muscles, and discs creates a strong supportive structure that has flexibility along with a highly functional range of motion. In addition to structural support and overall movement, the spinal column provides protection for the spinal cord and nerve roots, the most important component of spinal anatomy. The fact that much of your quality of life is dependent upon a healthy spine, it only makes sense to take a few moments to explore and learn more about this all important, core component of your body.

THE NUMBERS TO THE LEFT OF THE SPINE INDICATE SPINAL NERVE ROOTS AND THE
NUMBERS TO THE RIGHT OF THE SPINE INDICATE THE VERTEBRAL SPINAL SEGMENTS.

Spinal Regions

The spine is divided into four regions or sections. They are the cervical, thoracic, lumbar and sacral spines. Each region has its own specific characteristics and functions. The cervical, or "neck" region, starts at the base of the skull and ends a little above the shoulders. This spinal region by far has the greatest range of motion. The cervical spine's purpose is to support the weight of the head and allow for movement in all directions. The region of the spine found in the upper back is called the thoracic spine. This region of the spine begins at the base of the neck and ends at the bottom of the rib cage. The thoracic spine has the least amount of mobility because it attaches to the rib cage, which helps protect the abdominal organs. The lumbar spine, or "low back", is immediately below the thoracic spine and rests on top of the sacrum. The purpose of the lumbar region is primarily to support and bear the weight of the upper body. The final region is the sacrum, which is commonly referred to as the tail bone. This large bone is formed early in life by the fusion of five individual vertebra. The purpose of the sacrum is to provide a foundation for the entire spine and serve as an attachment point for the pelvic bones and coccyx. Together with the pelvic bones, they form a ring called the pelvic girdle that contains and supports the intestines, bladder, and internal sex organs.

Spinal Curves

When viewed straight on from the front or back, the normal spine is in a straight line, with each vertebra sitting directly on top of the other. When viewed from the side, the ideal spine has a definitive S-shaped curve. The curved shape allows for an even distribution of weight and flexibility of movement. The side curves of the spine also enable the body to maintain balance. A total of four different curves create the side view S-shape appearance; the cervical, thoracic, lumbar and sacral curves. The cervical and lumbar spines both have a curve known as a lordosis, which means their curve is concave in the back, or posteriorly. The thoracic and sacral curves both have a curve called a kyphosis, which means their curve is concave in the front, or anteriorly. The curves connect and work together like a coiled spring enabling the spine to absorb a significant amount of shock.

The Vertebrae

The term spine is commonly used interchangeably with the word "backbone," and not without reason: there are twenty four drum shaped back bones in the spine, known as vertebrae, which are stacked and center balanced on top of the sacrum and coccyx. There are seven cervical vertebrae, twelve thoracic vertebrae, and five lumbar vertebrae. The vertebrae and their foramen increase in size as they progress from the neck down to the low back. This is a wonderful design, as the weight bearing load of the lower back is much greater than the neck. Although the vertebrae have slightly different appearances as they range from the cervical spine to the lumbar spine, they generally have the same basic structures. The only exception to this rule is the first and second cervical vertebrae, as they are structurally unique and different from the others.

Each drum shaped vertebral body has two right and left bony projections called pedicles that extend posteriorly and ultimately join together to form the vertebral arch. The vertebral arch creates and serves as a safe tunnel or passageway for the spinal cord to travel down the spine. The vertebral arch formed by the pedicles also serves as a platform for several important bony projections. In all, seven bony processes extend from the vertebral arch: the tip or central spinous process, two lateral transverse processes, and the four articulating processes.

The spinous process that completes the formation of the vertebral arch extends posterior from each vertebra and can be felt through a person's skin. The transverse processes extend out on either side of each vertebra. Both the transverse and spinous processes are essentially levers where ligaments and tendons attach to the spine. These connection points allow for spinal movements to occur, much like a puppet with strings. The articular processes serve a much different role. There is a right and left articular process on the top and bottom of each individual vertebra on both sides of the spinous process. Their purpose is to form facet joints that connect the vertebrae to one another.

The Atlas and Axis

Facet Joints

The spinal joint created by the atlas, the ring shaped first cervical vertebra and axis, the second cervical vertebra is critical to a well functioning neck region. The union of the axis and atlas is referred to as the atlantoaxial joint. The atlas connects directly to the skull and gets its name from Atlas, the Greek god who supported the world on his shoulders. The axis is a typically shaped vertebra, but has centered superior pole like projection called the odontoid process. The unique ring shaped atlas connects with the second vertebra by way of the odontoid process, creating a dynamic pivot joint. This connection enables the head to perform turning, nodding and side bending movements in almost every direction.

The atlantoaxial joint sounds complicated, but can actually be visualized and recreated by using simple hand gestures. Take your right hand and form a circle or "ok" gesture with your thumb and index finger. Now take your left hand and make a "thumbs up" gesture and insert the left thumb into the circle created by the right two fingers. The right hand represents the atlas and the left hand represents the axis. Notice how the finger circle can pivot and rotate around in all directions without leaving the thumb of the left hand. This is very similar to how the atlas and axis work together.

This is the common name for zygapophyseal or apophyseal joints. Each vertebra has two sets of facets; a right and left set facing upward on the top of the vertebra and right and left set facing downward on the vertebra directly below. The facet joints are formed when the superior upward projecting surfaces of the lower vertebra's facets articulate and join together with the inferior downward projecting surfaces of the vertebra directly above. The hinge-like facet joints link the bones of the spine together in the posterior or back part of the spine allowing for stable motion of the spine to occur. They enable twisting, flexion and extension motions of the spine. The facet joints along with the intervertebral discs are directly responsible for the horizontal stability of the spine and are designed to restrict excessive movement, thereby protecting and limiting injury to the spinal cord.

Intervertebral Discs

Between each vertebra is a fibrocartilage cushion called an intervertebral disc. All of the discs work together to absorb the stress and shock the body incurs during movement and prevent the vertebrae from grinding against one another. The discs have two parts: the annulus fibrosis and the nucleus pulposus. The annulus is a sturdy tire-like structure that encases the gel-like nucleus pulposus. The annulus provides stability and helps to resist compressive stress because it is constructed with layers of sturdy elastic collagen fibers that are stacked and oriented at different angles similar to a radial tire. The nucleus pulposus, the center portion of the disc is a filled with a gel-like elastic substance that facilitates shock absorption. Together with the annulus, the nucleus pulposus efficiently and effectively transmits stress and weight from vertebra to vertebra. It is important to note that there is no disc between the first and second cervical vertebrae.

Intervertebral Foramina

The intervertebral foramina are openings, or "holes", on both sides of each vertebra located just in front of the transverse processes that enable the nerve roots to safely exit the spinal cord. Each foramen is formed by a groove in the bottom of one vertebra and a groove in the top of another. When the vertebrae stack on top of each other the grooves line up and a passageway is formed. The foramina provide a protective doorway for the nerve roots to exit the spinal column to form the peripheral nerves. The opening is usually pear or light bulb shaped. The size of the opening changes regularly as spinal movements can cause a narrowing and widening of the space.

Spinal Cord and Nerve Roots

The spinal cord is a slender cable like structure. Its diameter is about the same size as your little finger. This cord travels down and is protected within the spinal canal. The spinal cord begins immediately below the brain stem and ends relatively high up in the back at about the first or second lumbar vertebra. Thereafter, it continues in a looser bundle of nerve roots, called the cauda equina, down to the sacrum. The cauda equina, as its name indicates, is a group of nerves that resemble a horse's tail. Along the path of the spinal cord and the cauda equina the nerve roots extend off of the spinal cord and out of the spinal canal through intervertebral foramen. The nerve roots then form peripheral nerves that travel and carry brain impulses throughout the entire body.

Ligaments, Tendons and Muscles

Ligaments are fibrous bands of connective tissue that attach directly to bone. The ligaments of the spine connect the spinal bones together stabilizing and adding strength to the vertebral joint. Ligaments serve to limit movement, and protect the spine against hyperflexion or hyperextension injuries. Tendons also attach directly to the vertebra. The purpose of tendons is to serve as a medium that connects the spinal muscles to the vertebral bones. The muscles of the spine provide spinal stability and support. They also serve to flex, extend and rotate the spine. The muscular system of the spine is complex, with several different muscles playing important roles. The combination of the ligaments, tendons and muscles provides a natural brace, or wrap that protects the spine from injury.

When you look closely at the spine you can see how tough it is and how well designed it is to do its job. The spine is an amazingly strong and clever structure.

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