The Anatomy of the Spine

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The term "anatomy" in simplest terms means description of the shape or structure of an organism. In some ways spinal anatomy is like three-dimensional geography of your 'back bone'. With this introduction you will learn some of the basic aspects concerning the shape and structure of the normal human spine and how the different parts of the spine connect and interact.

The spine consists of solid bone blocks called vertebrae which are connected to each other by ligaments, small joints (facet joints) and soft discs to form a strong yet mobile structure that is held in good balance by powerful muscles. Introducing the concept of spine stability offers the challenge to deal with two mechanical requirements that seem to be opposed: rigidity and flexibility. In terms of analogy, one can examine the demands on a mast of a sailboat. The mast supports the sails and carries all the stress of a powerful wind. The mast is rigid, however, there is some flexibility, provided by the stays that are symmetrically attached to the hull. In a sense, the connective tissues, ligaments and muscles can be compared to a complex system of stays, distributed evenly on each side to provide balance and alignment of the spine. These muscles, adapt automatically and immediately to maintain balance and position by active contraction under the control of the nervous system. To meet the challenges of being mobile and carry heavy loads, the vertebrae vertebrae of the spine are not only connected by cushioning discs but also are aligned in a set of curvatures (cervical lordosis, thoracic kyphosis, lumbar lordosis) that permit energy absorption like a spring or coil.

When the body is in motion, every part of the spine reacts in some way. In the process of bending even with large amplitude the relationship between two vertebrae vertebrae must stay within limits which are pre-set by the biomechanical properties of the disc, ligaments and muscles. Each unit of two vertebrae with its interspaced disc, ligaments and facet joints is called a "mobile segment". Each mobile segment along the spine tolerates significant loads, and controlled levels of motion. The adapted shape of each vertebra as well as the viscoelasticity of the discs and ligaments supports the significant mechanical requirements at each mobile segment. Interestingly, most of spinal motion occurs in the upper cervical spine (neck area) and the lower back, or lumbar spine. The upper back (thoracic spine) has attachments to the rib cage and is thus much stiffer. Muscles of the back attach to all levels of the spine and have multiple functions, most importantly perhaps the role of balancing each other and to maintain the entire spinal structure in a permanent equilibrium.

Aside from the global role of balance and positioning of the spinal column, individual segments of the spine have some specific "duties". Of note, the spinal vertebrae are numbered according to zone and then numbered starting with #1 and going up in number as one counts down the spine. Thus the upper most vertebra is C1 (cervical #1) and the last lumbar vertebra is L5 (lumbar #5). There are 5 cervical, 12 thoracic, and 5 lumbar vertebrae.

When it comes to special "duties" of individual levels of the spine, consider for instance head rotation. This is made possible by the shape of the two first vertebrae in the cervical spine, which are linked together by a special set of ligaments providing rotation in the transverse plane around a swivel, which is the odontoid process of the second cervical vertebra, C2. The first cervical vertebra, C1, sits directly under the skull and articulates with the latter in flexion through the condyles (facet joints). Significant rotation of the neck is possible due to movement between the ring of C1 and the odontoid process of C2.

Aside from motion and stability the spine has one other very important role, it protects and houses the spinal cord and nerves that pass from the brain to the arms and legs. At the level of the skull where the brain sits, the nervous system connection run through the foramen magnum (passageway at the bottom of the skull) to the spinal cord. The spinal cord is located inside the spinal canal, a type of tunnel, located within the spinal column just behind the vertebrae. Protection of the spinal cord and nerves is insured by the bony and ligamentous structures that permit controlled motion of the spine but only up to a safe limit. In addition to protection inside the bony spinal canal, the cord and nerves are protected and nourished by an envelope (meninges and dura matter), which contains fluid (cerebrospinal fluid or CSF). From the spinal cord, individual nerve roots branch out and extend to various parts of the body to send signals back and forth from the brain.