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Structure and Function of the Low Back

Middle Column: Behind the vertebral bodies and discs, encased within the spinal canal, lie the spinal cord, which ends at the level of the higher lumbar vertebrae and below this run the nerve roots.

Illustration showing the middle column of the spine. © Louise Carrier 2007.

At each lumbar level, between the lumbar vertebrae, these nerve roots pass out of the spinal canal, and form a number of nerves that pass into the groin and leg, including the femoral nerve and the sciatic nerve. These nerves supply power to the leg muscles and receive sensation from the leg, such as the sensation of touch or pain from the skin. Within the spinal canal the spinal cord and nerve roots are bathed in a fluid called cerebo-spinal fluid and this is encased within a thin membrane. The bony spine serves to protect these delicate nervous structures.

Illustration showing a posterior view of the middle column of the spine as it is located within the body complete with nerves. © Louise Carrier 2007.

Posterior Column: Behind the spinal canal lie the facet joints, one on each side. The facet joints, by the way they are aligned, contribute to maintaining direction of movement between vertebrae but, in health, are not significant weight bearing joints.

Illustration showing the posterior column of the spine and how a facet joint is formed. © Louise Carrier 2007.

Inherently, the spinal column is unstable as it merely comprises a number of bones piled up on one another which are required to perform significant movements. To provide stability therefore there are a number of ligaments (strong, slightly elastic fibrous bands) that provide restriction to excessive movement by becoming taught when in a position of stretch.

Further stability is achieved through a corset of muscles that surround the spinal column and the abdominal wall. Some of these muscles join one vertebra to the one above and some traverse several vertebrae. These are termed segmental, stabilising muscles. There are bigger and stronger muscles that overlie these segmental muscles and rather than provide stability, these muscles are involved in the production of powerful spinal movements.