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Understanding How The Back Works

Tuesday, March 20, 2012 - 10:07

Contributing Author: Guy Slowik FRCS

The spine, which connects the skull to the pelvis, is also called the vertebral column. It consists of 24 segments of block-shaped bone called vertebrae and an additional 9 fused vertebrae that make up the lowest part of the spine, the sacrum and tailbone. Each vertebrae of the vertebral column has protruding bony areas for the attachment of muscles that are important for the spine to move. The spinal column protects the spinal cord and its emerging nerves that run down most of the length of the spine.

The vertebrae have two major functions:

  • To bear the weight of the body
  • To house the spinal cord or spinal nerve roots (cauda equina) within the spinal column

The spine is arranged in three natural curves:

  • The neck region or cervical spine, made up of 7 vertebrae - where the vertebrae curve forward.
  • The trunk region or thoracic spine, made up of 12 vertebrae - where the vertebral column curves backward, and to which the ribs attach
  • The low back region or lumbar spine, made up of 5 vertebrae - which curves forward in the same direction as the cervical spine.

When these curves are in their normal alignment, the body is in a balanced position. This distributes weight evenly throughout the vertebrae so one is in a less vulnerable position for strain and injury.

There are two major parts to each vertebra:

  • Vertebral body - The vertebral body is the front portion of the vertebrae. It is shaped like a cylinder and is greater in height than the back portion.
  • Vertebral arch - The vertebral arch is the back portion of the vertebrae. It is an irregularly shaped structure.

At the center of each vertebra is a hole, protected by the surrounding strong bone. Placed together, the central opening of each vertebra makes up the spinal canal through which the spinal cord, cauda equina, or spinal nerve roots pass. The spinal cord is the mass of nerve that connect the brain to the rest of the body.

Each vertebra has important bony projections called processes that provide sites for the attachment of ligaments and muscles that are important for the stability and movement of the spine.

  • The projections on either side of each vertebra are called transverse processes, and the ones at the back are called the spinous processes. The transverse processes are long and slender; the spinous processes are broad and thick.
  • The back portion of the vertebrae, behind the transverse processes, consists of an area of bone called the laminae.
  • On the back part of the vertebrae are two upper and two lower processes that form the joints connecting the back part of each vertebra. These are the facet joints. They are important for movement between each vertebra and for movements of the entire vertebral column as a unit.

The Discs Of The Back

Between each vertebra are spongy pads, like soft cushions, called discs - or more correctly, intervertebral discs. Each disc has a soft jelly-like center called the nucleus pulposus, which is surrounded by a fibrous outer envelope called the annulus fibrosis. Eighty percent of the disc is water, which is why it is so elastic. Together, a disc with the attached part of the vertebra above and below is considered an intervertebral joint. These joints allow the movement of the back.

Healthy discs are elastic and springy. They make up 20% to 25% of the total length of the vertebral column. Initially, the disc contains about 85% to 90% water, but this amount decreases to 65% with age, resulting in disc degeneration.

The Spinal Cord And The Lower Back

The nerves that come off the spinal cord are called nerve roots. These nerve roots pass through small openings on either side of the connecting vertebrae. Various nerve roots combine to form spinal nerves.

There are five pairs of lumbar (lower back) spinal nerves. The nerve roots that arise from the end of the spinal cord and continue down the spinal canal through the lower part of the spine looks like a "horse's tail" and are collectively named the cauda equina.

The Ligaments Of The Back

There are a series of ligaments that are important to the stability of the vertebral column. Important to the lumbar spine (lower back) are seven types of ligaments:

  • Anterior longitudinal ligaments and posterior longitudinal ligaments are associated with each joint between the vertebrae. The anterior longitudinal ligament runs along the front and outer surfaces of the vertebral bodies. The posterior longitudinal ligaments run within the vertebral canal along the back surface of the vertebral bodies.
  • The ligamentum flavum is located on the back surface of the canal where the spinal cord or caude equina runs.
  • The interspinous ligament runs from the base of one spinous process (the projections at the back of each vertebra) to another.
  • Intertransverse ligaments and supraspinous ligaments run along the tips of the spinous processes.
  • Joint-related structures called capsular ligaments also play an important role in stabilization and movement.

The Muscles Of The Lower Back

The muscles and tendons of the spine have been described as being a supporting system for the spine, much like a tent supported by guide ropes.

  • A group of back muscles called the erector spinae are an example of these muscles, which form on each side of the spine and consist of three columns. These muscles move the lower back, help straighten the back, provide resistance when a person is bending forward at the waist, and help a person return to the erect position.
  • The multifidus is another important muscle of the lumbar region. This muscle is thick and prominent in the lumbar spine and becomes smaller at its attachments high up the spine. It is an effective lever arm that allows the lumbar spine to bend backward.
  • The interspinales muscles, located on either side of the interspinous ligament, also are active in the backward bending of the lumbar spine.
  • The intertransversarii muscles attach to the transverse processes. These muscles are not only active in backward bending, but also in bending from side to side.
  • The intersegmental muscles are a series of muscles near the bottom of the spine that connect one intervertebral segment to another.
  • The abdominal muscles, located at the front and side of the abdomen, are very important in supporting and protecting the abdominal internal organs. They also play an important role in protecting movement of the vertebral column in backward bending, forward bending, and side bending.

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