Anatomy of the Spine

WHAT ARE THE MAIN ANATOMICAL REGIONS ON THE SPINE?

The spine is made up of a large number of spinal bones (vertebrae) which are linked together by intervertebral discs and facet joints.

The human spine is divided into the cervical (highest), thoracic, lumbar, sacral, and coccygeal (lowest) regions.

The vertebrae (spinal bones) in each region are numbered according to their region (letter C, T, L or S) and location (number).


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What are intervertebral discs?

The intervertebral discs are soft structures which act as shock absorbers between each of the vertebrae (bones) in the spine. A single disc sits between each vertebra. Each intervertebral disc has a strong outer ring of fibres (‘annulus’), and a soft, jelly-like centre (nucleus).

The central soft and juicy nucleus is a sphere-shaped structure that allows tilting, rotating, and gliding movements in the spine.

The nucleus functions as a primary shock absorber. It is a clear, jellylike material that is made up of 88 percent water in young adults.

As the body ages and/or degenerates, the amount of water in the nucleus reduces. The rest of the nucleus is made up of connective tissue cells, collagen fibres, and a small amount of cartilage. There are no blood vessels or nerves in the nucleus,

The annulus is a ring-like frame of fibres which connects each vertebral bone. It is the strongest part of the disc and it encloses the central nucleus and holds it under pressure to prevent it from rupturing.

What are the laminae?

The laminae are ‘shingles’ of bone lying over the back of the spinal canal. They meet in the midline, giving rise to the spinous process (the ridges that can be felt through the skin at the back of the spine). They are often removed (laminectomy) to decompress the nerves in the spinal canal.

What are the facet joints?

The facet joints are small joints on each side at the back of the spine. They allow movement between adjacent vertebrae and help to keep the spine stable. Facet joints form part of the roof of the spinal canal.  The area of the spinal canal immediately underneath the facet joint is known as the subarticular compartment or lateral recess. Spinal nerves run through these compartments.

Where do the spinal nerves run?

The spinal canal and intervertebral foraminae in the lumbar spine (lower back) are bony tunnels through which the spinal nerves (nerve roots) run.

When the size of these tunnels is reduced, there is less room for the spinal nerves. As a result, pressure on these structures may occur.

What about the spinal cord?

The spinal cord ends in the upper lumbar spine (usually at L1). The lower portion of the spinal cord is known as the ‘conus’ or ‘conus medullaris’. Pressure on the conus can cause impaired bowel and bladder control, and numbness around the anus and genitals (‘saddle anaesthesia’).

Below the ending of the spinal cord, the canal is occupied by the spinal nerves (also known as the ‘cauda equina’ which means ‘horses tail’, named after its appearance). These nerves run through the canal and then out through their respective intervertebral foraminae. These nerves supply function to the bladder, bowels, genitals, and legs.

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