Each region has a number of vertebral bones, typically there are 7 cervical vertebral bones, 12 thoracic vertebral bones, and 5 lumbar vertebral bones. The sacrum is a single, large fused bone. The coccyx is made of one or two small bones (occasionally, an individual may have one extra level or one bone less than normal. E.g. It is not unusual to see four or six lumbar levels).
C1 refers to the top cervical bone and L5 generally refers to the lowest lumbar level.
The vertebrae, the bones of the spine/vertebral bodies, are separated from each other by a disc. The discs are in the anterior (front) of the spine. Discs allow for movement shock absorption.
Nucleus Pulposus: is the interior of the vertebral disc. It is a jelly-like substance that helps absorb the axial load (i.e. forces pushing down on your body when you are sitting upright or standing).
Annulus Fibrosus: The annulus is a set of thicker/tougher/collagen-rich bands that surrounds and contains the nucleus pulposus in multiple concentric circles. Abnormalities in the annulus may cause pain because the annulus is supplied with pain-carrying nerves.
End Plate: the top and bottom portions of vertebral bodies are composed of thickened bone that connects with the intervertebral disc. If the vertebral disc is forced to carry a heavy load or becomes herniated, those forces may cause end-plate abnormalities. Such abnormalities may be seen in discograms or MRIs. These abnormalities may cause pain because the end plates are also supplied with pain-carrying nerves.
Posterior to (behind) each disc, and between each pair of vertebrae (where the upper and lower vertebral body, surrounding the disc, naturally meet to form a hole) is a foramen. One spinal root exits through each of these holes. The nerves go up the arms, chest and legs. The posterior parts of the vertebral bones are connected by small joints called facets.
Discs allow for flexibility and movement of the spine. They provide a cushion. If disc is not there then bone rubs against bone and it is loud and uncomfortable. When bones are fused together (no disc separation), corresponding range of motion is hindered.
Behind the facet joints and along the midline are spinous processes. The spinous processes are bumps which can easily be felt along the back of the neck, thoracic spine and lower back. The processes protect the spine.
In the center of all the vertebrae, which compose the spine, is the central canal which houses and protects the spinal cord (the grand-central station of nerves that carries signals to and from the brain).
The spinal cord itself is protected by the meninges – a tough layered tissue comprised of the dura mater (outer layer), arachoid mater, and pia mater (inside layer).
Nerve root runs up and down the entire spine. The higher up the injury the worse it can be. For example, Christopher Reeve sustained a severe cervical injury and became a quadriplegic. Pain runs up the nerve root (from the source of the pain) so higher-up injuries will cause numbness and damage to everything beneath it. Cauda Equina is at the end (lower portion) of the spinal canal. This nerve structure is called the cauda equina because it is shaped like a horses tail. When you get a herniation that affects the cauda equina, can get a variety of symptoms and if the lumbar herniation/injury is severe enough, possibly cauda-equina syndrome. Car accidents can also cause injury to the vertebral bone, these are referred to as modic changes.
The spinal cord stops around the L1/2 area in an area called the conus medullaris. All nerves beneath this level of the spinal cord enter through the thecal sac (also called the dural sac). The thecal sac offers significantly less protection to the nerve roots in the lower lumbar region.
So, this explains why more people suffer from lower-back (lumbar) pain compared to mid back (thoracic) or neck (cervical) pain? The nerve roots in the thoracic and cervical region enter through their respective foramen and plug directly into the spinal cord. However, the nerve roots that enter through the lumbar or sacral foramen enter the lesser-protected thecal sac and then up to the L1/2 in order to connect with the conus medullaris (lowest entry point into the spinal cord).