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In this section we discuss the anatomy of the human spine and what happens at a basic, structural, level when we experience back pain.
There are many things that can go wrong with the spine, but, in essence, up to 98% of all back pain is either caused by, or directly linked to, compression. That is the term used to describe the spine when it becomes squashed, or shortened from its natural length.
However, the spine is not a completely solid structure. If it were, it would be very difficult to compress. Instead, it is made up of individual vertebrae, or units of bone, that are stacked on top of each other, joined, or linked, by facet joints. Ultimately, it is the facet joints that become compressed and, as a result, the spine becomes squashed.
When this happens, we become stiff (a common symptom of back pain) because the joints are not able to move freely.
In addition to this, the compression reduces the space between the vertebrae, which, in turn, leads to compression of the intervertebral discs. If the pressure becomes too great, the disc ruptures, a condition commonly referred to as a herniated, or slipped disc (shown below).
Compression also causes the joints to rub together and, over time, they may become arthritic and painful (just like any other joint).
Finally, due to the way in which the spinal cord exits the vertebral column (a structure also known as the spinal column), compression can lead to nerve-root entrapment, a condition which is extremely painful.
Like any other form of pain, though, back pain is ultimately caused by damage to the human nervous system; the nerves simply produce pain in response to a harmful event (thereby alerting the brain of the problem). But, because the nervous system is responsible for many other activities, the effects of facet joint compression are wide ranging. Compression can also lead to referred pain (e.g. Sciatica), organ dysfunction and various other effects (e.g. Pins and Needles), as well as aggravating a whole host of bone conditions.
This section is designed to start at the beginning. We first examine the basic structure of the human spine, and then take a more detailed look at the individual vertebrae, what they look like and how they come together to form the spinal column. Next we look at the facet joints and explain how they lock the vertebrae together; we also examine what happens to the spine when they become compressed.
Finally, and we admit that things get slightly more complex at this point, we look at the human nervous system. We examine the nerves that exit the spinal column (and the brain), and divide the peripheral nervous system (PNS) into its various subcomponents. Next we discuss the function of the individual sub-systems, in terms of both sensory and motor components, and examine the symptoms of nerve-root compression.
The Spinal Column
The human spine consists of a number of vertebrae, or units of bone, arranged in a vertical structure, a structure known as the spinal column.
In new-born children, the column comprises 33 distinct, or physically separate, vertebrae. Over time, however, the units at the base of the column fuse together to form the coccyx, or “tailbone”, and the sacrum, which forms part of the pelvis.
As can be seen from the above picture, the column itself actually consists of five, distinct regions. These are known (from top to bottom) as the:
cervical spine (red)
thoracic spine (yellow)
lumbar spine (green)
coccyx (dark blue)
The mechanical function of the spine is two-fold: it must provide structural support for the body, and allow us to move freely in three dimensions (bending over and rotating as necessary); the neurological function of the spine, of course, is to innervate (or provide a nerve supply to) the rest of the body.
In order to understand how both of these functions are achieved, we first need to understand the structure of the individual vertebrae, and how they come together to form the column itself.
Each of the vertebrae in the top three sections of the spine (cervical, thoracic and lumbar) has the same, basic shape:
The main structures that are relevant to our discussion are listed below:
The body is the primary, weight-bearing structure. As we progress down the spinal column, each vertebra is loaded with more and more weight and, in order to accommodate this, the body of each unit becomes progressively larger (as do the vertebrae in general). For example, L4 is significantly larger than T3.
The body of each vertebra is separated from that of its neighbour by an inter-vertebral disc. The discs themselves are filled with a soft, inner core (the nucleus pulposus) and act as shock absorbers. A Slipped, or Herniated, Disc occurs when the inner core spurts out through a tear in the hard, outer layer (the annulus fibrosis).
The vertebral foramen is a hole through which the spinal cord passes (note: the plural of foramen is foramina). The foramina of all the vertebrae line up to form a continuous, three dimensional hole known as the spinal canal, which runs from the brain to the base of the spine.
Each vertebra comprises two laminae, thin plates of bone that extend from either side of the vertebral body and meet at the spinous process (the part of each vertebra you can feel, and see, just below the skin). Together, the laminae and the vertebral body surround the spinal cord and give rise to the vertebral foramen.
A vertebral process is essentially an outgrowth of bone that extends from one, or both, of the two laminae. The transverse and spinous processes provide an attachment for muscles in the back (as well as additional protection for the spinal cord).
The remaining four processes are extremely important, because they form the joints that lock the vertebrae together. They are known as the articular processes (from the Latin word articularis,meaning “of the joint”. The joints themselves are known as facet joints.