A Stack of Coins
You can think of the spine as a series of thick coins or cylinders with one stacked upon another. This stack starts at the pelvis where one specially-designed “coin” is wedge-shaped and lodged into the pelvis. It ends at the skull where the discs take the shape of tiny joints that the skull can sit upon.
The bones of the spine are more like Styrofoam with an egg shell over them. They create a special resilience and allows for a transfer of forces that could not occur if the spine was made of very hard discs. Because of this “soft” structure of the spine, fractured vertebrae generally do not shatter or break; instead, they compress and deform to absorb the shock.
The Invertebral Discs
What gives you the curves in your back? The hump of your chest and the sway of your lower back are actually caused by the soft tissue cushions between the discs, which are called the invertebral discs. This is your body’s system of shock absorbers between each of the vertebrae.
The invertebral disc function can be most similarly illustrated to that of a water balloon. On the outside, there is a tough surface lining called the annulus and on the inside, there is a jelly-like liquid. As a child, this nucleus is gel-like and rubbery, but as a person becomes older, the nucleus becomes drier, harder, and gristle-like. However, it still serves a vital function, separating the vertebrae so that motions such as twisting and jumping are absorbed and cushioned. The discs also absorb and lose water throughout the day. When a person lies down, the discs take on water and a person becomes taller. When a person walks and stands throughout a whole day, the water is squeezed out and height is lost.
We are going to review the 4 supporting structures within the spine which assist in maintaining overall stability.
Supporting Structure #1: The Facet Joint
This ingenious design component of the vertebrae reaches down to hook upon the vertebra below. This inferior articular process, also known as the lower joint process, starts at the top of the next vertebrae. The first reaches down to the second, the second to the third, and so on down to the pelvis where the hook on the fifth lumbar vertebrae hooks upon the pelvis for ultimate stability.
The facet joint is so named because a small “face”—much like the facet of a diamond—is formed. These vary in size from the top of the vertebrae to the bottom. When you bend or twist, these small joints rotate against one another from behind and the discs squirm a little in front to allow motion. When you combine seven cervical vertebrae, twelve thoracic vertebrae, and five lumbar vertebrae, a great deal of motion under stable and controlled conditions is possible.
Supporting Structure #2: Ligamentous Structure
This is another structure which consists of a long ligament that runs down the front and the back of the vertebrae.
Supporting Structure #3: Surrounding Muscles
The complex appearance of the vertebrae results mostly from the muscle attachment points. These attachment points are called transverse processes (side to side) and dorsal processes (going up the back). The dorsal process is over 1-2 inches long and is what you feel when you run your hand down someone’s spine. The muscles of the back and spine, coordinated by balance- and posture-controlling mechanisms of the body, hold the spine erect.
Supporting Structure #4: Abdomen and Chest
The final element of stability is the abdomen and chest. The front of the vertebrae “leans” on these structures. The ribs form a tight structural network around the vertebrae, so herniations of the thoracic discs are very uncommon. There are only five segments below the rib cage and this is where most herniations occur. The abdominal cavity is like a tightly enclosed pocket of air. The spinal column “leans” on the abdominal contents; this leaning is easier if the anterior abdominal wall is firm like a beach ball. This is one of the reasons we may recommend exercise to strengthen the abdomen.
Other Important Structures
The Spinal Cord and Its Elements
The spinal column has now been defines as a series of discs of coins stacked upon one another. The nerves travel down a hole at the back of each of these coins and the inferior articular processes arch from one vertebrae to the next. This creates a recess or a hole so that the spinal cord travels within a series of rings. These rings are broad and flattened so that they nearly touch one another. Only through the slits between these rings at the back of the vertebrae is any access to the spinal cord possible.
The spinal cord is beneath the long spike on the top of the vertebrae and is covered by skin and fat. This places the spinal cord down in the body approximately three inches and within bone.
Nerves travel between the vertebrae. They go out just beneath the point at which the inferior articular process leaves the disc of the vertebral body. They travel out, one after the other, and are numbered based on the level of the vertebra where they exit. For example, the fifth lumbar nerve comes out beneath the fifth lumbar vertebra. As they exit, they branch out much like the branches of a tree from the spinal lining and pass by the facet joints and the discs. You can now understand how a spur on the joint or the herniation of a disc might touch the nerve; this is why a person may experience leg pain.
The discs have as much feeling as your skin does. Small, thread-like nerves go to the discs and to the facet joints to monitor them. If a disc is damaged in a twisting injury, it can hurt and this pain will be felt in the back. However, in some cases, it may also be felt in the buttocks.
The spine is surrounded and supported by strong muscles, which are attached to the spinous processes all around the vertebrae. The body monitors the position of the spine and controls the spines so that the proper amount of flexion or straightness is created.
Muscles can spasm if structures are injured. Sometimes these muscle spasms are more severe than the actual pain from the skeletal structure itself, which results in having to treat the muscles as a separate entity.
In summary, the spine may be described as a series of coins that are hooked upon another from behind, each separated from the other by discs in front and small joints in the back. This combination of flexible cushioning discs and small joints allow for motion, but it is also rigid and stable in order to provide the structure on which our body can stand erect. The spine is also kept erect by the muscles around the spine which use the central balance system. Since the spine is a living structure, various parts of it can wear out and cause certain difficulties that we will now discuss.