A Conduit and Processor of Information

The spinal cord acts as a conduit of information between the brain and the body (Figure 1). Information travels along neurons (nerve cells) in the brain and body in the form of electrical impulses. These impulses are transmitted along axons (the long processes of neurons) and across synapses (the tiny gaps between adjacent neurons) to the spinal cord, which connects the brain with the sensation and movement systems in the rest of the body. 

This flow of electrical signals moves in both directions via the spinal cord. As well as carrying important information from our brain to our body, to orchestrate the movements of our muscles (and many other tasks), the spinal cord also carries information from the body back up to the brain. In this way, the brain obtains sensory information about our bodies, such as what we feel and the position of our limbs.

A further responsibility of the spinal cord is to integrate and compute the input of sensation and movement neurons in the brain and the body, to coordinate complex movements such as walking. This incredibly complex integration and computation of information in the spinal cord can be disturbed by compression and injury.
Figure 1: The spinal cord not only transmits the electrical signals travelling between the brain and the body, but also performs complex processing of these signals. Imagine if an electronic circuit board was mechanically disrupted: the intricate computations could no longer be completed as normal. When the spinal cord is subjected to mechanical injury, the electrical signals are disturbed.

Segmental Organisation of the Spinal Cord

The spinal cord is housed inside the spinal column, which runs all the way down the centre of your back (Figure 2). The cervical part of the spinal cord controls everything in your body from the neck downwards. 

The spinal cord is organised into segments (Figure 3). Each segment controls a specific muscle group and receives sensory signals from a specific area of the body. Damage to different segments of the cervical spine will result in deficits in different areas of the body. This is one of the reasons why people with DCM can experience a wide range of symptoms, depending on the level at which the spinal column is damaged.
Figure 2: The vertebral column runs from your skull all the way down the centre of your back to your coccyx. The cervical vertebrae are situated in your neck, and damage in this area can lead to DCM.
Figure 3: Side view of the spinal cord housed inside the spinal column. 

Degenerative Changes Induce Stress on the Spinal Cord

The spinal cord runs inside the spinal canal and is surrounded by bony and fibrous structures called vertebral bodies and ligaments (Figure 4). These structures form the spinal column. The spinal column is surrounded by muscles, which attach to the vertebral bones and various ligaments, allowing movements of the neck. Between the vertebral bodies, nerves exit the spinal cord to connect to the body where they control movement and sensation.
Figure 4: Anatomy of the spinal column.
It is this close relationship between the vertebrae and ligaments that predisposes the spinal cord to injury. Arthritic changes in the spinal column induce an inflammatory reaction in the affected bones, cartilage and ligaments. This results in protrusion of intervertebral discs, formation of bony spurs, and thickening of ligaments. Together these changes cause narrowing (stenosis) of the spinal canal and can injure the spinal cord in some people, leading to Degenerative Cervical Myelopathy (DCM) (Figure 5).
Figure 5: An illustration of the cervical spine, side view. It shows a disc prolapse between the 3rd (C3) and 4th (C4) vertebrae. This is causing compression of the spinal cord. A disc prolapse is an example of one of the pathological changes that can cause DCM. Often, these occur in combination with other degenerative changes. An isolated disc prolapse like this represents about 10% of DCM cases treated today. 

Strain to the Neck Causes Wear and Tear Arthritis

By the time you reach your thirties, steady ageing of spinal structures in the cervical spine starts to occur. This may be made worse through repetitive or heavy motion and can result in arthritis of the spinal column. A number of things can increase the risk of this, including:
Gender – osteoarthritis is more common in post-menopausal women
Excess Weight – accelerates wear and tear
Genetics – e.g. a family history of osteoarthritis
Associated Diseases – including infections, diabetes, and various forms of rheumatism

Slow-Motion Spinal Cord Injury

It is poorly understood how these changes cause injury and damage of the spinal cord. Traditionally, it was thought that gradual or intermittent compression of the spinal cord triggers damage, through mechanisms such as reduced blood flow, which starves cells of oxygen and nutrients, or inflammation or stretch on axons, which disturbs the flow of information in the spinal cord.

However, this is probably an oversimplification. For example, “compression” of the spinal cord is more often seen in healthy individuals without any features of DCM. Understanding these processes is an international research priority.