The spinal cord is seldom cut by injury unless the injury is due to a bullet or knife. In most cases, the spinal cord is compressed either slowly or rapidly by bone or disc displaced against the spinal cord. The extent and cause of damage depend on the speed of compression. Slow and prolonged pressure damages the spinal cord by blocking blood flow to the cord.
Spinal cord white matter is generally more resistant to ischemia (loss of blood flow) than brain. If the flow is compromised for longer than 10-20 minutes, the compressed tissue begins to die. Rapid compression of the cord is sometimes called “contusion”. The spinal cord is enclosed in dura, a relatively non-distensible membrane. Pressure on the dural sac displaces spinal cord tissue in the longitudinal direction, causing cellular elements in the spinal cord to be stretched and sheared. The spinal cord can be stretched or compressed without much damage as long as the compression rate is slower than 0.5 meters per second and the compression is not maintained for longer than 10-20 minutes.
However, when the compression rate exceeds 0.5 meters per second, axons break much like a rubber band will break when it is stretched too much and too rapidly. Animal studies have shown that axons that are closer to the center of the spinal cord are more likely to be broken because the maximum movement of tissue will be in the center of the cord. Also, larger myelinated axons are paradoxically more likely to break at the nodes of Ranvier between myelin segments because most of the stretching will be concentrated in these regions.