The spinal cord contains bundles of neurons. These conduct impulses, small electrical signals that carry signals to and from the brain to control the whole of the body. Neurons damaged by spinal cord injury are unable to conduct electrical impulses, and the result is disrupted communication between the brain and the rest of the body.
- Signals that originate in the brain and command muscles to move are unable to reach the muscles they control, which results in paralysis. Spinal cord injury is associated with many other changes in addition to paralysis. These include loss of bladder and bowel control, loss of sexual function and an inability to control body temperature.
- The injured neurons are unable to carry sensory information up the spinal cord from the skin and internal organs to the perception centres in the brain. Because these signals do not reach the brain, spinal cord damage prevents the injured person from feeling sensations such as temperature and pressure.
Because neurons in the spinal cord do not regrow naturally, the consequences of injury were, until recently, considered permanent. We now know that this need not be the case. Spinal Research funds scientists and clinicians throughout the world who are working together to repair and understand the damage caused by spinal cord injury.The spinal cord is normally protected by a series of ring-like bones, called vertebrae, that make up the spinal column. But when the spinal column is fractured, the vertebral bones can compress or bruise the fragile spinal cord. Most such injuries do not completely sever the spinal cord. Although the initial injury kills some neurons outright and damages others, many neurons remain intact, at least for a while. However, when neurons in the spinal cord and other areas of the central nervous system die, they release toxic chemicals that cause uninjured neurons in the surrounding area to die. This increases the overall damage and can double the size of the affected area in the first hours and days after injury. As the injury stabilises over the following weeks and months, the loss of neurons is accompanied by the formation of scar tissue in the damaged region. Myelin, the fatty insulating covering that surrounds neurons, and which is essential for rapid transmission of nerve signals, can also degenerate; the result is that that the remaining neurons are unable to transmit signals effectively.
Neurons in the spinal cord are carefully organised, depending on their function. This organisation means the long-term consequences of injury depend on the exact location of the damaged area (otherwise known as the level of the injury) and on its size.
Incomplete and complete injuries
There are two types of injury, called complete and incomplete.
- In a complete injury, the spinal cord is damaged across the whole of its width. This means that there is no function (either sensation or muscle control) below the level of injury. It also means that both sides of the body are affected equally.
- In an incomplete injury, the injury does not spread across the whole of the spinal cord; some areas away from the injury remain intact or at least intact enough to retain some function. People with incomplete injuries have some sensation and/or movement control below the level of injury. Frequently, one side of the injured person’s body is more affected than the other.
Spinal levels
Spinal levels are determined by the vertebrae that make up the spinal column. The higher the location of the injury in the spinal cord (ie closer to the head), the greater the proportion of the body affected. Thus, injuries in the neck area (called the cervical region) have the most extensive consequences. The cervical region contains 8 vertebrae, known as C1–C8, with C1 closest to the skull. The nerves that control movement of the muscles needed for breathing emerge from the spinal column at C4, so people with injuries at C4 or higher need a ventilator to breathe. Individuals with a C6 injury have little control over arm movement, apart from some ability to move their shoulders and flex their elbows, whereas those with an injury at C7 can move shoulders, elbows and wrists. 
Below the cervical level are the thoracic vertebrae (T1–T12). Injury in this region usually results in paralysis of the chest and abdominal muscles, whereas injury to the lumbar (L1–L5) and sacral (S1–S5) regions disrupts the control of leg and hip muscles.
However, the dysfunction associated with injury is not confined to tissues that are directly associated with the affected level. Injury at a particular level also disrupts the flow of signals that travel along the entire length of the cord. An injury prevents messages that originate in the brain from travelling below the damaged area and it also blocks the upward flow of sensory signals from below the wound to the brain.
Thus, injuries higher up the spinal cord cause relatively greater paralysis and dysfunction than lower spinal injuries: injuries in the cervical region cause paralysis in both the arms and then legs (known as tetraplegia or quadriplegia) whereas injuries in the thoracic region cause paralysis in the legs, which is called paraplegia.
