A spinal cord injury usually begins with a sudden, traumatic blow to the spine that fractures or dislocates the vertebrae. The damage begins at the moment of injury when displaced bone fragments, disc material, or ligaments bruise or tear into spinal cord tissue. Axons are cut off or damaged beyond repair, and neural cell membranes are broken. Blood vessels may rupture and cause heavy bleeding in the central grey matter, which can spread to other areas of the spinal cord over the next few hours.
Within minutes, the spinal cord swells to fill the entire cavity of the spinal canal at the injury level. This swelling cuts off blood flow, which also cuts off oxygen to spinal cord tissue. Blood pressure drops, sometimes dramatically, as the body loses its ability to self-regulate. As blood pressure lowers even further, it interferes with the electrical activity of neurons and axons. All these changes can cause a condition known as spinal shock that can last from several hours to several days.
Although there is some controversy among neurologists about the extent and impact of spinal shock, and even its definition in terms of physiological characteristics, it appears to occur in approximately half the cases of spinal cord injury, and is directly related to the size and severity of the injury. During spinal shock, even undamaged portions of the spinal cord become temporarily disabled and cannot communicate with the brain. Complete paralysis may develop, with loss of reflexes and sensation in the limbs.
The crushing and tearing of axons is just the beginning of the devastation that occurs in the injured spinal cord and continues for days. The initial physical trauma sets off a cascade of biochemical and cellular events that kills neurons, strips axons of their myelin insulation, and triggers an inflammatory immune system response. Days or sometimes, even weeks later, after this second wave of damage has passed, the area of destruction increases - sometimes to several segments above and below the original injury-as does the extent of the disability.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.