A neuron is a nerve cell that consists of a body, or soma; projections called dendrites; and a nerve fiber called an axon. The axons of each cell connect to other cells and serve as pathways throughout different parts of the brain. Following a brain injury, axon damage can occur directly from the force of an impact, or as a result of substances released by injured or dying cells. Swelling can also cause the axons to detach from their parent cells, leading to more damage. Diseases such as multiple sclerosis can cause degeneration of axons as well.
The nervous system relies on the connections between each cell to function properly. Diffuse axonal injury is one form of axon damage, in which the nerve fibers are physically disconnected. This often happens during high speed impacts, such as a car accident. The injury itself can disrupt the communication between different parts of the brain, since neurons are unable to interact. After an injury takes place, subsequent swelling can cause the axon to separate from the neuron.
Axon damage sometimes occurs as part of a cascading event. Once nerve cells are damaged and axons are disconnected, neurons release a high level of chemicals that build up to toxic levels. Other nerve cells die once the chemicals penetrate their deoxyribonucleic acid (DNA) and trigger cell death. Too much calcium can enter damaged neurons as well, and causes a swelling reaction that often causes axon damage. Medical studies have suggested that by reducing the level of calcium ions after a brain injury, cell death can be reduced.
Damage to neurons also occurs when axons are stretched. If stretched too quickly, the cytoskeleton of the axon can fail, leading to additional swelling and infusion of calcium into the cell. Inflammatory reactions associated with multiple sclerosis lead to axon damage and degeneration. The protective myelin sheath surrounding the axons often remains intact, so the condition can persist and go into remission multiple times.
After axon damage, nerve fibers can grow to replace the axons, but disruptions to this delicate process sometimes occur. High neurotransmitter levels, a lack of oxygen, or not enough blood flow to the damaged area can adversely affect the regeneration of nerve cells. The fibers can also be misdirected and connect in the wrong areas if the electrical activity in the brain is high enough. Serious long term effects can result, such as seizures, memory loss, and chronic pain.