Axonal damage can occur from brain injuries, diseases like multiple sclerosis, and high-speed impacts. The damage disrupts communication between neurons and can cause cell death. Regeneration of nerve fibers can be disrupted, causing long-term effects like seizures and memory loss.
A neuron is a nerve cell made up of a body or soma; projections called dendrites; and a nerve fiber called an axon. Each cell’s axons connect to other cells and serve as pathways to different parts of the brain. Following a brain injury, axonal damage can occur directly from the force of an impact or as a result of substances released from injured or dying cells. The swelling can also cause axons to detach from the parent cells, causing further damage. Diseases such as multiple sclerosis can also cause axon degeneration.
The nervous system relies on the connections between each cell to function properly. Diffuse axonal injury is a form of axonal injury, in which nerve fibers are physically disconnected. This often happens during high speed impacts, such as a car crash. The injury itself can disrupt communication between different parts of the brain, as the neurons are unable to interact. After an injury has occurred, subsequent swelling can cause the axon to separate from the neuron.
Axonal damage sometimes occurs as part of a cascade 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 leach into their deoxyribonucleic acid (DNA) and trigger cell death. Too much calcium can also enter damaged neurons and cause 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 rapidly, the cytoskeleton of the axon can fail, causing further swelling and infusion of calcium into the cell. The inflammatory reactions associated with multiple sclerosis lead to damage and degeneration of axons. 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 disruption to this delicate process sometimes occurs. High levels of neurotransmitters, lack of oxygen or insufficient blood flow to the damaged area can negatively 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, such as seizures, memory loss, and chronic pain, can occur.
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