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The National Institute of Neurological Disorders and Stroke (NINDS) conducts and supports research to better understand traumatic brain injury and the biological mechanisms underlying damage to the brain, to develop strategies and interventions to limit the primary and secondary brain damage that occurs within days of a head trauma, and to devise therapies to treat brain injury and help in long-term recovery of function.
NINDS grantees developed a device that simulates the condition of trauma in cells in culture. The device allows researchers to control the degree of injury to the cells and then study the biological and physiological responses those cells have to trauma. The device can also serve as a tool to test the effects of pharmaceutical agents.
The neuron is the main functional cell of the brain and nervous system, consisting of a cell body (soma), a tail or long nerve fiber (axon), and projections of the cell body called dendrites. The axons travel in tracts or clusters throughout the brain, providing extensive interconnections between brain areas.
One of the most pervasive types of injury following even a minor trauma is damage to the nerve cell's axon through shearing; this is referred to as diffuse axonal injury. This damage causes a series of reactions that eventually lead to swelling of the axon and disconnection from the cell body of the neuron. In addition, the part of the neuron that communicates with other neurons degenerates and releases toxic levels of chemical messengers called neurotransmitters into the synapse, damaging neighboring neurons through a secondary neuroexcitatory cascade. Therefore, neurons that were unharmed from the primary trauma suffer damage from this secondary insult. Many of these cells cannot survive the toxicity of the chemical onslaught and initiate programmed cell death, or apoptosis. This process usually takes place within the first 24 to 48 hours after the initial injury, but can be prolonged.
One area of research that shows promise is the study of the role of calcium ion influx into the damaged neuron as a cause of cell death and general brain tissue swelling. Calcium enters nerve cells through damaged channels in the axon's membrane. The excess calcium inside the cell causes the axon to swell and also activates chemicals, called proteases, that break down proteins. One family of proteases, the calpains, are especially damaging to nerve cells because they break down proteins that maintain the structure of the axon. Excess calcium within the cell is also destructive to the cell's mitochondria, structures that produce the cell's energy. Mitochondria soak up excess calcium until they swell and stop functioning. If enough mitochondria are damaged, the nerve cell degenerates. Calcium influx has other damaging effects: it activates destructive enzymes, such as caspases that damage the DNA in the cell and trigger programmed cell death, and it damages sodium channels in the cell membrane, allowing sodium ions to flood the cell as well. Sodium influx exacerbates swelling of the cell body and axon.
NINDS researchers have shown, in both cell and animal studies, that giving specialized chemicals can reduce cell death caused by calcium ion influx. Other researchers have shown that the use of cyclosporin A, which blocks mitochondrial membrane permeability, protects axons from calcium influx. Another avenue of therapeutic intervention is the use of hypothermia (an induced state of low body temperature) to slow the progression of cell death and axon swelling.
In the healthy brain, the chemical glutamate functions as a neurotransmitter, but an excess amount of glutamate in the brain causes neurons to quickly overload from too much excitation, releasing toxic chemicals. These substances poison the chemical environment of surrounding cells, initiating degeneration and programmed cell death. Studies have shown that a group of enzymes called matrix metalloproteinases contribute to the toxicity by breaking down proteins that maintain the structure and order of the extracellular environment. Other research shows that glutamate reacts with calcium and sodium ion channels on the cell membrane, leading to an influx of calcium and sodium ions into the cell. Investigators are looking for ways to decrease the toxic effects of glutamate and other excitatory neurotransmitters.
The brain attempts to repair itself after a trauma, and is more successful after mild to moderate injury than after severe injury. Scientists have shown that after diffuse axonal injury neurons can spontaneously adapt and recover by sprouting some of the remaining healthy fibers of the neuron into the spaces once occupied by the degenerated axon. These fibers can develop in such a way that the neuron can resume communication with neighboring neurons. This is a very delicate process and can be disrupted by any of a number of factors, such as neuroexcitation, hypoxia (low oxygen levels), and hypotension (low blood flow). Following trauma, excessive neuroexcitation, that is the electrical activation of nerve cells or fibers, especially disrupts this natural recovery process and can cause sprouting fibers to lose direction and connect with the wrong terminals.
Scientists suspect that these misconnections may contribute to some long-term disabilities, such as pain, spasticity, seizures, and memory problems. NINDS researchers are trying to learn more about the brain's natural recovery process and what factors or triggers control it. They hope that through manipulation of these triggers they can increase repair while decreasing misconnections.
NINDS investigators are also looking at larger, tissue-specific changes within the brain after a TBI. Researchers have shown that trauma to the frontal lobes of the brain can damage specific chemical messenger systems, specifically the dopaminergic system, the collection of neurons in the brain that uses the neurotransmitter dopamine. Dopamine is an important chemical messenger - for example, degeneration of dopamine-producing neurons is the primary cause of Parkinson's disease. NINDS researchers are studying how the dopaminergic system responds after a TBI and its relationship to neurodegeneration and Parkinson's disease.
The use of stem cells to repair or replace damaged brain tissue is a new and exciting avenue of research. A neural stem cell is a special kind of cell that can multiply and give rise to other more specialized cell types. These cells are found in adult neural tissue and normally develop into several different cell types found within the central nervous system. NINDS researchers are investigating the ability of stem cells to develop into neurotransmitter-producing neurons, specifically dopamine-producing cells. Researchers are also looking at the power of stem cells to develop into oligodendrocytes, a type of brain cell that produces myelin, the fatty sheath that surrounds and insulates axons. One study in mice has shown that bone marrow stem cells can develop into neurons, demonstrating that neural stem cells are not the only type of stem cell that could be beneficial in the treatment of brain and nervous system disorders. At the moment, stem cell research for TBI is in its infancy, but future research may lead to advances for treatment and rehabilitation.
In addition to the basic research described above, NINDS scientists also conduct broader based clinical research involving patients. One area of study focuses on the plasticity of the brain after injury. In the strictest sense, plasticity means the ability to be formed or molded. When speaking of the brain, plasticity means the ability of the brain to adapt to deficits and injury. NINDS researchers are investigating the extent of brain plasticity after injury and developing therapies to enhance plasticity as a means of restoring function.
The plasticity of the brain and the rewiring of neural connections make it possible for one part of the brain to take up the functions of a disabled part. Scientists have long known that the immature brain is generally more plastic than the mature brain, and that the brains of children are better able to adapt and recover from injury than the brains of adults. NINDS researchers are investigating the mechanisms underlying this difference and theorize that children have an overabundance of hard-wired neural networks, many of which naturally decrease through a process called neural pruning. When an injury destroys an important neural network in children, another less useful neural network that would have eventually died takes over the responsibilities of the damaged network. Some researchers are looking at the role of plasticity in memory, while others are using imaging technologies, such as functional MRI, to map regions of the brain and record evidence of plasticity.
Another area of research involves the development of improved rehabilitation programs for those who have disabilities from a TBI. The Congressional Children's Health Act of 2000 authorized the NINDS to conduct and support research related to TBI with the goal of designing therapies to restore normal functioning in cognition and behavior.
The original version of this article contained text from the NINDS public domain pages on TBI at http://www.ninds.nih.gov/health_and_medical/disorders/tbi_doc.htm and http://www.ninds.nih.gov/health_and_medical/pubs/tbi.htm
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