DIFFUSE BRAIN INJURY  [ back to Glossary Index ]
Diffuse Brain Injury occurs from rotational or torque forces upon the brain from sudden, rapid jerking of the head in a front to back or side to side motion. This is a white matter injury caused by sudden, stretching and straining of the long, narrow diameter nerve fibers known as axons, which transport messages back and forth across the brain (corpus callosum) and back and forth between the brain and spinal cord through the pons. Early research on this phenomenon in the 1950s and 1960s showed that very high speed impacts to the head (in the 70-80 mph range) could actually rip or "shear" large numbers of axons deep in the brain, which was associated with severe TBI and death. More recent research demonstrates that lower speed impact to the head (e.g. in a 30-40 mph crash) will not physically tear axons in two pieces, but will strain the axons. Such strain injuries precipitate onset of obstructive swellings in the affected axons (which appear about 48 hours post-trauma). The swelling shut down anterograde transport of nutrients down the axon necessary to keep it alive, and shut down retrograde transport of nerve impulses to the cell body which tells the cell nucleus there has been a disconnect to adjoining cells. These events lead to the death of the axon below the point of the swelling. This can kill the nerve cell or it can cut back the axon to a "growth cone" which may "sprout" new axonal "boutons" which may reconnect with some of the dendrites which were formerly part of the same brain circuit. The older a person gets the less "sprouting power" his axons have, and the more likely that damaged circuits stay damaged. Neurons that have been killed by "perturbation" of their cell walls or by loss of their axon are not replaced. The fetal brain reaches a maximum of 200 billion neurons which are gradually "pruned" back to about 100 billion by the time we are 3 years old. After that we never get new brain cells, and we are estimated to lose about 50,000 brain cells each day (especially if the swelling is close to the cell body) or in the progressive splitting of the axon with limited regrowth of axonal "sprouts" from the portion still attached to the neuron. Either way synapses are destroyed and lost forever or partially replaced over time.