The acute phase of mild traumatic brain injury is characterized by a distance-dependent neuronal hypoactivity

The consequences of mild traumatic brain injury (TBI) on neuronal functionality are only now being elucidated. We have now examined the changes in sensory encoding in the whisker-recipient barrel cortex and the brain tissue damage in the acute phase (24 hours) after induction of TBI (n=9), with sham controls receiving surgery only (n=5). Injury was induced using the lateral fluid percussion injury (LFPI) method, which causes a mixture of focal and diffuse brain injury. Both population and single cell neuronal responses evoked by both simple and complex whisker stimuli revealed a suppression of activity that decreased with distance from the locus of injury both within a hemisphere and across hemispheres, with a greater extent of hypoactivity in ipsilateral barrel cortex compared to contralateral cortex. This was coupled with an increase in spontaneous output in Layer 5a, but only ipsilateral to the injury site. There was also disruption of axonal integrity in various regions in the ipsilateral, but not contralateral hemisphere. These results complement our previous findings after mild diffuse-only TBI induced by the weight-drop impact acceleration method where, in the same acute post-injury phase we found a similar depth-dependent hypoactivity in sensory cortex. This suggests a common sequelae of events in both diffuse TBI and mixed focal/diffuse TBI in the immediate post-injury period that then evolve over time to produce different long-term functional outcomes.