Effect of mild traumatic brain injury on spontaneous activity of rat primary somatosensory cortex and its responsivity to vibrotactile stimulation

Abstract

While it has been previously demonstrated that concussion severity can be assessed using sensory tests of cortical functionality, the underlying neural mechanisms affected by concussion are still poorly understood. By using an animal model, it is possible to directly observe the neurophysiological effects of concussion, and thus shed light on the underlying changes in cortical functionality. In order to assess the effects of a single concussion, we recorded spike discharge activity of neurons in the rat primary somatosensory cortex prior to as well as 6-12hr and 78-86hr after a mild weight-drop impact-acceleration closed-head trauma. During the 6-12hr post-impact period, cortical spontaneous activity was elevated by 40% compared to the healthy control state, but its responsivity to vibrotactile stimulation was not significantly affected. However, the responsivity to vibrotactile stimulation did decline in the 78-86hr post-impact period. Also during this period, spontaneous activity in the middle and upper cortical layers was reduced by 35% below the healthy control state, but it remained high in the deep layers. We also recorded somatosensory cortical activity 6-12hr after delivering a second head trauma, identical to the one delivered 72hr prior. Although the two impacts mechanically were the same, the neurophysiological effect of the second impact was very different from that observed after the first impact: both the stimulus-evoked response and spontaneous activity were significantly reduced as compared to the same time period after the first impact. These findings demonstrate that mTBI alters the functional state of the somatosensory cortex in a post-injury time-dependent manner.