During a traumatic insult to the brain, tissue is subjected to large stresses at high rates which often surpass cellular thresholds leading to cell dysfunction or death. Cellular events that occur at the time of and immediately after an insult are poorly understood. Immediately following traumatic brain injury (TBI), the neuronal plasma membrane may become disrupted and potentiate detrimental pathways by allowing extracellular contents to gain access to the cytosol. In the current study, neuronal plasma membrane disruption was assessed in vivo following moderate unilateral controlled cortical impact (CCI) in rats using a normally cell-impermeant fluorescent compound as a plasma membrane permeability marker. This fluorescent dye was injected into the CSF and allowed to diffuse into the brain. TBI caused a widespread acute disruption of neuronal membranes which was significantly different compared to uninjured brains. Affected cells were present in cortex and hippocampal regions. These findings were complemented by an in vitro model of TBI where membrane disruption was quantified and its mechanisms elucidated. Permeability marker(s) were added to neuronal cultures before the insult as indicators for increases in plasma membrane permeability. The percentage of cells containing the permeability marker was dependent on the molecular mass, as smaller molecules gained access to a higher percentage of cells than larger ones. Permeability increases were also positively correlated with the rate at which the insult was applied. Membrane disruption was transient, evidenced by a robust resealing within the first minute after the insult (assessed by adding permeability markers at different time points following the mechanical insult). In addition, membrane resealing was found to be dependent on extracellular Ca²⁺, as chelation of the divalent ion abolished a significant amount of resealing. Neurons depend on a tightly regulated ion concentration gradient across the plasma membrane, and mechanically-induced changes in membrane permeability can affect action potential firing, axonal signal conduction, and synapse function. We have investigated the effects of mechanically-induced plasma membrane disruptions on neuronal network electrical activity. We have developed a multielectrode array system that allows the study of electrical activity before, during, and after a traumatic insult to neurons. Endogenous electrical activity of neuronal cultures presented a heterogeneous response following mechanical insult. Moreover, spontaneous firing dysfunction induced by injury outlasted the presence of membrane disruptions. This study provides a multi-faceted approach to elucidate the role of neuronal plasma membrane disruptions in TBI and its functional consequences.