LETTER Communicated by Steven Prescott Computer Modeling of Mild Axonal Injury: Implications for Axonal Signal Transmission Vladislav Volman Vladislav.Volman@l-3com.com L-3 Applied Technologies/Simulation, Engineering, and Testing, San Diego, CA 92121, U.S.A.; Computational Neurobiology Laboratory, Salk Institute for Biological Studies, La Jolla, CA 92037, U.S.A.; and Center for Theoretical Biological Physics, University of California at San Diego, La Jolla, CA 92093, U.S.A. Laurel J. Ng Laurel.Ng@l-3com.com L-3 Applied Technologies/Simulation, Engineering, and Testing, San Diego, CA 92121, U.S.A. Diffusion imaging and postmortem studies of patients with mild trau- matic brain injury (mTBI) of the concussive type are consistent with the observations of diffuse axonal injury to the white matter axons. Mechani- cal trauma to axons affects the properties of tetrodotoxin-sensitive sodium channels at the nodes of Ranvier, leading to axonal degeneration through intra-axonal accumulation of calcium ions and activation of calcium pro- teases; however, the immediate implications of axonal trauma regarding axonal functionality and their relevance to transient impairment of func- tion as observed in concussion remain elusive. A biophysically realistic computational model of a myelinated axon was developed to investigate how mTBI could immediately affect axonal function. Traumatized ax- ons showed alterations in signal propagation properties that nonlinearly depended on the level of trauma; subthreshold traumatized axons had decreased spike propagation time, whereas suprathreshold traumatized axons exhibited a slowdown of spike propagation and spike propaga- tion failure. Trauma had consistently reduced axonal spike amplitude. The susceptibility of an axon to trauma could be modulated by the func- tion of an ATP-dependent sodium-potassium pump. The results suggest a mechanism by which concussive mTBI could lead to the immediate impairment of signal propagation through the axon and the emerging dysfunctional neuronal information exchange. 1 Introduction Mild traumatic brain injury (mTBI) of the concussive type is probably one of the most frequently documented types of brain trauma (Cassidy et al., Neural Computation 25, 2646–2681 (2013) c  2013 Massachusetts Institute of Technology doi:10.1162/NECO_a_00491