The Pathobiology of Traumatically Induced Axonal Injury in Animals and Humans: A Review of Current Thoughts

Abstract

This manuscript provides a review of those factors involved in the pathogenesis of traumatically induced axonal injury in both animals and man. The review comments on the issue of primary versus secondary, or delayed, axotomy, pointing to the fact that in cases of experimental traumatic brain injury, secondary, or delayed, axotomy predominates. This review links the process of secondary axotomy to an impairment of axoplasmic transport which is initiated, depending upon the severity of the injury, by either focal cytoskeletal misalignment or axolemmal permeability change with concomitant cytoskeletal collapse. Data are provided to show that these focal axonal changes are related to the focal impairment of axoplasmic transport which, in turn, triggers the progression of reactive axonal change, leading to disconnection. In the context of experimental studies, evidence is also provided to explain the damaging consequences of diffuse axonal injury. The implications of diffuse axonal injury and its attendant deafferentation are considered by noting that with mild injury such deafferentation may lead to an adaptive neuroplastic recovery, whereas in more severe injury a disordered and/or maladaptive neuroplastic re-organization occurs, consistent with the enduring morbidity associated with severe injury. In closing, the review focuses on the implications of the findings made in experimental animals for our understanding of those events ongoing in traumatically brain-injured humans. It is noted that the findings made in experimental animals have been confirmed, in large part, in humans, suggesting the relevance of animal models for continued study of human traumatically induced axonal injury. Key words: diffuse axonal injury; delayed axotomy; cytoskeletal disruption; axoplasmic transport; deafferentation