Selective induction of ultrastructural (neurofilament) compaction in axons by means of a new head-injury apparatus

A new weight-drop head-injury apparatus is described that can produce a momentary depression of predetermined depth at a predetermined site of the elastic calvaria of scalped young adult rats. In Wistar rats weighing about 200 g, a 0.75-mm deep calvaria depression immediately caused ultrastructural (neurofilament) compaction in many long axon segments, which were diffusely scattered among non-compacted axons in a well-defined area of cortical layers IV and V under the impact site. Apart from these morphological changes and swollen astrocytic processes in their vicinity, the brain tissue appeared non-impaired. The blood pressure, intracranial pressure, heart rate and respiration rate had returned to the normal range in 1 min. Diffuse axonal swelling caused by impaired axonal transport, ultrastructural compaction in neuronal soma-dendrite domains, impression fracture and subarachnoid or subdural hemorrhages were observed only in rats with a calvaria depression of 1 mm or more. All these features create favorable circumstances for study of various problems that are closely related to the ultrastructural (neurofilament) compaction in axons, such as the fate of the affected axons.