Acute swelling of nodes of Ranvier caused by venoms which slow inactivation of sodium channels.

The venoms of the spider Phoneutria nigriventer and the scorpions Leiurus quinquestriatus and Centruroides sculpturatus cause acute, transient swelling of axons at nodes of Ranvier. The changes in the morphology of the node and paranode were studied in the mouse. Venom was injected into the sciatic nerve by means of a glass micropipette. After survival times ranging from 15 min to 3 weeks the nerves were examined by light and electron microscopy. The increase in nodal axoplasmic volume led within an hour to disruption of neurofilaments and microtubules, swelling of the paranodes and displacement of the terminal loops of myelin away from the node. Axonal calibre recovered within a few hours, but restoration of nodal width took several days and seemed to be accomplished by elongation and remodelling of the paranodal myelin. Occasional internodes were interrupted by node-like discontinuities in the myelin sheath. These developed within a few hours and persisted for the duration of the study.