Effects of Phoneutria nigriventer spider venom on mouse peripheral nerve

A physiological and morphological study has been made of the effects of the venom of the spider Phoneutria nigriventer on peripheral nerve and skeletal muscle of the mouse. Venom was injected either into the sciatic nerve, via a glass micropipette, or into the calf muscles. Repetitive firing of nerve fibres commenced within seconds of injection and caused fasciculation of muscles and irregular trains of end-plate potentials. These physiological disturbances were associated with swelling of the nodal and paranodal axoplasm and the development of vacuoles in the periaxonal space. The segments of internode between the vacuoles became attenuated, with an increase in the density of axoplasmic organelles and a reduction in axonal calibre. The repetitive firing ceased after 1-3 h with development of conduction block. Both the physiological and the morphological abnormalities gradually resolved and by 24 h nerve conduction and morphology had largely returned to normal. It seems likely that these short-lived abnormalities are a consequence of the influx of sodium ions at the nodes of Ranvier, caused by the action of the venom on sodium channels.