Effects of Latrodectus spider venoms on sensory and motor nerve terminals of muscle spindles

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Abstract
The effects of the venoms of the spiders L. mactans tredecimguttatus (black widow) and L. mactans hasselti (red black) on the sensory nerve terminals in muscle spindles were studied in the mouse. A sublethal dose of venom was injected into tibialis anterior and extensor digitorum longus muscles of 1 leg. After survival from 30 min-6 wk, muscles were examined in seiral paraffin sections impregnated with Ag or by EM. Sensory endings became swollen, some within 30 min, while over the next few hours there was progressive degeneration of annulospiral endings. By 24 h every spindle identified by light microscopy or EM was devoid of sensory terminals. Degenerated nerve endings were taken up into the sarcoplasm of intrafusal muscle fibers. Regeneration of sensory axons began within 24 h, new incomplete spirals were formed by 5 days and by 1 wk annulospiral endings were almost all normal in appearance. Intrafusal motor terminals underwent similar acute degenerative and regenerative changes. Evidently, intrafusal sensory and motor terminals were equally affected by Latrodectus venoms. Sensory nerve fibers possessed a capacity for regeneration equal to that of motor fibers and reinnervated intrafusal muscle fibers close to their original sites of innervation.