1. Membrane properties of the muscle fibre were studied in twitch motor system of sea-water elasmobranch (Taeniura lymma, Himantura uarnak and Pastinachus sephen) and teleost fish (Periophthalmodon barbarus, Tetradon immaculata, Hemiramphus welsby, Parexocoetus brachypterus and Conger labiatus).2. The resting potential of the elasmobranch fibre is mainly determined by the Cl- concentration difference between inside and outside the membrane whereas the K+ conductance is the determining factor in teleost fibres.3. The resting membrane of the elasmobranch fibre is permeable not only to Cl- ions but also several other anions (Br-, I-, NO3-, SCN-, ClO4-, ClO3-) of large limiting conductivities in the aqueous solution.4. The spike potential of the elasmobranch fibre always shows a significant overshoot in normal saline while no significant overshoot is generally found in teleost fibres.5. In both elasmobranchs and teleosts the spike is produced by the permeability increase of the membrane to Na+ ions and is effectively suppressed by tetrodotoxin at a concentration of 0·5-1·0 × 10-7 g/ml. of the external solution with one exception, i.e. the Na+ spike of Tetradon fibre is not suppressed by the toxin even when the concentration is above 5 × 10-4 g/ml.
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