[Sodium and potassium content of intrafusal muscle fibers and their resting membrane potential in different ionic media].

X-ray microanalysis revealed a high sodium concentration in intrafusal fibers of the frog-muscle whereas it appeared to be normal in extrafusal ones. Potassium concentration was practically the same both in intra- and extrafusal fibers which is in good agreement with the data obtained earlier with other techniques. Electrophysiological experiments demonstrated a similar behaviour of the resting MP found in both kinds of fibers in solutions with different ionic composition. The concentration of free sodium ions seems to be the same in intra- and extrafusal muscle fibers. The intrafusal fibers have believed to suggest a high amount of immobilized sodium.