The electrical responses of axons were used to monitor the time-course of a change in the concentration of an ion species in the fluid bathing the axons in connectives of isolated cockroach nerve cords. Initial exposure of the connectives to sodium-deficient Ringer resulted in a depletion of extra-axonal sodium which was much slower than the restoration observed on return of the sodium Ringer. It is suggested that this asymmetry could result from a sodium reservoir which delays the initial decline. Subsequent net inward and outward movements of sodium ions were rapid and symmetrical. Unlike sodium ions, lithium ions were apparently unable to reach the axon surfaces following sodium depletion. In view of the similar properties of sodium and lithium ions in many biological systems it is therefore unlikely that the sodium movements were passive. Instead, the results support the idea of net sodium transport by the perineurial and/or glial elements.

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