1. It has been found that the efflux of sodium from intact penultimate connectives of the ventral nerve cord of the cockroach, Periplaneta americana, can be represented by three components with half times of approximately 30 sec, 100 sec and 16 min.

2. The rate at which sodium escapes in the fast component is correlated with the height of the extraneuronal potential developed when a connective is exposed to a high-potassium bathing medium, the slower the rate of sodium escape the larger being the size of the extraneuronal potential. This gives support to the hypothesis that the extraneuronal potential is a diffusion potential brought about by restriction to ion movement imposed by the tight junctions at the inner ends of the perineurial clefts.

3. De-sheathing a connective greatly speeds up the rate of sodium efflux, the fast component being the most affected.

4. These results can be explained in terms of a three-compartment system, the properties of which are affected by stretching and de-sheathing.

5. It is concluded that the movement of ions is regulated at different levels of the C.N.S. but that the peripheral barrier (which is responsible for the production of the extraneuronal potential) may be the most important one in non-steady-state conditions.

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