Localized desensitization of Limulus photoreceptors produced by light or intracellular calcium ion injection.

Spots of light were used to measure the light sensitivity of spatially separated regions of single Limulus photoreceptors. The desensitization caused by irradiating part of the cell was largest in the irradiated region. The desensitization caused by intracellular calcium ion injection was largest near the infection site. The spread of desensitization away from the injection site suggests that calcium ion can diffuse over neuronal dimensions, but that the effective rate of diffusion is not so high as to abolish calcium gradients. The results are compatible with the previously proposed hypothesis that a rise in the intracellular calcium ion concentration mediates light adaptation.