The sensitivity to plane polarized light and the electrical interactions of photoreceptors were examined with intracellular and extracellular microelectrodes in excised compound eyes of the crayfish. There are two types of photoreceptor: each photoreceptor cell responds best to polarized light when the electric vector of the light is oriented in one of two orthogonal directions. Seven cells, representing each type, are grouped together to form ommatidia. In each ommatidium, cells that are sensitive to the same orientation of the electric vector of polarized light are coupled electrically. Cells having orthogonal polarized light sensitivities are not coupled. Nearly all cells studied were sensitive to orange light. A few cells of both types were found that were sensitive to blue light. Blue sensitive cells were not coupled to orange sensitive cells. The photocurrents of both cell types produced negative extracellular potentials which can be greater than 10 mV when measured near the photoreceptive membranes within ommatidia. Evidence suggests that the extracellular potentials produced by one type of cell can effectively reduce the receptor potentials recorded in the other cell type. It is proposed that such a mutual non synaptic interaction can make a cell more sensitive to the orientation of polarized light than would be predicted from the cell's differential absorption of polarized light (i.e. its dichroic ratio).
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