Is light receptor a dipole? Electrophysiological study in an arachnid

Retinal action potential (RAP) was recorded at different depths in the median eye of scorpion, using a glass micropipette electrode (tip diameter 2-3 mum). The amplitude of RAP was largest when the tip of the electrode touched the distal process of the receptor cell (RC). The polarity of RAP became reversed when the tip was inside the cell body. It is suggested that RC acted as a dipole with the outer segment being the current sink and the cell body, the source. This situation is similar to the findings in the light receptors of insects, crustaceans, molluscs and also in vertebrate retina.