Membrane origin of the fast photovoltage of squid retina.

When a bright light flash is absorbed by a small region in the outer segments of squid photoreceptors fixed in glutaraldehyde, a brief pulse of membrane current flows locally. The passive spreading of this current along the outer segments produces the photochemical component of the "early receptor potential." The source of the current lies electrically in parallel with the cell membranes and perhaps is located within them. Fixation with glutaraldehyde apparently does not reduce the resistance of the cell membrane to less than 5 percent of its value in live cells.