Electrical Signs of Information Flow in Photoreceptors

showed that single photons can excite dark-adapted human retinal rods, the mechanism which converts the tiny energy of a light quantum into a sensory message has been a puzzle. One of the most interest-ing parts of this puzzle is the system of communica-tion used by rods and cones to send messages indicating the arrival of photons in their outer seg-ments to their synapses with the visual neurons. It is widely believed that this system is at least partly an electrical one like that of nerve and muscle cells, because photoreceptors look like neurons and because only an electrical transmission system is thought o be fast enough to account for the short latency of vision. But it has been hard to demon-strate electrical transmission in photoreceptors experimentally; the cells are usually too small to yield good intracellular recordings of their mem-brane potentials, and external signs of electrical activity in them is often masked by cuffents from nearby neurons. Thus it seems useful to ask: Can the hypothesis of electrical transmission i photo-receptors be tested in other ways which make fewer demands on existing electrophysiological procedures? This paper is concerned with such an alternate approach based almost entirely upon analyzing the currents and voltages in the intercellular spaces around photoreceptors. In the first part of the paper, three important properties which the hypothesized signal-transmitting currents must possess if elec-trical transmission along the cells is to occur are deduced from general considerations of the struc-ture of photoreceptors and their efficiency as photon counters. Then, in the second part, experimental tests of the three deductions are applied to the receptor current of the squid retina. It is concluded that the receptor current has the correct magnitude, spatial distribution, and quantum efficiency to per-mit it to act as an information-carrying agent along squid photoreceptors.