The giant barnacle, Balanus nubilus, has three simple eyes, two lateral and one median. We have studied the convergence of the receptors of these ocelli by recording from the second-order cell (I-cell) and from the receptors' terminals. The I-cell's responses to illumination of the median and lateral eyes are similar in shape, dynamic range, and reversal potential, but the response to lateral input has a longer latency, a slower rise time, and a smaller amplitude. These differences primarily reflect the different voltage changes in the terminals of the decrementally conducting median and lateral receptors. Simultaneous recordings from the terminals of median and lateral receptors showed that the responses to light recorded in lateral terminals had a longer latency, a slower rise time, and smaller amplitude than signals in median terminals. The differences in the I-cell's responses to median and lateral input were essentially eliminated by stimulating the median and lateral ocellar nerves with extracellular suction electrodes positioned at equal distances from the receptors' terminals. The similarity of the I-cell's responses to median and lateral input suggests that lateral photoreceptors, like median receptors, contact the I-cell directly. No evidence was found for interaction between median and lateral receptors. Simultaneous fills of median and lateral receptors with cobalt showed minimal overlap between their terminal arbors. No voltage change was detected in the second receptor when the voltage in the first was changed with current pulses or when action potentials were elicited in the presence of tetraethylammonium ions. The absence of a detectable response in the terminals of one eye's receptors when the receptors of the other eye were stimulated with current or light suggests that there is no feedback from the I-cell to the receptors. Simultaneous illumination of the median and lateral eyes produced responses in the I-cell expected from two independent inputs. The first synaptic stage of the visual pathway in the barnacle is thus unusually simple, consisting of a small number of electrically isolated photoreceptors converging upon the same pair of second-order cells with no feedback interaction.
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