Fig. 5.

Individual primary photoreceptor neurons responded to ultraviolet and visible light. Activity from each of three different primary photoreceptor neurons from eye 2 (A), eye 3 (B) and eye 2 (C) from three different leeches recorded using a sharp intracellular electrode. The resting potentials in these three examples were −35 to −40 mV. Arrowheads indicate stimulus onset. In all cells (including the three shown here), there were strong responses to UVR. (A) In the neuron shown here there was no response to red light at any luminosity, and apparently equivalent responses to blue, green and UV light at the highest luminosity. As luminosity decreased, the response to UVR remained more robust than the others. (B) In the neuron shown here there was a response to red at 0 OD (|log 100% transmittance|), but the response quickly fell off with decreased luminosity. At 0 OD the cell seemed to have an equal response to blue, green and UV wavelengths, and this response decreased with decreased luminosity. (C) In the neuron shown here there was a response to red light similar to that in the receptor shown in B. Yet, with decreased luminosity the response to green remained more robust than to either blue or UV wavelengths.