Telodendrites of cone photoreceptors: structure and probable function.
Intracellular dye injection and compartmental modeling were used to analyze the structure and function of telodendrites of cones in the retina of the walleye. After identifying the spectral type of an impaled cone on the basis of its response to red and green light, horseradish peroxidase and/or Lucifer Yellow were injected for 1 to 25 minutes. In 38 of 58 recovered cells, dye spread into the telodendrites; so in many cases, the detailed pattern of the telodendritic arbor could be reconstructed from serial sections. Typically, five telodendritic processes, about 1 micron in diameter and 18 micron in length, radiated from the cone pedicle. A majority of the processes terminated at pedicles of neighboring cones. Some of the Lucifer Yellow injections provided evidence for electrical coupling between cones via telodendrites. Calculations from a compartmental model, based on the measured dimensions of cones and telodendrites, indicate that the signal arising in the inner segment spreads with little loss to the end of a telodendrite, whereas about half of the signal is lost in transmission from telodendrite to inner segment. Assuming that each contact point within the telodendritic network is an electrical synapse of 2,500 M omega, the model shows spatial interaction over a field of some 80 micron, which is comparable to that measured experimentally. Although our anatomical data indicate that orange- and green-sensitive cones may be interconnected via telodendrites, model calculations indicate that such connections do not appreciably distort the intrinsic spectral sensitivity of walleye cones. This outcome agrees with previous experimental results.
D000818:Animals; D003118:Color Perception / Q000502:physiology; D003712:Dendrites / Q000502:physiology; D004594:Electrophysiology; D005399:Fishes / Q000033:anatomy & histology*; D008959:Models, Neurological; D010786:Photoreceptor Cells / Q000166:cytology* / Q000502:physiology; D013194:Staining and Labeling; D009435:Synaptic Transmission; D014785:Vision, Ocular / Q000502:physiology