|
pubmed:abstractText |
1. Spectral sensitivity measurements using the suction electrode technique reveal three types of cone in the retina of the tiger salamander, showing maximum sensitivity at wavelengths 610 nm (red-sensitive cone), 444 nm (blue-sensitive cone) and below 400 nm (UV-sensitive cone). 2. The absolute sensitivities of red- and blue-sensitive cones to flashes of optimal wavelength are 0.022 and 0.33 pA photon-1 micron 2 respectively. 3. The time-to-peak of the dim flash response and the recovery of membrane current after a flash of any intensity are fastest in red-sensitive and slowest in blue-sensitive cones. 4. In blue- and UV-sensitive cones the flash response peaks progressively earlier as the flash strength is increased, as in rods. In red-sensitive cones, however, bright flash responses take longer to peak than dim flash responses. 5. In all three cone types, voltage clamping at -40 mV reduces the time-to-peak of the response to a bright flash, showing that the rising phase of the bright flash response is normally limited by the time constant of the cell. Under voltage clamp, all cones show a decrease in time-to-peak with increasing flash intensity. 6. Voltage clamping red-sensitive cones reveals two components of the rising phase of the response to a bright flash. Most of the current is rapidly suppressed by a bright flash, and represents the closure of light-sensitive channels. The residual current decays with a mean time constant of 20 ms, and is probably attributable to the decline of electrogenic Na(+)-Ca2+, K+ exchange. The amplitude of this exchange current suggests that the proportion of the dark current carried by calcium ions is greater in red-sensitive cones than in rods of the same species. 7. In UV-sensitive cones, a prominent oscillation of light-sensitive current is observed during the recovery from flashes of intermediate intensity. A similar, but slower and less prominent oscillation is usually seen in blue-sensitive cones. 8. When a red-sensitive cone is voltage clamped an oscillation similar to those in the other two cone types is revealed. An underswing of up to 2 pA is also observed after recovery from intermediate or bright flashes in the majority of red-sensitive cones, and voltage clamping increases the amplitude of this underswing.(ABSTRACT TRUNCATED AT 400 WORDS)
|
