The electrical light response of theLimulus ventral nerve photoreceptor, a superposition of distinct components — Observable by variation of the state of light adaptation

Summary

Light-initiated two component receptor potentials and the transmembrane currents which generate them were recorded from ventral nerve photoreceptors ofLimulus polyphemus using intracellular microelectrodes. The magnitude and temporal separation of the two components could be manipulated by using pairs of light pulses of variable duration, repetition rate and intensity; the optimal values of these parameters for component separation varied among preparations, but two components could be obtained fromall ventral nerve photoreceptors tested.

Light adaptation reduces the magnitude of the second component,C ₂, much more strongly than the first component,C ₁, so thatC ₂ can be completely suppressed whileC ₁ persists with a partially reduced magnitude. On the other hand, receptor potentials elicited from dark adapted or moderately light adapted ventral photoreceptors by intense light pulses are dominated by theC ₂ component. The light-initiated currents recorded under similar conditions, but voltage-clamped, are also dominated by theC ₂ component. It is conceivable that the plateau of an electrical response to a prolonged light stimulus is dominated by theC ₁ component, which is less sensitive to light adaptation, whereas the transient phase of the response is dominated byC ₂.

The light-initiated currents which generate componentsC ₁ andC ₂ of the receptor potentials were recorded while the ventral photoreceptor membrane potential was voltage clamped at various levels. The results indicate that the occurrence of the two components in the transient is not dependent on membrane voltage and that the reversal potentials of the two components do not differ significantly on the average in our measurements (6 experiments). Both, chord and slope conductances are much greater (by a factor of ca. 5) forC ₂ thanC ₁.