Multiple light‐evoked conductance changes in the photoreceptors of Hermissenda crassicornis

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Abstract
Light responses were recorded from the photoreceptors of H. crassicornis. The response to a flash is a complex potential change involving an initial depolarization, a hyperpolarization and a depolarizing tail. None of the response phases are due to synaptic interactions. Polarization of the membrane by extrinsic current indicates that 3 separate conductance changes are associated with the response. The initial depolarization and hyperpolarization are accompanied by conductance increases and the tail with a conductance decrease. The initial depolarization has a positive reversal potential and the hyperpolarizing and tail phase a reversal voltage more negative than resting potential. The different processes that give rise to the conductance changes have similar spectral sensitivities but are affected unequally by light adaptation. Strong light adaptation reduced the depolarizing phases more than the hyperpolarizing phase, so that following an adapting stimulus the cell responded to illumination with a pure hyperpolarization (isolated hyperpolarization). Removal of external Na+ greatly reduced the initial depolarization. In Na+-free sea water the cell responds to dim flashes with a slow depolarization (isolated tail) that involves a conductance decrease and has the same reversal potential as the hyperpolarizing response recorded from light adapted cells. The amplitude of the isolated hyperpolarization and tail varied inversely with the external K+ concentration. In Hermissenda photoreceptors light initiates processes that result in 3 distinct permeability changes. Following a brief flash there is: a rapid and transient increase in Na+ permeability responsible for the initial depolarization, a less rapid increase in K+ permeability responsible for the hyperpolarizing phase and a delayed decrease in K+ permeability that gives rise to the depolarizing tail.