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John Robson, Laura Frishman; The a-wave of the Electroretinogram: Importance of Axonal Currents. Invest. Ophthalmol. Vis. Sci. 2011;52(14):692.
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To assess the potential importance of the membrane currents of the rod axon in determining the waveform of the a-wave of the ganzfeld flash electroretinogram (ERG) recorded from the dark-adapted eye.
The trans-retinal voltage that is generated by photocurrents produced by stimuli of different strengths has been simulated using an electrical model of mammalian rods based on the measurements of Hagins et al (1970) and Arden (1976) in rat retina.
Simulations suggest that in addition to the contribution from the photocurrent flowing in the sub-retinal space, a substantial fraction of the recorded ERG is generated by the current that flows to hyperpolarize the axon and synaptic terminal of the rod in the outer nuclear layer (ONL). This current is comprised of a component proportional to the membrane voltage of the rod inner segment (related non-linearly to the photocurrent because of the voltage-dependent conductance of the inner-segment membrane) together with a capacitive component proportional to the rate of change of this voltage. These two components together contribute an initial transient negative "nose" to the a-wave that becomes a prominent feature of ERG responses to strong stimuli. Although the relative importance of these components has still to be determined, recordings in various species of the a-wave, including the initial recovery towards the baseline, are consistent with a-wave simulations having an initial transient component primarily related to capacitive currents in the ONL. The existence of these transient components is sufficient to explain the "negative ERG" waveforms seen when postreceptoral responses are genetically deleted in rodents, pharmacologically blocked in nonhuman primates or severely compromised in humans (e.g. in congenital stationary night blindness, central retinal vein occlusion, quinine toxicity).
Simulations indicate that transient currents in the ONL give the recorded a-wave of the mammalian ERG an initial negative "nose" that is absent from the outer-segment photocurrent of isolated rods. Interpretation of the amplitude and timing of the a-wave should take into account substantial effects attributable to capacitive currents in the rod axons as well as to currents related to the membrane voltage of the rod inner segments.
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