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Figure 5.22. A light-evoked efflux of K+ occurs in the inner and outer plexiform layers of the retina (right side of schematic). K+ enters the Müller cell at these sites and exits via high conductance K+ channels at the endfoot, thereby establishing a current source at the vitreous surface, and current sinks in the OPL and IPL. These spatial buffering currents (solid lines), carried by K+ intracellularly and by Na+ and Cl- extracellularly, are thought to generate the ERG b-wave. Note that, on this view, the current pathway underlying the b-wave potential is independent of currents generated by neurons engaged in the transmission of signals through the retina (line drawings on the right) (Newman, 1985). (Copyright 1985, Elsevier Science, reprinted with permission,)

Figure 5.22. A light-evoked efflux of K+ occurs in the inner and outer plexiform layers of the retina (right side of schematic). K+ enters the Müller cell at these sites and exits via high conductance K+ channels at the endfoot, thereby establishing a current source at the vitreous surface, and current sinks in the OPL and IPL. These spatial buffering currents (solid lines), carried by K+ intracellularly and by Na+ and Cl- extracellularly, are thought to generate the ERG b-wave. Note that, on this view, the current pathway underlying the b-wave potential is independent of currents generated by neurons engaged in the transmission of signals through the retina (line drawings on the right) (Newman, 1985). (Copyright 1985, Elsevier Science, reprinted with permission,)

the intracellular responses of Müller cells, Wen and Oakley (1990) found that 200 ^M Ba2+, which blocks K+ channels in Müller cells, reduced both the ERG b-wave and the Müller cell response. However, it did not suppress the K+ increase within the OPL that is thought to reflect the activity of depolarizing bipolar cells (Fig. 5.23A). These findings and those of related

Figure 5.21. A. Intracellular potential recordings from mudpuppy (Necturus) Müller cells show nearly identical latencies to the ERG b-wave at all stimulus intensities, but as expected, lack the a-wave and oscillatory potentials that appear in the ERG traces. However, the Müller cell responses do not replicate precisely the kinetics of the b-wave, at the higher intensities, the rise time appears slower and the potentials are more sustained (Miller and Dowling, 1970). (Copyright 1970 American Physiological Society, reprinted with permission.) B. Intensity-response data obtained with simultaneous recording of the ERG b-wave and the distal increase in K+. The two vertical axes have been adjusted arbitrarily, but scaling involved computations based on the relationship between the K+ increase and the resultant b-wave (Kline et al., 1985). (Copyright 1985 Elsevier Science, reprinted with permission.)

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