Fig. 1.

OFF and ON parasol cell light-evoked excitatory and crossover inhibitory synaptic conductances are largely rectified. (A) Dendritic morphology of OFF (left panel) and ON (right panel) parasol cells in the near retinal periphery (~5 mm eccentric from fovea). OFF and ON cells have relatively large somas and moderately branched, spine-laden dendritic trees that permit reliable targeting and identification in the in vitro retina. Scale bar = 50 µm (B) Intracellular voltage response (current clamp mode) from OFF (left) and ON (right) parasol cells to a large stimulus field (1-mm diameter 100% contrast, sinusoidally modulated at 5 Hz, two stimulus cycles shown; effective quanta ~1.1 × 10⁵ photons/s/µm²). The membrane potential depolarizes (spikes have been partially clipped to permit enlargement of membrane potential) and hyperpolarizes in-phase with light decrement and increment respectively for the OFF cell, and conversely for the ON cell. (C) Family of light-evoked postsynaptic currents for an OFF (left) and ON (right) parasol cell in response to the same stimulus as shown in B for ten holding potentials ranging from approximately −90 to +40 mV at ~15 mV intervals. Currents evoked near the chloride (−65 mV) and cation (0 mV) equilibrium potentials are indicated in blue and red, respectively. (D) Current-voltage (I–V) plots for time points at peak increment and decrement of stimulus indicated by red- and blue-filled circles below the current traces in (B). Reversal potential and slope were determined from linear fits (red and blue lines) to data points around the reversal potential. (E) Excitatory (blue) and inhibitory (red) synaptic conductances derived from slope and reversal potential of linear fits to I—Vs at 1.5-ms intervals across two stimulus cycles (see Materials and methods for details). Note that for both ON and OFF parasol cells, a rectified excitatory conductance underlies membrane depolarization during spiking phase of the light response and a very large and similarly rectified inhibitory conductance (“crossover inhibition”) is present during the hyperpolarizing response phase.