The interneuron that is postsynaptic to barnacle photoreceptors (the inverting, or I-cell) receives similar synaptic input from the receptors onto its two distinct and separate arbors. We compared the spread of light-evoked synaptic potentials from the proximal and distal arbors with the cell's soma in order to ascertain how well voltages spread in this cell. The proximal and distal arbors are connected by a commissural process ~200 μm in length and 2 μm in diameter. Voltages spreading from the distal arbor along this process were 20-60% of their original value, and delayed 15-20 ms, when recorded at the cell's soma. The reversal potential of the receptor's input to the distal arbor, determined by injecting current into the soma, appeared substantially negative (-150 mV) to that of the identical input to the proximal arbor (-80 mV). Assuming identical reversal potentials in the two arbors, this difference indicates that more current must be injected into the I-cell's soma to change the voltage of the distal arbor to a given potential than to change that of the proximal arbor to the same potential. Comparison of input from a lateral eye to the ipsilateral arbor with that from the median eye to the same arbor indicates that these two inputs are at an electrically equivalent distance from the soma. Uneven illumination of the eyes may, through local conductance changes, cause the arbors to function independently, even to the point where one arbor may depolarize when the other is hyperpolarized.
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