Description:
<jats:p>Photoreceptors in the isolated turtle retina of two
species of turtle, <jats:italic>Chelydra serpentina</jats:italic> and
<jats:italic>Pseudemus scripta elegans</jats:italic>, were penetrated with double-barrel
electrodes. Physiological responses were recorded through
one barrel and Neurobiotin tracer was injected from the
other. Intracellular injection of Neurobiotin revealed
patterns of tracer-coupled photoreceptors. Both the patterns
of tracer coupling and the electrophysiology suggest a
high degree of specificity of connections. Rods seem to
be coupled only to rods and green and red cones seem to
be coupled to cones of the same spectral type. Receptive-field
profiles, measured with a thin, sharply focused slit of
light, often had well-defined peaks and troughs in sensitivity.
We have taken advantage of this observation and used the
position of a peak in sensitivity to locate the position
on the retina of a coupled cell. In one rod, it was possible
to correlate physiological and morphological data and to
show that the peaks in the physiological receptive field
occurred at positions on the retina where there were dye-coupled
cells. This provides direct evidence that gap junctions
produce the physiological coupling between rods.</jats:p>