Connections of the small bipolar cells with the photoreceptors in the turtle. An electron microscope study of Golgi-impregnated, gold-toned retinas.

The connections between photoreceptors and the small bipolar cells with a Landolt club in the turtle retina (Pseudemys scripta elegans) were analyzed with the electron microscope after Golgi impregnation and gold toning. The dendritic tree of several bipolars was partially or completely reconstructed from continuous series of thin sections. All bipolar cells studied established both basal and invaginating junctions with cone pedicles; in invaginating junctions, their dendrites invariably ended as the central process of triads. Dendrites of this bipolar cell type make invaginating synapses, basal junctions, or a mixture of both. In turn, each pedicle found within the dendritic field of the bipolar cell contacts it with a mixture of invaginating and basal junctions; furthermore, each pedicle synapses with more than one dendrite from the same bipolar cell. All small bipolar cells examined also synapse with rod pedicles; the geometry of the connections and the types of synapses are the same for both rods and cones. The fact that small turtle bipolars can form both basal and invaginating contacts with a single cone indicates that the polarity of the postsynaptic response can not be predicted on the basis of the structure of the photoreceptor-to-bipolar cell synapse.