Connections between photoreceptors and horseradish peroxidase-injected bipolar cells in the carp retina.

We describe two physiologically and morphologically distinct types of on-center bipolar cell in the carp retina. These cells can be distinguished from one another in the following respects: (1) "type I" cells respond to moderate light intensities with a transient depolarization followed by a plateau, while the response of "type II" cells is approximately rectangular in shape; (2) latency of type II cell responses is shorter than that of type I cell responses; (3) mean diameter of type II cell dendritic fields (114 micron) is larger than that of type I cell dendritic fields (65 micron) in the outer plexiform layer; (4) mean diameter of type II cell bodies (7 micron) is smaller than that of type I cell bodies (9 micron); (5) axons of type II cells show extensive arborizations (51 micron), while those of type I cells show a single swelling with a few short collaterals (31 micron) in the proximal half of inner plexiform layer. HRP-injected on- and off-center bipolars were examined by electron microscopy. The dendrites of these bipolar cells invaginate into rod spherules and often occupy the central position of the triad. The dendrites of such bipolar cells also invaginate into cone pedicles, but in quite distinct patterns. Type I on-center bipolars do not send processes to the synaptic ribbons. In contrast, off-center bipolars and type II on-center bipolars send processes to the synaptic ribbons. The dendrites of off-center bipolars occupy the central position of the triad. Although type II on-center bipolar dendrites approach the ribbon very closely, there is usually a process from an off-center bipolar interposed between them.