Structure of the receptive fields of bipolar cells in the salamander retina

1. The receptive-field structure of bipolar cells in the salamander retina has been examined using isolated retinae from dark-adapted eyes. 2. Receptive-field mapping was carried out with a 25-microns diam spot of light whose wavelength and intensity was intended to stimulate rods rather than cones. 3. Both hyperpolarizing and depolarizing bipolar cells showed receptive fields having a single central point of maximum sensitivity from which sensitivity declined radially. Antagonistic surrounds could not be demonstrated using a small spot of light. 4. The diameter of receptive fields was found to vary between 374 and 662 micron, consistent with a single bipolar cell being effectively connected to 323-1,275 rods. 5. Lucifer yellow injections of bipolar cells revealed dendritic arbors whose greatest dimensions varied between 43 and 70 microns, consistent with a direct synaptic connection of between 10 and 24 rods to each bipolar cell. 6. We rule out signal spread within the rod network, extensive lateral ramification of rod process, nonlinearity of synaptic transmission, and light scatter, as possible explanations of large bipolar cell receptive fields. It seems likely, instead, that signals are extensively shared between bipolar cells.