Contacts between receptors and electrophysiologically identified neurones in the retina of the larval tiger salamander.

1. Following the intracellular recording of bipolar and horizontal cell responses, each unit was injected with horseradish peroxidase and a histochemical staining used to identify it at the level of the light and electron microscopes.2. Centre-depolarizing bipolar cells made contact with rods and cones at basal and ribbon junctions, the latter being fewer. Centre-hyperpolarizing bipolar cells made the same types of contacts with the receptors, but ribbon junctions predominated.3. It appears, therefore, that there is no fixed relationship between the sign of synaptic transmission from receptors to bipolar cells and the junctional features revealed by present methods for electron microscopy of tissue sections. Accordingly, the reason for the existence of more than one type of contact between receptors and bipolar cells remains to be determined.4. Light microscopy of the peroxidase-injected horizontal cells gave further support to the notion that type B responses are recorded from the cell body, and type A responses from the axon terminal, of a single type of horizontal cell. Electron microscopy showed that the processes (dendrites) originating from the cell body make ribbon and distal junctions with rods and cones, just as shown before for the axon terminals.5. As a result of these observations, it is possible to exclude one of two alternative hypotheses previously proposed to account for the properties of the surround responses recorded from the horizontal cell bodies.