The organization of the outer synaptic layer in the salamander retina was studied electronmicroscopically in serial sections of tissue prepared by conventional techniques or stained by the method of Golgi. Rod cell pedicles make ribbon junctions on cone cell processes, and rod cell processes invaginate cone pedicles without otherwise making any specialized contact with them. Horizontal cells make ribbon and distal junctions with the photoreceptor cell pedicles; a single horizontal cell may contact both rods and cones. Bipolar cells were observed to make either ribbon or basal junctions with the photoreceptor cell pedicles; in addition, certain processes believed to belong to bipolar cells make both ribbon and basal junctions with the same or different pedicles. A single bipolar cell may make contact with both rods and cones. Horizontal cells synapse on bipolar cell dendrites and on certain unidentified processes which in turn are also presynaptic to bipolar cells. Ascending branches of these processes invaginate deeply the rod and cone pedicles without otherwise engaging them in any junction. Horizontal cell processes are linked by two kinds of junctions: close membrane appositions, and contacts analogous to the distal junctions between horizontal cells and rod pedicles.
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.
3. Values of input resistance were derived from the voltage displacement induced by constant current pulses in darkness or at the peak of the photoresponse. The input resistance following illumination was also calculated from the effect of steady polarizing currents on the amplitude of the photoresponse.4. In darkness, the input resistance of the rod cells is time-and voltagedependent, but the voltage-current relations of most cells have a linear region which includes the physiological limits of membrane potential. At the peak of the photoresponse, the input resistance (slope of the linear region of the v-i relations) is decreased.5. Cone cells show approximately linear v-i relations. As reported by previous authors, illumination increases the input resistance.6. These results support the current view that the cone photoresponse is the consequence of a reduction in the permeability of channels which in darkness shunt the membrane. In rods, however, it appears that the main effect of illumination is to increase the permeability of the membrane to ions for which the equilibrium potential is more negative than the membrane potential in darkness.
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