Horizontal cells of the turtle retina. II. Analysis of interconnections between photoreceptor cells and horizontal cells by light microscopy.

Criteria were established whereby the chief and accessory members of double cones, red-, green-, and blue-sensitive single cones, and rods could be distinguished from one another in 1-micrometer sections through the retinas of Pseudemys scripta elegans and Chelydra serpentina. Criteria included the presence of absence of an oil droplet, the size, color, and radial position of the oil droplet, the size and shape of the inner and outer segments, and the shape orientation of the connection between the perikaryon and synaptic terminal of each receptor cell. Using these criteria, the color-specific interconnections between horizontal cells and receptor cells were analyzed using 1-micrometer serial sections through Golgi-stained examples of horizontal cells of each type. The results of this analysis are: (a) H1 cells bodies contact both the red- and green-sensitive members of double cones, as well as red- and green-sensitive single cones. (b) H1 axon terminals contact only the red-sensitive chief members of double cones, red-sensitive single cones, and rods. (c) H2-type cells contact only green- and blue-sensitive single cones. (d) H3-type cells contact only blue-sensitive single cones. (e) H4-type cells contact only the green-sensitive accessory members of double cones. With the exception of H1 cell bodies, all horizontal cells generally contacted every appropriate receptor cell type within their receptive field. A model for the generation of complex horizontal cell light responses is proposed and discussed.