Axons connecting somata and axon terminals of luminosity-type horizontal cells in the turtle retina: receptive field studies and intracellular injections of HRP.

Luminosity (L)-type horizontal cell responses to small and large spots of light in the turtle (Geoclemys) retina were recorded intracellularly, and each cell so studied was filled with horseradish peroxidase (HRP) by ionophoretic injection. Following histochemical reaction of the HRP with diaminobenzidine (DAB), light microscopic inspection demonstrated that this type of horizontal cell has a cell body connected to a tuberous axon terminal by a fine axon. By examining the location of the cell in relation to the linear array of electrode penetrations in each preparation, it was possible to determine whether a given recording was made in the cell body or in the axon terminal. The size of the receptive field of the cell body was significantly smaller than that of the axon terminals. Although the center-to-center distances between the cell body and the axon terminal of some L-type horizontal cells were as short as 50 micron, the length of the intervening axon was invariably longer and was at least 250 micron. Unless signals are amplified by some active mechanisms during their propagation along the axon, it is unlikely that current generated in the cell body spreads to the axon terminal through the thin axon, or in the opposite direction. The difference in receptive field sizes of cell body and of axon terminal may therefore indicate that these two structural subunits are electrically isolated.