Horizontal cell responses in the retina of the larval tiger salamander

The responses to light of horizontal cells were recorded intracellularly in the retina of the larval tiger salamander. 2. All the units studied had a large summation area and were hyperpolarized by circles of light of any wave-length centred on the recording electrode, but two types could be distinguished according to the properties of their receptive fields. Type A units were hyperpolarized following illumination of any portion of their receptive field, while type B units were not hyperpolarized by illumination of their surround unless the centre was simultaneously illuminated, stimulation of the surround alone resulting in either a small depolarization or virtually no response. 3. Procion yellow injections showed that type A responses are recorded from thick and long processes not directly continuous with an identifiable cell body, while type B responses originate from the cell body of cells that send very fine and tortuous processes towards the receptors. The histological observations also suggested that the type A units represent expansions or swellings of one or more of the fine processes originating from the type B units. Therefore, it seems possible that both types of units are just different parts of a single kind of horizontal cell, and that a majority of the dye injections failed to stain them simultaneously because of the small diameter of the connecting process. 4. The large summation area of type A units can be explained, just as for horizontal cells in other retinae, by supposing that they are electrically coupled to other units of the same type. The receptive field properties of type B units, however, can only be partly explained by electrical coupling, and then only if the existence of voltage-dependent junctions is postulated. Instead, the reversal of the polarity of responses to an annulus of light during steady illumination of the centre, plus the available electron microscopic evidence, suggest that the effect of the surround on the type B units is due to a chemical synaptic impingement from the type A units.