Morphology of physiologically identified bipolar cells in the retina of the tiger salamander, Ambystoma tigrinum.

Intracellular recordings of the light responses of bipolar cells were made in the isolated perfused retina of the larval tiger salamander by microelectrodes filled with a 1% solution of the enzyme horseradish peroxidase (HRP). Two classes of bipolar cell were identified in terms of their responses to luminous spots and annuli centered upon their receptive fields: on-center cells, which depolarized in response to a centered spot and hyperpolarized in response to a concentric annulus, and off-center cells, which responded with the opposite polarities. Physiologically identified cells were labelled with HRP by iontophoretic injection and prepared for light microscopy. Examples of each of the three classes of bipolar cell described in Cajal's study of the amphibian retina were found amongst the labelled cells. The only morphological feature found to correlate with the physiological classification was the stratum at which the axon of the cell arborized in the inner plexiform layer. The axons of on-center cells arborized in the more proximal sublamina b, while those of off-center cells arborized in the more distal sublamina a. This is consistent with earlier findings in the retinae of the cat and the carp.