The photoresponses of structurally identified amacrine cells in the turtle retina

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Abstract
Intracellular recordings were obtained from [Pseudemys scripta elegans] amacrine cells afterwards identified morphologically by horseradish peroxidase injection. There was a correlation between the time course of the photoresponses and the distribution of the cell processes across the inner plexiform layer (IPL). Cells producing the shortest duration, transient on-off photoresponses branched in a single, narrow stratum of the IPL (3-7 .mu.m across). Transient photoresponses with a longer time course were recorded from cells branching in a thicker stratum of IPL (up to 20 .mu.m), or from bistratified cells. Amacrine cells producing sustained center-on or center-off photoresponses were radially diffused across the whole IPL; therefore this type of photoresponse need not be associated with a specific cellular stratification within the IPL. The 2 main functional types of amacrine cell, i.e., transient on-off and sustained center-on and center-off, are subject to different structural organization of inputs than are the homologous physiological types of ganglion cells in this species, in the cat and in the carp. In a summary diagram the observed characteristics of the photoresponses are tentatively explained in terms of a non-homogeneous distribution of bipolar synaptic inputs along amacrine cell processes.