The Photoresponses of Structurally Identified Amacrine Cells in the Turtle Retina

Intracellular recordings were obtained from amacrine cells afterwards identified morphologically by horseradish peroxidase injection. There is a correlation between the time course of the photoresponses and the distribution of the cell processes across the inner plexiform layer (i.p.l.). Cells producing the shortest duration, transient `on-off' photoresponses branched in a single, narrow stratum of the i.p.l. (3-7 μ m across). Transient photoresponses with a longer time course were recorded from cells branching in a thicker stratum of i.p.l. (up to 20 μ m), or from bistratified cells. Amacrine cells producing sustained centre-on or centre-off photoresponses were radially diffused across the whole i.p.l.; therefore this type of photoresponse need not be associated with a specific cellular stratification within the i.p.l. It is concluded that the two main functional types of amacrine cell, i.e. transient on-off and sustained centre-on and centre-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.