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 μm across). Transient photoresponses with a longer time course were recorded from cells branching in a thicker stratum of i. p. l. (up to 20 μm), 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 term s of a non-homogeneous distribution of bipolar synaptic inputs along amacrine cell processes.