On and off pathways through amacrine cells in mammalian retina: the synaptic connections of "starburst" amacrine cells.

The neural architecture of on and off pathways in mammalian retina is described, including the development of ideas leading to an understanding of the bisublaminar organization of the inner plexiform layer of the retina which supports these two pathways. The complexities of bipolar cell contributions are contrasted with the relative simplicity of ganglion cell organization with regard to bisublaminar architecture, and a key role is described for internuncial amacrine cells as specific targets for bipolar cells. Two very different kinds of amacrine cell are considered and compared, both of which mediate bipolar input to ganglion cells. These are the rod (type II) amacrine cell, and the more recently discovered "starburst" amacrine cell, which is apparently cholinergic in function. As different as the wide-field starburst amacrine cells are from the narrow-field rod amacrine cells, they share important features. Both are interposed between bipolar and ganglion cells, and both have segregated regions of presynaptic boutons. They differ, however, in that rod amacrines may perform more specific functions related to receptive field center organization, while the functional role of starburst amacrines may be unrelated to receptive field properties of ganglion cells. The mirror-symmetry of type a and type b (off and on) starburst amacrine cells is described together with their synaptic circuitry. In contrast to the rod amacrine cell the output of starburst amacrines is exclusively to ganglion cells. Others have proposed a dual function for acetylcholine (ACh) in the retina. A unifying hypothesis is briefly sketched here which relates the pharmacology of ACh and the dendritic stratification of starburst amacrine cells to the form and function of ganglion cells. It is proposed that the amount of generalized synaptic excitation received from ACh/starburst amacrine cells by a particular type of ganglion cell is largely a function of co-stratification of the ganglion cell's dendrites with the distal boutons of starburst amacrine cells.