Second, many amacrine cells—perhaps a majority of the total number—perform find more some variety of vertical integration (the term is meant to contrast with lateral integration, as carried out by horizontal and wide-field amacrine cells). Only a small fraction of the 13 narrow field amacrine cell types found by MacNeil et al. (1999) were restricted to branching in narrow strata; the rest
communicate among several, sometimes all, of the layers of the IPL, like the cell shown in Figure 5. This means that they carry ON information into the OFF strata, and vice versa. This is termed crossover (for the crossing between ON and OFF layers) inhibition (because amacrine cells release GABA or glycine). It is the subject of very active investigation, which reveals a variety of interesting controls on the flow of information through the retina. The details are beyond the scope of this review, but an example is the finding
that some “excitatory” responses of ganglion cells to light are actually a release of amacrine mediated inhibition (Buldyrev et al., 2012; Demb and Singer, 2012; Farajian et al., 2011; Grimes et al., 2011; Molnar et al., 2009; buy JQ1 Nobles et al., 2012; Sivyer et al., 2010; Werblin, 2010). Third, most of the functions of amacrine cells are narrowly task-specific. An example is amacrine cell A17, a widely spreading neuron that places hundreds of electrotonically isolated synaptic boutons in contact with the output sites of the rod bipolar cell. At those points, the amacrine cell feeds back an inhibitory signal that improves the fidelity of information transmission by the rod bipolar
cell (Grimes et al., 2010; Sandell et al., 1989). This is the A17 cell’s primary, perhaps sole, task: and the A17 amacrine is in any case irrelevant to events that happen under daylight conditions. Another highly specialized amacrine cell, recently discovered in the ground squirrel retina, creates a specific receptive field property in a single type of ganglion cell (Chen and Li, 2012; Sher and DeVries, 2012). A blue-ON ganglion cell is well-known: it is excited by the blue-ON bipolar cell that selectively contacts blue cones. But electrophysiological recordings have encountered a blue-OFF ganglion cell, heptaminol inhibited when the stimulus lies at the short wavelength end of the spectrum. How can this happen if the only path through the retina is the blue-ON bipolar, carrying an excitatory signal? It turns out that a specific amacrine is driven directly by the blue-ON bipolar cell. The amacrine cell, like virtually all amacrine cells, is inhibitory to its postsynaptic partners. When excited by the blue-ON bipolar cell, this amacrine cell performs a sign inversion: it inhibits the ganglion cell upon which it synapses, thus creating a ganglion cell that is selective for blue stimuli and responds to a blue stimulus by slowing its firing—a blue-OFF ganglion cell. A final task-specific case is the role of the starburst amacrine cell.