Helga Kolb

1. General characteristics.

At every level of the retina there are reciprocal or feed-back loops in the circuitry so that certain neurons can interact laterally within the same layer, vertically from one layer to the other and indeed from the brain to the retina. The intra-layer feed-back loops are typically provided by neurons that use inhibitory neurotransmitters such as GABA, and have a function in sharpening the image by adding lateral inhibition or antagonism to receptive fields of the neurons, while the feed-back loops between the layers or from the brain are less clear in function. The latter loops tend to use neuromodulators as their transmitters and thereby have a more generalized effect on groups of neurons, or on the state of excitability of the neuron chains (adaptation for example).

2. Feedback in the outer retina.

As mentioned in previous chapters, it has been well demonstrated in turtle and fish retinas that cones receive an antagonistic reciprocal feed-back message from horizontal cells (Baylor et al., 1971), which serves to provide a restricted concentric receptive field for the individual cones. Evidence for feed-back synapses have been difficult to demonstrate either electrophysiologically or morphologically in the cones of the mammalian retinas. However, the rod axon terminals of HI horizontal cells ending in the rod spherules are seen to make small punctate chemical synapses, consisting a small cluster of vesicles at a single dense projection in the membrane, upon both the rod spherule (Fig. 2 and 3) and upon the rod bipolar cell dendrite (not illustrated) in human retina (Linberg and Fisher, 1988). A similar appearing small cluster of vesicles is occasionally seen in horizontal cell dendrites in the cone pedicle triads in human retina too (Fig. 1 and 3).

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Feedback Loops by Helga Kolb Webvision