| Description |
Visual cells respond to stimuli placed in restricted regions of visual space called their visual receptive fields. Visual output to the brain, relayed by retinal ganglion cell axons, results from neural processing within the retinal layers. The turtle retina is used here as a model to study the structural basis underlying ganglion cell complex receptive fields. The specific aims are: (1) to identify and quantify the synaptic content of the inner plexiform layer, where amacrine, bipolar and ganglion cells interact; (2) to improve techniques that combine retrograde marking of ganglion cell somata with intracellular dye staining; (3) to identify the ganglion cell types that project to the optic tectum, where most complex ganglion cells are expected to project; (4) to identify and characterize the cholinergic amacrine cells, postulated as the excitatory driving input onto complex ganglion cells; (5) to identify and quantify the synaptic inputs to an individual complex ganglion cell. An HRP-stained morphological type, G15, shown to have a complex receptive field (directionally selective), is analyzed by serial section electron microscopic reconstruction and postembedding immunocytochemistry for the inhibitory neurotransmitters GABA and glycine. It can be concluded from this study that: (1) amacrines constitute the principal input to ganglion cells, whereas bipolar output is mainly onto amacrine cells. Amacrine to amacrine synaptic contacts greatly outnumber other types of synapses. Stratum 2 of the inner plexiform layer has the highest overall absolute number of synapses, a feature that appears to be evolutionarily conserved; (2) most of the small, medium and large morphological retinal ganglion cell types project to the dorsolateral optic tectum; (3) the turtle retina contains three types of choline acetyltransferase-immunoreactive amacrine cells distinguished according to cell body location in the inner nuclear or ganglion cell layers and dendritic stratification levels in the inner plexiform layer; (4) synaptic input to dendrites near the G15 cell body is heavy and predominantly from amacrine cells. GABAergic inputs are scarce and glycinergic inputs were not found, nor were bipolar inputs in the periphery of the dendritic tree. Strong asymmetries thus appear in the distribution of synapses, which may play an important role in the genesis of complex, directionally selective receptive fields. Thus, this dissertation makes a contribution towards understanding how complex visual receptive fields are structurally formed in the submammalian retina. |
