Puller, C. | Search bioRxiv

bioRxiv2023

Spatial distribution and functional integration of displaced ipRGCs

The mammalian retina contains many distinct types of ganglion cells, which form mosaics to evenly tile the retina with cells of each type at each position of the visual field. It is well known that displaced retinal ganglion cells (dRGCs) exist with cell bodies in the inner nuclear layer, along with regularly placed RGCs with cell bodies in the ganglion cell layer. A prominent example of dRGCs are M1-type intrinsically photosensitive ganglion cells (ipRGCs) which exist in various species including humans and non-human primates. Little is known, however, about their spatial relationship with regularly placed ipRGCs. Here, we identified mouse ipRGC types M1, M2, and M4/sON[a] by immunohistochemistry and light microscopy to anatomically investigate the distribution of displaced and regularly placed cells. Reconstruction of immunolabeled dendritic mosaics from M1 and sON[a] RGCs indicated that dRGCs tiled the retina evenly with their regularly placed RGC partners. Multi-electrode array recordings revealed conventional receptive fields of displaced sON[a] RGCs which fit into the functional mosaic of their regularly placed counterparts. We further analyzed the RGC distributions across complete retinas. The analysis of regularly placed M1 ipRGCs and [a]RGCs revealed distinct density gradients where [~]16% and [~]8% occurred as dRGCs, respectively. The density distributions of dRGCs showed type-specific patterns which followed neither the global density distribution of all ganglion cells nor the local densities of corresponding cell types. Our study shows that the displacement of ganglion cell bodies into the inner nuclear layer occurs in a type-dependent manner, where dRGCs are positioned to form complete mosaics with their regularly placed RGC partners. Our data suggest that dRGCs and regularly placed RGCs serve the same functional role within their corresponding population of ganglion cells. Significance statementWe applied large-scale anatomical and electrophysiological experiments in mice to show that displaced intrinsically photosensitive retinal ganglion cells (ipRGCs) complete the mosaics of their regularly placed counterparts with their dendritic trees and receptive fields. Therefore, displaced ipRGCs likely serve the same functional role as corresponding regularly placed cells. The density distributions of displaced ipRGCs showed distinct, type-specific patterns. Interestingly, they followed neither the global density distribution of all ganglion cells nor the local densities of corresponding cell types.

neuroscience↗

bioRxiv2022

Saltatory axonal conduction in the avian retina

In contrast to most parts of the vertebrate nervous system, the ganglion cell axons in the retina typically lack any myelination. Ganglion cell axons of most species only become myelinated once they leave the retina to form the optic nerve. The avian retina is a well known exception in that ganglion cell axons are partly myelinated in the retinal nerve fiber layer. However, the functional and structural properties of myelination in the nerve fiber layer remain elusive. Here, we used large-scale multi-electrode array recordings in combination with immunohisto-chemistry and fluorescence microscopy of European quail and pigeon retinas to investigate myelination of retinal ganglion cell axons. Intraretinal myelination was accompanied by the formation of nodes of Ranvier. The internode length was positively correlated with the axon diameter. The variability of internode lengths along each axon was significantly smaller than across axons. Saltatory conduction of action potentials was observed in a large population of recorded cells. On average, myelinated axons had higher conduction velocities than unmyelinated axons. However, both groups showed a significant overlap at low velocities. The number of simultaneously active nodes was positively correlated with the conduction velocity. In contrast, the internode length and the time it took a node to activate were weak predictors for the conduction velocity. However, the conduction velocity was well described by the number of activated nodes, the internode length, and the activation time in concert. Significance StatementMyelination of axons serves saltatory signal conduction, which greatly decreases the time it takes for an action potential to travel along an axon. Retinal ganglion cell (RGC) axons, as part of the central nervous system, are usually devoid of myelin in mammals, whereas avian RGC axons are myelinated well before they enter the optic nerve. Using high resolution multi electrode arrays, we were able to image the saltatory propagation of a spike across an axon. Most axons with saltatory conduction were faster than non-saltatory axons. Surprisingly, a large number of saltatory axons had low conduction velocities. The signal conduction patterns were more diverse than expected. The velocity could be explained by the number of simultaneously activated nodes of Ranvier, the internode length and their activation time.

neuroscience↗