Abstract
Purpose :
There is a 1:1 relationship between cones and midget bipolar cells (Masri et al. 2020 JNeurosci 40:8132), providing a full-resolution representation of the visual image at the level of the bipolar terminals in the inner plexiform layer (IPL). The midget system is known for providing a 1:1 pathway from cones to midget retinal ganglion cells (RGC) to provide high-acuity vision in the central retina. However, as the major carrier of information from cones to the IPL, the midget bipolar cells are positioned to provide high spatial frequency contrast information to many other types of RGCs.
Methods :
Serial block-face scanning microscopy was used to reconstruct two ON midget bipolar cells, which were annotated completely for morphology and synaptic structures. Outgoing ribbon synapses and incoming conventional synapses were identified using classical electron microscopic criteria. Ribbon synapses were comprised of ribbons in the perimembranous cytoplasm of the bipolar cell and electron-dense, thick postsynaptic densities in the postsynaptic cells. Conventional synapses consisted of vesicles and associated material at the presynaptic membrane and an electron-dense postsynaptic density at the receiving cell’s membrane.
Results :
ON midget bipolar cell 22537 was found to have 34 conventional synapse inputs and 54 ribbon synapses, while cell 21873 was found to have 34 conventional synapse inputs and 64 ribbons (Figure 1). Because the ribbons usually synapse at dyads, each bipolar cell was associated with over 100 postsynaptic interactions. The majority of these were to cells other than the single midget RGC associated with each bipolar cell. Other partners included stellate varicose and starburst amacrine cells, parasol RGCs and many other, as of yet, unidentified contacts that were not midget RGCs.
Conclusions :
The present study of the central-inferior primate retina has revealed that midget bipolar cells in this region contact cells not only of the midget pathway but also provide high spatial frequency contrast information to a varietyof other circuits, including those involved in motion processing. Ongoing work will seek to identify the full complement of cells in synaptic contact with midget bipolar cells to fully understand the role of these remarkable cells in vision.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.