One may think that since we have two types of photoreceptors, we should have two types of bipolar cells as well. That's right: this is exactly how scientists classified these cells at first: rod and cone bipolar cells. These names were meant to tell us a lot about their connectivity – their connections with rods and cones.
As years passed scientists learnt that there still are variations in these 10 types of cells, so they started to create subtypes like type 3a, 3b and so on. At present they think that there are 14 different types of bipolar cells in the mouse retina, out of which only ONE type is a rod bipolar cell, all the rest belongs to the cone bipolar cell "family" (for review, see Euler et al., 2014).
|Behrens et al., 2016|
Although, that's still not all. The classification turned out to be imperfect since some cells break the rules! Cone bipolar cells, which were believed to contact cones exclusively, were found to have connections with rods (Behrens et al., 2016). Also, surprisingly, the only type of rod bipolar cell also turned out to have connections with some cones. Why is that? Well that's a question many scientists ask themselves worldwide. But trust me, science is evolving quickly, answers are coming soon!
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Thomas Euler, Silke Haverkamp, Timm Schubert & Tom Baden, Retinal bipolar cells: elementary building blocks of vision. Nature Reviews Neuroscience 15, 507–519 (2014) ; doi:10.1038/nrn3783
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