This video explains how the cells in the retina can produce different rates of firing in the retinal ganglion cells. Animation and voice-over by Laura Slusser.
Should specify in the beginning that the activation of the Off bipolar cell is in dark situation, because you are switching from light situation to dark situation without much context except the release of glutamate and the background.
Do horizontal cells contribute the opposing signal of both the center cones and surround cones, or only of the surround cones? Additionally, when we are considering an off-center bipolar cell with an off-center and on-surround, we maximize the depolarization of the cell. However, since the horizontal cells contribute the opposing signal to that of the surround, is it reasonable to say that the off-bipolars are maximally excited because of both the higher rate of Glutamate from the off-cones to the off-center bipolars AND from the opposite signal of the more inhibitory surround cones (contributed to by the horizontal cells of the surround)? Your video states that there is lowered inhibition by the horizontal cells in this case, but can that be thought of as increasing the excitability of the center cones?
Thanks a lot! Does the horizontal cells also release glycine? And do you have a reference for a close-up read on the receptive fields of bipolar cells? :)
Great question! Actually, the mechanism of the synapse from horizontal cells to rods/cones is not very well understood. One hypothesis is that the horizontal cells release GABA onto the photoreceptors (as shown in this video), but some scientists think it happens via ephaptic coupling or Pannexin 1, which is beyond the scope of this video. If you're looking for a good textbook, I'd recommend Principles of Human Physiology, 6th ed. by Cindy Stanfield. Have a great day, and good luck in all your endeavors!
Light-center means that the center photoreceptor is releasing less glutamate. Light-surround means that the surround photoreceptors are causing less inhibition. So we have less signal, but we also have less inhibition from the horizontal cell. For an ON bipolar cell, this results in a medium rate of firing for the RGC. From this information, you can't determine a discrete number for exactly how much it's firing, but the important part is how you compare it to different situations. Light-center, light-surround will produce less RGC firing than light-center, dark surround (maximum RGC firing). Light-center, light-surround will produce greater RGC firing than dark-center, light-surround (minimum RGC firing). I hope that helps, and have a great day!
Here's my attempt at this: - if it's an ON-bipolar cell, no light would mean the *center* photoreceptor would release glutamate and the bipolar cell would hyperpolarize because the cation channels would be closed (sending no signal to the ganglion cell); the *surround* photoreceptors would also release glutamate in the dark and the horizontal (interneuron) cells connecting them to the bipolar cell will inhibit their glutamate release, causing the bipolar cell actually to depolarize. So it would be a medium strength signal due to spatial summation. There will be both hyperpolarization of the center and depolarization of the surround. - if it's an OFF-bipolar cell, no light would create the opposite effect - the *center* would depolarize and the *surround* would hyperpolarize, resulting in a medium strength signal.
At least no paper i've read says that horizontal cells only inhibit cones in the center of their receptive field, so this model is incorrect according to what i know. But i can be wrong, not pretending to know it all or trying to be disrespectful
To better understand this subject you need to go back to the research conducted by Hubel and Wiesel during the second half of the 20th Century. That was the foundation that lead to an understanding for the visual system. Their initial paper was published back around 1960 when I was in Jr. High School. I became interested in the subject in the mid to late 1960s and built an "artificial retina" using an IBM 1620 Computer a couple of years later.
