today we're going to talk about bipolar cells and receptive fields in the retina let's start by looking at the photoreceptors shown here the first thing you should know is that photoreceptors release glutamate in the dark when light hits the photoreceptor it stops releasing glutamate remember this because a lot of physiology teachers and mcat style questions will try to trip you up on it the photoreceptor cells synapse with bipolar cells and horizontal cells for now let's look at the bipolar cell even without stimulation from the rods the bipolar cell will release a little bit of glutamate this will stimulate the retinal ganglion cell which sends signals to the brain the photoreceptor releases glutamate onto the bipolar cell and what happens next depends on the type of bipolar cell if it's an off bipolar cell this causes it to depolarize depolarization increases the release of glutamate on the retinal ganglion cell and the retinal ganglion cell fires faster however with an on bipolar cell the opposite happens when the photoreceptors release glutamate this causes the bipolar cell to hyperpolarize which reduces the release of glutamate onto the retinal ganglion cell therefore the retinal ganglion cell fires slower if it fires at all all right now let's take a look at the horizontal cell the horizontal cell receives input from the surround photoreceptors and sends the signal to the center photoreceptor it is an inhibitory interneuron so when it's stimulated by the surround photoreceptors it releases gaba onto the center photoreceptor this inhibits the release of glutamate which inhibits the signal now this is where things get interesting suppose you have an image where the center is bright and the surround is dark the surround photoreceptors release glutamate the center photoreceptor does not release glutamate this stimulates the horizontal cell which creates an inhibitory signal on the center photoreceptor this means that very little glutamate will reach the bipolar cell since we have an on bipolar cell the signal will be strongest when it's receiving the least amount of glutamate so the retinal ganglion cell will fire very quickly now let's look at the same configuration with an off-bipolar cell the opposite is going to happen since we have very little glutamate reaching the off bipolar cell the off bipolar cell will not send a very strong signal the retinal ganglion cell will fire very slowly now let's try a different image if we have a dark center with a bright surround the center photoreceptor will release glutamate while the surround photoreceptors will stop releasing glutamate this minimizes inhibition from the horizontal cell while maximizing the signal from the center photoreceptor we're going to get maximum signal on the bipolar cell and since this is an off-type bipolar cell the retinal ganglion cell is going to fire very quickly let's change things up one more time what if we have an image with a light center and a light surround well this is going to stop glutamate release from all of the photoreceptors we have little inhibition from the horizontal cell but we also have little stimulation from the center photoreceptor so the rate at which the retinal ganglion cell fires is going to be somewhere in the middle it's not going to be as fast as the dark center with the bright surround but it's not going to be as slow as the bright center with the dark surround for each of these situations the retinal ganglion cell fires at a different rate the brain interprets different rates as different patterns in real life it's a lot more complicated than this you'll have way more than three photoreceptors in a given receptive field but this gives you a basic understanding of how the receptive field in a retina works if you enjoyed this video please leave a comment and feel free to ask me any questions i'll see you in the next video and have a nice day