Cell Depolarization

To address this question: “Propose a possible explanation on why the light caused depolarization of the off-center ganglion cell?” we first have to describe the structure of the center -surround in the receptive fields of retinal. In the receptive field, there are cone photoreceptors that communicate with On and Off center bipolar cell and finally On and Off center ganglion cells that carry the action potential to the cortex. When light comes to the retina (eyes) it causes closing of channels in the photoreceptor, there is no Ca2+ (calcium) influx, therefore, no neurotransmitter release (glutamate). While in the dark, is the opposite to light, channels are open, permitting the release of glutamate into the cells. Figure 1, briefly shows the process of when light and no light is
Glutamate can cause depolarization (excite the cell) or hyperpolarize (inhibit the cell) in the retina depending on the type of channels. On and Off center bipolar cells have distinct glutamate receptor, G-protein coupled metabotropic and ionotropic, respectively. Therefore, in the presence of light, in the on center bipolar cell causes depolarization and in the off center bipolar cell hyperpolarization. Leading to an increase of discharges of action potential in the on center, while in the off-center ganglion cell there is a decrease. If light were to hit the surround of the receptive field, the mechanism/pathway would be the same as the one that was just explained, just a switch of place. However, that does not explain or provide a possible explanation on why the light caused depolarization of the off-center ganglion cell as is shown in Figure 2. This figure, confuse students, but it gives you the idea on how to approach and answer the question. Firs the circuit is missing neurons, and important ones such as on center bipolar and ganglion