In the vertebrate retina, photoreceptors make synaptic contacts with two kinds of second-order neurones, bipolar and horizontal cells. The photoreceptors and horizontal cells respond to light with a sustained hyperpolarization, while bipolar cells (in this article only the depolarizing type will be considered) are depolarized.
When the chemical transmission was blocked by an application of Ca⁺⁺-antagonists, horizontal cells were hyperpolarized (to E_K) and bipolar cells were depolarized, both to the level produced by a strong illumination. In a Na⁺-free medium, both types of cells were hyperpolarized. It is inferred from these observation that the transmitter from photoreceptors is continuously released in the dark and the release stops by illumination. It is further suggested that the transmitter substance depolarizes horizontal cells by increasing g_Na, and hyperpolarizes bipolar cells by decreasing g_Na.