Final answer:
In an active dark current situation within photoreceptors, there is depolarization and a sustained release of neurotransmitters. Light exposure leads to hyperpolarization of these cells, which in turn decreases neurotransmitter release.
Step-by-step explanation:
When the dark current in a visual receptor is very active, it means that there is a high level of depolarization occurring within the photoreceptors. In the absence of light, photoreceptor cells are depolarized due to the influx of sodium ions, which leads to a steady release of the neurotransmitter glutamate. In this depolarized state, the photoreceptors release neurotransmitters onto bipolar cells. However, upon exposure to light, the photoreceptors become hyperpolarized due to the action of the enzyme phosphodiesterase, which converts cyclic GMP (cGMP) into GMP, resulting in the closure of sodium channels and a decrease in the release of neurotransmitters.