What is the major effect of light on photoreceptor membranes?

The major effect of light on photoreceptor membranes is that they become less negative, which corresponds to an overall depolarization of the membrane potential. When light photons are absorbed by photopigments in the photoreceptors, a biochemical cascade is triggered, leading to the closure of ion channels that normally allow sodium ions to flow into the cell. Before exposure to light, photoreceptors are in a dark current state, maintaining a relatively depolarized membrane potential due to the influx of sodium ions.

With the exposure to light, this influx is halted, resulting in hyperpolarization. This means that the inside of the photoreceptor cell becomes less positive compared to its resting potential, which is typically around -40 mV, and it can reach values closer to -70 mV. This change in membrane potential is essential for signal transduction in the visual pathway, as it subsequently leads to a decrease in the release of neurotransmitters at the synaptic terminals, signaling the presence of light to bipolar and ganglion cells in the retina.

In summary, the correct answer illustrates the physiological response of photoreceptors to illumination, highlighting the critical role of phototransduction in visual processing.