Problem 11 - Entrance Test

This question delves into the specific molecular cascade of phototransduction. 1. Light absorption: A photon of light strikes rhodopsin (a pigment in rod cells), causing its retinal component to isomerize from 11-cis-retinal to all-trans-retinal. 2. Rhodopsin activation: This conformational change activates opsin, making rhodopsin an active enzyme. 3. G-protein activation: Activated rhodopsin activates a G-protein called transducin. 4. Enzyme activation: Activated transducin then activates a cGMP phosphodiesterase (PDE). 5. cGMP hydrolysis: PDE hydrolyzes cGMP (cyclic GMP) into 5'-GMP. 6. Channel closure: In the dark, cGMP levels are high, keeping cGMP-gated Na+ channels (and some Ca2+ channels) open, leading to a steady influx of positive ions and a depolarized state (around -40mV) with continuous glutamate release. When cGMP levels fall due to PDE activity in the light, these cGMP-gated cation channels close. 7. Hyperpolarization: The closure of these channels prevents the influx of Na+ and Ca2+ ions. This, coupled with the continued efflux of K+ ions through leak channels, leads to the hyperpolarization of the photoreceptor cell membrane (making it more negative, around -70mV). 8. Reduced neurotransmitter release: Hyperpolarization reduces the release of the inhibitory neurotransmitter (glutamate) from the photoreceptor onto the bipolar cells, thereby signaling the presence of light. Let's evaluate the options: A. Incorrect. Transducin activation leads to closure of Na+ channels, not opening. It is a G-protein but its activation is upstream of channel closure. B. Correct. The hydrolysis of cGMP by phosphodiesterase leads to a decrease in intracellular cGMP, which in turn causes the closure of cGMP-gated Na+ channels. This cessation of Na+ influx is the direct cause of hyperpolarization. C. Incorrect. The breakdown of rhodopsin is the initial step, but it doesn't directly activate a K+ channel to cause hyperpolarization. The primary channels involved are cGMP-gated cation channels. D. Incorrect. The photoreceptor hyperpolarizes and reduces its release of inhibitory neurotransmitter (glutamate). This reduction in inhibition allows certain bipolar cells to depolarize. This describes a subsequent event in the visual pathway, not the direct cause of photoreceptor hyperpolarization.