Final answer:
Phototransduction in the retina involves the conversion of light into a neural signal, initiated by a shape change of retinal within photopigments such as rhodopsin (A). This leads to a series of cellular events, temporarily rendering the photoreceptor cell less sensitive to light, a state known as bleaching. Enzymatic processes eventually restore the photopigment's sensitivity to light, allowing the cell to function normally.
Step-by-step explanation:
Photopigments and Phototransduction
The process of phototransduction in the retina begins when light causes a shape change of retinal in photoreceptors. This change from the 11-cis-retinal to the all-trans-retinal conformation activates the opsin proteins, leading to the activation of a G protein. The G protein then alters the membrane potential of the photoreceptor cell, resulting in decreased neurotransmitter release into the outer synaptic layer. A phenomenon known as bleaching occurs when the retinal molecule cannot revert to the 11-cis-retinal form, rendering it temporarily unresponsive to light. After a bright flash, negative afterimages are often perceived due to the bleaching of a large number of photopigments. The photoreceptors eventually recover as enzymatic changes reverse the photoisomerization, enabling the retinal to respond to light once more.
Photopigments like rhodopsin, found in rods, consist of opsin and retinal. Upon light exposure, retinal isomerizes, leading to a cascade that hyperpolarizes the photoreceptor cell by closing Na+ channels. This is in contrast to other sensory neurons which typically become depolarized in response to stimuli. Cones, on the other hand, contain photopigments with specific sensitivity to different wavelengths of light corresponding to red, green, and blue, enabling color vision.