Nad

less release

FIGURE 4 Steps in the phototransduction cascade of rods. (A) A rod photoreceptor in the depolarized state (blue) and in the hyperpolarized state (gray). Light changes 11-cis retinal to alltrans retinal, closes sodium channels, hyperpolarizes the rod, and decreases glutamate release. (B) Arrangement of molecules and components in the outer segment; four steps in the process of phototransduction are depicted: (1) In the dark-adapted state, cyclic guanidine monophosphate (cGMP) binds the sodium/calcium channel, allowing a continuous influx of sodium to keep the cell depolarized. (2) Light photoisomerizes the 11-cis retinal component of rhodopsin to all- trans retinal and allows rhodopsin to become meta-rhodopsin II. (3) Meta-rhodopsin II activates the G-protein transducin, which activates phosphodiesterase, which metabolizes cGMP and allows the sodium-calcium channel to close; closing the channel hyperpolarizes the rod (gray). (4) All-trans retinal diffuses away from the opsin and is carried by a series of retinal binding proteins to the retinal pigment epithelium (RPE), where isomerase converts it back to 11-cis retinal, which is carried back to the rod on retinal binding proteins where it binds to opsin and forms light-sensitive rhodopsin.

opsin

FIGURE 4 Steps in the phototransduction cascade of rods. (A) A rod photoreceptor in the depolarized state (blue) and in the hyperpolarized state (gray). Light changes 11-cis retinal to alltrans retinal, closes sodium channels, hyperpolarizes the rod, and decreases glutamate release. (B) Arrangement of molecules and components in the outer segment; four steps in the process of phototransduction are depicted: (1) In the dark-adapted state, cyclic guanidine monophosphate (cGMP) binds the sodium/calcium channel, allowing a continuous influx of sodium to keep the cell depolarized. (2) Light photoisomerizes the 11-cis retinal component of rhodopsin to all- trans retinal and allows rhodopsin to become meta-rhodopsin II. (3) Meta-rhodopsin II activates the G-protein transducin, which activates phosphodiesterase, which metabolizes cGMP and allows the sodium-calcium channel to close; closing the channel hyperpolarizes the rod (gray). (4) All-trans retinal diffuses away from the opsin and is carried by a series of retinal binding proteins to the retinal pigment epithelium (RPE), where isomerase converts it back to 11-cis retinal, which is carried back to the rod on retinal binding proteins where it binds to opsin and forms light-sensitive rhodopsin.

A second difference is that recycling of cone photopigments occurs within the cone and does not require involvement of the RPE.

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