The G protein (Fig. 8.6B) activates cyclic guanosine monophosphate phosphodiesterase, which is associated with the discs containing rhodopsin. This enzyme hydrolyzes cGMP to 5′-GMP, reducing the cGMP concentration in the cytoplasm of the rod. Consequently, the decrease in cGMP causes the closure of the cGMP-gated cation channels, resulting in hyperpolarization of the rod cell membrane and a decrease in the release of neurotransmitters. Hence, cGMP functions as a "second messenger" that translates the absorption of a photon by rhodopsin into a change in the membrane potential.

Overall, in all photoreceptors (cones experience a process similar to rod transduction), the absorption of light energy leads to (1) hyperpolarization of the photoreceptor and (2) a reduction in the release of neurotransmitters. Due to the short distance between the site of transduction and the synapse, modulation of neurotransmitter release occurs without the generation of an action potential.

Adaptation refers to the retina's ability to adjust its sensitivity according to the surrounding light conditions. This enables efficient operation of the retina in a wide range of lighting conditions and involves switching between the cone and rod systems for bright and low-light conditions, respectively.

CHAPTER 8 The Special Senses • Fig. 8.6 A depicts the flow of current in a rod in the dark, maintained by the Na+, K+ pump that keeps the rod depolarized. B illustrates the sequence of events that follow the absorption of light, leading to the reduction of cGMP. In the dark, cGMP keeps the Na+ channels open, but light absorption causes the closing of these channels and hyperpolarization of the rod. cGMP represents cyclic guanosine monophosphate; GC is guanylate cyclase; GTP is guanosine triphosphate; PDE is phosphodiesterase; Rh is rhodopsin; T is transducin.

Rhodopsin contains a chromophore called retinal, which is the aldehyde of retinol or vitamin A. Retinol is derived from carotenoids such as β-carotene, the orange pigment found in carrots. Humans cannot synthesize retinol and instead obtain it from food sources. Individuals with severe vitamin A deficiency experience "night blindness," a condition characterized by defective vision in low-light situations.

The exceptional sensitivity of rods, capable of signaling the capture of a single photon, is enhanced by an amplification mechanism where the photoactivation of only one rhodopsin molecule can activate hundreds of transducin molecules. Additionally, each phosphodiesterase molecule hydrolyzes thousands of cGMP molecules per second. Similar events occur in cones, but the membrane hyperpolarization occurs much more rapidly than in rods and requires thousands of photons.
Fig. 8.6B ). This G protein, in turn, activates cyclic guanosine monophosphate phosphodiesterase, which is associated with the rhodopsin-containing discs, hydrolyzes cGMP to 5′-GMP, and lowers the cGMP concentration in the rod cytoplasm. The reduction in cGMP leads to closing of the cGMP-gated cation channels, hyperpolarization of the rod cell membrane, and a reduction in the release of neurotransmitters. Thus cGMP acts as a “second messenger” to translate the absorption of a photon by rhodopsin into a change in membrane potential.
In sum, in all photoreceptors (cones undergo a process analogous to that described for rod transduction), capture of light energy leads to (1) hyperpolarization of the photoreceptor and (2) a reduction in the release of neurotransmitters. Because of the very short distance between the site of transduction and the synapse, the modulation of neurotransmitter release is accomplished without the generation of an action potential.
Adaptation refers to the ability of the retina to adjust its sensitivity according to ambient light. This ability allows the retina to operate efficiently over a wide range of lighting conditions, and it reflects a switching between the use of the cone and rod systems for brightand low-light conditions, respectively.
CHAPTER 8 The Special Senses •Fig. 8.6 A, Drawingofarodwiththeflowofcurrentinthedark.WiththeassistanceoftheNa+,K+pump,therodiskeptdepolarized.B, SequenceofthesecondmessengereventsthatfollowtheabsorptionoflightthroughthereductionofcGMP.BecausecGMPmaintainsopenNa+channelsinthedark,theresultsoflightabsorptionaretheclosingoftheNa+channelsandhyperpolarizationoftherod.cGMP,cyclicguanosinemonophosphate;GC,guanylatecyclase;GTP,guanosinetriphosphate;PDE,phosphodiesterase;Rh,rhodopsin;T,transducin.Passive Na+ influx Na+ current Rh T + + + – PDE GC GMP Light Disc A B cGMP GTP Hyperpolarization Membrane Membrane Na+ Na+ Ca++ K+ Active Na+ efflux
Rhodopsincontainsachromophorecalledretinal, whichisthealdehydeofretinol, orvitaminA.Retinolisderivedfromcarotenoids,suchasβ-carotene,theorangepigmentfoundincarrots.Likeothervitamins,retinolcannotbesynthesizedbyhumans;instead,itisderivedfromfoodsources.IndividualswithaseverevitaminAdeficiencysufferfrom“nightblindness,”aconditioninwhichvisionisdefectiveinlow-lightsituations.
Theextraordinarysensitivityofrods,whichcansignalthecaptureofasinglephoton,isenhancedbyanamplificationmechanisminwhichphotoactivationofonlyonerhodopsinmoleculecanactivatehundredsoftransducinmolecules.Inaddition,eachphosphodiesterasemoleculehydrolyzesthousandsofcGMPmoleculespersecond.Similareventsoccurincones,butthemembranehyperpolarizationoccursmuchmorequicklythaninrodsandrequiresthousandsofphotons.