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.