To be detected by the retina, light energy must be absorbed. This is the job primarily of the rods and cones (a small class of ganglion cells are also photosensitive) and is accomplished by visual pigment molecules located in their outer segments. For both rods and cones, the pigment molecule consists of a chromophore, 11-cis retinal, bound to an opsin protein. The visual pigment found in the outer segments of rods is rhodopsin, or visual purple (so named because it has a purple appearance when light has been absorbed). It absorbs light best at a wavelength of 500 nm. Three variants of visual pigment, resulting from the binding of different opsins to retinal, are found in cones (in most species, each cone expresses one of the three cone pigments). The cone pigments absorb best at 419 nm (blue), at 533 nm (green), and at 564 nm (red). However, the absorption spectrum of these visual pigments is broad so that they overlap considerably (
Fig. 8.5).
•Fig. 8.5 The Spectral Sensitivity of the Three Types of Cone Pigments and of the Rod Pigment (Rhodopsin) in the Human Retina. Notethatthecurvesoverlapandthattheso-calledblueandredconesactuallyabsorbmaximallyinthevioletandyellowranges,respectively.(DatafromSquireLR,etal[eds].Fundamental Neuroscience. SanDiego,CA:AcademicPress;2002.)
Despite the differences in spectral sensitivity, the transduction process is similar in rods and cones. The absorption of a photon by a visual pigment molecule leads to the isomerization of 11-cis retinal to all-trans retinal, release of the bond with the opsin, and conversion of retinal to retinol. These changes trigger a second messenger cascade that leads to a change in the electrical activity of the rod or cone (discussed later in this section).
The separation of all-trans retinal from opsin also causes both the loss of its ability to absorb light and bleaching (i.e., the visual pigment loses its color). In both rods and cones, regeneration of the visual pigment molecule is a multistep process: the all-trans retinal is transported to the retinal pigmented cell layer, where it is reduced to retinol, isomerized, and esterified back to 11-cis retinal. It is then transported back to the photoreceptor layer, taken up by outer segments, and recombined with opsin to regenerate the visual pigment molecule, which can again absorb light. There is evidence that cones also use a second pathway to regenerate visual pigment. This pathway is much more rapid and involves transport of the retinal molecule to and from the Müller cells (see
Fig. 8.2 ) rather than the pigmented epithelial cells. The potential importance of this more rapid pathway is discussed in the section “
Ultimately, the transduction process triggered by absorption of photons causes the photoreceptor to hyperpolarize. To understand this action and its consequences fully, it is necessary to know the baseline state of the photoreceptor in the dark (i.e., before it absorbs a photon). In darkness, photoreceptors are slightly depolarized (≈−40 mV) in relation to most neurons because cyclic guanosine monophosphate (cGMP)–gated cation channels in their outer segments are open (Fig. 8.6A ). These channels allow a steady influx of Na+ and Ca2+ . The resulting current is known as the dark current, and the depolarization it causes leads to the tonic release of the neurotransmitter glutamate at the photoreceptor’s synapses.
When light is absorbed in a rod (an equivalent sequence happens in cones), photoisomerization of rhodopsin activates a G protein called transducin (see