• T wo or three types of cone photoreceptor and a single type of rod photoreceptor are present in the norm al m am m alian retina. Som e non-m am m alian retinas have even m ore cone types
I . LIght mIcroscopy anduL structure of rods a tra ndcones.• In vertical sections of retina prepared forlight microscopy with the rods and conesnicely aligned, the rods and cones can bedistinguished rather easily.
2. o uter segmentgenera tIon.•It is from the base of the cilium that membraneevaginations and invaginations occur to producethe outer segment or the important visualpigment-bearing portion of the photoreceptor.Outer segments of both the rods and cones arisesfrom outpouching of the photoreceptor cellplasma membrane at this point.
3. VIsua pIgments and VIsuaL LtransductIon.•Vertebrate photoreceptors can respond to li ght by virtue oftheir containing a vi sual pigm ent em bed ded in the bilipidm em branous di scs that m ake up the outer segm ent. Thevi sual pigm ent consists of a protein called opsi n and achrom ophore d erived from vitam in A known as retinal.The vitam in A i s m anufactured from beta-carotene in thefood we eat, and the protein is m anufactured in thephotoreceptor cell. T he opsin and the chrom ophore arebound together and lie buried in the m em branes of theouter segm ent discs.
4. phagocytosIs of outer segments bypIgment epItheLIum.• T he s ta c ks o f d is cs co nt a in in gv is ua l p ig me n t mo le c ul es in t he ou te rs eg me nt s o f t he p ho t or ec e pt or s a rec on st an t ly r e ne we d. Ne w d is cs a r ea dd ed a t t he ba se o f t he ou te rs eg me nt at t h e ci li u m as di sc us s eda bo ve . A t th e s am e t im e o ld d is c s ar ed is pl ac e d up th e ou t er s e gm en t a nda re p in c he d o ff a t t he t i ps a nde ng ul fe d b y t he a pi c al p r oc es se s o ft he p ig m en t ep it he l iu m.
5. Different types of cone photorec eptor. •As we have seen from the morphological appearances described above, two basic types of photoreceptor, rods and cones, exist in the vertebrate retina. The rods are photoreceptors that contain the visual pigment - rhodopsin and are sensitive to blue-green light with a peak sensitivity around 500 nm wavelength of light. Rods are highly sensitive photoreceptors and are used for vision under dark- dim conditions at night. Cones contain cone opsins as their visual pigments and, depending on the exact structure of the opsin molecule, are maximally sensitive to either long wavelengths of light (red light), medium wavelengths of light (green light) or short wavelengths of light (blue light). Cones of different wavelength sensitivity and the consequent pathways of connectivity to the brain are, of course, the basis of color perception in our visual image.
6. Morphology of the S-cones. This is illustrated in the tangential section of the foveal cone mosaic where the hexagonal packing is distorted in many places by a larger-diameter cone (arrowed cones) breaking up the perfect mosaic into irregular subunits. The larger-diameter cones are S-cones. These cones have their lowest density in the foveal pit at 3-5% of the cones, reach a maximum density of 15% on the foveal slope (1 degree from the foveal pit) and then form an even 8% of the total population elsewhere in the retina
7. Densities of rods and cones in the human retina. It is important for our understanding of the organization of the visual connections for us to know the spatial distribution of the different cell types in the retina. Photoreceptors, we know, are organized in a fairly exact mosaic. As we saw in the fovea, the mosaic is a hexagonal packing of cones. Outside the fovea, the rods break up the close hexagonal packing of the cones but still allow an organized architecture with cones rather evenly spaced surrounded by rings of rods.
Rod sensitivity appears toThus in terms of densities of thedifferent photoreceptor populations in be bought at a price,the human retina, it is clear that the cone however, since rods aredensity is highest in the foveal pit and much slower to respond tofalls rapidly outside the fovea to a fairly light stimulation thaneven density into the peripheral retina.There is a peak of the rod cones. This is one reasonphotoreceptors in a ring around the why sporting events such asfovea at about 4.5 mm or 18 degreesfrom the foveal pit. The optic nerve baseball become(blind spot) is of course photoreceptor progressively more difficultfree. as daylight fails. Both electrical recordings and8. Rods and Night Vision. human observationsRods convey the ability to see at suggest that signals fromnight, under conditions of very rods may arrive as much asdim illumination. Animals with 1/10 second later than thosehigh densities of rods tend to be from cones under lightingnocturnal, whereas those with conditions where both canmainly cones tend to be diurnal. be simultaneously activated
Ultrastructure of rod and cone synaptic endings The job of the photoreceptor cell in the retina is to catch quanta of light in the visual pigment-containing membranes of the outer segment and pass a message, concerning numbers of quanta of light and sensitivities to the different wavelengths, to the next stage of integration and processing at the outer plexiform layer
10. Interphotoreceptor contacts at gap junctions. There also appears to be a pathway for crosstalk between cones and cones and cones and rods in the human retina. Cone pedicles have small projections from their sides or bases that pass to neighboring rod spherules and cone pedicles. Where these projections, called telodendria, meet they have a specialized junction known to be typical of electrical synaptic transmission. These are minute gap junctions.