Photoreceptors

[Light microscopy and ultrastructure] [Outer segment generation] [Visual pigments and visual transduction] [Phagocytosis of outer segments] [Different types of cones] [Morphology of S-cones] [Densities of rods and cones in human retina] [Rods and night vision] [Ultrastructure of synaptic endings] [Inter-photoreceptor contacts] [References]


5. Different types of cone photoreceptor.


Fig. 13. Photoreceptor types (59 K jpeg image)

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.


Three different cone mechanisms can be detected in behavioral, psychophysical and physiological testing. These mechanisms are the basis of so called trichromatic vision which most humans have. Where only one or two visual pigment bearing types of cone are present the vision is said to be monochromatic or dichromatic.

Fig. 14. Light spectra (59 K jpeg image)


Most mammalian species are dichromatic containing as well as rods only middle and short wavelength sensitive cones in their retinas. Primates and humans, birds, reptiles and fish are trichromatic, tetrachromatic and some even pentachromatic (the latter three vertebrate phyla).

Thus long, medium and short wavelength cones have been demonstrated to exist in human retina by photometric and psychophysical methods: L-cones (red) are known to be maximally sensitive to wavelengths peaking at 564nm, M-cones (green) at 533nm and S-cones (blue) at 437nm respectively (see spectra above) (Gouras, 1984, for a review).

To understand color vision and how the colored visual message is processed in the retina we need to be able to start with a morphological distinction of the three (or more) cone types, so they can then, hopefully be identified with any color specific connections they make, i.e. connections to bipolar, horizontal cell and finally ganglion cells of the retina. Fortunately, certain vertebrate species have distinctly different morphological cone types in their retinas and it has recently become possible to correlate these morphologies with spectral sensitivity. Now we can distinguish short, medium and long wavelength cones in retinas of some fish, frogs, birds, and reptiles (turtles) based on distinct morphological differences. Turtle retinas for example have colored oil droplets in their different spectral types of cone which identify them rather readily (see below) (see reviews by Kolb and Lipetz, 1991 and Ammermüller and Kolb, 1996).


Fig. 15. Turtle cone oil droplets (59 K jpeg image)

Fig. 16. Diagrammatic representation of turtle photoreceptors (59 K jpeg image)

However, primate and human retinas still contain cone types which look essentially the same morphologically, but here too with the latest anatomical techniques we are beginning to be able to see at least a difference between the short wavelength cone and the two longer wavelength cones. Specialized histochemical techniques (Marc and Sperling, 1977), dye uptake studies (DeMonasterio et al., 1981) or use of antibodies specific for visual pigments (Szel et al., 1988) have allowed identification of the different spectral types of cone in most mammalian species now. In primate retina antibodies against visual pigments stain outer segments of the L/M-cones together or the S-cones only.


Fig. 17. Primate cone mosaic (59 K jpeg image)

In the above anti-visual pigment antibody stained tissue, the S-cones stand out as the cones that are not stained because the antibody recognizes only L- and M-cone visual pigment. i.e. the brown stained cone profiles are L- and M-cone types while the unstained profiles surrounded by blue circles are the S-cones (Wikler and Rakic, 1990).

6. Morphology of the S-cones.

Recently, careful morphological studies have enabled us to distinguish the short wavelength specific (blue) cone from the medium and long wavelength specific cones in the human retina even without special antibody staining techniques (Ahnelt et al., 1987).

Fig. 18. Vertical section of Human S-cone (59 K jpeg image)



Thus, we now know that S-cones have longer inner segments that project further into subretinal space than longer wavelength cones. Their inner segment diameters do not vary much across the entire retina, thus they are fatter in the foveal area but thinner in the peripheral retina than longer wavelength cones. The S-cones also have smaller and morphologically different pedicles than the other two wavelength cones (Ahnelt et al., 1990). Furthermore, throughout the retina, the S-cones have a different distribution and do not fit into the regular hexagonal mosaic of cones typical of the other two types.

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 (Ahnelt et al., 1987).

Fig. 19. S-cones in foveal cone mosaic (59 K jpeg image)


Analogous information concerning relative distributions of the M- and L-cones in the human retina are not easily available because we cannot tell them apart by morphological features or even by anti-visual pigment staining. In the monkey retina, Marc and Sperling (1977), performed a colored light-dependent histochemical staining technique on freshly excised monkey eyes. They found that L-cones (red) occur at about 33% of the cones throughout the retina, while M-cones (green) peak in the fovea at 64% and vary between 52% and 59% elsewhere in the retina. However, caution still dictates the interpretation of these findings, for others have found the L-cones to outnumber the M-cones in fovea and perifoveal psychophysical testing paradigms (Cicerone and Nerger, 1989). The latest laser inferometry techniques (Roorda and Williams, 1999), measuring the distribution of the red and green cones in the living human fovea, show there to be considerable variation amongst individuals. Some have an equal distribution of L- and M-cones, but others have a larger number of red cones even up to the ratio of 2 L-cones:1 M-cones. Both Roorda and Williams (1999) data in human and Mollon and Bowmaker (1992) in monkey fovea show the patchwork nature of the L- and M-cone distribution (see chapter on midget pathways, Figure 5).

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. Thus in terms of densities of the different photoreceptor populations in the human retina, it is clear that the cone density is highest in the foveal pit and falls rapidly outside the fovea to a fairly even density into the peripheral retina (see graph and map below) (Osterberg, 1935; Curcio et al., 1987). There is a peak of the rod photoreceptors in a ring around the fovea at about 4.5 mm or 18 degrees from the foveal pit. The optic nerve (blind spot) is of course photoreceptor free (see below).


Fig. 20 Densitie of rods and cones along horizontal meridian (59 K jpeg image)

Fig. 21. Cone densities in human retina (59 K jpeg image)


8. Rods and Night Vision.

Rods convey the ability to see at night, under conditions of very dim illumination. Animals with high densities of rods tend to be nocturnal, whereas those with mainly cones tend to be diurnal. The nature of dim light is important both to physicists and to biologists. In 1905 Einstein proposed that light propagated only in discrete irreducible packets or quanta. This explained the non-classical features of the 'photoelectric effect', a process by which light releases electrons from metal surfaces, described by Heinrich Hertz in 1887. Rods are so sensitive that they actually detect single quanta of light, much as do the most sensitive of physical instruments. In 1942 Selig Hecht argued that human rods must be capable of detecting individual light quanta because light flashes so dim that only 1 in 100 rods were likely to absorb a quantum were yet reliably seen by careful observors. A century after the original discovery of the photoelectric effect it has become possible to record directly the minute electrical voltages in rods induced by absorption of individual light quanta. An excellent example is shown in the suction electrode recordings of monkey rods by Schneeweis and Schnapf (1995) below. Each dot in the figure below represents delivery of a very dim pulse of light containing only a few quanta. Voltage responses appear to come in 3 sizes: none, small, and large, representing the detection of 0, 1 or 2 quanta in each flash. The granularity of response to dim light stimuli is evident.


Fig. 22. Photovoltages recorded in monkey rods (78 K jpeg image)

Rod sensitivity appears to be bought at a price, however, since rods are much slower to respond to light stimulation than cones. This is one reason why sporting events such as baseball become progressively more difficult as daylight fails. Both electrical recordings and human observations suggest that signals from rods may arrive as much as 1/10 second later than those from cones under lighting conditions where both can be simultaneously activated (MacLeod, 1972).

9. 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 (see Transduction).


Fig. 23. EM picture of cone and rod endings (59 K jpeg image)

The information transmitting end of the cone cell is known as the pedicle and of the rod cell as the spherule. Cone pedicles are large, conical, flat end-feet (8-10 µm diameter) of the cone axon that lie more or less side by side on the same plane at the outer edge of the outer plexiform layer (OPL). The more numerous rod spherules, in contrast, are small round enlargements of the axon (3-5 µm diameter) or even extensions of the cell body. They lie packed between and above the cone pedicles. Both photoreceptor types' synaptic endings are filled with synaptic vesicles. At their synapses to second-order neurons (bipolar and horizontal cells), both rod spherules and cone pedicles exhibit dense structures known as synaptic ribbons pointing to the postsynaptic invaginated processes. In the cone pedicle approximately 30 of these ribbons occur and are associated with 30 "triads" of invaginated processes (Ahnelt et al., 1990). In the rod spherule 2 ribbons are associated with 4 invaginated second-order neurites.


Fig. 24 EM cone pedicle (59 K jpeg image)

Fig. 25. EM rod spherule (59 K jpeg image)


The cone "triad" of invaginated second-order processes consists typically of a central element which is a dendritic terminal of an invaginating bipolar cell (imb), and two lateral elements which are dendritic terminals of horizontal cells (HC) (Fig. 27). In addition, other varieties of bipolar cell have dendrites making synaptic contacts on the under surface of the cone pedicle at what were first called flat contacts (fmb) (Missotten, 1965; Dowling and Boycott, 1966; Kolb, 1970) (Fig. 27), but then were better characterized and defined by Lasansky (1971) as basal junctions (Fig. 26).


Fig. 26 EM of turtle cone pedicle and types of bipolar synaptic contacts (59 K jpeg image)

Fig. 27. Cone triad (59 K jpeg image)


Rods spherules have only two synaptic ribbons associated with two lateral elements that are horizontal cell axon terminals (HC) and two central invaginating dendrites of rod bipolar cells (rb) (Missotten, 1965; Dowling and Boycott, 1966; Kolb, 1970). There are no basal junctions on rod spherules.


Fig. 28. Rod triad (59 K jpeg image)


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 (Raviola and Gilula, 1975; Nelson et al., 1985).


Fig. 29. Gap junctions between photoreceptors (59 K jpeg image)

As many as 3-5 gap junctions occur on a single rod spherule from neighboring cone telodendria, and a single cone pedicle can have as many as 10 contacts to neighboring rods. Pedicles of S-cones do not have as many telodendrial gap junctions with either neighboring rods or cones (Ahnelt et al., 1990) and thus, this cone type remains relatively isolated in the cone mosaic, and, as we shall see later, remains isolated to the ganglion cell level too, due to connections with a specific 'S-cone bipolar cell'.

Direct interactions between different functional classes of photoreceptors were not anticipated based on any known or theoretical needs of the visual system. In fact such connections would appear to degrade spatial resolution, and, potentially, color perception by mixing signals from photoreceptors at different locations or with different photopigments. Nonetheless, in agreement with anatomical findings, mammalian cones appear to carry rod signals. The slow wave forms of rod signals are easily distinguished from the rapid wave forms of cone signals in voltage recordings from single cones. Examples of voltage responses from monkey cones are illustrated below in an illustration from Schneeweis and Schnapf (1995). Voltage records resulting from balanced red and green stimuli are illustrated.


Fig. 30. Voltage recording from monkey cone (59 K jpeg image)

Both records exhibit the same initial peak hyperpolarization in response to the brief stimulus, however the green stimulus (solid trace) also evokes a slower hyperpolarizing phase after the initial response which the red stimulus (dotted trace) does not. This latter electrical wave has the characteristics of a rod signal. Such signals have also been observed in cat cones (Nelson, 1977). One theory of the utility of this arrangement is that it allows rods to utilize neural pathways devoted to both cones and rods in sending visual information to the inner plexiform layer. Cone pathways may be tuned to faster temporal characteristics than rod pathways, and so by utilizing both pathways, rods may transmit a wider bandwidth of temporal information. There is evidence for two rod pathways with different dynamic signatures in perceptual experiments (Sharpe et al, 1989). Although the functional role of interreceptor junctions is still a matter of debate, they perhaps serve as a philosophical warning to studies of biological sensory systems: Not even the receptor cells themselves stand in isolation of the activity and influence of neighboring neurons.

11. References.

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[Light microscopy and ultrastructure] [Outer segment generation] [Visual pigments and visual transduction] [Phagocytosis of outer segments] [Different types of cones] [Morphology of S-cones] [Densities of rods and cones in human retina] [Rods and night vision] [Ultrastructure of synaptic endings] [Inter-photoreceptor contacts] [References]


Updated: February 2003