Photoreceptors and the receptor potential

Within an ommatidium, the lens focuses light onto the tips of photoreceptor cells, which are called retinula ('little retina') cells (see Fig. 4.5). There are usually eight or nine retinula cells per ommatidium. A retinula cell is elongated, with its longitudinal axis parallel to that of the ommatidium. Outermost is the cell body, and it includes a specialised structure, the rhab-domere, where phototransduction occurs. The cell membrane is folded into very regular finger-like projections, or microvilli, in a rhabdomere. The microvilli contain the photopigment, a rhodopsin molecule, that absorbs light and initiates the process of phototransduction. In most insects, including bees, locusts and dragonflies, the rhabdomeres of all the retinula cells of an ommatidium are adjacent to each other, creating a central rodlike structure, the rhabdom (see Fig. 4.5a). In most insects, all the retinula cells of one ommatidium share the same field of view, and all converge on the second-order neurons that are beneath their ommatidium. However, advanced dipteran flies, such as blowflies, have a different structure, where individual rhabdomeres remain separate (see Fig. 4.5b), and each photoreceptor of a rhabdom looks in a slightly different direction.

Light has a dual physical nature. The way in which it is diffracted when it passes through lenses and apertures is a characteristic of its wave-like properties. It also consists of discrete packets of energy, called photons. The amount of energy in a photon depends on the colour, or wavelength, of its light. Light is actually radiation that occupies a very narrow part of the electromagnetic spectrum, the part that animals can make use of with their eyes. Infrared radiation has a longer wavelength than visible light, and has insufficient energy to trigger phototransduction. At the other end of the visible spectrum, the amount of energy in an ultraviolet ray is so high that it is potentially damaging to biological molecules.

Photoreceptors are capable of responding to single photons of light, if the eye has been left in darkness for some time to maximise its sensitivity. In an insect, each photon gives rise to a discrete, depolarising potential, often called a bump because of its shape. Bumps are usually 1-2 mV high in intracellular recordings (Fig. 4.7a). In absolute darkness, bumps are very rare, and the evidence that each bump represents the absorption of a photon comes from a comparison between the statistical properties of the arrival of photons from a very dim light source at a retinula cell and of the occurrence of bumps in the cell. About 60 per cent of the photons that arrive at a facet generate a bump (Lillywhite, 1977).

In very dim light conditions, photoreceptor potentials consist of a series of discrete photon bumps. As light intensity increases, the bumps fuse together, and the photoreceptor response to a change in light intensity becomes much more smooth (Fig. 4.7b). In daylight, an insect retinula cell receives millions of photons a second. If each bump generated a signal of 1 mV this would lead to a signal of about 1000 V in the photoreceptor, which it is clearly incapable of generating. When exposed to light, the sensitivity of the photoreceptor response decreases quite dramatically, so that each additional photon generates a smaller response. This is called light adaptation. Conversely, adaptation to decreasing levels of ambient illumination, which causes sensitivity to increase, is called dark adaptation. Light and dark adaptation enable photoreceptor responses to be appropriate to the average ambient light intensity. A number of different mechanisms are involved, including changes in pigment-containing cells that regulate the arrival of light at a rhabdom (analogous to the pupil of vertebrate eyes), and changes in the sensitivity of transduction. It is important for photorecep-tors to be able to adjust their sensitivity to ambient lighting conditions, because these can vary enormously. In sunlight, each photoreceptor receives about 40 million photons per second, and this number decreases to 40000/s inside a room and 40/s in moonlight. Because natural light

Relative light intensity

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Relative light intensity

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Figure 4.7 Coding of light intensity by an insect photoreceptor cell. (a-c) Recordings from a dragonfly (Hemicordulia) retinula cell of the responses to three different levels of light stimuli, delivered in darkness. Extremely dim light (a) elicits two single photon bumps; moderately dim light (b) elicits a steady receptor potential in which small fluctuations, caused by the random nature of photon arrival, can still be discerned. Bright light (c) elicits a receptor potential with a sharp, initial peak, followed by a more sustained level. Notice how the voltage calibration changes in (a) to (c). (d) Intensity-response curves for a dragonfly retinula cell. The amplitude of the receptor potential is expressed as a percentage of the maximum response recorded, and light intensity is expressed on a logarithmic scale. The curve on the left (•-•-•) shows the peak size of the receptor potential to flashes of increasing intensity in a dark-adapted cell. The three curves to the right (i-i-i) are plots for cells adapted to particular levels of background illumination, indicated by the arrows. The row of dots connecting the bottoms of the four plots shows the steady-state amplitude of the receptor potential during different levels of sustained background illumination. (Recordings from Laughlin, 1981; copyright © 1981 Springer-Verlag.)

intensity varies so much, it is convenient to use a logarithmic scale to express it. On such a scale, each unit usually represents a tenfold change in light intensity. A change in intensity from 10 to 100 occupies the same space on such a scale as a change from 1000 to 10000 (10 is 101, or log1010 is 1; and 100 is 102, or log10 100 is 2, and so on).

One effect of light adaptation is that, at quite moderate light intensity, the response to a step increase in light consists of a sharp, initial depolarising peak, which quickly drops to a more sustained or plateau receptor potential (Fig. 4.7c). If the increase in light is maintained, the photoreceptor potential declines very gradually, but stabilises over a timescale of a few minutes. This means that a photoreceptor has both tonic and phasic characteristics. The sustained receptor potential that is produced in response to a long-lasting change in the overall level of illumination is a tonic characteristic, and the initial peak response, which emphasises a change in light intensity, is a phasic characteristic.

The second effect of adaptation is apparent in the way that information about light intensity is encoded as a particular value of receptor potential. This pattern of adaptation is usually examined quantitatively by plotting intensity-response curves, based on intracellular recording of the receptor potential (Fig. 4.7d). At the start of an experiment, the eye is kept in the fully dark adapted state and the sizes of the peak responses to steadily brighter flashes of light are recorded. The time intervals between successive flashes are kept long enough to ensure that the eye remains fully dark adapted throughout the experiment. The non-linear nature of bump summation is reflected in the sigmoid shape of the curve of peak receptor potential against a logarithmic scale of light intensity. For low intensities of stimulus, there is a gradual rise in the size of the receptor potential as stimulus intensity increases. The maximum intensity that the receptor is capable of signalling is about three log units (1000), on the horizontal axis of the graph. At this intensity, the response has saturated. However, over a range of intensities of three log units (a thousandfold change in absolute intensity) the size of the response is proportional to the logarithm of light intensity.

The next stage in the experiment is to superimpose the stimulus light on a steady, background level of illumination. The responses to superimposed test flashes are then recorded in the same way as before. Fig. 4.7d shows that background illumination shifts the intensity-response curve along the intensity axis, and the size of the shift depends on the intensity of the background light. At low intensities at which adaptation in the response waveform is small, the shift is also small. However, at higher intensities, the shift produced becomes larger. Hence, the range of intensities to which the cell responds is shifted to match the level of background illumination. This shift in the curves represents a loss in sensitivity: more light is now needed for the photoreceptor to generate the same voltage response. The curves keep approximately the same shape, indicating that the relation between test flash intensity and peak response remains much the same, and response amplitude changes in proportion to the logarithm of stimulus intensity. The extent of adaptation varies between types of eyes, and a very good illustration of this comes from recent research on adaptation in the eyes of different species of dipteran flies. This research shows that the physiology of photoreceptors is well correlated with the ecology and lifestyle of a particular species (Box 4.1).

Box 4.1. Fast and slow photoreceptors

Tipula, a slow-flying, nocturnal cranefly, and Sarcophaga, a fast-flying, diurnal fleshfly, are two of the 20 species of dipteran fly surveyed by Laughlin & Weckström (1993). The intracellular recordings in the figure are responses to 0.5s pulses of light delivered to dark-adapted eyes. The receptor potential in Tipula rises relatively slowly, and does not adapt during the light pulse, whereas that of Sarcophaga rises rapidly and, for the brighter pulses, adapts by decaying quickly from an initial peak to a more sustained level. Tipula is more sensitive to light than Sarcophaga, as shown by the intensity versus response curves and by the larger bump responses to individual photons in dim light. Photoreceptors of Sarcophaga are fast, and able to respond quickly to fluctuations in light as the insect flies around among twigs and leaves. Those of Tipula are too slow to help a fast-flying animal avoid colliding with objects, but are good at detecting light and dark at night time. Sarcophaga's photoreceptors are fast because they contain a special potassium channel. The lifestyle of Tipula means it does not need fast photoreceptors; in fact, to have them would be a significant metabolic cost to the animal. (Figure modified from Laughlin & Weckström (1993), copyright Springer-Verlag.)

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Relative light intensity

It is a common pattern in sensory receptors for the amplitude of the response to be proportional to the logarithm of stimulus intensity, and this makes excellent functional sense. First of all, the large range of light intensities to which an eye is exposed in the day-to-day life of an animal is compressed into a manageable scale. Second, responding in this way has the effect of making equal relative changes in light intensity generate equal absolute changes in the size of the receptor potential. One log unit on the light intensity scale is a tenfold change in absolute light intensity, which causes a change in receptor potential of about 35 per cent of its maximum amplitude. A tenfold change in light intensity is a very large change, and a photoreceptor would normally experience much smaller changes in intensity as it scans a natural image. Coding light intensity in this way enables the eye to recognise different objects because they are distinguishable by differences in the proportion of light that they reflect. The relative brightness of two objects is called their contrast. Contrasts of objects do not vary when ambient illumination changes. For instance, the contrast between the print and the white paper on this page will be the same whether you are reading the book in a dimly lit room or outside on a beach in bright sunlight. The black print will actually reflect more light on the beach than the white paper will when the book is read in the room. The signal that a photo-receptor generates, therefore, consists of a small, steady depolarising potential, which depends on the mean or background level of light, plus fluctuations of a few millivolts caused by viewing objects of different contrasts.

The receptor potential is not coded into spikes in an insect retinula cell, but is conducted by passive spread along the axon. Sometimes, the axon can be as long as 2 mm, but the cable properties of these axons are such that little of the signal is lost at the synaptic terminals.

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