Medical Physiology, 3rd Edition

Visual Transduction

The environment of most species is enveloped by light (Fig. 15-5). Animals have evolved a variety of mechanisms to transduce and detect light. Their brains analyze visual information to help them locate food, avoid becoming food, find a mate, navigate, and generally recognize distant objects. Light is an exceptionally useful source of information about the world because it is nearly ubiquitous and can travel far and fast and in straight lines with relatively little dispersion of its energy. The vertebrate eye, which we describe here, has two major components: an optical part to gather light and to focus it to form an image and a neural part (the retina) to convert the optical image into a neural code.

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FIGURE 15-5 Electromagnetic spectrum. AC, alternating current.

The optical components of the eye collect light and focus it onto the retina

The optical structures of the eye are among the most sophisticated of the specialized non-neural sensory endings, and they are often compared with a camera. As cameras have become more technologically sophisticated, the analogy has improved because the eye has systems to focus automatically, adjust its sensitivity for widely different light levels, move to track and stabilize a target, and even keep its surface washed and clear (obviously, cameras still have room for improvement). The similarity to a camera breaks down when we consider the retina, which is decidedly not like standard photographic film or electronic light detectors.

Figure 15-6A shows a cross section through the human eye. A ray of light entering the eye passes through several relatively transparent elements to reach the retina; these elements include a thin film of tears and then the cornea, the aqueous humor, the lens, and finally the vitreous humor. Tears are a surprisingly complex liquid, based on a plasma ultrafiltrate. They bathe the cornea in a layer that is <10 µm thick, keep it wet, and allow O2 to diffuse from the air to the corneal cells. Tears also contain lysozymes and antibodies to counter infection, a superficial oily layer that greatly slows evaporation and prevents spillage at the lid margins, and a thin mucoid layer to wet the surface of the cornea and to allow the tears to spread smoothly. Tears also help flush away foreign substances. The cornea is a thin, transporting epithelium that is devoid of blood vessels and has a cell structure specialized to maintain its high transparency. imageN15-2A The ciliary epithelium, a part of the ciliary body, constantly secretes aqueous humor, a protein-free ultrafiltrate of blood plasma, into the posterior chamber of the eye. The aqueous humor then flows between the iris and the anterior surface of the lens and reaches the anterior chamber through the pupil. This aqueous humor keeps the anterior portion of the eye slightly pressurized (~20 mm Hg), which helps maintain the eye's shape. The canals of Schlemm drain the aqueous humor. Excess pressure in the anterior chamber produces a disease called glaucoma. In the most common form of glaucoma, blockage of the canals of Schlemm leads to increased intraocular pressure. Pressure damages and kills ganglion cell axons at the optic disc, where they leave the eye and enter the optic nerve. The lens is an onion-like structure with closely packed columnar cells that are arranged in concentric shells and encased by a thin, tough, transparent capsule that is composed of epithelial cells. The cells of the lens have a high concentration of proteins called α-crystallins, which help increase the density of the lens and enhance its focusing power. The posterior chamber, which is filled with a gelatinous substance called vitreous humor, is also kept pressurized by the production of aqueous humor.

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FIGURE 15-6 The eye. A, Cross section of the right human eye, viewed from the top. B, Bending of light by a structure depends not only on the radius of curvature but also on the difference in the indices of refraction of the two adjoining media.

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The Cornea

Contributed by Mark Parker

The cornea is a transparent structure that consists of a collagen matrix called the stroma sandwiched between two layers of epithelia. The anterior corneal epithelium is a stratified squamous layer that faces the tear-film on the surface of the eye. The posterior corneal epithelium, or corneal endothelium, is a simple cuboidal cell layer that faces the aqueous humor in the anterior chamber of the eye. The stroma accounts for approximately 90% of the total thickness of the cornea. The transparency of the stroma depends on the maintenance of the appropriate hydration state or deturgescence of the collagen matrix. Disturbance of the structural array of collagen fibers within the matrix by dehydration or overhydration of the stroma results in increased scattering of light rays, a phenomenon called corneal opacification, and a consequent loss of vision. Both epithelial layers contribute toward maintaining stromal deturgescence. The anterior corneal epithelium protects the hydrated stroma from physical damage and prevents evaporation of fluid from the stroma. The corneal endothelium forms a leaky barrier between the stroma and the anterior chamber, allowing the paracellular leak of nutrient-rich fluid from the aqueous humor into the stroma. The endothelial cells pump ions from the stromal fluid into the aqueous humor via a transcellular route, thereby promoting the return of fluid from the stroma back into the anterior chamber, preventing corneal edema. Disruption of stromal collagen-structure or endothelial pump-function produces a group of diseases called corneal dystrophies.

References:

Edelhauser HF, Ubels JL. Cornea and Sclera. Kaufman PL, Alm A. Adler's Physiology of the Eye: Clinical Application. ed 10. Mosby Inc: St Louis; 2002.

The light must be focused to generate a clear optical image on the retina. This is accomplished by the cornea and, to a lesser extent, the lens. Focusing requires the path of the light to be bent, or refracted. Refraction can occur when light passes from a medium in which it travels relatively fast into a medium in which it travels relatively slowly, or vice versa. The index of refraction for a substance is essentially a measure of the speed of light within it; for example, light travels faster through air (index of refraction, 1.0003) than through the denser substance of the cornea (index of refraction, 1.376). Two things determine how much a light ray is refracted: the difference in the refractive indices of the two media and the angle between the incident light and the interface between the two media. Simple convex lenses use curved surfaces to control the refraction of light rays so that they converge (or focus) on a distant surface. The focal power (D) of one surface of a spherical lens is

image (15-1)

Here, n1 and n2 are the refractive indices of the first and second medium and r is the radius of curvature of the lens in meters. The unit of focal power is a diopter (1 D = 1 m−1). Focal power is the reciprocal of focal length. Thus, parallel light rays entering a 1-D lens are focused at 1 m and those entering a 2-D lens are focused at 0.5 m.

In the case of the eye, most of the focusing takes place at the interface between the air and the tear-covered anterior surface of the cornea because this region is where light encounters the greatest disparity in refractive indices on its path to the retina (see Fig. 15-6B). With a change of 0.376 in refractive index and a radius of outer curvature of 7.8 mm in a typical human cornea, the focal power is 48.2 D. The curvature on the inner surface of the cornea is reversed, so some focal power is lost as light passes into the aqueous humor. However, the change in refractive index at this surface is only 0.040, so the change is only −5.9 D. The lens of the eye, with convex curvature on both sides, has a potentially greater focal power than the cornea. However, because of the small difference in refractive index between the substance of the lens and the aqueous and vitreous humors surrounding it, the effective focal power of the lens is lower. The summed focal power of the optics of the relaxed eye is ~60 D, which allows it to focus light from distant objects onto the retina, the center of which is ~24 mm behind the surface of the cornea (Fig. 15-7A). The position of the retinal image is, of course, upside down relative to the object that produced it.

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FIGURE 15-7 How the eye focuses light. A distant point source of light generates parallel light rays at the surface of the cornea. A, In the normal eye the cornea and lens refract light to focus at a small spot on the retina. B, The myopic eye is abnormally long and focuses distant objects in front of the retina. C, The hyperopic eye is abnormally short and focuses distant objects behind the retina when the lens is relaxed. D, The eye with astigmatism (caused by abnormal curvature of the cornea or lens) cannot focus a point of light onto a small spot.

A normal resting eye is focused on distant objects, beyond ~7 m. If it were fixed in this position, it would be impossible to see objects that are close up. To focus objects that are closer than 7 m away, the eye needs to increase its focal power, a process called accommodation. The eye achieves this goal by changing the shape of the lens (see Fig. 15-6A). At rest, the lens is suspended around its edge by elastic zonal fibers that keep its capsule stretched and relatively flattened. To accommodate, the ciliary muscle fibers contract and release some of the tension in the zonal fibers. Relieved of the radial pull of its fibers, the lens becomes rounder. This increased curvature means increased focal power and a shift of the focal point closer to the eye (see Equation 15-1). There are limits to accommodation, of course, and they are strongly age dependent. Young children have the most pliable lenses and can increase their focal power up to 12 to 14 D. Their near point, the closest distance that they are able to focus, is about at the end of their nose.

With age, the lens becomes stiffer and less able to round up and accommodate. By age 30, the near point is ~10 cm, and by the mid-40s, it stretches beyond arm's length. The loss of accommodation with age is called presbyopia (from the Greek presbus [old] + ops [eye]); it is the reason that glasses for reading are unavoidable for almost everyone past middle age. Additional refractive flaws may be caused by an eye that is too long or short for its focusing power or by aberrations in the refracting surfaces of the eye. Myopia, or nearsightedness, occurs when the eye is too long; distant objects focus in front of the retina and appear blurred (see Fig. 15-7B). Hyperopia (or hypermetropia), or farsightedness, is a feature of eyes that are too short; even with the lens fully accommodated, a near object focuses behind the retina and appears blurry (see Fig. 15-7C). People with myopia can wear concave lenses that move the focal plane of all images back toward the retina. Those with hyperopia can wear convex lenses that move the focal plane forward. Astigmatism is caused by uneven curvature of the refractive surfaces of the eye (see Fig. 15-7D). As a result, a point source of light cannot be brought to a precise focus on the retina. The resultant diffuse focusing leads to blurring of the image. Most people with astigmatic vision can also wear lenses that compensate for the aberrant focusing properties of their eyes.

The iris is the colored structure that is visible through the window of the cornea. The iris's hue comes from pigments in its cells, but its function is to create and to adjust the round opening that it encircles—the pupil. The pupil is like the aperture of the camera, and the iris is the diaphragm that regulates the amount of light allowed to enter the eye. The iris has sphincter muscles, innervated by postganglionic parasympathetic fibers from the ciliary ganglion (see Fig. 14-4 and Fig. 15-8), that allow it to constrict (miosis).imageN15-3 The iris also has radially oriented muscles, innervated by postganglionic sympathetic fibers from the superior cervical ganglion (see Fig. 14-4), that allow it to dilate (mydriasis). Pupil size depends on the balance of the two autonomic inputs. The regulation of pupillary size by ambient light levels is called the pupillary light reflex (see Fig. 15-8). Light striking the retina stimulates fibers in the optic nerve that synapse in the brainstem in the nuclei of the posterior commissure. The next neuron projects to the Edinger-Westphal nuclei on both sides of the brain (see Fig. 14-5), stimulating preganglionic parasympathetic neurons that travel to both ciliary ganglia. These neurons activate postganglionic parasympathetic neurons that constrict both pupils. Thus, control of the pupils in the two eyes is “yoked”: an increase in light to only one eye causes its pupil to constrict (the direct light response), but it also causes an identical constriction in the other eye, even if that eye saw only constant light levels (the consensual light response). Pupillary responses serve two functions: (1) they regulate the total amount of light that enters the eye (over a range of ~16-fold), and (2) they affect the quality of the retinal image in the same way that the aperture affects the depth of focus of a camera (a smaller pupil diameter gives a greater depth of focus).

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FIGURE 15-8 Pupillary light reflex. The figure shows the parasympathetic pathways that lead to constriction of the pupils. Pupil diameter depends on the balance between these parasympathetic pathways as well as the sympathetic pathways shown in Figure 14-4. Note that most of the axons from the retinal ganglion cells synapse in the lateral geniculate nucleus (LGN; see Fig. 16-8).

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Importance of Pupil Size for Depth of Focus

Contributed by Barry Connors

Imagine that a photographer focuses a camera on an object by adjusting the focal power of the camera lens (see Equation 15-1). In principle, only objects at a particular distance from the camera will be in sharp focus. However, if the photographer stops down the diaphragm behind the lens of a camera (to high F values), only fairly parallel rays of light can enter the camera, so that the depth of focus is very broad. In other words, because the camera is receiving very little information concerning the distance to an object, objects at rather different distances from the camera may all appear to be more or less in focus.

Just the opposite happens when the photographer opens up the diaphragm (to very low F values). In this case, the depth of focus is very shallow, so that the only objects that are in sharp focus are those at a particular distance from the camera. Other objects that are closer or further from the camera will appear blurred to varying extents, depending on their distance from the point of optimal focus.

In the eye, miosis (pupillary constriction) is the equivalent of a stopped-down diaphragm (high F value), whereas mydriasis (pupillary dilation) is the equivalent of a fully open diaphragm (low F value). Thus, under conditions of mydriasis (e.g., with sympathetic stimulation or in response to low light levels), the depth of focus can be very shallow, which can cause close objects to appear blurred if the lens is unable to sufficiently accommodate (i.e., to increase its focal power). Blurring is especially problematic with increasing age (e.g., >40 years), when accommodation becomes progressively limited—a condition known as presbyopia.

Other peripheral structures are also essential to proper visual function. The most important are the extraocular muscles that control eye movements. Figure 15-6A shows two such muscles, the lateral and medial rectus muscles. The extraocular muscles determine the direction of gaze, the tracking of objects, and the coordination of the two eyes to keep their retinal images aligned as the eye, head, and visual world move about. Nuclei in the brainstem also control these tracking functions.

The retina is a small, displaced part of the CNS

The retina is a very thin (~200 µm thick in humans) sheet of tissue that lines the back of the eye and contains the light-sensitive cells, the photoreceptors. Photoreceptors capture photons, convert their light energy into chemical free energy, and ultimately generate a synaptic signal for relay to other visual neurons in the retina.

The retina is, histologically and embryologically, a part of the CNS. Not only does it transduce light into neural signals, but it also does some remarkably complex processing of visual information before passing it on to other regions of the brain. In addition to the photoreceptor cells, the retina has four additional types of neurons that form an orderly but intricate neural circuit (Fig. 15-9). One type, the ganglion cell, generates the sole output of the retina by sending its axons to the thalamus through the optic nerve (CN II).

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FIGURE 15-9 Neural circuits in the primate retina. Notice that the incoming light reaches intrinsically photosensitive retinal ganglion cells (ipRGCs) immediately, but hits rod and cone photoreceptor cells only after passing through several thin, transparent layers of other neurons. The pigment epithelium absorbs the light not absorbed by the photoreceptor cells and thus minimizes reflections of stray light. The ganglion cells communicate to the thalamus by sending action potentials down their axons. However, the photoreceptor cells and other neurons communicate by graded synaptic potentials that are conducted electrotonically.

The retina is a highly laminated structure. Through a quirk of evolution, the photoreceptors of the vertebrate eye—rods and cones—are on the outer surface of the retina, that is, the side facing away from the vitreous humor and incoming light. Thus, to reach the transducing cells, light has to first pass through all the retinal neurons. This path causes only minor distortion of image quality because of the thinness and transparency of the neural layers. This seemingly inverted arrangement may actually be an advantage for housekeeping of the eye. Rods and cones undergo a continuous process of renewal, sloughing off membrane from their outer segments and rebuilding them. They also demand a relatively high energy supply. Because they face the back of the eye, these photoreceptors are close to the pigment epithelium, which aids the renewal process, and to the blood vessels that supply the retina. These poorly transparent structures (i.e., pigment epithelium and blood vessels) are thus isolated from the light path. In fact, the pigment epithelium also absorbs photons that are not first captured by photoreceptors, before they can be reflected and degrade the visual image.

Each human eye has >100 × 106 photoreceptors but only 1 × 106 ganglion cells, which implies a high degree of convergence of information as it flows from the transducing cells to the output cells. Some of this convergence is mediated by a set of interneurons (i.e., cells that make synaptic connections only within the retina) called bipolar cells, which directly connect photoreceptors and ganglion cells in a mainly radial direction (see Fig. 15-9). The two remaining types of retinal neurons, horizontal cells and amacrine cells, are interneurons that mainly spread horizontally. Horizontal cells synapse within the outer layer of the retina and interconnect photoreceptors and bipolar cells to themselves and to each other. Horizontal cells often mediate interactions over a wide area of retina. Amacrine cells synapse within the inner layer of the retina and interconnect both bipolar cells and ganglion cells. The circuitry of the retina is much more complex than this picture implies. One hint of this complexity is that its four primary types of neurons are in turn divided into at least 10 to 20 distinct subtypes, each with different physiological and morphological features.

The thinness of the mammalian retina has an interesting biophysical consequence. Because signaling distances are so short, synaptic potentials can usually spread effectively within its neurons without the help of conventional action potentials. Electrotonic spread of potentials along the dendrites is generally enough. The main exceptions are the ganglion cells, which use action potentials to speed visual information along their axons to the thalamus, which is deep within the brain.

There are three primary types of photoreceptors: rods, cones, and intrinsically photosensitive ganglion cells

The two main types of photoreceptors, rods and cones, are named for their characteristic shapes (see Fig. 15-9). The human retina has only one type of rod, which is responsible for our monochromatic dark-adapted vision, and three subtypes of cones, which are responsible for the color-sensitive vision that we experience in brighter environments. Rods outnumber cones by at least 16 : 1, and each type of photoreceptor is spread in a distinct pattern across the retina.

The mammalian retina has a third type of light-sensitive cell, the intrinsically photosensitive retinal ganglion cell (ipRGC). This cell is a rare subtype of ganglion cell that expresses its own photopigment. Unlike the vast majority of ganglion cells, ipRGCs can respond to bright light even in the absence of input from rods and cones. The ipRGCs are involved in several nonimaging functions of the visual system.

In the central area of the primate retina is a small pit 300 to 700 µm in diameter (which accounts for 1 to 2.3 degrees of visual angle) called the fovea, which collects light from the center of our gaze (see Fig. 15-6). Several adaptations of the fovea allow it to mediate the highest visual acuity in the retina. Neurons of the inner layer of retina are actually displaced laterally to the side of the fovea to minimize light scattering on the way to the receptors. In addition, within the fovea, the ratio of photoreceptors to ganglion cells falls dramatically. Most foveal receptors synapse on only one bipolar cell, which synapses on only one ganglion cell (Fig. 15-10A). Because each ganglion cell is devoted to a very small portion of the visual field, central vision has high resolution. In other words, the receptive field of a foveal ganglion cell (i.e., the region of stimulus space that can activate it) is small. At the periphery, the ratio of receptors to ganglion cells is high (see Fig. 15-10B); thus, each ganglion cell has a large receptive field. The large receptive field reduces the spatial resolution of the peripheral portion of the retina but increases its sensitivity because more photoreceptors collect light for a ganglion cell. Foveal vision is purely cone mediated, and the sheet of foveal photoreceptors consists of only the smallest cones packed at the highest density (~0.3 µm from the center of one cone to another). Cone density falls to very low levels outside the fovea, and rod density rises. Both rods and cones mediate peripheral vision (i.e., nonfoveal vision, or vision at visual angles >10 degrees away from the center of the fovea and thus the center of gaze).

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FIGURE 15-10 Comparison of the synaptic connections and receptive fields in the fovea and periphery of the retina.

Rods and cones are elongated cells with synaptic terminals, an inner segment, and an outer segment (see Fig. 15-9). The synaptic terminals connect to the inner segment by a short axon. The inner segment contains the nucleus and metabolic machinery; it synthesizes the photopigments and has a high density of mitochondria. The inner segment also serves an optical function—its high density funnels photons into the outer segment. A thin ciliary stalk connects the inner segment to the outer segment. The outer segment is the transduction site, although it is the last part of the cell to see the light. Structurally, the outer segment is a highly modified cilium. Each rod outer segment has ~1000 tightly packed stacks of disk membranes, which are flattened, membrane-bound intracellular organelles that have pinched off from the outer membrane. Cone outer segments have similarly stacked membranes, except that they are infolded and remain continuous with the outer membrane. The disk membranes contain the photopigments—rhodopsin in rods and molecules related to rhodopsin in cones. Rhodopsin moves from its synthesis site in the inner segment through the stalk and into the outer segment through small vesicles whose membranes are packed with rhodopsin to be incorporated into the disks.

Rods and cones hyperpolarize in response to light

The remarkable psychophysical experiments of Hecht and colleagues in 1942 demonstrated that five to seven photons, each acting on only a single rod, are sufficient to evoke a sensation of light in humans. Thus, the rod is performing at the edge of its physical limits because there is no light level smaller than 1 photon. To detect a single photon requires a prodigious feat of signal amplification. As Denis Baylor has pointed out, “the sensitivity of rod vision is so great that the energy needed to lift a sugar cube one centimeter, if converted to a blue-green light, would suffice to give an intense sensation of a flash to every human who ever existed.”

Phototransduction involves a cascade of chemical and electrical events to detect, to amplify, and to signal a response to light. As do many other sensory receptors, photoreceptors use electrical events (receptor potentials) to carry the visual signal from the outer segment to their synapses. Chemical messengers diffusing over such a distance would simply be too slow. A surprising fact about the receptor potential of rods and cones is that it is hyperpolarizing. Light causes the cell's Vm to become more negative than the resting potential that it maintains in the dark (Fig. 15-11A). At low light intensities, the size of the receptor potential rises linearly with light intensity; but at higher intensities, the response saturates.

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FIGURE 15-11 Phototransduction. A, The experiment for which results are summarized here was performed on a red-sensitive cone from a turtle. A brief flash of light causes a hyperpolarization of the photoreceptor cell. The size of the peak and the duration of the receptor potential increase with the increasing intensity of the flash. At low light intensities, the magnitude of the peak increases linearly with light intensity. At high intensities, the peak response saturates, but the plateau becomes longer. B, A single rod has been sucked into a pipette, which allows the investigators to monitor the current. The horizontal white band is the light used to stimulate the rod. C, In the absence of light, Na+ enters the outer segment of the rod through cGMP-gated channels and depolarizes the cell. The electrical circuit for this dark current is completed by K+ leaving the inner segment. The dark current, which depolarizes the cell, leads to constant transmitter release. D, In the presence of light, Na+ can no longer enter the cell because cGMP levels are low, and the cGMP-gated channel closes. The photoreceptor cell thus hyperpolarizes, and transmitter release decreases. (A, Data from Baylor DA, Hodgkin AL, Lamb TD: The electrical response of turtle cones to flashes and steps of light. J Physiol 242:685–727, 1974; B, from Baylor DA, Lamb TD, Yau K-W: Responses of retinal rods to single photons. J Physiol 288:613–634, 1979.)

Hyperpolarization is an essential step in relaying the visual signal because it directly modulates the rate of transmitter release from the photoreceptor onto its postsynaptic neurons. This synapse is conventional in that it releases more transmitter—in this case glutamate—when its presynaptic terminal is depolarized and less when it is hyperpolarized. Thus, a flash of light causes a decrease in transmitter secretion. The upshot is that the vertebrate photoreceptor is most active in the dark.

How is the light-induced hyperpolarization generated? Figure 15-11B shows a method to measure the current flowing across the membrane of the outer segment of a single rod. In the dark, each photoreceptor produces an ionic current that flows steadily into the outer segment and out of the inner segment. This dark current is carried mainly by inwardly directed Na+ ions in the outer segment and by outwardly directed K+ ions from the inner segment (see Fig. 15-11C). Na+ flows through a nonselective cation channel of the outer segment, which light indirectly regulates, and K+ flows through a K+ channel in the inner segment, which light does not regulate. Na+ carries ~90% of the dark current in the outer segment, and Ca2+, ~10%. In the dark, Vm is about −40 mV. Na-K pumps, primarily located within the inner segments, remove the Na+ and import K+. An Na-Ca exchanger removes Ca2+ from the outer segment.

Absorption of photons leads to closure of the nonselective cation channels in the outer segment. The total conductance of the cell membrane decreases. Because the K+ channels of the inner segment remain open, K+ continues to flow out of the cell, and this outward current causes the cell to hyperpolarize (see Fig. 15-11D). The number of cation channels that close depends on the number of photons that are absorbed. The range of one rod's sensitivity is 1 to ~1000 photons.

Baylor and colleagues measured the minimum amount of light required to produce a change in receptor current (see Fig. 15-11B). They found that absorption of 1 photon suppresses a surprisingly large current, equivalent to the entry of >106 Na+ ions, and thus represents an enormous amplification of energy. The single-photon response is also much larger than the background electrical noise in the rod, as it must be to produce the rod's high sensitivity to dim light. Cones respond similarly to single photons, but they are inherently noisier and their response is only image the size of that in the rod. Cone responses do not saturate, even at the brightest levels of natural light. Cones also respond faster than rods.

Rhodopsin is a G protein–coupled “receptor” for light

How can a single photon stop the flow of 1 million Na+ ions across the membrane of a rod cell? The process begins when the photon is absorbed by rhodopsin, the light receptor molecule. Rhodopsin is one of the most tightly packed proteins in the body, with a density of ~30,000 molecules per square micrometer in the disk membranes. Thus, the packing ratio is 1 protein molecule for every 60 lipid molecules! One rod contains ~109 rhodopsin molecules. This staggering density ensures an optimized capture rate for photons passing through a photoreceptor. Even so, only ~10% of the light entering the eye is used by the receptors. The rest is either absorbed by the optical components of the eye or passes between or through the receptors. Rhodopsin has two key components: retinal and the protein opsin. Retinal is the aldehyde of vitamin A, or retinol (~500 Da). Opsin is a single polypeptide (~41 kDa) with seven membrane-spanning segments (Fig. 15-12A). It is a member of the superfamily of GPCRs (see pp. 51–52) that includes many neurotransmitter receptors as well as the olfactory receptor molecules.

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FIGURE 15-12 Rhodopsin, transducin, and signal transduction at the molecular level. A, The opsin molecule is a classic seven-transmembrane receptor that couples to transducin, a G protein. When the opsin is attached to retinal (magenta structure) via amino-acid residue 296 in the seventh (i.e., most C-terminal) membrane-spanning segment of opsin, the assembly is called rhodopsin. B, The absorption of a photon by 11-cis retinal causes the molecule to isomerize to all-trans retinal. C, After rhodopsin absorbs a photon of light, it activates many transducins. The activated α subunit of transducin (Gαt) in turn activates phosphodiesterase, which hydrolyzes cGMP. The resultant decrease in [cGMP]i closes cGMP-gated channels and produces a hyperpolarization (receptor potential). GMP, 5′-guanylate monophosphate; NCKX1, the Na+/(Ca2+-K+) exchanger (SLC24A1). (A, Data from Palczewsk K, Kumasaka T, Miyano, M et al: Crystal structure of rhodopsin: A G protein-coupled receptor. Science 289(5480):739–745, 2000. Reconstructed figure is courtesy of S. Filipek and K. Palczewski.)

To be transduced, photons are actually absorbed by retinal, which is responsible for rhodopsin's color. The tail of retinal can twist into a variety of geometric configurations, one of which is a kinked and unstable version called 11-cis retinal (see Fig. 15-12B). The cis form sits within a pocket of the opsin (comparable to the ligand-binding site of other GPCRs) and is covalently bound to it. However, because of its instability, the cis form can exist only in the dark. If 11-cis retinal absorbs a photon, it isomerizes within 1 ps to a straighter and more stable version called all-trans retinal. This isomerization in turn triggers a series of conformational changes in the opsin that lead to a form called metarhodopsin II, which can activate an attached molecule called transducin. Transducin carries the signal forward in the cascade and causes a reduction in Na+ conductance. Soon after isomerization, all-trans retinal and opsin separate in a process called bleaching; this separation causes the color to change from the rosy red of rhodopsin (rhodon is Greek for the color “rose”) to the pale yellow of opsin. The photoreceptor cell converts all-trans retinal to retinol (vitamin A), which then translocates to the pigment epithelium and becomes 11-cis retinal. This compound makes its way back to the outer segment, where it recombines with opsin. This cycle of rhodopsin regeneration takes a few minutes.

Transducin is so named because it transduces the light-activated signal from rhodopsin into the photoreceptor membrane's response (see Fig. 15-12C). Transducin was the first of the large family of GTP-binding proteins (G proteins; see p. 52) to be identified, and its amino-acid sequence is very similar to that of other G proteins (see Table 3-2). When it is activated by metarhodopsin, the α subunit of transducin exchanges a bound GDP for a GTP and then diffuses within the plane of the membrane to stimulate a phosphodiesterase that hydrolyzes cGMP to 5′-guanylate monophosphate.

cGMP is the diffusible second messenger that links the light-activated events of the disk membranes to the electrical events of the outer membrane. A key discovery by Fesenko and colleagues in 1985 showed that the “light-sensitive” cation channel of rods is actually a cGMP-gated cation channel (see pp. 169–172). This CNG channel was the first of its kind to be discovered (we have already discussed a similar channel in olfactory receptors). In the dark, a constitutively active guanylyl cyclase that synthesizes cGMP from GTP keeps cGMP levels high within the photoreceptor cytoplasm. This high [cGMP]i causes the cGMP-gated cation channels to spend much of their time open and accounts for the dark current (see Fig. 15-11C). Because light stimulates the phosphodiesterase and thus decreases [cGMP]i, light reduces the number of open cGMP-gated cation channels and thus reduces the dark current. The photoreceptor then hyperpolarizes, transmitter release falls, and a visual signal is passed to retinal neurons.

Strong amplification occurs along the phototransduction pathway. The absorption of 1 photon activates 1 metarhodopsin molecule, which can activate ~700 transducin molecules within ~100 ms. These transducin molecules activate phosphodiesterase, which increases the rate of cGMP hydrolysis by ~100-fold. One photon leads to the hydrolysis of ~1400 cGMP molecules by the peak of the response, thus reducing [cGMP] by ~8% in the cytoplasm around the activated disk. This decrease in [cGMP]i closes ~230 of the 11,000 cGMP-gated channels that are open in the dark. As a result, the dark current falls by ~2%.

The cGMP-gated channel has additional interesting properties. It responds within milliseconds when [cGMP]i rises, and it does not desensitize in response to cGMP. The concentration-response curve is very steep at low [cGMP]i because opening requires the simultaneous binding of three cGMP molecules. Thus, the channel has switch-like behavior at physiological levels of cGMP. Ion conductance through the channel also has steep voltage dependence because Ca2+ and Mg2+ strongly block the channel (as well as permeate it) within its physiological voltage range. This open-channel block (see Fig. 7-20D) makes the normal single-channel conductance very small, among the smallest of any ion channel; the open channel normally carries a current of only 3 × 10−15 A (3 fA)! The currents of ion channels are inherently “noisy” as they flicker open and closed. However, the 11,000 channels—each with currents of 3 fA—summate to a rather noise-free dark current of 11,000 channels × 3 fA per channel = 33 pA. In contrast, if 11 channels—each with currents of 3 pA—carried the dark current of 33 pA, the 2% change in this signal (0.66 pA) would be smaller than the noise produced by the opening and closing of a single channel (3 pA). Thus, the small channels give the photoreceptor a high signal-to-noise ratio.

The [cGMP]i in the photoreceptor cell represents a dynamic balance between the synthesis of cGMP by guanylyl cyclase and the breakdown of cGMP by phosphodiesterase. Ca2+, which enters through the relatively nonselective cGMP-gated channel, synergistically inhibits the guanylyl cyclase and stimulates the phosphodiesterase. These Ca2+ sensitivities set up a negative-feedback system. In the dark, the incoming Ca2+ prevents runaway increases in [cGMP]i. In the light, the ensuing decrease in [Ca2+]i relieves the inhibition on guanylyl cyclase, inhibits the phosphodiesterase, increases [cGMP]i, and thus poises the system for channel reopening.

When a light stimulus terminates, the activated forms of each component of the transduction cascade must be inactivated. One mechanism of this termination process appears to involve the channels themselves. As described in the preceding paragraph, closure of the cGMP-gated channels in the light leads to a fall in [Ca2+]i, which helps replenish cGMP and facilitates channel reopening. Two additional mechanisms involve the proteins rhodopsin kinase and arrestin. Rhodopsin kinase phosphorylates light-activated rhodopsin and allows it to be recognized by arrestin. Arrestin, an abundant cytosolic protein, binds to the phosphorylated light-activated rhodopsin and completely terminates its ability to activate transducin.

The eye uses a variety of mechanisms to adapt to a wide range of light levels

The human eye can operate effectively over a 1010-fold range of light intensities, which is the equivalent of going from almost total darkness to bright sunlight on snow. However, moving from a bright to a dark environment, or vice versa, requires time for adaptation before the eye can respond optimally. Adaptation is mediated by several mechanisms. One mechanism mentioned above is regulation of the size of the pupil by the iris, which can change light sensitivity by ~16-fold. That still leaves the vast majority of the range to account for. During dark adaptation, two additional mechanisms with very different time courses are evident, as we can see from a test of the detection threshold for the human eye (Fig. 15-13). The first phase of adaptation is finished within ~10 minutes and is a property of the cones; the second takes at least 30 minutes and is attributed to the rods. A fully dark-adapted retina, relying on rods, can have a light threshold that is as much as 15,000 times lower than a retina relying on cones. In essence, then, the human eye has two retinas in one, a rod retina for low light levels and a cone retina for high light levels. These two systems can operate at the same time; when dark adapted, the rods can respond to the lowest light levels, but cones are available to respond when brighter stimuli appear.

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FIGURE 15-13 Effect of dark adaptation on the visual threshold. The subject was exposed to light at a level of 1600 millilumens and then switched to the dark. The graph is a plot of the time course of the subject's relative threshold (on a log scale) for detecting a light stimulus. (Data from Hecht S, Shlaer S, Smith EL, et al: The visual functions of the complete color blind. J Gen Physiol 31:459–472, 1948.)

The rapid and slow phases of adaptation that are discussed in the preceding paragraph have both neural and photoreceptor mechanisms. The neural mechanisms are relatively fast, operate at relatively low ambient light levels, and involve multiple mechanisms within the neuronal network of the retina. The photoreceptor mechanisms involve some of the processes that are described in the previous section. Thus, in bright sunlight, rods become ineffective because most of their rhodopsin remains inactivated, or bleached. cGMP-gated channels are closed and thus Ca2+ entry is blocked, so [Ca2+]i falls to a few nanomolar as Ca2+ is removed by the Na+/(Ca2+-K+) exchanger NCKX1 (SLC24A1; see Table 5-4). After returning to darkness, the rods slowly regenerate rhodopsin and become sensitive once again. However, a component of the cGMP system also regulates photoreceptor sensitivity. In the dark, when baseline [cGMP]i is relatively high, substantial amounts of Ca2+ enter through cGMP-gated channels. The resultant high [Ca2+]i (several hundred nanomolars) inhibits guanylyl cyclase and stimulates phosphodiesterase, thereby preventing [cGMP]i from rising too high. Conversely, when background light levels are high, this same feedback system causes baseline [cGMP]i to remain high so that [cGMP]i can fall in response to further increases in light levels. Otherwise, the signal-transduction system would become saturated. In other words, the photoreceptor adapts to the increased background light intensity and remains responsive to small changes. Additional adaptation mechanisms regulate the sensitivity of rhodopsin, guanylyl cyclase, and the cGMP-gated channel. Clearly, adaptation involves an intricate network of molecular interactions.

Color vision depends on the different spectral sensitivities of the three types of cones

The human eye responds only to a small region of the electromagnetic spectrum (see Fig. 15-5), but within it, we are exquisitely sensitive to the light's wavelength. We see assorted colors in a daytime panorama because objects absorb some wavelengths while reflecting, refracting, or transmitting others. Different sources of light may also affect the colors of a scene; the light from tungsten bulbs is reddish, whereas that of fluorescent bulbs is bluish.

Research on color vision has a long history. In 1801, Thomas Young first outlined the trichromatic theory of color vision, which was championed later in the 19th century by Hermann von Helmholtz. These investigators found that they could reproduce a particular sample hue by mixing the correct intensities of three lights with the primary hues blue, green, and red. They proposed that color vision, with its wide range of distinct, perceived hues, is based on only three different pigments in the eye, each absorbing a different range of wavelengths. Microspectrophotometry of single cones in 1964 amply confirmed this scheme. Thus, although analysis of color by the human brain is sophisticated and complex, it all derives from the responses of only three types of photopigments in cones.

Our sensitivity to the wavelength of light depends on the retina's state of adaptation. When it is dark adapted (also called scotopic conditions), the spectral sensitivity curve for human vision is shifted toward shorter wavelengths compared with the curve obtained after light adaptation (photopic conditions; Fig. 15-14A). The absolute sensitivity to light can also be several orders of magnitude higher under scotopic conditions (see Fig. 15-13). The primary reason for the difference in these curves is that rods are doing the transduction of dim light under dark-adapted conditions, whereas cones transduce in the light-adapted eye. As we would predict, the spectral sensitivity curve for scotopic vision is quite similar to the absorption spectrum of the rods' rhodopsin, with a peak at 500 nm.

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FIGURE 15-14 Sensitivity of vision and photoreceptors at different wavelengths of light. A, The graph shows the results of a psychophysical experiment. Under dark-adapted (scotopic) conditions, the human eye is maximally sensitive at ~500 nm. Under light-adapted (photopic) conditions, the eye is maximally sensitive at ~560 nm. B, The spectral sensitivity of rods (obtained with a spectrophotometer) peaks at ~500 nm; that of the three types of cones peaks at ~420 nm for the S (blue) cone, ~530 nm for the M (green) cone, and ~560 nm for the L (red) cone; and that of melanopsin peaks at ~475 nm. Each absorbance spectrum has been normalized to its peak sensitivity. (A, Data from Knowles A: The biochemical aspects of vision. In Barlow HB, Mollon JD [eds]: The Senses. Cambridge, UK, Cambridge University Press, 1982, pp 82–101; B, rhodopsin data from Dartnell HJ, Bowmaker JK, Mollon JD: Microspectrophotometry of human photoreceptors. In Mollon JD, Sharpe LT [eds]: Colour Vision. London, Academic Press, 1983, pp 69–80; melanopsin data from Matsuyama T, Yamashita T, Imamoto Y, Shichida Y: Photochemical properties of mammalian melanopsin. Biochemistry 51:5454–5462, 2012.)

The spectral sensitivity of the light-adapted eye depends on the photopigments in the cones. Humans have three different kinds of cones, and each expresses a photopigment with a different absorbance spectrum. The peaks of their absorbance curves fall at ~420, 530, and 560 nm, which correspond to the violet, yellow-green, and yellow-red regions of the spectrum (see Fig. 15-14B). The three cones and their pigments were historically called blue, green, and red, respectively. They are now more commonly called S, M, and L (for short, medium, and long wavelengths); we use this terminology here. Because the absolute sensitivity of the short-wavelength cone is only one tenth that of the other two, the spectral sensitivity of photopic human vision is dominated by the two longer-wavelength cones (compare the spectral sensitivity functions in Fig. 15-14A with the absorbance spectra of the cones in Fig. 15-14B).

Single cones do not encode the wavelength of a light stimulus. If a cone responds to a photon, it generates the same response regardless of the wavelength of that photon. A glance at Figure 15-14B shows that each type of cone pigment can absorb a wide range of wavelengths. The pigment in a cone is more likely to absorb photons when their wavelength is at its peak absorbance, but light hitting the cone on the fringe of its absorbance range can still generate a large response if the light's intensity is sufficiently high. This property of response univariance is the reason that vision in an eye with only one functioning pigment (e.g., scotopic vision using only rods) can only be monochromatic. With a single pigment system, the distinction between different colors and between differences in intensity is confounded. Two different cones (as in most New World monkeys), each with a different but overlapping range of wavelength sensitivities, remove much of the ambiguity in encoding the wavelength of light stimuli. With three overlapping pigments (as in Old World monkeys and humans), light of a single wavelength stimulates each of the three cones to different degrees, and light of any other wavelength stimulates these cones with a distinctly different pattern. Because the nervous system can compare the relative stimulation of the three cone types to decode the wavelength, it can also distinguish changes in the intensity (luminance) of the light from changes in its wavelength.

Color capabilities are not constant across the retina. The use of multiple cones is not compatible with fine spatial discrimination because of wavelength-dependent differences in the eye's ability to focus light, known as chromatic aberration, and because very small objects may stimulate only single cones. The fovea has only M and L cones, which limits its color discrimination in comparison to the peripheral portions of the retina but leaves it best adapted to discriminate fine spatial detail (Box 15-1).

Box 15-1

Inherited Defects in Color Vision

Inherited defects in color vision are relatively common, and many are caused by mutations in visual pigment genes. For example, 8% of white males and 1% of white females have some defect in their L or M pigments caused by X-linked recessive mutations. A single abnormal pigment can lead to either dichromacy (the absence of one functional pigment) or anomalous trichromacy (a shift in the absorption spectrum of one pigment relative to normal), often with a consequent inability to distinguish certain colors. Jeremy Nathans and colleagues found that men have only one copy of the L pigment gene; but located right next to it on the X chromosome, they may have one to three copies of the M pigment gene. He proposed that homologous recombination could account for the gene duplication, loss of a gene, or production of the hybrid L-M genes that occur in red-green color blindness. Hybrid L-M pigments have spectral properties intermediate between those of the two normal pigments, probably because their opsins possess a combination of the traits of the two normal pigments.

Lack of two of the three functional cone pigments leads to monochromacy. The number of people who have such true color blindness is very small, <0.001% of the population. For example, S-cone monochromacy is a rare X-linked disorder in which both L and M photopigments are missing because of mutations on the X chromosome. The S pigment gene is on chromosome 7.

The four different human visual pigments have a similar structure. The presence of retinal and the mechanisms of its photoisomerization are essentially identical in each. The main difference is the primary structure of the attached protein, the opsin. M and L opsins share 96% of their amino acids. Pairwise comparisons among the other opsins show only 44% or lower sequence similarity, however. Apparently, the different amino-acid structures of the opsins affect their charge distributions in the region of the 11-cis retinal and shift its absorption spectrum to give the different pigments their specific spectral sensitivities.

The ipRGCs have unique properties and functions

The ipRGC, the third retinal photoreceptor, differs from rods and cones in fundamental ways. First, instead of expressing rhodopsin or cone opsins, ipRGCs use a related but unique light-sensitive protein called melanopsin that is most sensitive in the blue part of the spectrum (~475 nm; see Fig. 15-14B). Second, ipRGCs depolarize in response to light, and if the stimulus is strong enough, they generate action potentials. Rods and cones hyperpolarize when illuminated and never generate action potentials. Third, to reach threshold, the ipRGCs require very bright light—several orders of magnitude stronger than the threshold for cone responses. Fourth, ipRGCs take seconds to respond, and they quite faithfully maintain their responses even when light levels are sustained for hours.

Most of the molecular components of melanopsin-mediated phototransduction in ipRGCs are unknown. However, ipRGCs resemble invertebrate photoreceptors much more than they resemble rods and cones. Melanopsin seems to activate a G protein of the Gq family (see Table 3-2), which stimulates PLC to produce IP3 and diacylglycerol (DAG); this causes TRP channels to open and depolarizes the neuron. When light hits rods and cones it reduces [cGMP]i, causing CNG channels to close and hyperpolarizing the photoreceptor (see p. 368). Ironically, although ipRGCs are by definition intrinsically photosensitive they also receive some input from rods and cones via synapses of bipolar cells and amacrine cells.

What are the functions of ipRGCs? With their slow, maintained responses to relatively bright light, they seem well equipped for reporting the levels of ambient daytime illumination. With their very large receptive fields, sparse numbers, and sluggishness, ipRGCs are not well adapted to the image-forming functions we usually think of as “vision”—processes that cones and rods perform so well. In fact, ipRGCs help to mediate a variety of non–image-forming processes that are reflexive and subconscious. The body maintains circadian rhythms for most physiological processes; some ipRGCs send axons to the suprachiasmatic nuclei in the hypothalamus to ensure that its circadian clock is synchronized with the light-dark cycles of day and night. The ipRGCs mediate the pupillary light reflex by sending axons to the pretectal nuclei (see Fig. 15-8). Activity of ipRGCs inhibits the production of melatonin by the pineal gland. They also provide photic information to some of the brain's sleep-wake regulatory systems.