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Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.

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Molecular Cell Biology. 4th edition.

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Section 21.6Sensory Transduction

The nervous system receives input from a large number of sensory receptors (see Figure 21-6). Photoreceptors in the eye, taste receptors on the tongue, odorant receptors in the nose, and touch receptors on the skin monitor various aspects of the outside environment. Stretch receptors surround many muscles and fire when the muscle is stretched. Internal receptors monitor the levels of glucose, salt, and water in body fluids. The nervous system, the brain in particular, processes and integrates this vast barrage of information and coordinates the response of the organism.

Since the “language” of the nervous system is electric signals, each of the many types of receptor cells must convert, or transduce, its sensory input into an electric signal. A few sensory cells are themselves neurons that generate action potentials in response to stimulation. However, most are specialized epithelial cells that do not generate action potentials but synapse with and stimulate adjacent neurons that then generate action potentials (see Figure 21-1c). The key question that we will consider is how a sensory cell transduces its input into an electric signal.

Mechanoreceptors and Some Other Sensory Receptors Are Gated Cation Channels

Some sensory receptors are gated Na+ or Na+/Ca2+ channels that open in response to various stimuli; activation of such receptors causes an influx of ions, leading to membrane depolarization. Examples include the stretch and touch receptors that are activated by stretching of the cell membrane; these have been identified in a wide array of cells, ranging from vertebrate muscle and epithelial cells to yeast, plants, and even bacteria.

The cloning of genes encoding touch receptors began with the isolation of mutant strains of the nematode Caenorhabditis elegans that were insensitive to touch. Three of the genes in which mutations were isolated — MEC4, MEC6, and MEC10 — encode three similar subunits of a Na+ channel in the touch-receptor cells. These gated channels are necessary for touch sensitivity and may open directly in response to mechanical stimulation. Similar kinds of channels are found in prokaryotes and lower eukaryotes; by opening in response to membrane stretching, these channels may play a role in osmoregulation and the control of a constant cell volume. It is thought that such receptors were among the first sensory receptors to evolve, though no homologs have yet been identified in mammals.

Of the four taste stimuli used by vertebrates (salty, sweet, bitter, and sour), the “receptors” for salt are the best understood. These are simply ungated Na+ channels in the apical membrane of taste-receptor cells; elevation of the extracellular Na+ causes entry of Na+ through these channels and thus depolarization of the membrane. A different sensory protein is the receptor for capsaicin*, the molecule that makes chili peppers seem hot. In this case, the receptor is a capsaicin-gated Na+/Ca2+ channel that is found in many sensory pain neurons. When activated, the influx of Na+ and Ca2+ ions depolarizes these cells, initiating a nerve impulse that goes to the brain. Interestingly, the capsaicin receptor is also activated by elevated temperatures (≈48°C) that produce pain, possibly explaining why we perceive foods that contain capsaicin as “hot.”

More often, though, the connection between a sensory receptor protein and an ion channel is indirect; the sensory receptor activates a G protein that, in turn, directly or indirectly induces the opening or closing of ion channels. The light-sensing cells (photoreceptors) in the mammalian retina function in this manner, as do the chemical-sensing cells (odorant receptors) in the nose. We will discuss the light-sensing system in some detail, as it is one of the bestunderstood sensory systems and also illustrates how a sensory system can adapt to varying intensities of stimuli, in this case the level of ambient light.

Visual Signals Are Processed at Multiple Levels

The human retina contains two types of photoreceptors, rods and cones, that are the primary recipients of visual stimulation. Cones are involved in color vision, while rods are stimulated by weak light like moonlight over a range of wavelengths. In bright light, such as sunlight, the rods become inactive, for reasons that we will discuss.

The photoreceptors synapse on layer upon layer of interneurons that are innervated by different combinations of photoreceptor cells. Some, like bipolar and ganglion cells, are in the retina (Figure 21-43); others are in several places in the brain. Different interneurons have receptive fields of different shapes; that is, they are stimulated by only certain photoreceptors. This property allows distinct patterns of light to be recognized. Ganglion cells transmit distinct visual characteristics in multiple parallel pathways; such parallel processing allows the separation of different aspects of the visual stimulus, such as color, form, and motion. All of these signals are processed and interpreted by the part of the brain called the visual cortex.

Figure 21-43. Some of the cells in the neural layer of the human retina.

Figure 21-43

Some of the cells in the neural layer of the human retina. The outermost layer of cells (in the rear of the eyeball) forms a pigmented epithelium in which the tips of the rod and cone cells are buried. Light focused from the lens passes through all of (more...)

Neurons in each subdivision of the cortex are responsive to different aspects of an object in the visual field, such as its orientation or its movement in a particular direction, its color, or its depth. At a variety of levels, from the retina up to the higher areas of the visual cortex, certain interneurons are activated optimally in response to direct stimulation of their receptive field but are inhibited by stimulation coming from just outside their receptive field. This phenomenon, called center-surround, allows the appreciation of areas of contrast between adjacent areas of the visual field. Neurons in the visual system do not form a picture in the brain, but somehow we can interpret all of these multiple, parallel signals generated by “seeing” an object.

The Light-Triggered Closing of Na+ Channels Hyperpolarizes Rod Cells

Membrane disks in the outer segments of rod cells contain rhodopsin, a light-sensitive protein also known as visual purple (Figure 21-44). In the dark, the membrane potential of a rod cell is about −30 mV, considerably less than the resting potential (−60 to −90 mV) typical of neurons and other electrically active cells. As a consequence of this depolarization, rod cells in the dark are constantly secreting neurotransmitters, and the bipolar neurons with which they synapse are continually being stimulated. Absorption of a pulse of light by rhodopsin in the outer segment of a rod cell causes the electric potential across the plasma membrane to become slightly more negative, to about −35 mV (Figure 21-45). This light-induced hyperpolarization in the outer segment spreads to the synaptic body and causes a decrease in neurotransmitter release.

Figure 21-44. (a) Diagram of the structure of a human rod cell.

Figure 21-44

(a) Diagram of the structure of a human rod cell. At the synaptic body, the rod cell forms a synapse with one or more bipolar neurons. Rhodopsin is a light-sensitive transmembrane protein located in the flattened membrane disks of the outer segment. (b) (more...)

Figure 21-45. A brief pulse of light causes a transient hyperpolarization of the rod-cell membrane.

Figure 21-45

A brief pulse of light causes a transient hyperpolarization of the rod-cell membrane. The membrane potential is measured by an intracellular microelectrode (see Figure 21-7).

The depolarized state of the plasma membrane of resting, dark-adapted rod cells is due to the presence of a large number of open nonselective ion channels that admit Na+ and Ca2+ as well as K+. The effect of light is to close these channels, causing the membrane potential to become more negative (see Figure 21-10). The more photons absorbed, the more channels are closed, the fewer Na+ ions cross the membrane from the outside, the more negative the membrane potential becomes, and the less neurotransmitter is released.

Remarkably, a single photon absorbed by a resting rod cell produces a measurable response, a hyperpolarization of about 1 mV, which in amphibians lasts a second or two. Humans are able to detect a flash of as few as five photons; these dim flashes have a maximum effect within ≈150 ms and the response returns to baseline within ≈200 ms. A single photon blocks the inflow of about 10 million Na+ ions due to the closure of hundreds of channels. Only about 30 – 50 photons need to be absorbed by a single rod cell in order to cause half-maximal hyperpolarization. The photoreceptors in rod cells, like many other types of receptors, exhibit the phenomenon of adaptation. That is, more photons are required to cause hyperpolarization if the rod cell is continuously exposed to light than if it is kept in the dark.

Let us now turn to three key questions: how is light absorbed; how is the signal transduced into the closing of ion channels; and how does the rod cell adapt to large variations in light intensity?

Absorption of a Photon Triggers Isomerization of Retinal and Activation of Opsin

The photoreceptor in rod cells, rhodopsin, consists of the transmembrane protein opsin covalently bound to the light-absorbing pigment 11-cis-retinal (Figure 21-46). Opsin has seven membrane-spanning α helices, similar to other receptors that interact with transducing G proteins. Rhodopsin is localized to the thousand or so flattened membrane disks that make up the rod’s outer segment; a human rod cell contains about 4×107 rhodopsin molecules.

Figure 21-46. Rhodopsin, the photoreceptor in rod cells, is formed from 11-cis-retinal and opsin, a transmembrane protein.

Figure 21-46

Rhodopsin, the photoreceptor in rod cells, is formed from 11-cis-retinal and opsin, a transmembrane protein. Absorption of light causes rapid photoisomerization of the cis-retinal to the trans isomer, forming the unstable intermediate meta-rhodopsin II, (more...)

The pigment 11-cis-retinal absorbs light in the visible range (400 – 600 nm). The primary photochemical event is isomerization of the 11-cis-retinal moiety in rhodopsin to all-trans-retinal, which has a different conformation than the cis isomer; thus the energy of light is converted into atomic motion. The unstable intermediate in which opsin is covalently bound to all-trans-retinal is called meta-rhodopsin II, or activated opsin. The light-induced formation of activated opsin is both extremely efficient and rapid. At 500 nm, the wavelength of maximum rhodopsin absorption, ≈20 percent of the photons that strike the retina lead to a signal-transduction event, an efficiency comparable to that of the best photomultiplier tubes. Of the 57 kcal/mol of energy of photons of 500 nm, 27 kcal/mol (47 percent) is stored in the activated meta-rhodopsin II intermediate, making it an effective and reliable trigger of the next signaling step. An absorbed photon triggers opsin activation in less than 10 ms. In contrast, the spontaneous isomerization of 11-cis-retinal is extremely slow — about once per thousand years. Because there is very little spontaneous activation of opsin, the system has a very good ratio of signal to noise.

Activated opsin is unstable and spontaneously dissociates, releasing opsin and all-trans-retinal. In the dark, all-trans-retinal is converted back to 11-cis-retinal in several reactions catalyzed by enzymes in rod-cell membranes; the cis isomer can then rebind to opsin, re-forming rhodopsin.

Cyclic GMP Is a Key Transducing Molecule in Rod Cells

Rod outer segments contain an unusually high concentration of 3,5-cyclic GMP (cGMP), about 0.07 mM. This nucleotide is the key transducing molecule linking activated opsin to the closing of cation channels in the rod-cell plasma membrane. Formation of cGMP from GTP is catalyzed by guanylate cyclase, a reaction that appears to be unaffected by light. However, light triggers activation of a cGMPspecific phosphodiesterase that is present in rod outer segments:

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As a result of this reaction, the cGMP concentration decreases upon illumination. Injection of cGMP into a rod cell depolarizes the cell membrane, and the effect is potentiated if an analog of cGMP that cannot be hydrolyzed is injected. Thus the high level of cGMP present in the dark acts to keep nucleotide-gated cation channels open; the light-induced decrease in cGMP leads to channel closing and membrane hyperpolarization.

Figure 21-47 depicts how light absorption by rhodopsin is coupled to activation of cGMP phosphodiesterase by transducin (Gτ), a signal-transducing G protein that is found only in rods. Like other trimeric G proteins, transducin has three subunits — Gτα, Gβ, and Gγ — and cycles between active and inactive states (see Figure 20-19). In the resting (dark) state, the α subunit has a tightly bound GDP (Gτα· GDP) and is incapable of affecting cGMP phosphodiesterase. Light-activated opsin catalyzes the exchange of free GTP for a GDP on the α subunit of transducin and the subsequent dissociation of Gτα· GTP from the β and γ subunits. Free Gτα· GTP then activates cGMP phosphodiesterase. (This pathway is similar to activation of adenylate cyclase by some Gs protein – coupled receptors; see Figure 20-16.) A single molecule of activated opsin in the disk membrane can activate 500 transducin molecules, which in turn activate cGMP phosphodiesterase; this is the primary stage of signal amplification in the visual system.

Figure 21-47. Coupling of light absorption by rhodopsin to activation of cGMP phosphodiesterase in rod cells.

Figure 21-47

Coupling of light absorption by rhodopsin to activation of cGMP phosphodiesterase in rod cells. In dark- adapted rod cells, a high level of cGMP acts to keep nucleotide-gated nonselective cation channels open and the membrane depolarized compared with (more...)

As with the α subunits of other G proteins, Gτα has an inherent GTPase activity, which slowly converts active Gτα· GTP back to inactive Gτα· GDP. Hydrolysis of GTP is accelerated when Gτα· GTP is bound to the phosphodiesterase and enhanced further by the action of a GTPase-activating protein (GAP) specific for Gτα· GTP (see Figure 20-22). Thus, in mammals Gτα remains in the active GTP-bound state only for a fraction of a second; cGMP phosphodiesterase rapidly becomes inactivated and the cGMP level gradually rises to its dark-adapted level. This allows rapid responses of the eye toward moving or changing objects. Once re-formed, Gτα· GDP combines with Gβ and Gγ, thus regenerating trimeric transducin.

Direct support for the role of cGMP in rod-cell activity has been obtained in patch-clamping studies using isolated patches of rod outer-segment plasma membrane, which contains abundant cGMP-gated cation channels. When cGMP is added to the cytosolic surface of these patches, there is a rapid increase in the number of open ion channels. The effect occurs in the absence of protein kinases or phosphatases, and cGMP acts directly on the channels to keep them open, indicating that these are nucleotide-gated channels. The channel protein contains four subunits each of which is able to bind a cGMP molecule (Figure 21-27b). Three or four cGMP molecules must bind per channel in order to open it; this allosteric interaction makes channel opening very sensitive to small changes in cGMP levels. Light closes the channels by activating cGMP phosphodiesterase, which acts to lower the level of cGMP.

Rod Cells Adapt to Varying Levels of Ambient Light

Cone cells are insensitive to low levels of illumination, and the activity of rod cells is inhibited at high light levels. Thus when we move from daylight into a dimly lighted room, we are initially blinded. As the rod cells slowly become sensitive to the dim light, we gradually are able to see and distinguish objects. This process of adaptation permits a rod cell to perceive contrast over a 100,000-fold range of ambient light levels; as a result, differences in light levels, rather than the absolute amount of absorbed light, are used to form visual images.

One process contributing to visual adaptation affects the cGMP level and the affinity of gated cation channels for the nucleotide. Light, as we noted, causes a reduction in cGMP levels; this leads to a closing of cGMP-gated channels that admit both Na+ and Ca2+ ions. Because Ca2+ is continuously extruded from the cells by Na+/Ca2+ antiporters (see Figure 15-3b), the Ca2+ concentration in the cytosol falls at high ambient light levels. This drop in Ca2+ concentration “resets” the light-sensing system to a new, higher baseline level by two mechanisms:

  • The enzyme that synthesizes cGMP is stimulated at low but not high concentrations of Ca2+ by a Ca2+-sensing protein. Thus the light-induced drop in Ca2+ leads to an increase in cGMP concentration, causing the cGMP-gated Na+/Ca2+ channels to remain open for longer periods.
  • At high cytosolic Ca2+ concentrations, a calmodulin-like protein binds to the cGMP-gated Na+/Ca2+ channels, reducing their affinity for cGMP. Conversely, a drop in Ca2+ causes the channels to bind cGMP more tightly and thus tend to remain open longer.

Both of these mechanisms favor opening of the cGMP-gated channels at high ambient light levels, so that a greater increase in light level is necessary to hydrolyze sufficient cGMP to close the same number of channels, and to generate the same visual signal than if the cells had not been exposed to light. In other words, at high ambient light, rod cells become less sensitive to small changes in levels of illumination.

A second process, affecting the protein opsin itself, participates in adaptation of rod cells to varying ambient light levels and also prevents overstimulation of the rod cell at very high ambient light (Figure 21-48). A rod-cell enzyme, rhodopsin kinase, phosphorylates light-activated opsin (O*) but not dark-adapted rhodopsin (O). Each opsin molecule has seven phosphorylation sites; the more sites that are phosphorylated, the less able O* is to activate transducin and thus induce closing of cGMP-gated cation channels. Because the extent of O* phosphorylation is proportional to the amount of time each opsin molecule spends in the lightactivated form, it is a measure of the background level of light. Under high light conditions, phosphorylated opsin is abundant and transducin activation is reduced; thus, a greater increase in light level will be necessary to generate a visual signal. When the level of ambient light is reduced, the opsins become dephosphorylated and transducin activation increases; in this case, fewer additional photons will be necessary to generate a visual signal.

Figure 21-48. Role of opsin phosphorylation in adaptation of rod cells to changes in ambient light levels.

Figure 21-48

Role of opsin phosphorylation in adaptation of rod cells to changes in ambient light levels. Light-activated opsin (opsin*), but not dark-adapted rhodopsin, is a substrate for rhodopsin kinase. The extent of opsin* phosphorylation is directly proportional (more...)

At high ambient light (such as noontime outdoors), the level of opsin phosphorylation is such that the protein arrestin binds to opsin. Arrestin binds to the same site on opsin as does transducin, totally blocking activation of transducin and causing a shutdown of all rod-cell activity. The mechanism by which rod-cell activity is controlled by rhodopsin kinase is similar to adaptation (or desensitization) of the β-adrenergic receptor to high levels of hormone (see Figure 20-47). Indeed, rhodopsin kinase is very similar to β-adrenergic receptor kinase (BARK), the enzyme that phosphorylates and inactivates only the ligand-occupied β-adrenergic receptor, and each kinase can phosphorylate the other’s substrate. Moreover, a homolog of arrestin binds to BARK-phosphorylated β-adrenergic receptors, blocking their interaction with Gs proteins.

Color Vision Utilizes Three Opsin Pigments

There are three classes of cone cells in the human retina. Each contains a different rhodopsin photopigment and absorbs light at a different wavelength (Figure 21-49). One absorbs mainly blue light, one green, and one red. As in rods, the relative amount of light absorbed by each class of cones is translated into electrical signals that are transmitted to the brain. There the overall pattern of absorbed light of different wavelengths is converted into what we perceive as color. All cone opsins bind the same retinal as found in rods, and the three cone opsins are similar to rod opsin and to each other. The unique absorption spectra of the three cone rhodopsins are due to different amino acid side chains that contact the retinal on the inside of rhodopsin and that affect its ability to absorb light of different wavelengths.

Figure 21-49. The absorption spectra of the three human opsins responsible for color vision.

Figure 21-49

The absorption spectra of the three human opsins responsible for color vision. Individual cone cells express one of the three cone opsins. The spectra were determined by measuring in a microspectrophotometer the light absorbed by individual cone cells (more...)

Study of cone opsins have led to molecular explanations of the different types of color blindness in humans. The “blue” opsin gene is located on human chromosome 7, while the “red” and “green” opsin genes are located next to each other, head-to-tail, on the X chromosome. The sequences of the red and green opsin genes are 98 percent identical, indicating that they arose from an evolutionary recent gene duplication. Furthermore, new world monkeys have only a single opsin gene on their X chromosome, whereas old world monkeys, which are more closely related to humans, have two. Two adjacent and almost identical genes can be expected to recombine unequally during gamete formation rather frequently, resulting in X chromosomes with only a green or only a red opsin gene. This results in red-green color blindness, a phenotype found in about 8 percent of males, who have a single X chromosome, but in only 0.64 percent of females, who have two X chromosomes.

Remarkably, because of polymorphisms in the red opsin genes, even individuals with “normal” color vision see colored objects differently. For instance, an alanine or serine may be located at position 180 of red opsin; this position is in the middle of the fourth membrane-spanning α helix, a region that contacts the bound retinal. The absorption maximum of the alanine form is ≈530 nm and of the serine form is ≈560 nm. Thus, individuals with serine at position 180 have a higher sensitivity to red light than others (see Figure 21-49); they “see” colors differently due to the change in a single nucleotide.

A Thousand Different G Protein – Coupled Receptors Detect Odors

The visual system functions efficiently with only four types of related photoreceptors, three cone opsins and one rod opsin. In contrast, the olfactory system utilizes a thousand homologous odorant receptors in responding to the millions of different chemicals we can smell.

Signal transduction in the olfactory system also differs from that in the visual system. The olfactory epithelium lining the air cavities in the nose contains thousands of sensory neurons, which have modified cilia extending from their apical (outward-facing) surface (Figure 21-50a). Each individual neuron expresses only one specific odorant receptor in the ciliary membrane and thus “senses” only one or a few odorants. All receptors are coupled to a single type of G protein, Golf, unique to olfactory epithelia. Stimulation of Golf, like Gs, activates adenylate cyclase, leading to an increase in the level of cAMP, which then binds to and opens a cAMP-gated cation channel unique to olfactory epithelia. (This channel is similar in structure to the cGMP-gated Na+/Ca2+ channel in the visual system; see Figure 21-27b). Opening of the cAMP-gated channel induces depolarization of the olfactory-cell membrane (rather than the hyperpolarization induced by activation of rhodopsin), initiating an action potential that is transmitted along the axon to the brain. Mice in which the gene encoding this cAMP-gated channel is disrupted are anosmic (cannot detect any odorant), attesting to the importance of the cAMP signaling pathway in olfaction.

Figure 21-50. General organization of the vertebrate olfactory system.

Figure 21-50

General organization of the vertebrate olfactory system. (a) The olfactory epithelium, lining the nasal air passages, contains olfactory neurons each of which expresses only one type of the ≈1000 different odorant receptors. These receptors are present (more...)

To isolate genes encoding odorant receptors, researchers first identified sequences of amino acids that were conserved in many other known G protein – coupled receptors. Assuming that odorant receptors were also coupled to G proteins, the workers prepared primers for the polymerase chain reaction (PCR) that would allow amplification of cDNA sequences encoding novel G protein – coupled receptors. These sequences then were used to screen a cDNA library prepared from olfactory epithelia, leading to identification and cloning of several hundred receptor genes.

The diversity of odorant receptors is entirely encoded in the nuclear genome; there is no evidence that somatic recombination contributes to this diversity, as it does in the immune system. In situ hybridization has shown that each of the receptor genes is expressed in only a few of the millions of olfactory epithelial cells, as might be expected for a receptor that binds a specific kind of odorant. For many years, researchers could not determine which receptor bound which odorant molecule(s). Recent experiments in which one particular receptor is overexpressed in olfactory epithelial cells have provided an approach for matching up particular receptors and odorants. Cells expressing the recombinant receptor are exposed to many candidate odorants and the effects on membrane potential are monitored. In one such study, the receptor stimulated by n-octanal (CH3(CH2)6CHO) was found to be unaffected by other molecules including the closely related octanoic acid (CH3(CH2)6COOH) and octanol (CH3(CH2)6CH2OH). Thus, odorant receptors indeed can distinguish closely related smelly substances.

Neurons expressing a given odorant receptor are dispersed throughout the olfactory epithelium, and thus are able to sample all of the air in the nasal passages. Importantly, axons from neurons expressing each type of odorant receptor project to the same segment of the olfactory bulb, the part of the brain that collects signals from olfactory sensory neurons (Figure 21-50b,c). All the sensory neurons that respond to a given odorant are thought to synapse with one or a group of interneurons that sum these signals and transmit them to other parts of the brain. Apparently, the same receptors that bind odorants at one end of a sensory neuron in some way “target” the axons such that they synapse with only one set of interneurons. (Formation of such topographic maps, which also occurs in the visual system, is discussed in Chapter 23.) The brain determines which odorant receptors have been activated by examining the spatial pattern of electric activity in the olfactory bulb.

It is striking that so many different odorant receptors were selected during evolution. How each olfactory sensory neuron “chooses” to express only one of the thousand odorant-receptor genes is an interesting problem currently being studied.


  •  Sensory transduction systems convert signals from the environment — light, taste, sound, touch, smell — into electric signals. These signals are collected, integrated, and processed by the central nervous system.
  •  The receptors that detect touch and stretch, heat, and capsaicin are gated cation channels that open in response to these stimuli. In contrast, the receptors that detect light or odor are coupled to G proteins.
  •  The retina contains rod cells, which respond to weak monochromatic light over a range of wavelengths, and three classes of cone cells, which respond to colors in bright light. Rhodopsin, the photoreceptor in rods and cones, occurs in four forms; each comprises one of four homologous opsin proteins linked to 11-cis-retinal.
  •  In rod cells, the light-induced isomerization of the 11-cis-retinal moiety in rhodopsin produces activated opsin, which then activates the signal-transducing G protein transducin (Gτ) by catalyzing exchange of free GTP for bound GDP on the subunit. Gτα· GTP, in turn, activates cGMP phosphodiesterase, which acts to lower the cGMP level. This reduction leads to closing of cGMP-gated Na+/Ca2+ channels, hyperpolarization of the membrane, and release of less neurotransmitter (see Figure 21-47).
  •  As the ambient light increases, the Ca2+ level in rod cells decreases, stimulating formation of cGMP and increasing the affinity of cGMP-gated Na+/Ca2+ channels for ligand. Phosphorylation of light-activated opsin and subsequent binding of arrestin to phosphorylated opsin inhibits its ability to activate transducin (see Figure 21-48). As a result of these mechanisms, a greater increase in light level is necessary to generate a visual signal at high light levels than at low levels, permitting rod cells to function over a 100,000-fold range of illumination.
  •  Each sensory neuron in the olfactory epithelium expresses a single type of odorant receptor, which “senses” only one or a few odorants.
  •  Stimulation of odorant receptors, which are coupled to Golf, leads to activation of adenylate cyclase. The resulting increase in cAMP opens cAMP-gated cation channels causing depolarization of the cell membrane and generation of an action potential.
  •  The axon of each odorant receptor is targeted to a particular point in the olfactory bulb, such that all cells expressing the same receptor synapse with one set of interneurons (see Figure 21-50). The spatial pattern of electric activity in the olfactory bulb reflects which receptors have been stimulated.



Capsaicin is so potent that its effects are calibrated by the spice industry as Scoville heat units, a scale developed by Wilbur Scoville in 1912. He calibrated the potency of peppers by diluting alcoholic extracts until he could just detect the pungency after placing a drop on his tongue. According to this scale, Bell peppers have a potency of <1 unit; jalapeño peppers, 103; habenero peppers, 105; and pure capsaicin > 107. [See D. Clapham, 1997, Nature 389:763.]

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By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21661