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131 Sensory Receptors and the CNS Sensory Pathways All animals gain information about the external and internal environment through sensory pathways that involve four basic steps: reception, transduction, transmission, and perception. Sensory reception is a process in which specialized structures called sensory receptors detect a stimulus. Some sensory receptors sense external stimuli, like pressure, temperature, chemicals, or light levels, while others detect internal stimuli, like blood pressure and oxygen levels. Ion channels in the plasma membrane respond to the stimulus by opening or closing, which changes the relative internal and external ion concentrations. As a result, the membrane potential changes through a process called sensory transduction. If the change in membrane potential is sufficiently large, an action potential is generated. Neurons carry the action potential to the central nervous system (CNS) in a process called transmission. Perception, the awareness of a stimulus, occurs at the brain.

Sensory receptors are present on neurons, or on cells associated with neurons. Sensory neurons have specialized dendrites that contain sensory receptors. For example, sensory receptors in skin have specialized structures called lamellae that deform in response to pressure, resulting in the sense of touch (Figure 1a). Other sensory receptors, such as those responsible for the sense of taste, are found in specialized epithelial cells that form synapses with neurons (Figure 1b). Regardless of cell type, a stimulus causes ion channels to open or close, which in turn causes the membrane to depolarize or hyperpolarize. If the membrane depolarizes enough, an action potential is typically generated. If the membrane becomes hyperpolarized, generation of an action potential is typically inhibited. Many sensory receptors generate action potentials even in the absence of a stimulus. In this case, a stimulus changes the rate at which action potentials occur. Neurons containing sensory receptors directly transmit the action potential to the CNS. Epithelial cells containing sensory receptors release synaptic vesicles in response to a stimulus. Neurotransmitter travels across the synaptic cleft and binds receptors on the sensory neuron, which generates an action potential that is transmitted to the CNS.

http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145683/1 1/4 4/7/2015 Sensory Receptors and the CNS | Principles of Biology from Nature Education Figure 1: Sensory receptors may be present on neurons or epithelial cells associated with neurons. (a) Lamellae are modified dendrites of neurons present in skin that are able to detect pressure. (b) Sensory cells in taste buds form synapses with neurons. © 2014 Nature Education All rights reserved.

Test Yourself

Explain how sensory pathways are regulated through membrane ion permeability.

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Many sensory receptors can detect very weak stimuli. For example, the human eye is capable of detecting a few photons of light. During transduction, the signal may be strengthened through a process called amplification. Amplification often involves signal transduction pathways. The signal transduction pathway that occurs when sugar binds a taste sensory receptor is shown in Figure 2. In this pathway, binding of sugar molecules to G protein­coupled receptors causes a signal transduction cascade that causes potassium channels to close and calcium channels to open. The membrane depolarizes, and an action potential is generated. As a result, synaptic vesicles fuse with the plasma membrane.

Figure 2: Signal transduction pathways result in amplification. Binding of a sugar molecule to a taste sensory receptor, which is located in a taste bud, results in a signal transduction cascade that amplifies the http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145683/1 2/4 4/7/2015 Sensory Receptors and the CNS | Principles of Biology from Nature Education signal. © 2014 Nature Education All rights reserved. If a stimulus continues without changing, the sensitivity of sensory receptors eventually changes through a reversible process called sensory adaptation. Sensory adaptation enables an animal to adjust to changing environmental conditions. For example, sensory adaptation allows the human eye to adjust to different light levels. When light intensity decreases, the pupil increases in size and light sensory receptors, called rods and cones, become more sensitive to light. The reverse process occurs when light intensity increases. Sensory adaptation also allows an animal to ignore stimuli that might otherwise be distracting. For example, sensory adaptation allows a person to tune out the hum of a refrigerator so they can focus on a conversation. Test Yourself

Compare the process of amplification to the process of sensory adaptation. What is the advantage of each process?

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Perception of sensory information. Through a process called integration, a number of separate weak stimuli can be aggregated so that they are perceived as a single strong stimulus by the CNS. Integration begins in the sensory cell when a number of separate stimuli are added together to depolarize the membrane sufficiently to generate an action potential. Integration continues during transmission, when many action potentials generated by a single sensory receptor are added into a single signal. Integration also occurs in the CNS, where signals from multiple sensory receptors are integrated into a single signal. As a result of integration, many small stimuli are perceived as a single strong stimulus. For example, if many auditory sensory cells generate many action potentials in rapid succession, the resulting integrated signal is perceived as a single, loud sound. If only a few auditory sensory cells generate a few action potentials, the integrated signal is perceived as a quiet sound. Sensory neurons all transmit action potentials to the CNS, yet various types of sensory information are perceived differently. This is because neurons associated with a particular sense transmit information to a specific area of the brain. For example, sensory information from the olfactory neurons in the nose travels to the olfactory bulb in the brain, which integrates the information and transmits the perception of smell. Sensory information from the ears travels to the auditory cortex, which integrates the information and transmits the perception of sound. Once the sense is identified, the information is transmitted to other parts of the brain so that it can be interpreted. For example, information about a perceived odor is transmitted to the amygdala, a region of the brain that stores emotional memories. If the odor was perceived during a prior emotional event, it can evoke both a memory of the event in which the odor was previously perceived and the emotion associated with the memory.

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WHY DOES THIS TOPIC MATTER? http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145683/1 3/4 4/7/2015 Sensory Receptors and the CNS | Principles of Biology from Nature Education Stem Cells Stem cells are powerful tools in biology and medicine. What can scientists do with these cells and their incredible potential?

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131 Sensory Receptors and the CNS Types of Sensory Receptors Receptors are classified into five different groups based on the type of stimulus to which they respond: mechanoreceptors, chemoreceptors, electromagnetic receptors, thermoreceptors, and nociceptors (pain receptors).

Mechanoreceptors. Mechanoreceptors detect forms of mechanical energy, including pressure and sound. Deformation of mechanoreceptors causes ion channels to open or close. Some mechanoreceptors in the skin, called lamellae, are modifications that occur at the tip of dendrites. Lamellae consist of many membrane layers that deform in response to pressure, which opens or closes ion channels and results in the sense of touch. The frequency of action potentials generated depends on the level of distortion of the membrane layers. Another type of mechanoreceptors found in dendrites that wrap around the base of hair follicles is able to detect movement of the hair.

Figure 3: Skin sensory receptors. Skin contains various types of receptors, including mechanoreceptors, temperature receptors, and pain receptors. Mechanoreceptors close to the skin surface are able to detect light pressure. Receptors deeper in the skin are only activated in response to strong pressure. © 2014 Nature Education All rights reserved. The mechanoreceptors in the human ear that are responsible for the sense of hearing are hair­like cilia that bend in response to pressure from a sound wave. When the cilia bend one way, the cell membrane becomes more permeable to ions and becomes depolarized. When the cilia bend the opposite way, ion permeability decreases and the membrane becomes http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145683/2 1/7 4/7/2015 Sensory Receptors and the CNS | Principles of Biology from Nature Education hyperpolarized. Fish and many aquatic amphibians have lateral line canals running along each side of the body (Figure 4). The canals are lined with mechanoreceptors that have sensory hairs contained in a gelatinous dome called a cupola. Water that enters the lateral canal through pores moves the hairs, generating an action potential. Fish and aquatic amphibians use the lateral line to sense swimming speed, the direction and strength of water current, and the presence of other moving animals, including predators and prey.

Figure 4: The lateral line canals of fish contain mechanoreceptors that detect water movement. Fish and some aquatic amphibians use lateral lines to detect movement of water. © 2011 Nature Education All rights reserved. Specialized mechanoreceptors present in blood vessels are used to regulate blood pressure. These receptors, which are called baroreceptors, are stimulated in response to increased blood flow. Another type of internal mechanoreceptor that detects stretching in skeletal muscle is used to ensure that muscle fibers do not stretch to the point that damage occurs.

Chemoreceptors. Chemoreceptors are able to detect the presence of certain molecules present in the air, water, food, and in the body. Chemoreceptors are responsible for both olfaction, which is the sense of smell, and gustation, which is the sense of taste. In terrestrial animals, olfaction occurs when small airborne molecules called odorants bind to protein receptors present on the http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145683/2 2/7 4/7/2015 Sensory Receptors and the CNS | Principles of Biology from Nature Education surface olfactory sensory cells. Gustation occurs when molecules present in solution called tastants bind protein receptors present on gustatory sensory cells. Aquatic animals are not exposed to airborne odorants and therefore do not have distinct senses of taste and smell. There are many different odorant receptors, and each one is specific for a particular molecule. The number of odorant receptors varies considerably from species to species. For example, mice have about 1,200 odorant chemoreceptors, whereas humans have about 400. As a result, mice have a more keen sense of smell than humans. In humans, olfactory receptors are primarily located in the nose, but the location of olfactory receptors can vary among species. For example in many insects, the antennae and mouthparts are olfactory organs. Hairs present on these organs contain dendrites that are olfactory sensory receptors. Gustatory receptors are often located in the mouth. Chemoreceptors are also used to detect pheromones, which are molecules that used for communication between different members of the same species. The male silkworm (Bombyx mori) detects a pheromone secreted by females and follows the scent to find a mate. Adult sea lampreys (Petromyzon marinus) appear to follow pheromones to navigate during migration, as they detect pheromones secreted by the larvae of the species and follow the pheromone trail to find their way back to suitable breeding sites. The sea lamprey is an invasive species that parasitizes fish, and studies are being conducted to determine if the population could be controlled by application of a pheromone that disrupts the migration route of adults (Figure 5). Internal chemoreceptors are responsible for maintaining homeostasis. For example, chemoreceptors in the mammalian brain can detect changes in salt concentration of the blood and signal thirst when the levels become too high.

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Figure 5: Sea lampreys use pheromones for navigation during migration. (a) The sea lamprey (Petromyzon marinus) is an invasive species that parasitizes fish. (b) The lamprey has a specialized mouth that allows it to latch onto and suck the blood out of their prey, which typically die from blood loss. (a) Jacana/Science Source. (b) Gary Meszaros/Science Source.

Electromagnetic receptors. http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145683/2 4/7 4/7/2015 Sensory Receptors and the CNS | Principles of Biology from Nature Education Electromagnetic receptors detect electromagnetic energy, such as electricity, magnetism, and light. Sensory receptors that detect light are called photoreceptors. Photoreceptors are found in visual organs that vary in complexity. One of the simplest, eyespots found in planaria (Planaria spp) can differentiate between light and dark (Figure 6). Planaria, which prefer darkness, orient themselves so the light level detected is at a minimum and the same for both eyespots. Once the animal is oriented in this direction, it swims away from the light. Insects and some worms have compound eyes, with hundreds or even thousands of light detectors. Compound eyes are particularly good at detecting motion. Many insects have superb color vision and some, like bees, can see ultraviolet light invisible to humans. Vertebrates, some jellyfish, spiders, worms, marine worms, octopi, and squid have single­lens eyes which, unlike compound eyes, are able to focus and therefore provide a sharper image.

Figure 6: Light detection in a planarian. A planarian turns its body until light levels are at a minimum and the same for both eyespots. Once the animal is oriented in this direction, it swims away from the light. © 2012 Nature Education All rights reserved. Some species of fish can generate electric currents and then use electromagnetic receptors to find prey that disrupt the currents. Other animals, such as sharks, rays, and even duck­billed platypuses (Ornithorhynchus anatinus), do not give off electricity but have electromagnetic receptors that detect the impulses given off by the muscles of their prey.

Some snakes, such as the North American rattlesnake (Crotalus spp), use infrared sensors to detect the body heat of their prey. Many species, including butterflies (Figure 7), salamanders, lobsters, turtles, birds, bats, and whales are believed to use a substance called magnetite to aid in migration. The magnetite allows these organisms to detect Earth's magnetic fields and serves as a navigating compass as they migrate towards breeding or wintering grounds.

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Figure 7: Migratory butterflies use magnetite to help navigate. Monarch butterflies (Danaus plexippus) can travel more than 4,000 kilometers across the United States to winter in California and Mexico. Experimental evidence suggests the butterflies use Earth's magnetic field at least in part to help determine their migratory route. Courtesy of Gene Nieminen/USFWS.

Thermoreceptors. Thermoreceptors open or close ion channels in response to changes in temperature. Some thermoreceptors are also activated by chemicals. For example, capsaicin, the chemical that make hot peppers taste hot, binds to and activates a receptor on the tongue that is also activated by heat. Thermoregulators, or animals that regulate their body temperature, have thermoreceptors in the brain that mediate a physiological or behavioral response to maintain temperature homeostasis. For example, in endotherms (animals that maintain body temperature through metabolism), a drop in body temperature causes an increase in metabolism. In ectotherms (animals that do not regulate body temperature through metabolism), a change in body temperature may cause the animal to move to a new location; for example, a cold lizard may move from the shade to the sun.

Pain receptors. Pain receptors, also called nociceptors, are sensory receptors that respond to injurious stimuli such as extreme heat, extreme pressure, and certain chemicals that might cause damage to tissues. In humans, many naked dendrites from other types of sensory receptors, including thermoreceptors, mechanoreceptors, and chemoreceptors, are able to sense painful stimuli. Test Yourself

What are the five categories of sensory receptors in animals?

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IN THIS MODULE

Sensory Pathways Types of Sensory Receptors Summary Test Your Knowledge

WHY DOES THIS TOPIC MATTER?

Stem Cells Stem cells are powerful tools in biology and medicine. What can scientists do with these cells and their incredible potential?

PRIMARY LITERATURE

Brain preplay anticipates the future Preplay of future place cell sequences by hippocampal cellular assemblies. View | Download

An artificial self­assembling retina from stem cells Self­organizing optic­cup morphogenesis in three­dimensional culture. View | Download

Innovation in Cannabis medicine Cannabinoid potentiation of glycine receptors contributes to cannabis­induced analgesia. View | Download

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Out Of Balance Learn about medical conditions that involve dysfunction in the vestibular system

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131 Sensory Receptors and the CNS

Test Your Knowledge

1. What term is used to describe the movement of neural signals to the central nervous system?

transmission reception transduction perception None of the answers are correct.

2. What is the definition of sensory adaptation?

Sensory adaptation is the process through which the response of a sensory system is reduced through continuous stimulation. Sensory adaptation is the process through which a signal is strengthened. Sensory adaptation is the process through which a stimulus is converted to a neural signal. Sensory adaptation is the process through which multiple small stimuli are combined into a single, perceived strong stimulus. None of the answers are correct.

3. Complete the following sentence: The process through which a sensory stimulus is converted into a neural signal is called...

transduction. reception. transmission. perception. None of the answers are correct.

4. Which of the following statements about the sense of taste is/are true?

Gustation is the sense of taste. The sense of taste involves detection of dissolved molecules. In aquatic animals, the sense of taste is indistinguishable from the sense of smell. Chemoreceptors detect molecules called tastants that are associated with a particular taste. All the statements are true.

5. Which of the following is true about the sense of smell?

Odors are sensed by mechanoreceptors, which are activated by the movement of air in the nose. The integration process in the brain is not complicated because there are so few distinct odors detectable. Unlike other senses, sensory adaptation does not occur with the sense of smell. Sensory integration occurs in the olfactory bulb of the brain, which then transmits the integrated information to other parts of the brain. In humans, gustation and olfaction are identical processes because they both involve chemoreception for producing the sensory signal.

6. Which of the following statements about sensory pathways is true?

Epithelial cells that contain sensory receptors form synapses with neurons. http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145683/4 1/2 4/7/2015 Sensory Receptors and the CNS | Principles of Biology from Nature Education Epithelial cells that contain sensory receptors do not generate action potentials. All cells that contain sensory receptors are neurons. All cells that contain sensory receptors are epithelial cells. None of the answers are correct.

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IN THIS MODULE

Sensory Pathways Types of Sensory Receptors Summary Test Your Knowledge

WHY DOES THIS TOPIC MATTER?

Stem Cells Stem cells are powerful tools in biology and medicine. What can scientists do with these cells and their incredible potential?

PRIMARY LITERATURE

Brain preplay anticipates the future Preplay of future place cell sequences by hippocampal cellular assemblies. View | Download

An artificial self­assembling retina from stem cells Self­organizing optic­cup morphogenesis in three­dimensional culture. View | Download

Innovation in Cannabis medicine Cannabinoid potentiation of glycine receptors contributes to cannabis­induced analgesia. View | Download

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Out Of Balance Learn about medical conditions that involve dysfunction in the vestibular system

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132 Auditory and Balance System

The auditory system is responsible for detecting sound. In insects, hairs differing in length and stiffness detect vibrations of different frequencies so the insect can detect sound. The auditory system of mammals consists of ears and brain structures that perceive sound. The mammalian auditory system is closely associated with the vestibular system, which is responsible for the sense of balance.

The Human Auditory System The human ear consists of three regions: the outer ear, the middle ear, and the inner ear. The outer ear includes pinnae (singular pinna), flaps of cartilage that funnel sound, and the auditory canal, which transmits sound to the middle ear. The tympanic membrane separates the outer ear from the middle ear, which contains three tiny bones called the malleus, incus, and stapes. Sound waves cause the tympanic membrane to vibrate, which causes the bones to move. When the stapes moves, it presses against another membrane called the oval window. The oval window is associated with a fluid­filled chamber of the inner ear called the cochlea. The cochlea contains cells with hair­like sensory receptors that are able to detect sound. The inner ear also contains a series of interconnected fluid­filled chambers that detect movement and orientation. A canal called the Eustachian tube allows air to move between the middle ear and the pharynx, which equalizes pressure in the middle ear.

Figure 1: The human ear. The mammalian ear includes the outer, middle, and inner ear. Sound is collected by the pinna and funneled into the auditory canal. The sound waves cause the tympanic membrane to vibrate, which moves bones in http://www.nature.com/principles/ebooks/principles­of­biology­104015/29155732/1 1/8 4/7/2015 Auditory and Balance System | Principles of Biology from Nature Education the middle ear. One of these bones, called the stapes, presses against the oval window, which is a membrane associated with the cochlea of the inner ear. The cochlea contains cells with hair­like sensory receptors that detect vibrations, which occur when the stapes presses against the oval window. Signals are transmitted to the brain via the auditory nerve. The semicircular canals are able to sense head position and momentum. © 2012 Nature Education All rights reserved.

The cochlea processes auditory signals. The cochlea is a snail­shaped organ that senses vibrations associated with sound and transmits this information to the brain (Figure 2). The cochlea has three fluid­filled chambers: the vestibular canal, the tympanic canal, and the cochlear duct. The cochlear duct sits between the vestibular canal and the tympanic canal. The vestibular canal and tympanic canal are filled with a fluid called perilymph, which is similar in ionic composition to other body fluids. The fluid in the cochlear duct, called endolymph, has a high potassium content and low sodium content unlike most other bodily fluids. The significance of this fluid composition will be discussed in the next section. The tectorial membrane separates the vestibular canal from the cochlear duct, and the basilar membrane separates the tympanic canal from the cochlear duct. The organ of Corti is a sound­sensing structure inside the cochlear duct that rests on the basilar membrane. Hair cells embedded in the organ of Corti have cilia that move when the basilar membrane moves. A space called the tunnel of Corti separates two types of hair cells: inner hair cells and outer hair cells. Inner hair cells, which are arranged in a single row, are the primary cells involved in sound reception. Inner hair cells synapse with sensory neurons that join the auditory nerve (also called the cochlear nerve), which transmits information to the brain. Outer hair cells, which are arranged in three rows, synapse with afferent neurons that receive input from the brain. Outer hair cells are believed to modulate the stiffness of the tectorial membrane, which serves to amplify the sound. Scanning electron micrographs of hair cells are shown in Figure 3.

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Figure 2: Cross­section of the cochlea. The cochlea is a snail­shaped organ with two fluid­filled canals: a vestibular canal and a tympanic canal. The cochlear duct runs between the two canals. The organ of Corti, which contains sound­detecting hair cells, rests on the basilar membrane of the cochlear duct. © 2014 Nature Education All rights reserved.

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Figure 3: Micrographs of the organ of Corti. (a) Outer hair cells (OHC) are separated from inner hair cells (IHC) by supporting pillar cells (PC). The hair­like cilia can be visualized at the top of both the OHC and the IHC. (b) In this view IHC are further magnified. © 2004 Nature Publishing Group Modified from Frolenkov, G. I., et al., Genetic insights into the morphogenesis of inner ear hair cells. Nature Reviews Genetics 5, 489–498 (2004) doi:10.1038/nrg1377. Used with permission. Test Yourself

Cilia on hair cells are sensory receptors that detect sound vibrations. Which class of receptors is able to detect sound vibrations? (The five types of sensory receptors, which were described in a separate module, are: mechanoreceptors, chemoreceptors, electromagnetic receptors, thermoreceptors, and nociceptors).

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Hair cells in the organ of Corti contain mechanoreceptors that sense sound. The tips of hair cells in the human ear have cilia that touch the tectorial membrane (Figure 3). The cilia contain ion channels that allow the passage of positively charged monovalent ions, such as sodium and potassium, in and out of the cell. When the cilia are still, some of the ion channels are open and others are closed. Because the endolymph surrounding the hair cells has a high potassium ion concentration relative to the inside of the cell, potassium tends to enter the cells, generating action potentials at a slow, steady rate. When the basilar membrane moves in response to a sound vibration, the cilia are pushed against the tectorial membrane, which causes them to move back and forth. Movement in one direction causes more monovalent ion channels to open, and movement in the other direction causes more monovalent ion channels to close. Thus, the membrane potential changes as the hair cells move back and forth. When more monovalent ion channels are open, the membrane depolarizes, which causes voltage­gated calcium channels to open. Calcium floods the cell, which stimulates the release of neurotransmitter into a chemical synapse with a neuron. As a result, the rate of action potential generation by the neuron increases. When more monovalent ion channels are closed, the membrane hyperpolarizes and the rate of action potential generation decreases. Micrographs of hair cells are shown in Figure 4.

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Figure 4: Cilia on hair cells in the organ of Corti bend in response to sound vibrations. Vibration of the basilar membrane push cilia against the tectorial membrane, causing the cilia to move back and forth. Movement of cilia in one direction causes monovalent ion channels to open, and movement in the other direction causes monovalent ion channels to close (monovalent ion channels not shown). When monovalent ion channels open, potassium floods the cell and causes the membrane to depolarize, which causes voltage­gated calcium channels to open. The resulting influx of calcium into the cell triggers the fusion of synaptic vesicles with the plasma membrane, which releases neurotransmitter into the synaptic cleft. Neurotransmitter binds receptors on sensory neurons, generating an action potential. When monovalent ion channels close, the membrane becomes hyperpolarized, and neurotransmitter release is inhibited. © 2014 Nature Education All rights reserved. Test Yourself

Speculate how hearing might be affected if the membrane inside the cochlea ruptures so that the endolymph mixes with the perilymph.

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How the cochlea distinguishes pitch and volume. Sound travels as a mechanical wave that can be defined by two basic parameters: amplitude and wavelength (Figure 5a). The amplitude, or height, http://www.nature.com/principles/ebooks/principles­of­biology­104015/29155732/1 6/8 4/7/2015 Auditory and Balance System | Principles of Biology from Nature Education

of a sound wave is correlated with the volume (loudness) of a sound. Louder sounds cause the basilar membrane to vibrate more vigorously, which triggers more frequent action potentials. The wavelength, or width, of a wave, is correlated with pitch. Wavelength is closely associated with frequency, which is defined as the number of waves that pass a stationary point per unit time. Shorter waves are able to pass a stationary point faster than longer waves, and for this reason frequency increases as wavelength decreases. Frequency is often measured in Hertz (Hz), which has units of cycles per second. High­frequency waves produce high­pitch sounds and low­frequency waves produce low­pitch sounds. Test Yourself

Which of the following is associated with a higher pitch sound, a sound wave with a long wavelength or a sound wave with a short wavelength?

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The ear is able to detect pitch because the basilar membrane becomes thinner and more flexible toward the middle of the cochlea. As the basilar membrane becomes thinner, the frequency it vibrates in response to decreases (Figure 5b). Sound waves enter the cochlea through the round window and travel inward through the vestibular canal to the middle of the cochlea, called the apex. Sound waves are propagated back out through the tympanic canal, until they exit through the round window.

Figure 5: The cochlea is able to sense the volume and pitch of a sound. (A) Loudness (volume) depends on the amplitude of a sound wave, and (B) pitch depends on frequency. (C) The basilar membrane varies in stiffness, and therefore different regions are able to detect different pitches. © 2014 Nature Education All rights reserved. http://www.nature.com/principles/ebooks/principles­of­biology­104015/29155732/1 7/8 4/7/2015 Auditory and Balance System | Principles of Biology from Nature Education Test Yourself

What would happen if the basilar membrane near the apex became stiffer?

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Loud sounds can cause hearing loss. Noise­induced hearing loss occurs because loud sounds over­stimulate hair cells, which can damage cilia or kill the cells. Noise­induced hearing loss may occur rapidly from exposure to a very loud sound such as an explosion, or it may occur gradually through exposure to moderately loud sounds. Personal listening devices can produce sounds loud enough to cause hearing loss through sustained exposure.

Cochlear implants allow auditory perception for certain types of deafness. Cochlear implants are devices that stimulate the auditory nerve in people who are deaf due to damage or abnormal development of hair cells. The implant consists of a small microphone that picks up sounds from the environment, a speech processer that selects the sounds picked up by the microphone, and a receiver implanted in the skull that converts these sounds into electrical signals that are transmitted to the auditory nerve. The signals can then travel normally to the brain for processing and response. The implant does not restore hearing; instead, it gives a representation of sound so the wearer of the implant can interpret and respond to audio stimuli. Implants are most effective when they are inserted into young children or adults who have recently become deaf. Adults were born deaf often have deficits in speech perception and the ability to interpret speech; therefore, they are unable to understand speech even with the device.

IN THIS MODULE

The Human Auditory System The Vestibular System Summary Test Your Knowledge

PRIMARY LITERATURE

How a jaw became an ear Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont. View | Download

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132 Auditory and Balance System The Vestibular System Two of fluid­filled chambers of the inner ear, the utricle and saccule, detect tilting of the head and acceleration. These chambers contain hair cells that have cilia projecting into a gelatinous material called the otolithic membrane. Heavy calcium carbonate crystals called otoliths that are embedded in the top of the otolithic membrane tend to move when an animal tilts its head or accelerates, which causes cilia of the hair cells to move and stimulates sensory neurons. Three semicircular canals, which are connected to the utricle, detect rotational movements. Each semicircular canal also contains hair cells with cilia embedded in a gelantenous cap called a cupula. When fluid inside the canal moves, the hairs bend, which stimulates sensory neurons. The three semicircular canals are oriented at different angles relative to one another and are therefore able to detect movement in all directions. Continuous movement of fluid in the semicircular canals leads to the sensation of dizziness. The organs of the vestibular system are illustrated in Figure 6.

Figure 6: Organs of the vestibular system. http://www.nature.com/principles/ebooks/principles­of­biology­104015/29155732/2 1/2 4/7/2015 Auditory and Balance System | Principles of Biology from Nature Education The vestibular system consists of the utricle, the saccule and three semicircular canals. The semicircular canals contain hair cells that are embedded in a gelatinous cap called a cupula. The endolymph moves during head rotation, pushing against the cupula and causing the hair cells to bend. The utricle and saccule contain hair cells that are embedded in the gelatinous otolithic membrane. Acceleration or head tilt causes heavy otoliths on top of the otolithic membrane to move. Hair cells of the utricle, saccule and semicircular canals form synapses with sensory neurons connected to the vestibular nerve. © 2014 Nature Education All rights reserved. Figure Detail

CHARGE syndrome is a genetic disorder that can cause deficits in the auditory and vestibular systems. CHARGE syndrome is a rare, autosomal­dominant disorder that results from abnormal development of the nervous system. The acronym CHARGE refers to various defects that occur in the disease (Coloboma (a hole) in the eye, Heart defects, Atresia (blockage) of the nasal passage, Retardation of growth and/or development, Genital and/or urinary abnormalities, and Ear abnormalities and deafness). One consequence of this disorder is that the semicircular canals often do not develop properly (Figure 7). Because of developmental abnormalities of the inner ear, individuals with CHARGE syndrome often have hearing loss and balance problems.

Figure 7: Computerized tomography scan of the semicircular canals. a) In a normal newborn, semicircular canals, which are rich in minerals, can be visualized using computerized tomography. The canals, which are inside the black circle, are ring­shaped. b) In an infant with CHARGE syndrome, the semicircular canals are absent. © 2007 Nature Publishing Group Sanlaville D. & Verloes, A. CHARGE syndrome: an update. European Journal of Human Genetics 15, 389–399 (2007) doi:10.1038/sj.ejhg.5201778. Used with permission.

IN THIS MODULE

The Human Auditory System The Vestibular System Summary Test Your Knowledge

PRIMARY LITERATURE

How a jaw became an ear Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont. View | Download

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132 Auditory and Balance System

Test Your Knowledge

1. Which of the following structures is located in the outer ear?

auditory nerve semicircular canals stapes cochlea pinna

2. Which of the following is the correct order of ear structures through which sound travels?

Round window, tympanic membrane, oval window, vestibular canal, tympanic canal tympanic membrane, oval window, vestibular canal, tympanic canal, round window tympanic membrane, round window, vestibular canal, tympanic canal, oval window tympanic canal, oval window, vestibular canal, tympanic membrane, round window tympanic membrane, oval window, tympanic canal, vestibular canal, round window

3. Which structures are located in the cochlea?

tympanic canal, basilar membrane, vestibular canal pinna, auditory canal, Eustachian tube malleus, incus, stapes auditory nerve, semicircular canal, auditory canal None of the answers are correct.

4. Which of the following statements about the human ear is/are true?

The organ of Corti contains sensory hair cells that detect sound. Pinnae are structures of the outer ear that funnel sound into the ear. The malleus, incus, and stapes are bones of the inner ear that move in response to sound vibrations. The auditory nerve carries sensory signals from hair cells to the brain. All the answer choices are correct.

5. Describe what happens when the cilia on hair cells bend in response to movement caused by a sound vibration.

The hairs are pulled upward by the tympanic membrane, which causes ion channels to open and the membrane to become depolarized. The hairs move back and forth. Movement of the hairs in one direction causes ion channels to open, and movement in the other direction causes the channels to close. When the ion channels open, calcium enters the cell and the membrane depolarizes. When the ion channels close, the membrane becomes hyperpolarized. The hairs move back and forth. Movement of the hairs in one direction causes ion channels to open, and movement in the other direction causes the channels to close. When the ion channels open, sodium enters the cell and the membrane depolarizes. When the ion channels close, the membrane becomes hyperpolarized. The hairs move back and forth. Movement of the hairs in one direction causes ion channels to open, and movement in the other direction causes the channels to close. When the ion channels open, potassium enters the cell and the membrane depolarizes. When the ion channels close, the membrane becomes hyperpolarized. The hairs bend, which causes ion channels to close. The membrane becomes hyperpolarized. http://www.nature.com/principles/ebooks/principles­of­biology­104015/29155732/4 1/2 4/7/2015 Auditory and Balance System | Principles of Biology from Nature Education

6. Which of the following components comes in contact with cilia on hair cells?

sensory neurons tympanic membrane stapes tectorial membrane oval window

7. A person who had a habit of listening to loud music through headphones took a hearing test. He found that he had lost 20% of his hearing from noise­induced trauma because many cilia on hair cells had broken off. Why did this cause hearing loss?

Without cilia, the hair cells cannot detect vibrations due to sound waves. The endolymph cannot vibrate without cilia, and this vibration is necessary to trigger a signal via the auditory nerve. The cochlea cannot transmit sound vibrations from the middle to the inner ear without cilia. Without cilia, sound waves cannot move from the vestibular canal, through the cochlea, and to the tympanic canal. None of the answers are correct.

8. Which of the following statements about inner ear organs is true?

The cochlea detects rotational movement, and the utricle and saccule detect tilting and acceleration. The utricle and saccule detect rotational movement, and the semicircular canals detect tilting and acceleration. The semicircular canals detect rotational movement, and the utricle and saccule detect tilting and acceleration. The utricle detects rotational movement, and the semicircular canals and saccule detect tilting and acceleration. The saccule detects rotational movement, and the utricle and cochlea detect tilting and acceleration.

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IN THIS MODULE

The Human Auditory System The Vestibular System Summary Test Your Knowledge

PRIMARY LITERATURE

How a jaw became an ear Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont. View | Download

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133 Vision

Our sense of vision is so important to us that we have given the part of the electromagnetic spectrum our eyes can see a special name: visible light. Yet some animals can see electromagnetic radiation that cannot be detected by human eyes. Bees for example can see ultraviolet light, which enables them to see patterns on flowers that are invisible to humans. Some snakes can see infrared light, which enables them to see the warmth of prey. Some animals, such as birds, have the ability to see magnetic fields and polarized light (light waves that travel in a single direction). Visual acuity and the ability to see in different environmental conditions also vary among species. Birds of prey can see moving prey at a great distance. Cats have a layer at the back of their eye that reflects light and enables them to see in the dark. Yet, regardless of the differences in what animals are able to see, the process of photoreception — the detection of light, is fundamentally the same in all animals. Photoreception is possible because specialized electromagnetic receptors called photoreceptors are able to detect light. Photoreceptors contain pigments that change shape when they are struck by a photon of light, which triggers a neural response.

Light­Detecting Organs Photoreceptors are clustered in light­detecting organs. The simplest type of light­detecting organ, called an eyespot, contains photoreceptors that can distinguish light from dark. Flatworms such as Planaria spp. use two eyespots on the top of their head to move toward darkness, where they are less likely to be detected by predators (Figure 1). In more complex animals, photoreceptors are integrated with other cell types to form eyes.

Figure 1: Planarian eyespots. Planaria have eyespots that allow them to detect light. Eric V. Grave/Science Source.

Insects and crustaceans (which are both in the phylum Arthropoda), as well as some segmented worms (phylum Annelida), have compound eyes, which consist of hundreds to thousands of individual light detectors called ommatidia (singular ommatidium) (Figure 2). Each ommatidium has a separate lens that focuses light. Sensory information from different ommatidia is integrated in the brain to form a visual image. Compound eyes are excellent at detecting motion, which is why it is so tricky to swat a fly. However, compound eyes cannot focus images.

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Figure 2: Compound eyes. The compound eyes on this robber fly (Holcocephala fusca) consist of many facets called ommatidia. Each ommatidium has a separate lens to focus light. Thomas Shahan/Science Source.

Vertebrates and many invertebrates such as some mollusks, worms, jellyfish, and spiders have single­lens eyes, or simple eyes. In many single­lens eyes, light enters through a small opening called the pupil. An iris contracts or expands to adjust the amount of light entering the pupil. A lens refracts, or bends, the light so that it hits the retina, the region of the eye that contains photoreceptor cells. In invertebrates, the lens moves back and forth to focus the image. In vertebrates, the lens changes shape to focus the image. The photoreceptor cells on the retina send signals through the optic nerve to the brain. Although they are remarkably similar in structure, the single­lens eyes of vertebrates and invertebrates appear to have evolved independently. Evidence for this convergent evolution includes the fact that blood vessels and nerve cells enter from the back of the retina in some invertebrates (e.g., squid), whereas they enter from the front of the retina in vertebrates.

Structure of the human eye. The human eye consists of multiple layers (Figure 3, left). The sclera is connective tissue that covers the eye. The sclera forms a transparent layer at the front of the eye called the cornea. The cornea, which has a convex shape, acts as an immovable lens that refracts light entering the eye so that it is directed toward the light­sensing region called the retina. A mucous membrane layer called the conjunctiva surrounds the sclera and keeps the eye lubricated. A thin, pigmented layer called the uvea lies inside the sclera. The pigments in the uvea prevent reflection of light within the eye. The uvea consists of three regions: the iris, the ciliary body, and the choroid. The iris, which is the colored portion of the eye, contains smooth muscles that expand and contract to adjust the amount of light entering the eye. The ciliary body surrounds the iris and has muscles that adjust the shape of the lens to focus the light. The rest of the uvea, called the choroid, provides oxygen and nourishment to cells of the retina, which forms the layer inside the choroid. The lens, which consists of transparent protein, is located behind the iris. A watery substance called aqueous humor fills the space in front of the lens. The ciliary body produces aqueous humor and provides nourishment to the lens and cornea. If the ducts that drain the aqueous humor become blocked, http://www.nature.com/principles/ebooks/principles­of­biology­104015/29155738/1 2/6 4/7/2015 Vision | Principles of Biology from Nature Education pressure in the eye can build, a condition called glaucoma. Left untreated, glaucoma can result in blindness. The vitreous humor is a gel­like substance that fills the inside of the eye behind the lens. Test Yourself

Which parts of the eye are involved in focusing light?

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The retina consists of photoreceptors that detect light and neurons that transmit sensory information to the optic nerve (Figure 3, right). The outermost layer of the retina, called the pigment epithelium, consists of a single layer of cells that provide nourishment to photoreceptor cells in the layer above. Photoreceptor cells in most vertebrates are modified neurons called rods and cones. Rods, which are tall and thin, are much more sensitive to light than cones and are responsible for night vision. Cones, which have a conical shape, provide color vision. The layer of the retina above the rods and cones contains two other types of neurons called bipolar cells and horizontal cells. Above the bipolar and horizontal cells is a layer of neurons called amacrine cells, and above the amacrine cells is a layer of neurons called ganglion cells. Bipolar cells, horizontal cells, and amacrine cells transmit information between the rods and cones and the ganglion cells. Ganglion cells in turn transmit information to the optic nerve, which extends from the back of the eye to the brain.

Figure 3: Parts of the human eye. In the human eye, a lens focuses light on a photosensitive layer called the retina. The pupil controls the amount of light entering the eye. © 2012 Nature Education All rights reserved. Test Yourself

The ratio of rods to cones varies among species. Compared to species that are active during http://www.nature.com/principles/ebooks/principles­of­biology­104015/29155738/1 3/6 4/7/2015 Vision | Principles of Biology from Nature Education

the day, do you think species that are active at night would have more or fewer rods? Why?

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In all vertebrate animals, the region of the eye where the optic nerve exits the eye is called the optic disk. There are no photoreceptors in the optic disk, which creates a blind spot. We are usually unaware of the blind spot because the brain is good at filling in missing parts of images. To test your blind spot, make an "R" and an "L" about six inches apart on a piece of paper (R to the left and L to the right). Bring the paper about six inches from your face and close one eye. If you close your left eye, focus on the L, and if you close your right eye, focus on the R. Slowly move the paper away with your eye focused on the letter. Eventually when the paper is about an arm's length away, the letter you are not focused on will enter your blind spot and disappear.

A single­lens eye shares similarities with a microscope. A single­lens eye shares many common features with a microscope. In both cases, light that illuminates an object passes through an aperture called an iris that contracts or expands to adjust the amount of light that passes through. A compound microscope typically contains two lenses that refract light: an objective lens and an ocular lens. The focus knob moves the objective lens up and down, which focuses the image. Likewise, the single­ lens eye has a cornea and a lens that both refract light. The eye focuses the image by changing the shape of the lens. In both the microscope and the eye, the lens inverts the image.

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Figure 4: A light microscope is similar to the simple eye. Both a light microscope and a simple eye have an iris that adjusts light levels and lenses that refract and focus the image. © 2011 Nature Education All rights reserved.

IN THIS MODULE

Light­Detecting Organs Through Focusing, the Eye Can View Objects Near and Far Color Vision Visual Signal Transduction and Transmission Future Perspectives Summary Test Your Knowledge

WHY DOES THIS TOPIC MATTER?

Stem Cells Stem cells are powerful tools in biology and medicine. What can scientists do with these cells and their incredible potential?

PRIMARY LITERATURE

Fruit flies use spatial memory to navigate Visual place learning in Drosophila http://www.nature.com/principles/ebooks/principles­of­biology­104015/29155738/1 5/6 4/7/2015 Vision | Principles of Biology from Nature Education

melanogaster. View | Download

An artificial self­assembling retina from stem cells Self­organizing optic­cup morphogenesis in three­dimensional culture. View | Download

Gene therapy restores color vision in monkeys Gene therapy for red–green colour blindness in adult primates. View | Download

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133 Vision Color Vision The human eye has two main types of photoreceptors: rods and cones. Rods, which are able to detect very low levels of light, are used for night vision. Rods cannot detect color, which is why images appear gray in dim lighting. Cones require more light to become activated, but are able to detect different colors. Three types of cones are found in the human retina called S­cones, M­ cones, and L­cones. Each cone type contains a pigment that is optimized to absorb light within a certain range of wavelengths. S­cones have pigments that absorb visible light of a relatively short wavelength, which appears blue. M­cones have pigments that absorb light of a medium wavelength, which appears green. L­cones absorb light of a relatively long wavelength, which appears red. The absorption spectra for the pigments found in the different cone cells overlaps, which means more than one cone can be simultaneously stimulated. When this happen, intermediate colors are perceived. The human retina contains about 5 million cones and 100 million rods. However, the density of the two types of photoreceptors varies in different parts of the retina. The center of the visual field, called the fovea, has exclusively cones. Cones, which require more light for activation, are concentrated in the fovea because light is brightest here. In daylight, visual acuity is best at the center of field of vision because light from this part of the field tends to strike cones. Rods become more plentiful toward the periphery of the retina. Most primates, fishes, amphibians, reptiles, and birds have excellent color vision. However, the retinas of some nocturnal mammals contain mostly rod photoreceptors. As a result, these animals are able to see well at night but have poor color vision.

IN THIS MODULE

Light­Detecting Organs Through Focusing, the Eye Can View Objects Near and Far Color Vision Visual Signal Transduction and Transmission Future Perspectives Summary Test Your Knowledge

WHY DOES THIS TOPIC MATTER?

Stem Cells Stem cells are powerful tools in biology and medicine. What can scientists do with these cells and their incredible potential?

PRIMARY LITERATURE

Fruit flies use spatial memory to navigate Visual place learning in Drosophila melanogaster. View | Download

An artificial self­assembling retina from stem cells Self­organizing optic­cup morphogenesis in three­dimensional culture. View | Download http://www.nature.com/principles/ebooks/principles­of­biology­104015/29155738/3 1/2 4/7/2015 Vision | Principles of Biology from Nature Education

Gene therapy restores color vision in monkeys Gene therapy for red–green colour blindness in adult primates. View | Download

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133 Vision Through Focusing, the Eye Can View Objects Near and Far Through a process called accommodation, the lens of the human eye changes shape to focus light rays from objects at varying distances. When the eye focuses on a close object, the lens becomes nearly round. When the eye focuses on a distant object, the lens becomes much flatter. However, if the curvature of the lens or the shape of the eyeball is wrong, the image cannot be focused. Nearsightedness, or myopia, occurs when the cornea or lens is too curved or the eyeball is too long (Figure 5, left). As a result, light from a distant object is refracted too much and comes into focus in front of the retina, causing the object to appear blurry. Nearsightedness can be corrected with concave lenses that cause light rays to converge further back in the eye or by surgically reducing the curvature of the cornea. Farsightedness, or hyperopia, occurs when the cornea or lens is too flat or the eyeball is too short (Figure 5, middle). As a result, light from a close object is not refracted enough and comes into focus behind the retina. Farsightedness can be corrected with convex lenses or by increasing the curvature of the cornea. Astigmatism is a condition in which vision at any distance is blurred because the cornea or lens is irregularly shaped (Figure 5, right). Astigmatism often occurs in conjunction with near­ or far­ sightedness. Like near­ and farsightedness, astigmatism can be corrected by surgically changing the curve of the cornea or with corrective lenses.

Figure 5: Cause of nearsightedness, farsightedness, and astigmatism. In the nearsighted eye, light from distant objects is focused in front of the retina. In the farsighted eye, light from distant objects is focused behind the retina. In the astigmatic eye, light from any distance is out of focus. © 2011 Nature Education All rights reserved.

Test Yourself

Bifocals have two lenses, one that is concave and one that is convex. One lens is meant to correct myopia and the other corrects hyperopia. Which lens corrects which condition?

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IN THIS MODULE

Light­Detecting Organs Through Focusing, the Eye Can View http://www.nature.com/principles/ebooks/principles­of­biology­104015/29155738/2 1/2 4/7/2015 Vision | Principles of Biology from Nature Education Objects Near and Far Color Vision Visual Signal Transduction and Transmission Future Perspectives Summary Test Your Knowledge

WHY DOES THIS TOPIC MATTER?

Stem Cells Stem cells are powerful tools in biology and medicine. What can scientists do with these cells and their incredible potential?

PRIMARY LITERATURE

Fruit flies use spatial memory to navigate Visual place learning in Drosophila melanogaster. View | Download

An artificial self­assembling retina from stem cells Self­organizing optic­cup morphogenesis in three­dimensional culture. View | Download

Gene therapy restores color vision in monkeys Gene therapy for red–green colour blindness in adult primates. View | Download

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133 Vision Visual Signal Transduction and Transmission The photoreceptor in both rods and cones, called rhodopsin, is composed of a pigment called retinal and a protein called opsin. Different types of opsin molecules absorb different wavelengths of light. Rhodopsin is embedded in a highly folded membrane found in both rods and cones. When a photon of light strikes rhodopsin, the retinal pigment changes from a cis­ to a trans­ configuration. The activated rhodopsin activates a signal transduction cascade that causes sodium channels to close. As a result the membrane, which is normally depolarized, becomes hyperpolarized, inhibiting the release of neurotransmitter. Glutamate, the neurotransmitter found in rods and cones, has an inhibitory effect on some neurons and a stimulatory effect on others. Unlike most neurons, which produce an all­or­none action potential, rods and cones produce a graded potential that varies with the level of activation. Thus, the amount of neurotransmitter released varies with light intensity: maximum neurotransmitter release occurs in darkness, and neurotransmitter releases decreases with increasing light intensity. This graded release of neurotransmitter enables the eye to distinguish different levels of light. Besides the photoreceptors, the retina contains four additional neural cell types: horizontal cells, bipolar cells, amacrine cells, and ganglion cells (see Figure 6). Sensory information from rods and cones is transmitted to the ganglion cells through bipolar cells. However, signal transmission may be direct or indirect. In direct transmission, the signal passes from a photoreceptor to a bipolar cell to a ganglion. In indirect transmission the signal may pass through a horizontal cell before being transmitted to a bipolar cell, or it may pass through an amacrine cell from the bipolar cell. When this happens, the horizontal cells and amacrine cells modulate the signal. For example, horizontal cells suppress signal transmission from photoreceptors that are only mildly stimulated but not from those that are strongly stimulated, which filters out noise and sharpens the image. Some amacrine cells modulate sensitivity to light, which enables the eye to function under variable light conditions.

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Figure 6: A rod cell forms a synapse with a bipolar cell. A rod cell (red) forms a synapse with a bipolar cell (green). The outer segment of the rod cell is stained purple. © 2010 Nature Publishing Group Markus, A. Speedy rod signaling. Nature Neuroscience 13, 410 (2010) doi:10.1038/nn0410­410. Used with permission. Bipolar cells, horizontal cells, and amacrine cells produce graded potentials similar to the ones produced by rods and cones. However, ganglion cells produce action potentials. Action potentials, which are all­or­none events, occur only if the membrane becomes sufficiently depolarized. Axons of ganglion cells join the optic nerve, which transmits sensory information from the eye to the brain. The optic nerves of the two eyes converge in a region called the optic chiasm. At the optic chiasm, images from the left field of view of both eyes are transmitted to the right side of the brain, and images from the right field of view are transmitted to the left side of the brain (Figure 7). In this manner, visual information from the two eyes is integrated. From the optic chiasm the axons travel to the lateral geniculate nuclei (LGN), which is part of the thalamus. In the lateral geniculate nuclei, ganglion axons form synapses with neurons that travel to the primary visual cortex of the occipital lobe. The primary visual cortex is able to form a map of the image because different parts of the cortex are stimulated by different parts of the image: the left side processes information from the right field of view, and the right side processes information from the left field of view. The upper part of the visual cortex processes information from the lower field of view, and the lower part process information from the upper field of view. Thus the image, which was inverted by the lenses of the eyes, is flipped to the upright orientation by the primary visual cortex.

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Figure 7: Processing of visual information occurs in the visual cortex. The optic nerves split at the optic chiasm and then travel to the lateral geniculate nucleus. In the lateral geniculate nucleus, axons from ganglion cells form synapses with visual cortex neurons, which inverts the upside down image formed in the retina. © 2012 Nature Education All rights reserved.

IN THIS MODULE

Light­Detecting Organs Through Focusing, the Eye Can View Objects Near and Far Color Vision Visual Signal Transduction and Transmission Future Perspectives Summary Test Your Knowledge

WHY DOES THIS TOPIC MATTER?

Stem Cells Stem cells are powerful tools in biology and medicine. What can scientists do with these cells and their incredible potential?

PRIMARY LITERATURE

Fruit flies use spatial memory to navigate Visual place learning in Drosophila melanogaster. View | Download

An artificial self­assembling retina from stem cells Self­organizing optic­cup morphogenesis in three­dimensional culture. View | Download

Gene therapy restores color vision in monkeys Gene therapy for red–green colour blindness in adult primates. View | Download

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133 Vision

Test Your Knowledge

1. What does rhodopsin respond to in all types of photoreceptors in the eyes?

color motion pressure light electricity

2. When light passes into a human eye, which of the following structures does it strike last?

pupil vitreous humor cornea iris retina

3. Complete the following sentence: Animals with compound eyes see images through...

a single lens like the human eye. a single ommatidium with hundreds to thousands of lenses. hundreds to thousands of ommatidia, each with its own lens. a single facet each with hundreds to thousands of lenses. None of the answers are correct.

4. Which of the following statements about visual processing is true?

At the optic chiasm, all the axons from ganglions of the left eye travel to the right side of the brain, and all the axons from ganglions of the right eye travel to the left side of the brain. At the optic chiasm, all the axons from ganglions of the left eye travel to the left side of the brain and all the axons from ganglions of the right eye travel to the right side of the brain. At the optic chiasm, images from the left field of view of each eye are transmitted to the right side of the brain and images from the right field of view are transmitted to the left side of the brain. At the optic chiasm, images from the left field of view of each eye are transmitted to the left side of the brain and images from the right field of view are transmitted to the right side of the brain. None of the answers are correct.

5. Complete the following sentence: The outermost layer of the retina in which rods and cones are embedded is called the...

vitreous humor. choroid. pigment epithelium. sclera. None of the answers are correct.

6. Complete the following sentence: The only type of cells in the retina that produce http://www.nature.com/principles/ebooks/principles­of­biology­104015/29155738/7 1/2 4/7/2015 Vision | Principles of Biology from Nature Education an action potential are the...

photoreceptor cells. bipolar cells. ganglion cells. horizontal cells. None of the answers are correct.

7. If an organism lost the function of the ciliary body, which specific function would not occur in the eye?

The lens and cornea would become dry. The lens could not focus. The pupil could not expand and contract. The lens could not focus and the lens and cornea would become dry. The lens could not focus, the pupil could not expand and contract, and the lens and cornea would become dry.

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IN THIS MODULE

Light­Detecting Organs Through Focusing, the Eye Can View Objects Near and Far Color Vision Visual Signal Transduction and Transmission Future Perspectives Summary Test Your Knowledge

WHY DOES THIS TOPIC MATTER?

Stem Cells Stem cells are powerful tools in biology and medicine. What can scientists do with these cells and their incredible potential?

PRIMARY LITERATURE

Fruit flies use spatial memory to navigate Visual place learning in Drosophila melanogaster. View | Download

An artificial self­assembling retina from stem cells Self­organizing optic­cup morphogenesis in three­dimensional culture. View | Download

Gene therapy restores color vision in monkeys Gene therapy for red–green colour blindness in adult primates. View | Download

SCIENCE ON THE WEB

How do Scientists Study Olfaction? Learn about odorant brain activation maps from Michael Leon's lab

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134 Taste and Smell The Senses of Taste and Smell The aroma of pizza wafting from a neighbor's door may draw you toward food. A bitter taste may cause you to spit out poison before it causes harm. Humans, like other animals, rely on their ability to detect chemicals in the environment to find safe sources of food and avoid harmful substances. Many animals also use chemical cues to detect the presence of a predator, find a mate, or navigate. The sense of smell, called olfaction, involves detecting molecules present in the air. The sense of taste, called gustation, involves detecting molecules in solution. Animals that live in water do not have distinct senses of taste and smell. In both olfaction and gustation, sensory receptors called chemoreceptors bind molecules and trigger a series of events that leads to a neural response. Perception of olfaction and gustation are closely associated in the brain, which is why food doesn't taste right when olfactory receptors in your nose are blocked by a head cold. Other senses may also come into play in identifying foods. For example, vegetable oil has very little flavor, but the tongue can sense its presence by its thick, slippery texture. In humans, most olfactory receptors are found in the nose and most gustatory receptors are found on the tongue. However, these receptors may be found in other areas. For example, taste receptor genes are expressed throughout the digestive tract. Scientists believe receptors found in locations other than the tongue may regulate appetite and metabolism. Sperm have olfactory receptors that enable them to swim toward an egg. In insects, olfactory receptors are typically found on the antennae, while gustatory receptors can be found on mouthparts, antennae, and feet (Figure 1).

Figure 1: Insects such as the fruit fly (Drosophila melanogaster) can taste with their feet. Fruit fly olfactory receptors are located on the antennae and on an organ near the mouth called the maxillary palp. Gustatory receptors are located on the mouthparts and legs and in other locations. Robert Noonan/Science Source.

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134 Taste and Smell Gustation: the Sense of Taste Gustation involves the ability to detect molecules present in solution called tastants. In humans, five general types of tastants have been identified: sweet, sour, bitter, salty, and umami. Umami is a Japanese word that describes the pleasant, savory flavor of certain meats and cheeses. The amino acid glutamate, which is found in many proteins, is the tastant responsible for umami flavor. Monosodium glutamate (MSG), a food additive that enhances savory flavor, binds to umami receptors on the tongue. The surface of the tongue is covered with bumps called papillae (singular papilla). Different types of papillae, each with a characteristic shape and size, exist on different parts of the tongue (Figure 2). Taste buds are located in depressions surrounding the papillae. Each taste bud contains a bundle of taste receptor cells, which are specialized epithelial cells. Hair­like projections from the taste sensory receptor cells extend into a space above the taste bud. An opening in the epithelium, called a taste pore, allows tastant molecules dissolved in saliva to enter the opening and bind chemoreceptors, called gustatory receptors, on these hair­like projections. Taste sensory receptor cells form synapses with sensory neurons. Each taste receptor cell expresses a single type of gustatory receptor capable of binding one of the five types of tastants. Previously, scientists thought that each type of gustatory receptor was clustered in a specific region of the tongue. However, more recent evidence indicates that each taste bud contains cells that are able to detect all five types of tastants.

Figure 2: The location of taste buds on the human tongue. Taste buds are located in depressions associated with papillae. An opening in the epithelium, called a taste pore, allows tastants to bind receptors on taste sensory receptor cells. Each taste bud contains sensory receptor capable of detecting sweet, bitter, sour, salty, and umami flavors. © 2014 Nature Education All rights reserved.

http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145703/2 1/3 4/7/2015 Taste and Smell | Principles of Biology from Nature Education Types of taste receptor. In humans, only one receptor type is responsible for detecting tastants with a sweet flavor, such as table sugar. Humans also have a single receptor type capable of detecting umami flavor. In contrast, more than 30 types of receptors capable of detecting tastants with a bitter flavor have been identified. The receptors for sweet, umami and bitter flavors are G protein­ coupled receptors (Figure 3). Binding of tastant to the G protein­coupled receptor causes a signal transduction cascade, which in turn causes ion channels to open. The resulting membrane depolarization stimulates the release of neurotransmitter into the synaptic cleft. The neurotransmitter binds a sensory neuron, which induces an action potential that is sent to the brain. Receptors that detect sour tastants belong to family of protein channels called transient receptor potential (TRP) channels. TRP channels are non­ selective cation channels. Upon binding of tastant to the TRP channel, the ion channel opens and allows cations such as sodium, calcium and magnesium to cross the plasma membrane. Some TRP channels also act as thermoreceptors that sense heat. For example, TRPVI, the receptor that binds capsaicin (the molecule that makes hot peppers taste hot) is also activated by heat. TRPVI also acts as a nociceptor (pain receptor) responsible for the painful, burning sensation associated with food that is spicy or hot in temperature. Emerging evidence indicates that the epithelial sodium channel (ENaC) is responsible for detection of sodium in table salt. However, scientists have not yet determined which receptors are responsible for sensing potassium and calcium ions. Receptors for other tastants have yet to be identified, and some investigators believe that new gustatory receptors may still be discovered that cannot be classified into the five known categories.

Figure 3: G protein­coupled receptor tastant­signaling pathway. Sweet, umami, and bitter tastants bind G protein­coupled receptors, which triggers a signal transduction cascade.

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Because of a mutation, an animal lacks papillae on the tip of its tongue. How might this affect the animal's ability to detect different types of tastes?

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Perception of taste depends on the type of gustatory cell activated. To humans, phenyl­B­D­glucopyranoside (PBDG) has an unpleasant, bitter taste. However mice, which normally avoid bitter chemicals, will drink water with PBDG dissolved in it. For this reason, scientists believe mice are unable to taste PBDG. In 2005, Ken Mueller and his team produced genetically engineered mice that expressed the human PBDG receptor in bitter taste cells and found that these mice avoided drinking water containing PBDG. In a second group of mice, the PBDG receptor was expressed in sweet taste cells. These mice preferred PBDG water to plain water, presumably because it tasted sweet to them. These results indicate that the taste perceived depends on the type of taste cell activated.

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134 Taste and Smell Olfaction: the Sense of Smell Olfaction involves the detection of airborne molecules called odorants. Odorant molecules can travel across a long distance, and for this reason olfaction is used for a variety of purposes, such as communication among members of a species or detecting distant food sources. Odorants such as the one produced by the striped skunk (Mephitis mephitis) can also be used to repel potential predators (Figure 4).

Figure 4: Odorants can be used for defense. The striped skunk (Mephitis mephitis) squirts a foul­smelling liquid that discourages potential predators. Greg Dimijian/Science Source.

The sensory cells responsible for olfaction are specialized neurons located in the epithelium surrounding the nasal cavity (Figure 5). These neurons have dendrites that extend into the nasal cavity and axons that extend into the olfactory bulb of the brain. The dendrites in the nasal cavity contain odorant receptors that are a type of G protein­coupled receptors. Binding of odorant molecules to the G protein­coupled receptors triggers a signal transduction cascade that stimulates the production of cAMP. The cAMP opens calcium and sodium channels in the plasma membrane. Calcium and sodium flow into the cell, which produces a membrane potential. If the membrane becomes sufficiently depolarized, an action potential is generated and transmitted to the olfactory bulb in the brain. In the olfactory bulb, the olfactory neurons form synapses with other neurons that send action potentials to other parts of the brain such as the amygdala, which stores and retrieves memories associated with odors.

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Figure 5: Olfactory neurons are responsible for the sense of smell. Olfactory neurons, which are located in the epithelium surrounding the nasal cavity, have dendrites extending into the nasal cavity and axons extending into the olfactory bulb of the brain. The dendrites contain G protein­coupled receptors that bind odorant molecules. © 2014 Nature Education All rights reserved.

Each olfactory receptor binds a particular molecule type. There are many different olfactory receptors; in fact, in vertebrates, the olfactory receptor constitutes the largest gene family. Humans have about 900 olfactory receptor genes and mice have 1,500. The ability to smell differs among species. For example, a dog's sense of smell is about 10,000 times as acute as a human's. In part, this is because a dog has about forty times as many odorant receptors in its nose than a human does. The dog's airway passage is also designed such that airflow for olfaction is separated from airflow for breathing, which improves sensitivity. Test Yourself

In mammals, how does olfaction differ from gustation?

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134 Taste and Smell

Test Your Knowledge

1. In mammals, what must an odorant bind to for it to be detected?

a gustatory receptor on a sensory neuron the epithelium surrounding the nasal cavity an olfactory bulb an odorant receptor on a sensory epithelial cell an odorant receptor on a sensory neuron

2. Which of the following correctly describes chemoreceptors?

They are receptor proteins that are able to bind chemicals. They are involved in olfaction but not gustation. They are enzymes that are able to bind chemicals. They are involved in gustation but not olfaction. None of the answers are correct.

3. Which term is synonymous with the sense of taste?

gustation olfaction assimilation action potential nociception

4. Which of the following statements about taste buds is true?

A taste bud contains a bundle of sensory cells, and each sensory cell responds to one type of tastant. Taste buds that detect one type of tastant are clustered in a particular region on the tongue. Taste buds are located on top of papillae. A taste bud contains a bundle of sensory cells, and all the sensory cells in a taste bud respond to the same type of tastant. None of the answers are correct.

5. Which of the following statements about tastants is true?

Umami receptors are able to bind any type of amino acid found in proteins. Monosodium glutamate binds sour receptors on the tongue. Five types of tastant are known, but more might be discovered as this is an active area of research. Over 900 tastant receptor genes have been found in humans. All receptors that bind tastants are G protein­coupled receptors.

6. Which of the following statements about olfaction and gustation are true?

In humans, there are far more olfactory receptors than gustatory receptors. Animals that live in water do not have distinct senses of smell and taste. Olfaction and gustation both involve chemoreceptors. Perception of smell and taste are closely associated in the brain. All the answer choices are correct.

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7. Which of the following statements about human taste receptors is true?

Taste receptors are found on neurons. All taste receptors are associated with ion channels. The signal transduction pathway for all tastants begins with the G protein­coupled receptor. Taste receptors are found only on the tongue. One receptor binds molecules with a sweet taste and one receptor binds molecules with an umami taste, but there are more than 30 different receptors for bitter taste.

8. Which of the following statements about gustation and olfaction is true?

All vertebrate animals have distinct senses of gustation and olfaction. Sensory cells for both olfaction and gustation are specialized epithelial cells that form synapses with sensory neurons. Sensory cells for both olfaction and gustation are neurons. Sensory cells for gustation are neurons, whereas sensory cells for olfaction are specialized epithelial cells that form synapses with sensory neurons. Sensory cells for olfaction are neurons, whereas sensory cells for gustation are specialized epithelial cells that form synapses with sensory neurons.

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