The body has millions of sense organs. They fall into

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1 CHPTER 15 SENSE ORGNS CHPTER OUTLINE Sensory Receptors, 448 Receptor Response, 448 Distribution of Receptors, 449 Classification of Receptors, 449 Classification by Location, 449 Classification by Stimulus Detected, 449 Classification by Structure, 450 Free Nerve Endings, 450 Encapsulated Nerve Endings, 451 Special Senses, 453 Sense of Smell, 453 Olfactory Receptors, 453 Olfactory Pathway, 454 Sense of Taste, 455 Taste Buds, 455 Neuronal Pathway for Taste, 456 Sense of Hearing and Balance: The Ear, 456 External Ear, 456 Middle Ear, 457 Inner Ear, 457 Cochlea and Cochlear Duct, 457 Sense of Hearing, 459 Vestibule and Semicircular Canals, 460 Sense of Balance, 460 Vision: The Eye, 464 Structure of the Eye, 464 Coats of the Eyeball, 464 Cavities and Humors, 466 Muscles, 467 ccessory Structures, 467 The Process of Seeing, 469 Formation of Retinal Image, 469 The Role of Photopigments, 470 Neuronal Pathway of Vision, 472 Cycle of Life, 473 The Big Picture, 473 Mechanisms of Disease, 473 Case Study, 477 KEY TERMS adaptation chemoreceptor cochlea crista ampullaris equilibrium exteroceptors gustatory labyrinth macula mechanoreceptor nociceptor olfactory photoreceptor proprioceptor refraction retina rhodopsin thermoreceptor visceroceptors The body has millions of sense organs. They fall into two main categories: general sense organs and special sense organs. Of these, by far the most numerous are the general sense organs, or receptors. Receptors function to produce the general, or somatic, senses (such as touch, temperature, and pain) and to initiate various reflexes necessary for maintaining homeostasis. Special sense organs function to produce the special senses (vision, hearing, balance, taste, and smell), and they too initiate reflexes important for homeostasis. In this chapter we begin with a description of receptors and follow with information related to the special senses. SENSORY RECEPTORS Sense organs called sensory receptors make it possible for the body to respond to stimuli caused by changes occurring in our external or internal environment. This function is crucial to survival. The abilities to see and hear, for example, may provide the necessary warning to help us avoid injury from dangers in our external environment. Internal sensations ranging from pain and pressure to hunger and thirst help us maintain homeostasis of our internal environment. RECEPTOR RESPONSE The general function of receptors is to respond to stimuli by converting them to nerve impulses. Receptors are often de- 448

2 Sense Organs Chapter scribed as the specialized dendritic endings or end organs of sensory neurons. s a rule, different types of receptors respond to different types of stimuli. They are specialized in that a particular type of receptor responds, under normal physiological conditions, to a particular type of stimulus and is less able or unable to respond to others. Heat receptors, for example, do not respond to light or stretch stimuli. When an adequate stimulus acts on a receptor, a potential develops in the receptor s membrane. It is called a receptor potential. The receptor potential is a graded response, graded to the strength of the stimulus (see Chapter 12, p. 355). When a receptor potential reaches a certain threshold, it triggers an action potential in the sensory neuron s axon. These impulses then travel over sensory pathways to the brain and spinal cord, where they are interpreted as a particular sensation, such as heat or cold, or they initiate some type of reflex action, such as withdrawal of a limb from a painful stimulus. Receptors often exhibit a functional characteristic known as adaptation. daptation means that the magnitude of the receptor potential decreases over a period in response to a continuous stimulus. s a result, the rate of impulse conduction by the sensory neuron s axon also decreases. So too does the intensity of the resulting sensation. familiar example of adaptation is feeling the touch of your clothing when you first put it on and soon not sensing it at all. Touch receptors adapt rapidly. In contrast, the proprioceptors in our muscles, tendons, and joints adapt slowly. s long as stimulation of them continues, they continue sending impulses to the brain. DISTRIBUTION OF RECEPTORS Receptors responsible for the special senses of smell, taste, vision, hearing, and equilibrium are grouped into localized areas (nasal mucosa or tongue) or into such complex organs as the eye and ear. The general sense organs consist of microscopic receptors widely distributed throughout the body in the skin, mucosa, connective tissues, muscles, tendons, joints, and viscera. Sensations produced by these receptors are often called the somatic senses. Their distribution is not uniform in all areas. In some, it is very dense; in others, it is sparse. The skin covering the fingertips, for instance, contains many more receptors to touch than does the skin on the back. simple procedure, the two-point discrimination test, demonstrates this fact. subject reports the number of touch points felt when an investigator touches the skin simultaneously with two points of a compass. If the skin on the fingertip is touched with the compass points barely one eighth of an inch apart, the subject senses them as two points. If the skin on the back is touched with the compass points this close together, they will be felt as only one point. Unless they are an inch or more apart, they cannot be discriminated as two points. Why this difference? Because touch receptors are so densely distributed in the fingertips that two points very close to each other stimulate two different receptors they are sensed as two points. The situation is quite different in the skin on the back. There, touch receptors are so widely scattered that two points have to be at least an inch apart to stimulate two receptors and be felt as two points. CLSSIFICTION OF RECEPTORS Receptors can be classified according to (1) their location in the body, (2) the specialized stimulus that causes them to respond, and (3) their structure. CLSSIFICTION BY LOCTION Three groups or classes of receptors can be identified by their location: 1. Exteroceptors 2. Visceroceptors (or interoceptors) 3. Proprioceptors Exteroceptors, as the name implies, are located on or very near the body surface and respond most frequently to stimuli that arise external to the body itself. Receptors in this group are sometimes called cutaneous receptors because of their placement in the skin. However, the special sense organs, which are described later in the chapter, are also classified as exteroceptors. Examples of exteroceptors include those that detect pressure, touch, pain, and temperature. Visceroceptors (interoceptors) are located internally, often within the substance of body organs (viscera), and when stimulated provide information about the internal environment. They are activated by stimuli such as pressure, stretching, and chemical changes that may originate in such diverse internal organs as the major blood vessels, intestines, and urinary bladder. Visceroceptors are also involved in mediating sensations such as hunger and thirst. Proprioceptors are a specialized type of visceroceptors. They are less numerous and generally more specialized than other internally placed receptors, and their location is limited to skeletal muscle, joint capsules, and tendons. These proprioceptors provide us with information about body movement, orientation in space, and muscle stretch. ctivation of two types of proprioceptors, called tonic and phasic receptors, allows us to orient our body in space and provides us with positional information about specific body parts while at rest or during movement. The firing of the nonadapting tonic receptors allows us to locate, for example, our arm, hand, or foot at rest without having to look. Phasic receptors are rapidly adapting receptors, so they are triggered only when there is a change in position. Phasic receptors therefore permit us to feel the changing position of our body parts during continuous movement. CLSSIFICTION BY STIMULUS DETECTED Receptors are frequently classified into five categories based on the types of stimuli that activate them: 1. Mechanoreceptors. Mechanoreceptors are activated by mechanical stimuli that in some way deform or change the position of the receptor, resulting in the generation of a receptor potential. Examples include

3 450 Unit 3 Communication, Control, and Integration Box 15-1 FYI Referred Pain The pain from stimulation of nociceptors in deep structures is frequently referred to surface areas. Referred pain is the term for this phenomenon. Pain originating in the viscera and other deep structures is generally interpreted as coming from the skin area whose sensory fibers enter the same segment of the spinal cord as the sensory fibers from the deep structure. For example, sensory fibers from the heart enter the first to fourth thoracic segments, and so do sensory fibers from the skin areas over the heart and on the inner surface of the left arm. Pain originating in the heart is referred to those skin areas, but the reason for this is not clear. pressure applied to the skin or to blood vessels, or caused by stretch or pressure in muscle, tendon, or lung tissue. 2. Chemoreceptors. Receptors of this type are activated by either the amount or the changing concentration of certain chemicals. Our senses of taste and smell depend on chemoreceptors. Specialized chemoreceptors in the body also sense the concentration of such specific chemicals as hydrogen ions (ph) and blood glucose. 3. Thermoreceptors. Thermoreceptors are activated by changes in temperature. 4. Nociceptors. This type of receptor is activated by intense stimuli of any type that results in tissue damage. The cause may be a toxic chemical, intense light, sound, pressure, or heat. The sensation produced is one of pain. 5. Photoreceptors. This type of receptor is found only in the eye. Photoreceptors respond to light stimuli if the intensity is great enough to generate a receptor potential. CLSSIFICTION BY STRUCTURE Regardless of their location or how they are activated, the general (somatic) sensory receptors may be classified anatomically as either 1. Free nerve endings, or 2. Encapsulated nerve endings Refer often to Figure 15-1 and Table 15-1 as you read about the general sensory receptors that are described in the following paragraphs. Free Nerve Endings Free nerve endings are the simplest, most common, and most widely distributed sensory receptors. They are located both on the surface of the body (exteroceptors) and in the deep visceral organs (visceroceptors). The term nociceptor is used to describe these slender sensory fibers, which, in most cases, terminate in small swellings called dendritic knobs.nociceptors serve as the primary sensory receptors for pain. Brain tissue is unique in that it lacks these free (sensory) nerve endings altogether and is therefore incapable of sensing painful stimuli. Just the opposite is true of many deep visceral organs, in which free nerve endings responsible for mediating pain (nociceptors) are present. Pain is one of the few sensations that can be evoked in these areas. In addition to mediating painful stimuli, typical or slightly modified free nerve endings are also responsible for other sensations, including itching, tickling, touch, movement, and mechanical stretching. In addition, we now know that free nerve endings are the primary receptors mediating sensations of heat and cold. (Some authorities continue to identify Krause s end bulbs and Ruffini s corpuscles, which are described below, as secondary thermoreceptors.) It is important to stress that in responding to powerful stimuli of any kind, the generation of a receptor potential in free nerve endings most often results in the sensation of pain often the first indication of injury or disease. Nerve fibers that carry pain impulses from free nerve ending receptors to the brain can be divided into two types acute () and chronic (B) fibers. cute pain () fibers mediate sharp, intense, and generally localized pain sensations, whereas chronic (B) fibers are associated with less intense but more persistent, pain often described as dull or aching.

4 Sense Organs Chapter Tendon Type Ib sensory fiber Krause s end bulb Free nerve endings Pacini s corpuscle Merkel endings (Merkel s disc) Meissner s corpuscle Muscle fibers (extrafusal fibers) Ruffini s corpuscle (Ruffini ending) Golgi tendon organ Capsule Perimysium of muscle fiber bundle Connective tissue capsule Efferent motor fiber Type II sensory ending Type I sensory endings Type II sensory ending α Efferent motor fiber B Intrafusal fibers Nuclear bag fibers Nuclear chain fibers Neuromuscular spindle Figure 15-1 Somatic sensory receptors., Exteroceptors. B, Proprioceptors. See also Figure 6-1 (p. 161), which shows a microscopic section of the skin. Root hair plexuses are delicate, weblike arrangements of free nerve endings that surround hair follicles and detect hair movement (see Figure 15-1). Flattened or disc-shaped variations of free nerve endings, called Merkel discs, are responsible for mediating sensations of light or discriminative touch (see Figure 15-1). Use of the term discriminative touch means simply that the sensation, which may be very subtle, can be located exactly on the skin surface. The term crude touch implies that the sensation is easily recognized, but its exact location is hard to determine. Encapsulated pacinian corpuscles, described below, mediate sensations of crude touch. Merkel discs are delicate mechanoreceptors that are more easily deformed than pacinian corpuscles and thus are capable of generating an action potential when exposed to minimal stimulation. This is because they are not encapsulated and because of their more superficial placement in the epidermis of the skin. The detection of discriminative touch by Merkel discs is yet another example of the relationship between form and function. Encapsulated Nerve Endings The six types of encapsulated nerve endings have in common some type of connective tissue capsule that surrounds their terminal or dendritic end. In addition, most of these specialized receptors are primary mechanoreceptors, that is, they are most often activated by a mechanical or deforming type of stimulus. Encapsulated receptors vary in size and anatomical characteristics, as well as in numbers and distribution throughout the body (see Figure 15-1 and Table 15-1). Touch and Pressure Receptors. Meissner s corpuscle is shown in Figure These structures are relatively large and superficially placed ovoid- or egg-shaped mechanoreceptors. When deformed by a mechanical type of stimulus, this type of receptor, sometimes called a tactile corpuscle,mediates sensations of discriminative touch and low-frequency vibration. Meissner s corpuscles are especially numerous in hairless skin areas, such as the nipples, fingertips, and lips. Two anatomical variants of Meissner s corpuscles called Krause s end bulbs and Ruffini s corpuscles are also mechanoreceptors. Krause s end bulbs are egg shaped, like Meissner s corpuscles, but they are somewhat smaller and have fewer and less tightly coiled dendritic endings within their covering capsule. These receptors are more numerous in mucous membranes than in skin and are sometimes called mucocutaneous corpuscles. They are involved in discriminative touch and low-frequency vibration. lthough free nerve endings are the primary thermoreceptors, some evidence continues to suggest that Krause s end bulbs are also stimulated by temperatures below 65 F (18 C). For this reason, they are sometimes listed as secondary cold receptors. Ruffini s corpuscles (see Figure 15-1) are also considered variants of Meissner s corpuscles, but they have a more flattened capsule and are more deeply located in the dermis of the skin. These receptors mediate sensations of crude and persistent touch. Because they are slow adapting, they permit the skin of the fingers to remain sensitive to deep pressure for long periods. The ability to grasp an object, such as the steering wheel of a car, for long periods of time and still be

5 452 Unit 3 Communication, Control, and Integration Table 15-1 Classification of Somatic Sensory Receptors By Structure By Location and Type By ctivation Stimulus By Sensation or Function Free Nerve Endings Nociceptors Merkel discs Root hair plexuses Both exteroceptors and visceroceptors most body tissues Exteroceptors Exteroceptors lmost any noxious stimulus; temperature change; mechanical Light pressure; mechanical Hair movement; mechanical Pain; temperature; itch; tickle; stretching Discriminative touch Sense of deflection type movement of hair Nociceptors Merkel discs Root hair plexuses Encapsulated Nerve Endings Touch and pressure receptors Meissner s corpuscle Krause s corpuscle Ruffini s corpuscle Pacinian corpuscle Exteroceptors; epidermis, hairless skin Mucous membranes Dermis of skin, exteroceptors Dermis of skin, joint capsules Light pressure, mechanical Mechanical; thermal? Mechanical; thermal? Deep pressure, mechanical Discriminative touch; lowfrequency vibration Touch; low-frequency vibration Crude and persistent touch Deep pressure; high-frequency vibration; stretch Stretch receptors Muscle spindles Golgi tendon receptors Skeletal muscle Musculotendinous junction Stretch, mechanical Force of contraction and tendon stretch, mechanical Sense of muscle length Sense of muscle tension Ruffini s corpuscle Meissner s corpuscle Krause s end bulb Muscle spindles Pacinian corpuscle Golgi tendon receptors

6 Sense Organs Chapter able to sense its presence between the fingers depends on these receptors. In addition to their ability to respond to mechanical stimuli for extended periods, Ruffini s corpuscles are also considered by some authorities to be secondary thermoreceptors (heat receptors) that are stimulated by temperature in the range of 85 to 120 F (29 C to 49 C). Pacinian corpuscles are large mechanoreceptors, which, when sectioned, show thick laminated connective tissue capsules. They are found in the deep dermis of the skin especially in the hands and feet and are also numerous in joint capsules throughout the body. Pacinian corpuscles respond quickly to sensations of deep pressure, high-frequency vibration, and stretch. lthough sensitive and quick to respond, these receptors adapt quickly, and the sensations they evoke seldom last for long. Stretch Receptors. The most important stretch receptors are associated with muscles and tendons and are classified as proprioceptors. Two types of stretch receptors, called muscle spindles and Golgi tendon receptors,operate to provide the body with information concerning muscle length and the strength of muscle contraction (Figure 15-1, B). natomically, each muscle spindle consists of a discrete grouping of about 5 to 10 modified muscle fibers called intrafusal fibers,which are surrounded by a delicate capsule. These fibers have striated ends that are capable of contraction but are devoid of contractile filaments in their central areas where, instead, there are several nuclei surrounded by clear cytoplasm and two types of sensory nerve fibers that are described below. The muscle spindles can be found lying between and parallel to the regular muscle fibers called extrafusal fibers. Both ends of each spindle are connected or anchored to connective tissue elements within the muscle mass. Two types of sensory (afferent) nerve fibers are found encircling the central area of each spindle. Both large diameter and rapidly conducting type Ia and slower conducting small diameter type II fibers encircle the clear central area of each spindle. When stretching occurs, afferent impulses from these sensory neurons pass to the spinal cord and are relayed to the brain, providing a mechanism to monitor changes in muscle length. The striated ends of the muscle spindle fibers (intrafusal fibers) are capable of contraction when stimulated by efferent impulses generated in gamma motor neurons, whereas regular muscle fibers (extrafusal fibers) are stimulated to contract by efferent impulses generated in alpha motor neurons. Both types of neurons are located in the anterior gray horn of the spinal cord. Muscle spindles are stimulated if the length of a relaxed muscle is stretched and exceeds a certain limit. The result of stimulation is a stretch reflex that shortens a muscle or muscle group, thus aiding in the maintenance of posture or the positioning of the body or one of its extremities in a way that may be opposed by the force of gravity (see Figure 11-17, p. 327). We do this unconsciously, because these receptors do not produce a particular sensation such as pain, heat, or cold. Golgi tendon organs, like muscle spindles, are proprioceptors. They are located at the point of junction between muscle tissue and tendon. Each Golgi tendon organ consists of dendrites of afferent (sensory) nerves called type Ib nerve fibers, which are associated with bundles of collagen fibers from the tendon and surrounded by a capsule. These receptors act in a way opposite that of muscle spindles. They are stimulated by excessive muscle contraction and when activated they cause muscles to relax. This response, called a Golgi tendon reflex,protects muscles from tearing internally or pulling away from their tendinous points of attachment to bone because of excessive contractile force. The different types of somatic sense receptors, their locations, and their functions are summarized in Table Classify receptors into four groups based on the type of stimuli that activate them. 2. Distinguish between the special and the general, or somatic, senses. 3. Name the three types of sense receptors classified according to location in the body. 4. Name the receptors associated with pain, touch, pressure, and stretch responses. SPECIL SENSES The special senses are characterized by receptors grouped closely together or located in specialized organs. The senses of smell (olfaction), taste, hearing, equilibrium, and vision are considered the special senses. SENSE OF SMELL OLFCTORY RECEPTORS The olfactory epithelium consists of yellow-colored epithelial support cells, basal cells and specialized bipolar type olfactory receptor neurons. These neurons have unique olfactory cilia,which touch the surface of the olfactory epithelium lining the upper surface of the nasal cavity. The olfactory receptor neurons are chemoreceptors.they are unique because they are replaced on a regular basis by germinative basal cells in the olfactory epithelium. Receptor potentials are generated in olfactory receptor neurons when gas molecules or chemicals dissolved in the mucus covering the nasal epithelium (Figure 15-2) bind to receptors in the membrane of the olfactory receptor neurons. Gentle, random movement of the cilia helps mix the covering mucus and increases its efficiency as a solvent. The olfactory epithelium is located in the most superior portion of the nasal cavity (see Figure 15-2). Functionally, this is a poor location because a great deal of inspired air flows around and down the nasal passageways without contacting the olfactory receptor cells. The location of these receptors explains the necessity for sniffing, or drawing air forcefully up into the nose, to smell delicate odors. The olfactory receptors are extremely sensitive, that is, capable of being stimulated by even very slight odors caused by

7 454 Unit 3 Communication, Control, and Integration just a few molecules of a particular chemical. lthough humans have a sense of smell far less keen than many animals, some individuals can distinguish several thousand different odors, and most of us can easily identify at least several hundred. Examples of well-known primary scents include putrid, floral, peppermint, and musky odors. Combinations of two or more of these or other primary scents produce the wide array of odors we can identify. Rapid adaptation of olfactory sensations in the face of continuous stimulation is due to the inhibition of action potentials by specialized granule cells in the olfactory bulbs more than by actual fatigue of receptor cells in the olfactory epithelium. Current research suggests that hundreds of physiologically different kinds of olfactory receptor neurons are present in the olfactory epithelium. Identification of different odors may be the result of stimulation of differing combinations of these receptors or by differing levels of stimulation of the entire olfactory epithelium. OLFCTORY PTHWY If the level of odor-producing chemicals dissolved in the mucus surrounding the olfactory cilia reaches a threshold Olfactory bulb Fibers of olfactory nerve Frontal Cribriform plate of ethmoid bone Olfactory tract bone Nasal cavity Olfactory recess Nasopharynx Palate P S I Corpus callosum Intermediate olfactory area B Medial olfactory area Frontal bone Lateral olfactory area Olfactory bulb Fibers of olfactory nerve Nasal cavity Nasal bone Mitral cell ssociation neuron Granule cell Olfactory tract Olfactory bulb P S Cribriform plate Foramen I xon Basal cell C Olfactory epithelium Supporting cell Olfactory neuron Mucous layer on epithelial surface Dendrite Cilia Olfactory vesicle Figure 15-2 Olfaction. Location of olfactory epithelium, olfactory bulb, and neuronal pathways involved in olfaction., Midsagittal section of the nasal area shows the locations of major olfactory sensory structures. B, Major olfactory integration centers of the brain. C, Details of the olfactory bulb and olfactory epithelium.

8 Sense Organs Chapter level, a receptor potential and, then, an action potential will be generated and passed to the olfactory nerves in the olfactory bulb. From there, the impulse passes through the olfactory tract and into the thalamic and olfactory centers of the brain for interpretation, integration, and memory storage. The sense of smell can create powerful and long-lasting memories. Memories coupled with unique sensory inputs, especially distinctive odors, often persist from early childhood to death. Dental office smells, baby smells, kitchen smells, and new car smells are examples of olfactory triggers that often bring back memories of events that occurred years earlier. In addition to the olfactory cortex and thalamic areas of the brain, components of the limbic system, including the angulate gyrus and hippocampus, play a key role in coupling olfactory sense inputs to both short- and long-term Olfactory tract Olfactory bulbs Tongue S I P Figure 15-3 Taste sensory cortex (parietal lobe) Pons Fornix Spinal cord Thalamus and processing center mygdaloid nucleus Hippocampus Facial nerve (VII) Primary olfactory area (temporal lobe) Temporal lobe Glossopharyngeal nerve (IX) Relationship of olfactory and gustatory pathways. memory. Our sense of smell and taste are closely related. Note in Figure 15-3 that the neuronal inputs from both olfactory and gustatory (taste) receptors travel in several common areas of the brain. Ultimately, olfactory sensations are produced in the (smell) sensory cortex located in the temporal lobes and taste sensations result from stimulation of the (taste) sensory cortex in the parietal lobes. SENSE OF TSTE TSTE BUDS The taste buds are the sense organs that respond to gustatory, or taste, stimuli. lthough a few taste buds are located in the lining of the mouth and on the soft palate, most are associated with small, elevated projections on the tongue, called papillae (pa-pil-e) (Figure 15-4, and B). Fungiform, circumvallate, and foliate papillae contain taste buds. Threadlike filiform papillae do not contain taste buds but allow us to experience food texture and feel. lthough the different varieties of papillae are distributed differently across the tongue surface, there is no regional difference in where a particular taste can be detected. For example, the long-held belief that bitter taste was localized at the back of the tongue with its high concentration of circumvallate papillae (see Figure 15-4, ), or that sweet taste is detected best at the tip of the tongue is simply not true. No tongue or taste map accurately indicates regional areas of particular sensitivity to different tastes. ll tastes can be detected in all areas of the tongue that contain taste buds. Taste buds house the chemoreceptors responsible for taste. They are stimulated by chemicals, called testants, dissolved in the saliva. Each grapelike taste bud contains 50 to 125 of these chemoreceptors, called gustatory cells, which are surrounded by a supportive epithelial cell capsule. Tiny B D C P R L Figure 15-4 The tongue., Dorsal surface and regions sensitive to various tastes. B, Section through a papilla with taste buds on the side. C, Enlarged view of a section through a taste bud. D, Scanning electron micrograph of the tongue surface showing the papillae in detail.

9 456 Unit 3 Communication, Control, and Integration cilia-like structures, called gustatory hairs, extend from each of the gustatory cells and project into an opening called the taste pore, which is bathed in saliva (Figure 15-4, C). The sense of taste depends on the creation of a receptor potential in gustatory cells. Only then can an action potential be generated and a nerve impulse relayed to the brain for interpretation. Generation of a receptor potential begins when specialized areas called G protein receptor sites on the cell membranes or porelike ion channels in the covering gustatory hairs bind to taste-producing chemicals (testants) in the saliva. The nature and concentration of the chemicals that bind to either the G protein receptor sites or ion channels determine how fast the receptor potential is generated. Taste cells appear similar structurally, and all of them can respond at least in some degree to most taste-producing chemicals. Functionally, however, each taste bud responds most effectively to only one of four primary taste sensations: sour, sweet, bitter, and salty. Our ability to detect many different flavors, or tastes, is due to combinations of the four primary sensations and to interaction with the sense of smell. The number of primary taste sensations (such as the pure, or primary, olfactory scents) is likely to increase with expanded research efforts. Metallic taste, for example, may be a fifth primary taste sensation rather than a mixture of other flavors. In addition, Japanese researchers recently have suggested that yet another primary taste, called umami, results from stimulation of G protein receptor sites by amino acids, such as glutamate, found in protein-rich foods such as meat and fish. The exact mechanism by which a chemical testant binds to a particular G protein receptor site or ion channel on a gustatory hair is unknown. Chemical structure plays a part but is not the only factor involved, because substances that are very different chemically, such as artificial sweeteners, chloroform, and table sugar, produce a sweet taste. Some chemical compounds and specific ions, however, are definitely associated with specific tastes. Sour (H ) and salty (Na and Cl ) tastes activate ion channels, whereas sweet and bitter tastes result from stimulation of G protein receptor sites. s chemoreceptors, the taste buds, like olfactory receptors, tend to be quite sensitive but fatigue easily. Very low levels of taste-producing chemicals are required to generate a receptor potential. However, adaptation often begins within a few seconds after a taste sensation is first noticed and is generally complete in a few minutes. NEURONL PTHWY FOR TSTE The taste sensation begins with creation of a receptor potential in the gustatory cells of a taste bud. The generation and propagation of an action potential, or nerve impulse, then transmit the sensory input to the brain. We generally think of taste in terms of the primary sensations of sour, sweet, bitter, and salty (and perhaps metallic and umami). However, touch, texture, and temperature are also involved in taste and whether we sense it as pleasant, neutral, or unpleasant. Nervous impulses generated in the anterior two thirds of the tongue travel over the facial (VII) nerve, whereas those generated from the posterior one third are conducted by fibers of the glossopharyngeal (IX) nerve. third cranial nerve, the vagus (X) nerve, plays a minor role in taste. It contains a few fibers that carry taste sensation from a limited number of taste buds located in the walls of the pharynx and on the epiglottis. ll three cranial nerves carry impulses into the medulla oblongata. Relays then carry the impulses into the thalamus and then into the taste, or gustatory, area of the cerebral cortex in the parietal lobe of the brain (see Figure 15-3). 1. List the special senses. 2. Discuss how a receptor potential is generated in olfactory cells. 3. Discuss how a receptor potential is generated in gustatory cells. 4. Trace the path of a nervous impulse carrying (1) olfactory and (2) gustatory sense information from its point of origin to that area of the brain where interpretation occurs. 5. List the four primary taste sensations. 6. Locate on the tongue the area where specific tastes are best detected. SENSE OF HERING ND BLNCE: THE ER The ear has dual sensory functions. In addition to its role in hearing, it also functions as the sense organ of balance, or equilibrium. The stimulation, or trigger, responsible for hearing and balance involves activation of specialized mechanoreceptors called hair cells. Sound waves and movement are the physical forces that act on hair cells to generate receptor potentials, and then nerve impulses, which are eventually perceived in the brain as sound or balance. The ear is divided into three anatomical parts: external ear, middle ear, and inner ear (Figure 15-5). The structures illustrated in Figure 15-5 are not drawn to scale. Instead, the smaller components of the middle and inner ear are enlarged so that they can be more easily identified. In addition, this type of artistic rendering makes it easier to see the anatomical relationships of these tiny elements to one another and to adjacent structures. EXTERNL ER The external ear has two divisions: the flap, or modified trumpet, on the side of the head, called the auricle or pinna, and the tube leading from the auricle into the temporal bone, named the external auditory meatus (ear canal). This canal is about 3 cm long and travels, in general, an inward, forward, and downward direction, although the first portion of the tube slants upward and then curves downward. Because of this curve in the auditory canal, in adults, the auricle should be pulled up and back to straighten the tube when medications are dropped into the ear. Modified sweat glands in the auditory canal secrete cerumen (waxlike substance), which occasionally becomes impacted and may cause pain and temporary deafness. The tympanic membrane (eardrum) stretches across the inner end of the auditory canal, separating it from the middle ear.

10 Sense Organs Chapter External ear Middle ear Inner ear uricle (pinna) (Not to scale) Temporal bone External auditory meatus Tympanic membrane Semicircular canals Oval window Facial nerve Vestibular nerve Cochlear nerve coustic nerve (VIII) Cochlea Vestibule Round window uditory tube Malleus Incus Stapes S uditory ossicles L I M Figure 15-5 The ear. External, middle, and inner ear. (natomical structures are not drawn to scale.) MIDDLE ER The middle ear (tympanic cavity), a tiny epithelial-lined cavity hollowed out of the temporal bone, contains the three auditory ossicles: the malleus, incus, and stapes (see Figure 15-5). The names of these very small bones describe their shapes (hammer, anvil, and stirrup). The handle of the malleus is attached to the inner surface of the tympanic membrane, whereas the head attaches to the incus, which in turn attaches to the stapes. There are several openings into the middle ear cavity: one from the external auditory meatus, covered with the tympanic membrane; two into the internal ear, the oval window (into which the stapes fits), and the round window, which is covered by a membrane; and one into the auditory (eustachian) tube. Posteriorly the middle ear cavity is continuous with numerous mastoid air spaces in the temporal bone. The clinical importance of these middle ear openings is that they provide routes for infection to travel. Head colds, for example, especially in children, may lead to middle ear or mastoid infections via the nasopharynx-auditory tube, middle ear mastoid path. The auditory, or eustachian, tube is composed partly of bone and partly of cartilage and fibrous tissue and is lined with mucosa. It extends downward, forward, and inward from the middle ear cavity to the nasopharynx, or pharyngotympanic tube (the part of the throat behind the nose). The auditory tube serves a useful function: it makes possible equalization of pressure against inner and outer surfaces of the tympanic membrane and therefore prevents membrane rupture and the discomfort that marked pressure differences produce. The way the auditory tube equalizes tympanic membrane pressure is this: when you swallow or yawn, air spreads rapidly through the open tube. tmospheric pressure then presses against the inner surface of the tympanic membrane. Because atmospheric pressure is continually exerted against its outer surface, the pressures are equal. INNER ER The inner ear is also called the labyrinth because of its complicated shape. It consists of two main parts, a bony labyrinth and, inside this, a membranous labyrinth. The bony labyrinth consists of three parts: vestibule, cochlea, and semicircular canals (Figure 15-6). The membranous labyrinth consists of the utricle and saccule inside the vestibule, the cochlear duct inside the cochlea, and the membranous semicircular canals inside the bony ones. The vestibule (containing the utricle and saccule) and the semicircular canals are involved in balance; the cochlea is involved in hearing. The term endolymph is used to describe the clear and potassium-rich fluid that fills the membranous labyrinth. Perilymph, a fluid similar to cerebrospinal fluid, surrounds the membranous labyrinth and therefore fills the space between this membranous tunnel and its contents and the bony walls that surround it (see Figure 15-6). Cochlea and Cochlear Duct The word cochlea, which means snail, describes the outer appearance of this part of the bony labyrinth. When sectioned, the cochlea resembles a tube wound spirally around a cone-shaped core of bone, the modiolus. The modiolus houses the spiral ganglion, which consists of cell bodies of the first sensory neurons in the auditory relay. Inside the cochlea lies the membranous cochlear duct the only part of the internal ear concerned with hearing. This structure is shaped

11 458 Unit 3 Communication, Control, and Integration like a somewhat triangular tube. It forms a shelf across the inside of the bony cochlea, dividing it into upper and lower sections all along its winding course (see Figure 15-6). The upper section (above the cochlear duct) is called the scala vestibuli,whereas the lower section below the cochlear duct is called the scala tympani. The roof of the cochlear duct is known as the vestibular membrane (Reissner s membrane). The basilar membrane is the name of the floor of the cochlear duct. It is supported by bony and fibrous projections from the wall of the cochlea. Perilymph fills the scala vestibuli and scala tympani, and endolymph fills the cochlear duct. The hearing sense organ, named the organ of Corti, rests on the basilar membrane throughout the entire length of the cochlear duct. The structure of the organ of Corti consists of supporting cells, as well as of important hair cells that project into the endolymph and are topped by an adherent gelatinous membrane called the tectorial membrane. Dendrites of the sensory neurons, whose cell bodies lie in the spi- Box 15-2 HELTH MTTERS Cochlear Implants Recent advances in electronic circuitry are being used to correct some forms of nerve deafness. If the hairs on the organ of Corti are damaged, nerve deafness results even if the vestibulocochlear nerve is healthy. new surgically implanted device can improve this form of hearing loss by eliminating the need for the sensory hairs. s you can see in the Figure, a transmitter just outside the scalp sends external sound information to a receiver under the scalp (behind the auricle). The receiver translates the information into an electrical code that is relayed down an electrode to the cochlea. The electrode, wired to the organ of Corti, stimulates the vestibulocochlear nerve endings directly. Thus even though the cochlear hair cells are damaged, sound can be perceived. L Receiver Transmitter Electrode S I M Semicircular canals Perilymph space Vestibular (Reissner s) membrane Utricle (in vestibule) Endolymph (within membrane) mpulla Vestibular nerve Cochlear nerve Modiolus Scala vestibuli B Cochlear duct Saccule (in vestibule) L Oval window S I Round window M Cochlear duct Cochlea Scala tympani Tectorial membrane Basilar membrane Hair cells Supporting cells Organ of Corti Figure 15-6 The inner ear., The bony labyrinth (orange) is the hard outer wall of the entire inner ear and includes semicircular canals, vestibule, and cochlea. Within the bony labyrinth is the membranous labyrinth (purple), which is surrounded by perilymph and filled with endolymph. Each ampulla in the vestibule contains a crista ampullaris that detects changes in head position and sends sensory impulses through the vestibular nerve to the brain. B, The inset shows a section of the membranous cochlea. Hair cells in the organ of Corti detect sound and send the information through the cochlear nerve. The vestibular and cochlear nerves join to form the eighth cranial nerve.

12 Sense Organs Chapter ral ganglion in the modiolus, have their beginnings around the bases of the hair cells of the organ of Corti. xons of these neurons extend to form the cochlear nerve (a branch of the eighth cranial nerve) to the brain. They conduct impulses that produce the sensation of hearing. Sense of Hearing Sound is created by vibrations that may occur in air, fluid, or solid material. When we speak, for example, the vibrating vocal cords create sound waves by producing vibrations in air passing over them. Numerous terms are used to describe sound waves. The height, or amplitude, of a sound wave determines its perceived loudness, or volume. The number of sound waves that occur during a specific time unit (frequency) determines pitch. Our ability to hear sound waves depends in part on volume, pitch, and other acoustic properties. Sound waves must be of sufficient amplitude to initiate movement of the tympanic membrane and have a frequency that is capable of stimulating the hair cells in the organ of Corti at some point along the basilar membrane. The basilar membrane is not the same width and thickness throughout its length. Because of this structural fact, different frequencies of sound will cause the basilar membrane to vibrate and bulge upward at different places along its length. Two bulges are shown in Figure High-frequency sound waves cause the narrow portion of the basilar membrane near the oval window to vibrate, whereas low frequencies vibrate the membrane near the apex of the cochlea, where it is considerably wider and thicker. This ability of sound waves of differing frequency to vibrate and cause a bulge, or upward displacement, of the basilar membrane at differing points along its length explains how specific groups of hair cells respond to specific frequencies of sound. When a particular portion of the basilar membrane bulges upward, the cilia on hair cells attached to that particular area are stimulated, and, ultimately, sound of a particular pitch is perceived. Our perception of different degrees of loudness of the same sound is determined by the amplitude, or movement, of the basilar membrane at any particular point along its length. The higher the upward bulge, the more the cilia on the attached hair cells are bent or stimulated. This causes an increase in perceived loudness. The moving wave of perilymph caused by upward displacement of the basilar membrane is soon dampened as it moves through the cochlea. Hearing results from stimulation of the auditory area of the cerebral cortex. First, however, sound waves must be projected through air, bone, and fluid to stimulate nerve endings and set up impulse conduction over nerve fibers. Pathway of Sound Waves. Sound waves in the air enter the external auditory canal with aid from the pinna. t the inner end of the canal, they strike against the tympanic membrane, setting it in vibration. Vibrations of the tympanic membrane move the malleus, whose handle attaches to the membrane. The head of the malleus attaches to the incus, and the incus attaches to the stapes. So when the malleus vibrates, it moves the incus, which moves the stapes against the oval window into which it fits so precisely. t this point, fluid conduction of sound waves begins. When the stapes moves against the oval window, pressure is exerted inward into the perilymph in the scala vestibuli of the cochlea. This starts a ripple in the perilymph that is transmitted through Figure 15-7 Effect of sound waves on cochlear structures. Sound waves strike the tympanic membrane and cause it to vibrate. This causes the membrane of the oval window to vibrate, which causes the perilymph in the bony labyrinth of the cochlea and the endolymph in the membranous labyrinth of the cochlea, or cochlear duct, to move. This movement of endolymph causes the basilar membrane to vibrate, which in turn stimulates hair cells on the organ of Corti to transmit nerve impulses along the cranial nerve. Eventually, nerve impulses reach the auditory cortex and are interpreted as sound.

13 460 Unit 3 Communication, Control, and Integration the vestibular membrane (the roof of the cochlear duct) to endolymph inside the duct and then to the organ of Corti and to the basilar membrane that supports the organ of Corti and forms the floor of the cochlear duct. From the basilar membrane the ripple is next transmitted to and through the perilymph in the scala tympani and finally expends itself against the round window like an ocean wave as it breaks against the shore but on a much reduced scale. The steps involved in hearing are shown in Figure Neuronal Pathway of Hearing. Dendrites of neurons whose cell bodies lie in the spiral ganglion and whose axons make up the cochlear nerve terminate around the bases of the hair cells of the organ of Corti, and the tectorial membrane adheres to their upper surfaces. The movement of the hair cells against the adherent tectorial membrane somehow stimulates these dendrites and initiates impulse conduction by the cochlear nerve to the brainstem. Before reaching the auditory area of the temporal lobe, impulses pass through relay stations in nuclei in the medulla, pons, midbrain, and thalamus. 1. List the three anatomical divisions of the ear. 2. Identify the three auditory ossicles. 3. Name the divisions of both the membranous and bony labyrinths. 4. Name the sense organ responsible for hearing. Vestibule and Semicircular Canals The vestibule constitutes the central section of the bony labyrinth. Look again at Figure Notice that the bony labyrinth opens into the oval and round windows from the middle ear, as well as the three semicircular canals of the internal ear. The utricle and saccule are the membranous structures within the vestibule. Both have walls of simple cuboidal epithelium and are filled with endolymph. Three semicircular canals, each in a plane approximately at right angles to the others, are found in each temporal bone (see Figure 15-6). Within the bony semicircular canals and separated from them by perilymph are the membranous semicircular canals. Each contains endolymph and connects with the utricle inside the bony vestibule. Near its junction with the utricle each canal enlarges into an ampulla. Sense of Balance The sense organs involved in the sense of balance, or equilibrium, are found in the vestibule and semicircular canals. The sense organs located in the utricle and saccule function in static equilibrium a function needed to sense the position of the head relative to gravity or to sense acceleration or deceleration of the body, as would occur when seated motionless in a vehicle that was increasing or decreasing in speed. The sense organs associated with the semicircular canals function in dynamic equilibrium a function needed to maintain balance when the head or body itself is rotated or suddenly moved. Static Equilibrium. small and highly specialized patchlike strip of epithelium, called the macula (MK-u-lah), is found in both the utricle and saccule (Figure 15-8, ). It is specialized sensory epithelium containing receptor hair cells and supporting cells covered with a gelatinous matrix. Movements of the macula provide information related to head position or acceleration. ction potentials are generated by movement of the hair cells, which occurs when the position of the head relative to gravity changes. Otoliths tiny ear stones composed of protein and calcium carbonate are located within the matrix of the macula (Figure 15-8, B). Now note the relative positions of the utricular and saccular maculas in Figure 15-8,. The two maculas are oriented almost at right angles to each other: the one in the utricle is parallel to the base of the skull, and the one in the saccule is perpendicular. Changing the position of the head produces a change in the amount of pressure on the otolith-weighted matrix, which, in turn, stimulates the hair cells (Figure 15-8, C and D). This stimulates the adjacent receptors of the vestibular nerve. Its fibers conduct impulses to the brain that produce a sense of the position of the head and also a sensation of a change in the pull of gravity, for example, a sensation of acceleration. In addition, stimulation of the macula evokes righting reflexes,muscular responses to restore the body and its parts to their normal position when they have been displaced. Impulses from proprioceptors and from the eyes also activate righting reflexes. Interruption of the vestibular, visual, or proprioceptive impulses that initiate these reflexes may cause disturbances of equilibrium, nausea, vomiting, and other symptoms. Dynamic Equilibrium. Dynamic equilibrium depends on the functioning of the crista ampullaris, located in the ampulla of each semicircular canal. This specialized structure is a form of sensory epithelium that is similar in many ways to the maculas. Each cone-shaped crista is composed of many hair cells, with their processes embedded in a gelatinous cap, called the cupula (Figure 15-9, and B). The cupula is not weighted with otoliths and does not respond to the pull of gravity. It serves, instead, much like a float that moves with the flow of endolymph in the semicircular canals. Like the maculas, the semicircular canals are placed nearly at right angles to each other. This arrangement enables detection of movement in all directions. s the cupula moves, it bends the hairs embedded in it, producing first a receptor and then an action potential that passes through the vestibular portion of the eighth cranial nerve to the medulla oblongata and, from there, to other areas of the brain and spinal cord for interpretation, integration, and response. When a person spins (Figure 15-9, C to E), the semicircular canals move with the body, but inertia keeps the endolymph in them from moving at the same rate. The cupula therefore moves in a direction opposite to head movement until after the initial movement stops. Dynamic equilibrium is thus able to detect changes both in the direction and in the rate at which movement occurs.