Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition II. Biological Control Systems 9. The Sensory Systems © The McGraw−Hill Companies, 2001 Association Cortex and Perceptual Processing The cortical association areas (Figure 9–8) are brain areas that lie outside the primary cortical sensory or motor areas but are adjacent to them. The association areas are not considered part of the sensory pathways but rather play a role in the progressively more com- plex analysis of incoming information. Although neurons in the earlier stages of the sen- sory pathways are associated with perception, infor- mation from the primary sensory cortical areas is elab- orated after it is relayed to a cortical association area. The region of association cortex closest to the primary sensory cortical area processes the information in fairly simple ways and serves basic sensory-related func- tions. Regions farther from the primary sensory areas process the information in more complicated ways, in- cluding, for example, greater input from areas of the brain serving arousal, attention, memory, and lan- guage. Some of the neurons in these latter regions also receive input concerning two or more other types of sensory stimuli. Thus, an association-area neuron re- ceiving input from both the visual cortex and the “neck” region of the somatosensory cortex might be concerned with integrating visual information with sensory information about head position so that, for example, a tree is understood to be vertical even though the viewer’s head is tipped sideways. Fibers from neurons of the parietal and temporal lobes go to association areas in the frontal lobes that are part of the limbic system. Through these connec- tions, sensory information can be invested with emo- tional and motivational significance. Further perceptual processing involves not only arousal, attention, learning, memory, language, and emotions, but also comparing the information pre- sented via one type of sensation with that of another. For example, we may hear a growling dog, but our perception of the event and our emotional response vary markedly, depending upon whether our visual system detects the sound source to be an angry animal or a loudspeaker. Factors That Affect Perception We put great trust in our sensory-perceptual processes despite the inevitable modifications we know to exist. Some of the following factors are known to affect our perceptions of the real world: 1. Afferent information is influenced by sensory receptor mechanisms (for example by adaptation), and by processing of the information along afferent pathways. 2. Factors such as emotions, personality, experience, and social background can influence perceptions so that two people can witness the same events and yet perceive them differently. 3. Not all information entering the central nervous system gives rise to conscious sensation. Actually, this is a very good thing because many unwanted signals are generated by the extreme 232 PART TWO Biological Control Systems Spinal cord Touch Temperature Thalamus and brainstem Cerebral cortex Temperature Touch Specific ascending pathways Nonspecific ascending pathway FIGURE 9–7 Diagrammatic representation of two specific sensory pathways and a nonspecific sensory pathway. Auditory cortex Frontal lobe association area Parietal lobe association area Somatosensory cortex Visual cortex Occipital lobe association area Temporal lobe association area FIGURE 9–8 Areas of association cortex. Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition II. Biological Control Systems 9. The Sensory Systems © The McGraw−Hill Companies, 2001 In summary, for perception to occur, the three processes involved—transducing stimulus energy into action potentials by the receptor, transmitting data through the nervous system, and interpreting data— cannot be separated. Sensory information is processed at each synapse along the afferent pathways and at many levels of the central nervous system, with the more complex stages receiving input only after it has been processed by the more elementary systems. This hierarchical processing of afferent information along individual pathways is an important organizational principle of sensory systems. As we shall see, a second important principle is that information is processed by parallel pathways, each of which handles a limited as- pect of the neural signals generated by the sensory transducers. A third principle is that information at each stage along the pathway is modified by “top- down” influences serving emotions, attention, mem- ory, and language. Every synapse along the afferent pathway adds an element of organization and con- tributes to the sensory experience so that what we per- ceive is not a simple—or even an absolutely accurate— image of the stimulus that originally activated our re- ceptors. We turn now to how the particular characteristics of a stimulus are coded by the various receptors and sensory pathways. Primary Sensory Coding The sensory systems code four aspects of a stimulus: stimulus type, intensity, location, and duration. Stimulus Type Another term for stimulus type (heat, cold, sound, or pressure, for example) is stimulus modality. Modali- ties can be divided into submodalities: Cold and warm are submodalities of temperature, whereas salt, sweet, bitter, and sour are submodalities of taste. The type of sensory receptor activated by a stimulus plays the pri- mary role in coding the stimulus modality. As mentioned earlier, a given receptor type is par- ticularly sensitive to one stimulus modality—the ade- quate stimulus—because of the signal transduction mechanisms and ion channels incorporated in the re- ceptor’s plasma membrane. For example, receptors for vision contain pigment molecules whose shape is transformed by light; these receptors also have intra- cellular mechanisms by which changes in the pigment molecules alter the activity of membrane ion channels and generate a neural signal. Receptors in the skin have neither light-sensitive molecules nor plasma- membrane ion channels that can be affected by them; thus, receptors in the eyes respond to light and those in the skin do not. 233 The Sensory Systems CHAPTER NINE sensitivity of our sensory receptors. For example, under ideal conditions the rods of the eye can detect the flame of a candle 17 mi away. The hair cells of the ear can detect vibrations of an amplitude much lower than those caused by blood flow through the ears’ blood vessels and can even detect molecules in random motion bumping against the ear drum. It is possible to detect one action potential generated by a certain type of mechanoreceptor. Although these receptors are capable of giving rise to sensations, much of their information is canceled out by receptor or central mechanisms, which will be discussed later. In other receptors’ afferent pathways, information is not canceled out—it simply does not feed into parts of the brain that give rise to a conscious sensation. For example, stretch receptors in the walls of some of the largest blood vessels monitor blood pressure as part of reflex regulation of this pressure, but people have no conscious awareness of their blood pressure. 4. We lack suitable receptors for many energy forms. For example, we cannot directly detect ionizing radiation and radio or television waves. 5. Damaged neural networks may give faulty perceptions as in the bizarre phenomenon known as phantom limb, in which a limb that has been lost by accident or amputation is experienced as though it were still in place. The missing limb is perceived to be the “site” of tingling, touch, pressure, warmth, itch, wetness, pain, and even fatigue, and it is felt as though it were still a part of “self.” It seems that the sensory neural networks in the central nervous system that exist genetically in everyone and are normally triggered by receptor activation are, instead, in the case of phantom limb, activated independently of peripheral input. The activated neural networks continue to generate the usual sensations, which are perceived as arising from the missing receptors. Moreover, somatosensory cortex undergoes marked reorganization after the loss of input from a part of the body so that a person whose arm has been amputated may perceive a touch on the cheek as though it were a touch on the phantom arm; because of the reorganization, the arm area of somatosensory cortex receives input normally directed to the face somatosensory area. 6. Some drugs alter perceptions. In fact, the most dramatic examples of a clear difference between the real world and our perceptual world can be found in illusions and drug- and disease- induced hallucinations, where whole worlds can be created. Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition II. Biological Control Systems 9. The Sensory Systems © The McGraw−Hill Companies, 2001 All the receptors of a single afferent neuron are preferentially sensitive to the same type of stimulus. For example, they are all sensitive to cold or all to pressure. Adjacent sensory units, however, may be sensitive to different types of stimuli. Since the re- ceptive fields for different modalities overlap, a sin- gle stimulus, such as an ice cube on the skin, can give rise simultaneously to the sensations of touch and temperature. Stimulus Intensity How is a strong stimulus distinguished from a weak one when the information about both stimuli is relayed by action potentials that are all the same size? The fre- quency of action potentials in a single receptor is one way, since as described earlier, increased stimulus strength means a larger receptor potential and a higher frequency of action-potential firing. In addition to an increased firing rate from indi- vidual receptors, receptors on other branches of the same afferent neuron also begin to respond. The action potentials generated by these receptors propagate along the branches to the main afferent nerve fiber and add to the train of action potentials there. Figure 9–9 is a record of an experiment in which increased stim- ulus intensity to the receptors of a sensory unit is reflected in increased action-potential frequency in its afferent nerve fiber. In addition to increasing the firing frequency in a single afferent neuron, stronger stimuli usually affect a larger area and activate similar receptors on the endings of other afferent neurons. For example, when one touches a surface lightly with a finger, the area of skin in contact with the surface is small, and only receptors in that skin area are stimulated. Pressing down firmly increases the area of skin stimulated. This “calling in” of receptors on additional afferent neurons is known as recruitment. Stimulus Location A third type of information to be signaled is the loca- tion of the stimulus—in other words, where the stim- ulus is being applied. (It should be noted that in vision, hearing, and smell, stimulus location is interpreted as arising from the site from which the stimulus originated rather than the place on our body where the stimulus was actually applied. For example, we interpret the sight and sound of a barking dog as occurring in that furry thing on the other side of the fence rather than in a spe- cific region of our eyes and ears. More will be said of this later; we deal here with the senses in which the stimulus is located to a site on the body.) The main factor coding stimulus location is the site of the stimulated receptor. The precision, or acuity, with which one stimulus can be located and differen- tiated from an adjacent one depends upon the amount of convergence of neuronal input in the specific as- cending pathways. The greater the convergence, the less the acuity. Other factors affecting acuity are the size of the receptive field covered by a single sensory unit and the amount of overlap of nearby receptive fields. For example, it is easy to discriminate between two adjacent stimuli (two-point discrimination) ap- plied to the skin on a finger, where the sensory units are small and the overlap considerable. It is harder to do so on the back, where the sensory units are large and widely spaced. Locating sensations from internal organs is less precise than from the skin because there are fewer afferent neurons in the internal organs and each has a larger receptive field. It is fairly simple to see why a stimulus to a neu- ron that has a small receptive field can be located more precisely than a stimulus to a neuron with a large re- ceptive field (Figure 9–10). The fact is, however, that even in the former case one cannot distinguish exactly where within the receptive field of a single neuron a 234 PART TWO Biological Control Systems 40 mmHg 60 mmHg 100 mmHg 140 mmHg 180 mmHg Time Pressure (mmHg) Action potentials 180 120 60 FIGURE 9–9 Action potentials from an afferent fiber leading from the pressure receptors of a single sensory unit as the receptors are subjected to pressures of different magnitudes. Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition II. Biological Control Systems 9. The Sensory Systems © The McGraw−Hill Companies, 2001 stimulus has been applied; one can only tell that the afferent neuron has been activated. In this case, receptive-field overlap aids stimulus localization even though, intuitively, overlap would seem to “muddy” the image. Let us examine in the next two paragraphs how this works. An afferent neuron responds most vigorously to stimuli applied at the center of its receptive field be- cause the receptor density—that is, the number of receptors in a given area—is greatest there. The re- sponse decreases as the stimulus is moved toward the receptive-field periphery. Thus, a stimulus activates more receptors and generates more action potentials if it occurs at the center of the receptive field (point A in Figure 9–11). The firing frequency of the afferent neuron is also related to stimulus strength, however, and a high frequency of impulses in the single affer- ent nerve fiber of Figure 9–11 could mean either that a moderately intense stimulus was applied to the cen- ter at A or that a strong stimulus was applied to the periphery at B. Thus, neither the intensity nor the lo- cation of the stimulus can be detected precisely with a single afferent neuron. Since the receptor endings of different afferent neurons overlap, however, a stimulus will trigger ac- tivity in more than one sensory unit. In Figure 9–12, neurons A and C, stimulated near the edge of their re- ceptive fields where the receptor density is low, fire at a lower frequency than neuron B, stimulated at the center of its receptive field. In the group of sensory 235 The Sensory Systems CHAPTER NINE (a) (b) Central nervous system Central nervous system Stimulus A Stimulus B FIGURE 9–10 The information from neuron a indicates the stimulus location more precisely than does that from neuron b because a’s receptive field is smaller. Central nervous system Afferent neuron Action-potential frequency ABC ABC Point of stimulation FIGURE 9–11 Two stimulus points, A and B, in the receptive field of a single afferent neuron. The density of nerve endings around area A is greater than around B, and the frequency of action potentials in response to a stimulus in area A will be greater than the response to a similar stimulus in B. FIGURE 9–12 A stimulus point falls within the overlapping receptive fields of three afferent neurons. Note the difference in receptor response (that is, the action-potential frequency in the three neurons) due to the difference in receptor distribution under the stimulus (low receptor density in A and C, high in B). Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition II. Biological Control Systems 9. The Sensory Systems © The McGraw−Hill Companies, 2001 units in Figure 9–12, a high action-potential frequency in neuron B occurring simultaneously with lower fre- quencies in A and C permits a more accurate localiza- tion of the stimulus near the center of neuron B’s re- ceptive field. Once this location is known, the firing frequency of neuron B can be used to indicate stimu- lus intensity. Lateral Inhibition The phenomenon of lateral in- hibition is, however, far more important in localiza- tion of the stimulus site than are the different sensi- tivities of receptors throughout the receptive field. In lateral inhibition, information from afferent neurons whose receptors are at the edge of a stimulus is strongly inhibited compared to information from the stimulus’s center. Thus, lateral inhibition increases the contrast between relevant and irrelevant information, thereby increasing the effectiveness of selected path- ways and focusing sensory-processing mechanisms on “important” messages. Figure 9–13 shows one neu- ronal arrangement that accomplishes lateral inhibi- tion. Lateral inhibition can occur at different levels of the sensory pathways but typically happens at an early stage. Lateral inhibition can be demonstrated by press- ing the tip of a pencil against your finger. With your eyes closed, you can localize the pencil point precisely, even though the region around the pencil tip is also indented and mechanoreceptors within this region are activated (Figure 9–14). Exact localization occurs be- cause the information from the peripheral regions is removed by lateral inhibition. Lateral inhibition is utilized to the greatest degree in the pathways providing the most accurate localiza- tion. For example, movement of skin hairs, which we can locate quite well, activates pathways that have sig- nificant lateral inhibition, but temperature and pain, which we can locate only poorly, activate pathways that use lateral inhibition to a lesser degree. Stimulus Duration Receptors differ in the way they respond to a con- stantly maintained stimulus—that is, in the way they undergo adaptation. The response—the action-potential frequency—at the beginning of the stimulus indicates the stimulus strength, but after this initial response, the frequency differs widely in different types of receptors. Some re- ceptors respond very rapidly at the stimulus onset, but, after their initial burst of activity, fire only very slowly or stop firing all together during the remainder of the stimulus. These are the rapidly adapting receptors; they are important in signaling rapid change (for ex- ample, vibrating or moving stimuli). Some receptors adapt so rapidly that they fire only a single action potential at the onset of a stimulus—an on response— 236 PART TWO Biological Control Systems + + + + + + + Action potentials in interneuron Interneurons Afferent neurons Action potentials in afferent neuron Excitatory synapses Inhibitory synapses A B A B Key FIGURE 9–13 Afferent pathways showing lateral inhibition. The central fiber at the beginning of the pathway (bottom of figure) is firing at the highest frequency and inhibits, via inhibitory neurons A, the lateral neurons more strongly than the lateral pathways inhibit it, via inhibitory neurons B. Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition II. Biological Control Systems 9. The Sensory Systems © The McGraw−Hill Companies, 2001 while others respond at the beginning of the stimulus and again at its removal—so-called on-off responses. The rapid fading of the sensation of clothes pressing on one’s skin is due to rapidly adapting receptors. Slowly adapting receptors maintain their re- sponse at or near the initial level of firing regardless of the stimulus duration (Figure 9–15). These receptors signal slow changes or prolonged events, such as oc- cur in the joint and muscle receptors that participate in the maintenance of upright posture when standing or sitting for long periods of time. Central Control of Afferent Information All sensory signals are subject to extensive control at the various synapses along the ascending pathways before they reach higher levels of the central nervous system. Much of the incoming information is reduced or even abolished by inhibition from collaterals from other neurons in ascending pathways (lateral inhibi- tion, discussed earlier) or by pathways descending from higher centers in the brain. The reticular forma- tion and cerebral cortex, in particular, control the in- put of afferent information via descending pathways. The inhibitory controls may be exerted directly by synapses on the axon terminals of the primary affer- ent neurons (an example of presynaptic inhibition) or indirectly via interneurons that affect other neurons in the sensory pathways (Figure 9–16). 237 The Sensory Systems CHAPTER NINE Pencil Inhibition Skin (a) (b) Area of inhibition of afferent information Excitation Effect on action-potential frequency Area of receptor activation Area of sensation (c) Area of excitation Rapidly adapting Slowly adapting Stimulus intensity Stimulus intensity Action potentials Action potentials Time Time Excitatory neuron Inhibitory neuron + + + FIGURE 9–14 (a) A pencil tip pressed against the skin depresses surrounding tissue. Receptors are activated under the pencil tip and in the adjacent tissue. (b) Because of lateral inhibition, the central area of excitation is surrounded by an area where the afferent information is inhibited. (c) The sensation is localized to a more restricted region than that in which mechanoreceptors are actually stimulated. FIGURE 9–15 Rapidly and slowly adapting receptors. The top line in each graph indicates the action-potential firing of the afferent nerve fiber from the receptor, and the bottom line, application of the stimulus. FIGURE 9–16 Descending pathways may control sensory information by directly inhibiting the central terminals of the afferent neuron (an example of presynaptic inhibition) or via an interneuron that affects the ascending pathway by inhibitory synapses. Arrows indicate the direction of action-potential transmission. Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition II. Biological Control Systems 9. The Sensory Systems © The McGraw−Hill Companies, 2001 In some cases (for example, in the pain pathways), the afferent input is continuously inhibited to some de- gree. This provides the flexibility of either removing the inhibition (disinhibition) so as to allow a greater degree of signal transmission or of increasing the in- hibition so as to block the signal more completely. We conclude our general introduction to sensory system pathways and coding with a summary of the general principles of the organization of the sensory systems (Table 9–1). We now present the individual systems. I. Sensory processing begins with the transformation of stimulus energy into graded potentials and then into action potentials in nerve fibers. II. Information carried in a sensory system may or may not lead to a conscious awareness of the stimulus. Receptors I. Receptors translate information from the external world and internal environment into graded potentials, which then generate action potentials. a. Receptors may be either specialized endings of afferent neurons or separate cells at the end of the neurons. b. Receptors respond best to one form of stimulus energy, but they may respond to other energy forms if the stimulus intensity is abnormally high. c. Regardless of how a specific receptor is stimulated, activation of that receptor always leads to perception of one sensation. Not all receptor activations lead, however, to conscious sensations. II. The transduction process in all sensory receptors involves—either directly or indirectly—the opening SECTION A SUMMARY or closing of ion channels in the receptor. Ions then flow across the membrane, causing a receptor potential. a. Receptor-potential magnitude and action-potential frequency increase as stimulus strength increases. b. Receptor-potential magnitude varies with stimulus strength, rate of change of stimulus application, temporal summation of successive receptor potentials, and adaptation. Neural Pathways in Sensory Systems I. A single afferent neuron with all its receptor endings is a sensory unit. a. Afferent neurons, which usually have more than one receptor of the same type, are the first neurons in sensory pathways. b. The area of the body that, when stimulated, causes activity in a sensory unit or other neuron in the ascending pathway of that unit is called the receptive field for that neuron. II. Neurons in the specific ascending pathways convey information to specific primary receiving areas of the cerebral cortex about only a single type of stimulus. III. Nonspecific ascending pathways convey information from more than one type of sensory unit to the brainstem reticular formation and regions of the thalamus that are not part of the specific ascending pathways. Association Cortex and Perceptual Processing I. Information from the primary sensory cortical areas is elaborated after it is relayed to a cortical association area. a. The primary sensory cortical area and the region of association cortex closest to it process the information in fairly simple ways and serve basic sensory-related functions. b. Regions of association cortex farther from the primary sensory areas process the sensory information in more complicated ways. c. Processing in the association cortex includes input from areas of the brain serving other sensory modalities, arousal, attention, memory, language, and emotions. Primary Sensory Coding I. The type of stimulus perceived is determined in part by the type of receptor activated. All receptors of a given sensory unit respond to the same stimulus modality. II. Stimulus intensity is coded by the rate of firing of individual sensory units and by the number of sensory units activated. III. Perception of the stimulus location depends on the size of the receptive field covered by a single sensory unit and on the overlap of nearby receptive fields. Lateral inhibition is a means by which ascending pathways emphasize wanted information and increase sensory acuity. IV. Stimulus duration is coded by slowly adapting receptors. 238 PART TWO Biological Control Systems 1. Specific sensory receptor types are sensitive to certain modalities and submodalities. 2. A specific sensory pathway codes for a particular modality or submodality. 3. The ascending pathways are crossed so that sensory information is generally processed by the side of the brain opposite the side of the body that was stimulated. 4. In addition to other synaptic relay points, all ascending pathways, except for those involved in smell, synapse in the thalamus on their way to the cortex. 5. Information is organized such that initial cortical processing of the various modalities occurs in different parts of the brain. 6. Ascending pathways are subject to descending controls. TABLE 9–1 Principles of Sensory System Organization Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition II. Biological Control Systems 9. The Sensory Systems © The McGraw−Hill Companies, 2001 _ V. Information coming into the nervous system is subject to control by both ascending and descending pathways. sensory system specific ascending pathway sensory information somatic receptor sensation somatosensory cortex perception visual cortex sensory receptor auditory cortex stimulus nonspecific ascending pathway stimulus transduction polymodal neuron adequate stimulus cortical association area receptor potential modality adaptation recruitment sensory pathway acuity ascending pathway lateral inhibition sensory unit rapidly adapting receptor receptive field slowly adapting receptor SECTION A KEY TERMS 1. Distinguish between a sensation and a perception. 2. Describe the general process of transduction in a receptor that is a cell separate from the afferent neuron. Include in your description the following terms: specificity, stimulus, receptor potential, neurotransmitter, graded potential, and action potential. 3. List several ways in which the magnitude of a receptor potential can be varied. 4. Describe the relationship between sensory information processing in the primary cortical sensory areas and in the cortical association areas. 5. List several ways in which sensory information can be distorted. 6. How does the nervous system distinguish between stimuli of different types? 7. How is information about stimulus intensity coded by the nervous system? 8. Make a diagram showing how a specific ascending pathway relays information from peripheral receptors to the cerebral cortex. SECTION A REVIEW QUESTIONS 239 The Sensory Systems CHAPTER NINE SPECIFIC SENSORY SYSTEMS SECTION B Somatic Sensation Sensation from the skin, muscles, bones, tendons, and joints is termed somatic sensation and is initiated by a variety of somatic receptors (Figure 9–17). Some respond to mechanical stimulation of the skin, hairs, and underlying tissues, whereas others respond to temper- ature or chemical changes. Activation of somatic recep- tors gives rise to the sensations of touch, pressure, warmth, cold, pain, and awareness of the position of the body parts and their movement. The receptors for vis- ceral sensations, which arise in certain organs of the tho- racic and abdominal cavities, are the same types as the receptors that give rise to somatic sensations. Some or- gans, such as the liver, have no sensory receptors at all. Each sensation is associated with a specific recep- tor type. In other words, there are distinct receptors for heat, cold, touch, pressure, limb position or movement, and pain. After entering the central nervous system, the afferent nerve fibers from the somatic receptors synapse on neurons that form the specific ascending pathways going primarily to the somatosensory cor- tex via the brainstem and thalamus. They also synapse on interneurons that give rise to the nonspecific path- ways. For reference, the location of some important as- cending pathways is shown in a cross section of the spinal cord (Figure 9–18a), and two are diagrammed as examples in Figure 9–18b and c. Note that the pathways cross from the side where the afferent neurons enter the central nervous system to the opposite side either in the spinal cord (Figure 9–18b) or brainstem (Figure 9–18c). Thus, the sensory pathways from somatic receptors on the left side of the body go to the somatosensory cortex of the right cere- bral hemisphere, and vice versa. In the somatosensory cortex, the endings of the ax- ons of the specific somatic pathways are grouped ac- cording to the location of the receptors giving rise to the pathways (Figure 9–19). The parts of the body that are most densely innervated—fingers, thumb, and lips—are represented by the largest areas of the so- matosensory cortex. There are qualifications, however, to this seemingly precise picture: The sizes of the ar- eas can be modified with changing sensory experience, and there is considerable overlap of the body-part rep- resentations. Touch-Pressure Stimulation of the variety of mechanoreceptors in the skin (see Figure 9–17) leads to a wide range of touch- pressure experiences—hair bending, deep pressure, vibrations, and superficial touch, for example. These mechanoreceptors are highly specialized nerve end- ings encapsulated in elaborate cellular structures. The details of the mechanoreceptors vary, but generally the nerve endings are linked to collagen-fiber networks within the capsule. These networks transmit the Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition II. Biological Control Systems 9. The Sensory Systems © The McGraw−Hill Companies, 2001 mechanical tension in the capsule to ion channels in the nerve endings and activate them. The skin mechanoreceptors adapt at different rates, about half adapting rapidly (that is, they fire only when the stimulus is changing), and the others adapt- ing slowly. Activation of rapidly adapting receptors gives rise to the sensations of touch, movement, and vibration, whereas slowly adapting receptors give rise to the sensation of pressure. In both categories, some receptors have small, well-defined receptive fields and are able to provide precise information about the contours of objects in- denting the skin. As might be expected, these recep- tors are concentrated at the fingertips. In contrast, other receptors have large receptive fields with obscure boundaries, sometimes covering a whole finger or a large part of the palm. These receptors are not involved in detailed spatial discrimination but signal informa- tion about vibration, skin stretch, and joint movement. Sense of Posture and Movement The senses of posture and movement are complex. The major receptors responsible for these senses are the muscle-spindle stretch receptors, which occur in skele- tal muscles and respond both to the absolute magni- tude of muscle stretch and to the rate at which the stretch occurs (to be described in Chapter 12). The senses of posture and movement are also supported by vision and the vestibular organs (the “sense organs of balance,” described later). Mechanoreceptors in the joints, tendons, ligaments, and skin also play a role. The term kinesthesia refers to the sense of movement at a joint. Temperature There are two types of thermoreceptors in the skin, each of which responds to a limited range of temper- ature. Warmth receptors respond to temperatures be- tween 30 and 43°C with an increased discharge rate upon warming, whereas receptors for cold are stimu- lated by small decreases in temperature. It is not known how heat or cold alter the endings of the ther- mosensitive afferent neurons to generate receptor potentials. Pain A stimulus that causes (or is on the verge of causing) tissue damage usually elicits a sensation of pain. Re- ceptors for such stimuli are known as nociceptors. They respond to intense mechanical deformation, excessive heat, and many chemicals, including neuropeptide transmitters, bradykinin, histamine, cytokines, and prostaglandins, several of which are released by dam- aged cells. These substances act by combining with spe- cific ligand-sensitive ion channels on the nociceptor plasma membrane. 240 PART TWO Biological Control Systems Skin surface Dermis Epidermis A – Tactile (Meissner’s) corpuscle (light touch) B – Tactile (Merkle’s) corpuscles (touch) C – Free terminal (pain) D – Lamellated (Pacinian) corpuscle (deep pressure) E – Ruffini corpuscle (warmth) D E A A C C B FIGURE 9–17 Skin receptors. Some nerve fibers have free endings not related to any apparent receptor structure. Thicker, myelinated axons, on the other hand, end in receptors that have a complex structure. (Not drawn to scale; for example, Pacinian corpuscles are actually four to five times larger than Meissner’s corpuscles.) [...]... through the lever action of the middle-ear bones The amount of energy transmitted to the inner ear can be lessened by the contraction of two small skeletal muscles in the middle ear that alter the tension of the tympanic membrane and the Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition II Biological Control Systems © The McGraw−Hill Companies, 2001 9 The Sensory Systems The. .. on input from the vestibular system despite the fact that the vestibular organs are sometimes called the sense organs of balance The third use of vestibular information is in providing conscious awareness of the position and acceleration of the body, perception of the space surrounding the body, and memory of spatial information 257 Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth... parts of the body in somatosensory cortex, although there is actually much overlap between the cortical regions The left half of the body is represented on the right hemisphere of the brain, and the right half of the body is represented on the left hemisphere Several of these chemicals are secreted by cells of the immune system (described in Chapter 20) that have moved into the injured area In fact, there... again The difference between the pressure of molecules in zones of compression and rarefaction determines the wave’s amplitude, which is related to the loudness of the sound; the greater the amplitude, the louder the sound The frequency of vibration of the sound source (that is, the number of zones of compression or rarefaction in a given time) determines the pitch we hear; the faster the vibration, the. .. cells in the pathways that give rise to the sense of smell, lie in a small patch of membrane, the olfactory epithelium, in the upper part of the nasal cavity (Figure 9 44 a) These cells are specialized afferent neurons that have a single enlarged dendrite that extends to the surface of the epithelium Several long nonmotile cilia extend out from the tip of the dendrite and lie along the surface of the olfactory... et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition II Biological Control Systems © The McGraw−Hill Companies, 2001 9 The Sensory Systems The Sensory Systems CHAPTER NINE b The vibrating membrane causes movement of the three small middle-ear bones; the stapes vibrates against the oval-window membrane c Movements of the oval-window membrane set up pressure waves in the fluid-filled... the back of the eye, turn the eyes inward toward the nose Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition II Biological Control Systems © The McGraw−Hill Companies, 2001 9 The Sensory Systems The Sensory Systems CHAPTER NINE b' a a' b FIGURE 9–23 Refraction (bending) of light by the lens system of the eye For simplicity, we show light refraction only at the surface of. .. waves of pressure there The wall of the scala vestibuli is largely bone, and there are only two paths by which the pressure waves can be dissipated One path is to the helicotrema, where the waves pass around the end of the cochlear duct into the scala tympani and back to the round-window membrane, which is then bowed out into the middle ear cavity However, most of the pressure is transmitted from the. .. epithelium (Figure 9 44 b) where they are bathed in mucus The cilia contain the receptor proteins (binding sites) for olfactory stimuli The axons of the neurons form the olfactory nerve, which is cranial nerve I For an odorous substance (that is, an odorant) to be detected, molecules of the substance must first diffuse into the air and pass into the nose to the region of the olfactory epithelium Once there,... accommodation is a function of the lens, not the cornea 245 Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition 246 II Biological Control Systems 9 The Sensory Systems © The McGraw−Hill Companies, 2001 PART TWO Biological Control Systems Since the lens must be elastic to assume a more spherical shape during accommodation for near vision, the increasing stiffness of the lens that . receptors on the left side of the body go to the somatosensory cortex of the right cere- bral hemisphere, and vice versa. In the somatosensory cortex, the endings of the ax- ons of the specific. represented on the right hemisphere of the brain, and the right half of the body is represented on the left hemisphere. Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth. because of the shapes of their light-sensitive tips. Note in Figure 9–27 that the light-sensitive por- tion of the photoreceptor cells the tips of the rods and cones—faces away from the incoming