Human Physiology an Integrated Approach

Sensory Physiology

General Properties of Sensory Systems 10 Receptors Are Sensitive to Particular Forms of Energy Sensory Transduction Converts Stimuli into Graded Potentials A Sensory Neuron Has a Receptive Field The CNS Integrates Sensory Information Coding and Processing Distinguish Stimulus Properties Somatic Senses Pathways for Somatic Perception Project to the Cortex and Cerebellum Touch Receptors Respond to Many Diff erent Stimuli Temperature Receptors Are Free Nerve Endings Nociceptors Initiate Protective Responses Pain and Itching Are Mediated by Nociceptors Chemoreception: Smell and Taste Olfaction Is One of the Oldest Senses Taste Is a Combination of Five Basic Sensations Taste Transduction Uses Receptors and Channels The Ear: Hearing Hearing Is Our Perception of Sound Sound Transduction Is a Multistep Process The Cochlea Is Filled with Fluid Sounds Are Processed First in the Cochlea Auditory Pathways Project to the Auditory Cortex Hearing Loss May Result from Mechanical or Neural Damage Nature does not The Ear: Equilibrium communicate with The Vestibular Apparatus Provides Information about Movement man by sending and Position encoded messages. The Semicircular Canals Sense Rotational Acceleration — Oscar Hechter, in Biology The Otolith Organs Sense Linear Acceleration and Head Position and Medicine into the Equilibrium Pathways Project Primarily to the Cerebellum 21st Century, 1991 The Eye and Vision The Skull Protects the Eye Background Basics Light Enters the Eye through the Pupil The Lens Focuses Light on the Retina Summation Phototransduction Occurs at the Retina Second messenger systems Photoreceptors Transduce Light into Electrical Signals Threshold Signal Processing Begins in the Retina G proteins Plasticity Tonic control Membrane potential Graded potentials Neurotransmitter Vestibular release hair cells

343 Sensory Physiology

magine floating in the dark in an indoor tank of buoyant Table salt water: there is no sound, no light, and no breeze. Th e air Information Processing by the Sensory Division 10.1 Iand water are the same temperature as your body. You are in a sensory deprivation chamber, and the only sensations you Stimulus Processing Usually Conscious are aware of come from your own body. Your limbs are weight- less, your breath moves in and out eff ortlessly, and you feel your Special Senses Somatic Senses heart beating. In the absence of external stimuli, you turn your Vision Touch awareness inward to hear what your body has to say. In decades past, fl otation tanks for sensory deprivation were Hearing Temperature a popular way to counter the stress of a busy world. Th ese facili- ties are hard to fi nd now, but they illustrate the role of the aff er- Taste Pain ent division of the nervous system: to provide us with information about the environment outside and inside our bodies. Sometimes Smell Itch we perceive sensory signals when they reach a level of conscious Equilibrium Proprioception awareness, but other times they are processed completely at the subconscious level ( Tbl. 10.1 ). Stimuli that usually do not reach Stimulus Processing Usually Subconscious conscious awareness include changes in muscle stretch and ten- Somatic Stimuli Visceral Stimuli sion as well as a variety of internal parameters that the body monitors to maintain homeostasis, such as blood pressure and pH. Th e Muscle length and Blood pressure responses to these stimuli constitute many of the subconscious re- tension fl exes of the body, and you will encounter them as we explore the processes that maintain physiological homeostasis. Proprioception Distension of gastrointestinal tract In this chapter we are concerned primarily with sensory stimuli whose processing reaches the conscious level of perception. Blood glucose concentration Th ese stimuli are those associated with the special senses of vision, Internal body temperature

RUNNING PROBLEM Osmolarity of body fluids Lung inflation Ménière’s Disease On December 23, 1888, Vincent Van Gogh, the legendary pH of cerebrospinal fluid French painter, returned to his room in a boardinghouse in Arles, France, picked up a knife, and cut off his own ear. A pH and oxygen content of blood local physician, Dr. Felix Ray, examined Van Gogh that night and wrote that the painter had been “assailed by auditory hallucinations” and in an eff ort to relieve them, “mutilated hearing, taste, smell, and equilibrium, and the somatic senses of himself by cutting off his ear.” A few months later, Van Gogh touch, temperature, pain, itch, and proprioception. Propriocep- committed himself to a lunatic asylum. By 1890, Van Gogh tion , which is defi ned as the awareness of body movement and po- was dead by his own hand. Historians have postulated sition in space, is mediated by muscle and joint sensory receptors that Van Gogh suff ered from epilepsy, but some American called proprioceptors and may be either unconscious or conscious. neurologists disagree. They concluded that the painter’s If you close your eyes and raise your arm above your head, you are strange attacks of dizziness, nausea, and overwhelming aware of its position because of the activation of proprioceptors. tinnitus (ringing or other sounds in the ears), which he described in desperate letters to his relatives, are more We fi rst consider general properties of sensory pathways. consistent with Ménière’s disease, a condition that aff ects the We then look at the unique receptors and pathways that distin- inner ear. Today, Anant, a 20-year-old college student, will be guish the diff erent sensory systems from one another. examined by an otolaryngologist (ear-nose-throat specialist) to see if his periodic attacks of severe dizziness and nausea are caused by the same condition that might have driven Van General Properties Gogh to suicide. of Sensory Systems All sensory pathways have certain elements in common. They begin with a stimulus, in the form of physical energy that acts on a sensory receptor. Th e receptor is a transducer

344 Sensory Physiology that converts the stimulus into an intracellular signal, usu- million associated parts, and the human eye has about 126 mil- ally a change in membrane potential. If the stimulus is above lion sensory receptors. threshold, action potentials pass along a sensory neuron to the central nervous system, where incoming signals are integrated. Some stimuli pass upward to the cerebral cortex, Receptors Are Sensitive to Particular where they reach conscious perception, but others are acted on subconsciously, without our awareness. At each synapse Forms of Energy along the pathway, the nervous system can modulate and Receptors in the sensory system vary widely in complexity, shape the sensory information. ranging from the branched endings of a single sensory neuron Sensory systems in the human body vary widely in com- to complex, highly organized cells such as photoreceptors. Th e plexity. Th e simplest systems are single sensory neurons with simplest receptors consist of a neuron with naked (“free”) nerve branched dendrites that function as receptors, such as pain and endings ( Fig. 10.1 a). In more complex receptors, the nerve itch receptors. Th e most complex systems include multicellular endings are encased in connective tissue capsules (Fig. 10.1 b). sense organs , such as the ear and the eye. The cochlea of the Th e axons of both simple and complex receptors may be myelin- ear contains about 16,000 sensory receptors and more than a ated or unmyelinated. 10

SENSORY RECEPTORS

(a) Simple receptors are neurons (b) Complex neural receptors have nerve (c) Most special senses receptors are cells with free nerve endings. They endings enclosed in connective tissue capsules. that release neurotransmitter onto sensory may have myelinated or This illustration shows a Pacinian corpuscle, neurons, initiating an action potential. The unmyelinated axons. which senses touch. cell illustrated is a hair cell, found in the ear.

Stimulus Stimulus Stimulus

Free nerve endings Enclosed nerve Specialized receptor ending cell (hair cell) Layers of connective tissue Synaptic vesicles Synapse

Unmyelinated axon

Myelinated axon Myelinated axon

Cell body Cell body

Cell body of sensory neuron

Fig. 10.1

345 Sensory Physiology

Th e special senses have the most highly specialized receptors. The receptors for smell are neurons, but the other four Table 10.2 special senses use non-neural receptor cells that synapse onto Types of Sensory Receptors sensory neurons. Th e hair cell of the ear, shown in Figure 10.1c, is an example of a non-neural receptor. When activated, the hair Type of Receptor Examples of Stimuli cell releases a neurotransmitter that initiates an action potential Chemoreceptors Oxygen, pH, various in the associated sensory neuron. Both neural and non-neural organic molecules such as receptors develop from the same embryonic tissue. glucose Non-neural accessory structures are critical to the opera- tion of many sensory systems. For example, the lens and cornea Mechanoreceptors Pressure (baroreceptors), cell stretch of the eye help focus incoming light onto photoreceptors. Th e (osmoreceptors), vibration, hairs on our arms help somatosensory receptors sense move- acceleration, sound ment in the air millimeters above the skin surface. Accessory structures oft en enhance the information-gathering capability Photoreceptors Photons of light of the sensory system. Thermoreceptors Varying degrees of heat Receptors are divided into four major groups, based on the type of stimulus to which they are most sensitive ( Tbl. 10.2 ). Chemoreceptors respond to chemical ligands that bind to the receptor (taste and smell, for example). Mechanoreceptors are specifi c for one form of energy, they can respond to most respond to various forms of mechanical energy, including pres- other forms if the intensity is high enough. Photoreceptors of sure, vibration, gravity, acceleration, and sound (hearing, for the eye respond most readily to light, for instance, but a blow example). Th ermoreceptors respond to temperature, and pho- to the eye may cause us to “see stars,” an example of mechanical toreceptors for vision respond to light. energy of suffi cient force to stimulate the photoreceptors. Sensory receptors can be incredibly sensitive to their preferred form of stimulus. For example, a single photon of light Concept Check Answers: End of Chapter stimulates certain photoreceptors, and a single odorant molecule may activate the chemoreceptors involved in the sense of smell. 1 . What advantage do myelinated axons provide? Th e minimum stimulus required to activate a receptor is known 2. What accessory role does the outer ear (the pinna) play in the auditory as the threshold , just as the minimum depolarization required system? to trigger an action potential is called the threshold. 3. For each of the somatic and visceral stimuli listed in Table 10.1 , which of How is a physical or chemical stimulus converted into a the following receptor types is the appropriate transducer: mechano-, change in membrane potential? The stimulus opens or closes chemo-, photo-, or thermoreceptors? ion channels in the receptor membrane, either directly or in- directly (through a second messenger). In most cases, channel opening results in net infl ux of Na + or other cations into the receptor, depolarizing the membrane. In a few cases, the response Sensory Transduction Converts Stimuli to the stimulus is hyperpolarization when K+ leaves the cell. In the case of vision, the stimulus (light) closes cation channels to into Graded Potentials hyperpolarize the receptor. How do receptors convert diverse physical stimuli, such as light Th e change in sensory receptor membrane potential is a or heat, into electrical signals? Th e fi rst step is transduction , the graded potential called a receptor potential. In some cells, the conversion of stimulus energy into information that can be pro- receptor potential initiates an action potential that travels along cessed by the nervous system. In many receptors, the opening or the sensory fi ber to the CNS. In other cells, receptor potentials closing of ion channels converts mechanical, chemical, thermal, infl uence neurotransmitter secretion by the receptor cell, which or light energy directly into a change in membrane potential. in turn alters electrical activity in an associated sensory neuron. Some sensory transduction mechanisms include signal transduction and second messenger systems that initiate the change in membrane potential. A Sensory Neuron Has a Receptive Field Each sensory receptor has an adequate stimulus, a par- Somatic sensory and visual neurons are activated by stimuli that ticular form of energy to which it is most responsive. For exam- fall within a specifi c physical area known as the neuron’s recep- ple, thermoreceptors are more sensitive to temperature changes tive field. For example, a touch-sensitive neuron in the skin than to pressure, and mechanoreceptors respond preferentially responds to pressure that falls within its receptive fi eld. In the to stimuli that deform the cell membrane. Although receptors

346 Sensory Physiology

RECEPTIVE FIELDS OF SENSORY NEURONS

(a) Convergence creates large receptive fields. (b) Small receptive fields are found in more sensitive areas.

Compass with points separated by 20 mm

The receptive fields of three When fewer neurons converge, primary sensory neurons secondary receptive fields are overlap to form one large much smaller. secondary receptive field. Skin surface

Skin surface

Primary sensory neurons 10

Convergence of primary Secondary neurons allows simultaneous sensory subthreshold stimuli to sum neurons at the secondary sensory neuron and initiate an action potential.

Two stimuli that fall within the same The two stimuli activate separate secondary receptive field are perceived pathways to the brain. The two as a single point, because only one points are perceived as distinct signal goes to the brain. Therefore, stimuli and hence there is there is no two-point discrimination. two-point discrimination.

Fig. 10.2

simplest case, one receptive fi eld is associated with one sensory placed within 20 mm of each other are interpreted by the brain neuron (the primary sensory neuron in the pathway), which as a single pinprick. In these areas, many primary neurons con- in turn synapses on one CNS neuron (the secondary sensory verge on a single secondary neuron, so the secondary receptive neuron ). (Primary and secondary sensory neurons are also fi eld is very large ( Fig. 10.2 a). known as fi rst-order and second-order neurons .) Receptive fi elds In contrast, more sensitive areas of skin, such as the fi n- frequently overlap with neighboring receptive fi elds. gertips, have smaller receptive fi elds, with as little as a 1:1 re- In addition, sensory neurons of neighboring receptive lationship between primary and secondary sensory neurons fields may exhibit convergence, in which multiple presynap- ( Fig. 10.2 b). In these regions, two pins separated by as little as tic neurons provide input to a smaller number of postsynaptic 2 mm can be perceived as two separate touches. neurons ( Fig. 10.2 ). Convergence allows multiple simultaneous subthreshold stimuli to sum at the postsynaptic (secondary) neuron. When multiple primary sensory neurons converge The CNS Integrates Sensory Information on a single secondary sensory neuron, their individual recep- Sensory information from much of the body enters the spinal cord tive fi elds merge into a single, large secondary receptive fi eld, as and travels through ascending pathways to the brain. Some sensory shown in Figure 10.2 a. information goes directly into the brain stem via the cranial nerves. Th e size of secondary receptive fi elds determines how sen- Sensory information that initiates visceral refl exes is integrated in sitive a given area is to a stimulus. For example, sensitivity to the brain stem or spinal cord and usually does not reach conscious touch is demonstrated by a two-point discrimination test. In perception. An example of an unconscious visceral reflex is the some regions of skin, such as that on the arms and legs, two pins control of blood pressure by centers in the brain stem.

347 Sensory Physiology

Each major division of the brain processes one or more closely linked to memory and emotion. Most people have expe- types of sensory information ( Fig. 10.3 ). For example, the rienced encountering a smell that suddenly brings back a fl ood midbrain receives visual information, and the medulla oblon- of memories of places or people from the past. gata receives input for sound and taste. Information about bal- One interesting aspect of CNS processing of sensory in- ance and equilibrium is processed primarily in the cerebellum. formation is the perceptual threshold , the level of stimulus Th ese pathways, along with those carrying somatosensory in- intensity necessary for you to be aware of a particular sen- formation, project to the thalamus, which acts as a relay and sation. Stimuli bombard your sensory receptors constantly, processing station before passing the information on to the but your brain can fi lter out and “turn off ” some stimuli. You cerebrum. experience a change in perceptual threshold when you “tune Only olfactory { olfacere, to sniff } information is not routed out” the radio while studying or when you “zone out” dur- through the thalamus. Th e sense of smell, a type of chemorecep- ing a lecture. In both cases, the noise is adequate to stimulate tion, is considered to be one of the oldest senses, and even the sensory neurons in the ear, but neurons higher in the pathway most primitive vertebrate brains have well-developed regions dampen the perceived signal so that it does not reach the con- for processing olfactory information. Information about odors scious brain. travels from the nose through the fi rst cranial nerve and olfac- Decreased perception of a stimulus, or habituation, is tory bulb to the olfactory cortex in the cerebrum. Perhaps it is accomplished by inhibitory modulation. Inhibitory modula- because of this direct input to the cerebrum that odors are so tion diminishes a suprathreshold stimulus until it is below

SENSORY PATHWAYS IN THE BRAIN

Most pathways pass through the thalamus on their way to the cerebral cortex.

Primary somatic Gustatory cortex sensory cortex

Olfactory cortex

Olfactory bulb Auditory cortex Visual cortex

1 Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. Eye 2 Cerebellum Most sensory pathways project 2 Nose 1 Thalamus Sound to the thalamus. The thalamus modifies and relays information to cortical centers. Brain stem Equilibrium 3 Equilibrium pathways project 3 primarily to the cerebellum. Tongue

FIGURE QUESTION Which sensory pathways shown Somatic do not synapse in the thalamus? senses

Fig. 10.3

348 Sensory Physiology the perceptual threshold. It oft en occurs in the secondary and larization of the receptor. Th e blow to the eye that causes us to higher neurons of a sensory pathway. If the modulated stimulus “see” a fl ash of light is another example of labeled line coding. suddenly becomes important, such as when the professor asks you a question, you can consciously focus your attention and Location of the Stimulus The location of a stimulus is also overcome the inhibitory modulation. At that point, your con- coded according to which receptive fi elds are activated. Th e sen- scious brain seeks to retrieve and recall recent sound input from sory regions of the cerebrum are highly organized with respect your subconscious so that you can answer the question. to incoming signals, and input from adjacent sensory receptors is processed in adjacent regions of the cortex. Th is arrangement preserves the topographical organization of receptors on the Coding and Processing Distinguish skin, eye, or other regions in the processing centers of the brain. Stimulus Properties For example, touch receptors in the hand project to a spe- If all stimuli are converted to action potentials in sensory neu- cific area of the cerebral cortex. Experimental stimulation of rons and all action potentials are identical, how can the central that area of the cortex during brain surgery is interpreted as a nervous system tell the diff erence between, say, heat and pres- touch to the hand, even though there is no contact. Similarly, sure, or between a pinprick to the toe and one to the hand? Th e one type of the phantom limb pain reported by amputees occurs when secondary sensory neurons in the spinal cord become hy- attributes of the stimulus must somehow be preserved once the 10 stimulus enters the nervous system for processing. Th is means peractive, resulting in the sensation of pain in a limb that is no that the CNS must distinguish four properties of a stimulus: longer there. (1) its nature, or modality, (2) its location, (3) its intensity, and Auditory information is an exception to the localization rule, however. Neurons in the ears are sensitive to diff erent fre- (4) its duration. quencies of sound, but they have no receptive fi elds and their S e n s o r y M o d a l i t y Th e modality of a stimulus is indicated by activation provides no information about the location of the which sensory neurons are activated and by where the pathways sound. Instead, the brain uses the timing of receptor activation of the activated neurons terminate in the brain. Each receptor to compute a location, as shown in Figure 10.4 . type is most sensitive to a particular modality of stimulus. For A sound originating directly in front of a person reaches example, some neurons respond most strongly to touch; others both ears simultaneously. A sound originating on one side respond to changes in temperature. Each sensory modality can be subdivided into qualities. For instance, color vision is divided into red, blue, and green according to the wavelengths that most The Brain Uses Timing Differences to Localize Sound strongly stimulate the diff erent visual receptors. Source In addition, the brain associates a signal coming from of sound a specifi c group of receptors with a specifi c modality. Th is 1:1 association of a receptor with a sensation is called labeled line Sound takes longer coding . Stimulation of a cold receptor is always perceived as to reach right ear. cold, whether the actual stimulus was cold or an artifi cial depo-

RUNNING PROBLEM

Ménière’s disease—named for its discoverer, the nineteenth- century French physician Prosper Ménière—is associated with a build-up of fl uid in the inner ear and is also known as endolymphatic hydrops {hydro- , water}. Symptoms of Ménière’s disease include episodic attacks of vertigo, nausea, and tinnitus, accompanied by hearing loss and a feeling of fullness in the ears. Vertigo is a false sensation of spinning Signals coming movement that patients often describe as dizziness. Left from the left reach Right the brain first. Q1: In which part of the brain is sensory information about equilibrium processed?

Top view of head Fig. 10.4

349 Sensory Physiology

reaches the closer ear several milliseconds before it reaches the localized. In the visual system, lateral inhibition sharpens our other ear. Th e brain registers the diff erence in the time it takes perception of visual edges. for the sound stimuli to reach the two sides of the auditory cor- Th e pathway in Figure 10.5 also is an example of popula- tex and uses that information to compute the sound’s source. tion coding, the way multiple receptors function together to Lateral inhibition , which increases the contrast between send the CNS more information than would be possible from a activated receptive fi elds and their inactive neighbors, is another single receptor. By comparing the input from multiple receptors, way of isolating the location of a stimulus. Figure 10.5 shows the CNS can make complex calculations about the quality and this process for a pressure stimulus to the skin. A pin pushing spatial and temporal characteristics of a stimulus. on the skin activates three primary sensory neurons, each of which releases neurotransmitters onto its corresponding sec- Concept Check Answer: End of Chapter ondary neuron. However, the three secondary neurons do not all respond 4 . In Figure 10.5, what kind(s) of ion channel might open in neurons A and in the same fashion. Th e secondary neuron closest to the stimu- C that would depress their responsiveness: Na+, K+, Cl -, or Ca2+? lus (neuron B) suppresses the response of the secondary neurons lateral to it (that is, on either side), where the stimulus is weaker, and simultaneously allows its own pathway to proceed Intensity and Duration of the Stimulus The intensity of a without interference. The inhibition of neurons farther from stimulus cannot be directly calculated from a single sensory the stimulus enhances the contrast between the center and the neuron action potential because a single action potential is “all- sides of the receptive field, making the sensation more easily or-none.” Instead, stimulus intensity is coded in two types of

LATERAL INHIBITION

Lateral inhibition enhances contrast and makes a stimulus easier to perceive. The responses of primary sensory neurons A, B, and C are proportional to the intensity of the stimulus in each receptor field. Secondary sensory neuron B inhibits secondary neurons A and C, creating greater contrast between B and its neighbors.

Stimulus Stimulus Pin

Skin

ABC

Tonic level Primary neuron A B C response is proportional to stimulus strength.

Primary of action potentials Frequency sensory neurons

Pathway closest to Secondary the stimulus inhibits neurons neighbors.

- -

ABC

Inhibition of lateral Tonic level Tertiary neurons enhances neurons perception of stimulus.

A B C of action potentials Frequency Fig. 10.5

350 Sensory Physiology

information: the number of receptors activated (another exam- Similarly, the duration of a stimulus is coded by the du- ple of population coding) and the frequency of action potentials ration of action potentials in the sensory neuron. In general, a coming from those receptors, called frequency coding . longer stimulus generates a longer series of action potentials Population coding for intensity occurs because the thresh- in the primary sensory neuron. However, if a stimulus persists, old for the preferred stimulus is not the same for all receptors. some receptors adapt, or cease to respond. Receptors fall into Only the most sensitive receptors (those with the lowest thresh- one of two classes, depending on how they adapt to continuous olds) respond to a low-intensity stimulus. As a stimulus in- stimulation. creases in intensity, additional receptors are activated. Th e CNS Tonic receptors are slowly adapting receptors that fi re rap- then translates the number of active receptors into a measure of idly when fi rst activated, then slow and maintain their fi ring as stimulus intensity. long as the stimulus is present ( Fig. 10.7 a). Pressure-sensitive For individual sensory neurons, intensity discrimination baroreceptors, irritant receptors, and some tactile receptors and begins at the receptor. If a stimulus is below threshold, the pri- proprioceptors fall into this category. In general, the stimuli that mary sensory neuron does not respond. Once stimulus intensity activate tonic receptors are parameters that must be monitored exceeds threshold, the primary sensory neuron begins to fire continuously by the body. action potentials. As stimulus intensity increases, the receptor In contrast, phasic receptors are rapidly adapting recep- potential amplitude (strength) increases in proportion, and the tors that fi re when they fi rst receive a stimulus but cease fi ring if frequency of action potentials in the primary sensory neuron the strength of the stimulus remains constant (Fig. 10.7 b). Pha- 10 increases, up to a maximum rate ( Fig. 10.6 ). sic receptors are attuned specifi cally to changes in a parameter.

CODING FOR STIMULUS INTENSITY AND DURATION

Longer or stronger stimuli release more neurotransmitter.

Transduction site Trigger zone Myelinated axon Cell body Axon terminal

Stimulus

Amplitude 20 0 -20 -40 Threshold Duration -60 -80 (a) Moderate 0 5 10 0 5 10 0 5 10 Membrane potential (mV) Stimulus Time (sec)

20 0 -20 -40 -60 -80 (b) Longer and 0 5 10 0 5 10 0 5 10 Stronger Stimulus Membrane potential (mV) Receptor potential Receptor potential Frequency of action Neurotransmitter 12 3 4 strength and is integrated at the potentials is proportional release varies with duration vary with trigger zone. to stimulus intensity. the pattern of action the stimulus. Duration of a series of potentials arriving action potentials is at the axon terminal. proportional to stimulus duration.

Fig. 10.6

351 Sensory Physiology

RECEPTOR ADAPTATION

Receptors adapt to sustained stimulus. (a) Tonic receptors are slowly (b) Phasic receptors rapidly adapt to a adapting receptors that respond constant stimulus and turn off. for the duration of a stimulus.

Stimulus Stimulus

Receptor

Receptor potential

Axon of sensory neuron Action potentials in sensory neuron Time Time

Fig. 10.7

Once a stimulus reaches a steady intensity, phasic receptors response to loud noises, thus decreasing the sound signal before adapt to the new steady state and turn off . Th is type of response it reaches auditory receptors. allows the body to ignore information that has been evaluated To summarize, the specifi city of sensory pathways is estab- and found not to threaten homeostasis or well-being. lished in several ways: Our sense of smell is an example of a sense that uses phasic 1 Each receptor is most sensitive to a particular type of receptors. For example, you can smell your cologne when you stimulus. put it on in the morning, but as the day goes on your olfactory 2 A stimulus above threshold initiates action potentials in a receptors adapt and are no longer stimulated by the cologne sensory neuron that projects to the CNS. molecules. You no longer smell the fragrance, yet others may 3 Stimulus intensity and duration are coded in the pattern of comment on it. action potentials reaching the CNS. Adaptation of phasic receptors allows us to fi lter out ex- 4 Stimulus location and modality are coded according to traneous sensory information and concentrate on what is new, which receptors are activated or (in the case of sound) by diff erent, or essential. In general, once adaptation of a phasic the timing of receptor activation. receptor has occurred, the only way to create a new signal is to 5 Each sensory pathway projects to a specifi c region of the either increase the intensity of the excitatory stimulus or remove cerebral cortex dedicated to a particular receptive field. the stimulus entirely and allow the receptor to reset. Th e brain can then tell the origin of each incoming signal. Th e molecular mechanism for sensory receptor adaptation depends on the receptor type. In some receptors, K+ c h a n n e l s in the receptor membrane open, causing the membrane to repo- Concept Check Answers: End of Chapter larize and stopping the signal. In other receptors, Na + c h a n n e l s quickly inactivate. In yet other receptors, biochemical pathways 5 . How do sensory receptors communicate the intensity of a stimulus to alter the receptor’s responsiveness. the CNS? Accessory structures may also decrease the amount of 6. What is the adaptive significance of irritant receptors that are tonic stimulus reaching the receptor. In the ear, for example, tiny instead of phasic? muscles contract and dampen the vibration of small bones in

352 Sensory Physiology

Somatic Senses ability of the patient to communicate with the surgeon during this process has expanded our knowledge of brain regions There are four somatosensory modalities: touch, propriocep- tremendously. tion, temperature, and nociception , which includes pain and Experiments can also be done on nonhuman animals by itch. stimulating peripheral receptors and monitoring electrical activity in the cortex. We have learned from these experiments Pathways for Somatic Perception Project that the more sensitive a region of the body is to touch and other stimuli, the larger the corresponding region in the cortex. Inter- to the Cortex and Cerebellum estingly, the size of the regions is not fi xed. If a particular body Receptors for the somatic senses are found both in the skin and part is used more extensively, its topographical region in the in the viscera. Receptor activation triggers action potentials in cortex will expand. For example, people who are visually handi- the associated primary sensory neuron. In the spinal cord, many capped and learn to read Braille with their fi ngertips develop primary sensory neurons synapse onto interneurons that serve an enlarged region of the somatosensory cortex devoted to the as the secondary sensory neurons. Th e location of the synapse fi ngertips. between a primary neuron and a secondary neuron varies ac- In contrast, if a person loses a fi nger or limb, the portion cording to the type of receptor ( Fig. 10.8 ). of the somatosensory cortex devoted to the missing structure Neurons associated with receptors for nociception, tem- begins to be taken over by sensory fi elds of adjacent structures. 10 perature, and coarse touch synapse onto their secondary neu- Reorganization of the somatosensory cortex “map” is an ex- rons shortly aft er entering the spinal cord. In contrast, most fi ne ample of the remarkable plasticity of the brain. Unfortunately, touch, vibration, and proprioceptive neurons have very long sometimes the reorganization is not perfect and can result in axons that project up the spinal cord all the way to the medulla. sensory sensations, including pain, that the brain interprets as All secondary sensory neurons cross the midline of the being located in the missing limb (phantom limb pain). body at some point, so that sensations from the left side of Contemporary research in this fi eld now uses noninvasive the body are processed in the right hemisphere of the brain and imaging techniques, such as functional magnetic resonance im- vice versa. The secondary neurons for nociception, tempera- aging (fMRI) and positive emission tomography (PET) scans to ture, and coarse touch cross the midline in the spinal cord, then watch brains at work. Both techniques measure the metabolic ascend to the brain. Fine touch, vibration, and proprioceptive activity of neurons, so that more active areas of neuronal activity neurons cross the midline in the medulla. become highlighted and can be associated with their location. In the thalamus, all secondary sensory neurons synapse onto tertiary sensory neurons , which in turn project to the somatosensory region of the cerebral cortex. In addition, many Touch Receptors Respond to Many sensory pathways send branches to the cerebellum so that it can use the information to coordinate balance and movement. Diff erent Stimuli The somatosensory cortex is the part of the brain that Touch receptors are among the most common receptors in the recognizes where ascending sensory tracts originate. Each sen- body. Th ese receptors respond to many forms of physical contact, sory tract has a corresponding region of the cortex, so that all such as stretch, steady pressure, fluttering or stroking move- sensory pathways for the left hand terminate in one area, all ment, vibration, and texture. They are found both in the skin pathways for the left foot terminate in another area, and so on ( Fig. 10.10 ) and in deeper regions of the body. ( Fig. 10.9 ). Within the cortical region for a particular body Touch receptors in the skin come in many forms. Some part, columns of neurons are devoted to particular types of are free nerve endings, such as those that respond to noxious receptors. stimuli. Others are more complex. Most touch receptors are dif- For example, a cortical column activated by cold recep- fi cult to study because of their small size. However, Pacinian tors in the left hand may be found next to a column activated corpuscles, which respond to vibration, are some of the larg- by pressure receptors in the skin of the left hand. Th is columnar est receptors in the body, and much of what we know about so- arrangement creates a highly organized structure that maintains matosensory receptors comes from studies on these structures. the association between specifi c receptors and the sensory mo- Pacinian corpuscles are composed of nerve endings encap- dality they transmit. sulated in layers of connective tissue (see Fig. 10.1 b). Th ey are Some of the most interesting research about the somato- found in the subcutaneous layers of skin and in muscles, joints, sensory cortex has been done on patients during brain surgery and internal organs. Th e concentric layers of connective tissue for epilepsy. Because brain tissue has no pain fi bers, this type in the corpuscles create large receptive fi elds. of surgery can be performed with the patient awake under lo- Pacinian corpuscles respond best to high-frequency vibra- cal anesthesia. The surgeon stimulates a particular region of tions, whose energy is transferred through the connective tissue the brain and asks the patient about sensations that occur. Th e

353 Sensory Physiology

SOMATOSENSORY PATHWAYS

4 Sensations are perceived 4 in the primary somatic sensory cortex.

3 3 Sensory pathways synapse in the thalamus. THALAMUS

MEDULLA

2 2 Fine touch, vibration, and proprioception pathways cross the midline in the medulla.

Fine touch, proprioception, vibration

KEY 1 1 Pain, temperature, and Nociception, coarse touch cross the Primary sensory neuron temperature, midline in the spinal cord. Secondary sensory neuron coarse touch Tertiary neuron

SPINAL CORD

FIGURE QUESTION A blood clot damages sensory tracts passing through the lower right side of the medulla. Tell whether the following sensations would be abnormal on the right side (ipsilateral) or left (contralateral) side of the body.

(a) pain (b) proprioception (c) temperature

PRIMARY SENSORY SECONDARY SENSORY SYNAPSE WITH TERTIARY SENSORY

Fine touch, Primary sensory Secondary sensory neuron crosses proprioception, neuron synapses in the midline of body in medulla. vibration medulla. Synapse with tertiary Tertiary sensory neuron sensory neuron in the terminates in somatosensory Irritants, Primary sensory neuron Secondary sensory neuron crosses thalamus. cortex. temperature, synapses in dorsal horn midline of body in spinal cord. coarse touch of spinal cord.

Fig. 10.8

354 Sensory Physiology

receptors are stimulated by temperatures in the range extending THE SOMATOSENSORY CORTEX from normal body temperature (37 °C) to about 45 °C. Above Each body part is represented next to the area of the sensory cortex that temperature, pain receptors are activated, creating a sensa- that processes stimuli for that body part. This mapping was created tion of painful heat. Th ermoreceptors in the brain play an im- by two neurosurgeons, W. Penfield and T. Rasmussen, in 1950 and is called a homunculus (little man). portant role in thermoregulation. Th e receptive fi eld for a thermoreceptor is about 1 mm in diameter, and the receptors are scattered across the body. Th ere are considerably more cold receptors than warm ones. Tempera- ture receptors slowly adapt between 20 and 40 °C. Th eir initial response tells us that the temperature is changing, and their sustained response tells us about the ambient temperature. Outside the 20–40 °C range, where the likelihood of tissue damage is The amount of space greater, the receptors do not adapt. Th ermoreceptors use a fam- on the somatosensory ily of cation channels called transient receptor potential or TRP cortex devoted to each body part is proportional channels. to the sensitivity of that part. 10 Nociceptors Initiate Protective Responses Nociceptors {nocere, to injure} are receptors that respond to a variety of strong noxious stimuli (chemical, mechanical, or thermal) that cause or have the potential to cause tissue damage. Ac- tivation of nociceptors initiates adaptive, protective responses, such as the reflexive withdrawal of a hand from a hot stove touched accidentally. Nociceptors are not limited to the skin. Discomfort from overuse of muscles and joints helps us avoid Thalamus additional damage to these structures. Two sensations may be perceived when nociceptors are activated: pain and itch. Nociceptors are sometimes called pain receptors, even though pain is a perceived sensation rather than a stimulus. Nociceptive pain is mediated by free nerve endings whose ion channels are sensitive to a variety of chemical, mechanical, and Sensory signals thermal stimuli. For example, the membrane channels called from left side of body vanilloid receptors respond to damaging heat from a stove or other source, as well as to capsaicin, the chemical that makes Cross section of the right cerebral hemisphere hot chili peppers burn your mouth. (Vanilloid receptors are also and sensory areas of the cerebral cortex called transient receptor potential V 1 o r TRPV1 c h a n n e l s , i n t h e Fig. 10.9 same family as the thermoreceptor channels.) At the opposite end of the temperature spectrum, researchers recently identifi ed capsule to the nerve ending, where the energy opens mechani- a membrane protein that responds both to cold and to menthol, cally gated ion channels. Pacinian corpuscles are rapidly adapt- one reason mint-fl avored foods feel cool. ing phasic receptors, and this property allows them to respond Nociceptor activation is modulated by local chemicals that to a change in touch but then ignore it. For example, you notice are released upon tissue injury, including K+, histamine, and your shirt when you fi rst put it on, but the touch receptors soon prostaglandins released from damaged cells; serotonin released adapt. Properties of the remaining touch receptors depicted in from platelets activated by tissue damage; and the peptide sub- Figure 10.10 —Meissner’s corpuscles, Ruffini corpuscles, and stance P , which is secreted by primary sensory neurons. Th ese Merkel receptors—are summarized in the table of that fi gure. chemicals, which also mediate the infl ammatory response at the site of injury, either activate nociceptors or sensitize them by lowering their activation threshold. Increased sensitivity to pain Temperature Receptors Are Free Nerve Endings at sites of tissue damage is called infl ammatory pain . Temperature receptors are free nerve endings that terminate in Nociceptors may activate two pathways: (1) refl exive protec- the subcutaneous layers of the skin. Cold receptors are sensitive tive responses that are integrated at the level of the spinal cord primarily to temperatures lower than body temperature. Warm and (2) ascending pathways to the cerebral cortex that become conscious sensation (pain or itch). Primary sensory neurons from

355 Sensory Physiology

SENSORY RECEPTORS IN THE SKIN

Merkel receptors Meissner's corpuscle sense steady pressure responds to flutter and and texture. stroking movements. Hair Free nerve ending

Free nerve ending of Free nerve ending nociceptor responds of hair root senses Hair root to noxious stimuli. hair movement. Sensory nerves Pacinian corpuscle carry signals to senses vibration. spinal cord.

Ruffini corpuscle responds to skin stretch.

RECEPTORSTIMULUS LOCATION STRUCTURE ADAPTATION

Free nerve endings Temperature, noxious Around the hair roots and Unmyelinated Variable stimuli, hair movement under surface of skin nerve endings

Meissner’s corpuscles Flutter, stroking Superficial layers Encapsulated in Rapid of skin connective tissue

Pacinian corpuscles Vibration Deep layers of skin Encapsulated in Rapid connective tissue

Ruffini corpuscles Stretch of skin Deep layers of skin Enlarged nerve Slow endings

Merkel receptors Steady pressure, Superficial layers Enlarged nerve Slow texture of skin endings

Fig. 10.10

nociceptors terminate in the dorsal horn of the spinal cord (see refl ex causes the leg to contract and move the foot away from Fig. 10.8 ). Th ere they synapse onto secondary sensory neurons the stimulus. Th e frog is unable to feel pain because the brain, that project to the brain or onto interneurons for local circuits. which translates sensory input into perception, is not func- Irritant responses that are integrated in the spinal cord tional, but its protective spinal refl exes are intact. initiate rapid unconscious protective reflexes that automati- cally remove a stimulated area from the source of the stimulus. For example, if you accidentally touch a hot stove, an automatic Pain and Itching Are Mediated by Nociceptors withdrawal refl ex causes you to pull back your hand even be- Afferent signals from nociceptors are carried to the CNS in fore you are aware of the heat. Th is is one example of a spinal two types of primary sensory fibers: Ad (A-delta) fibers, and refl ex. C fi bers ( Tbl. 10.3 ). Th e most common sensation carried by Th e lack of brain involvement in many protective refl exes these pathways is perceived as pain, but when histamine or has been demonstrated in the classic “spinal frog” preparation, some other stimulus activates a subtype of C fi ber, we perceive in which the animal’s brain has been destroyed. If the frog’s foot the sensation we call itch ( pruritus ). Itch comes only from no- is placed in a beaker of hot water or strong acid, the withdrawal ciceptors in the skin and is characteristic of many rashes and

356 Sensory Physiology

Table Classes of Somatosensory Nerve Fibers 10.3

Fiber Type Fiber Characteristics Speed of Conduction Associated With

A b (beta) Large, myelinated 30–70 m/sec Mechanical stimuli

Ad (delta) Small, myelinated 12–30 m/sec Cold, fast pain, mechanical stimuli

C Small, unmyelinated 0.5–2 m/sec Slow pain, heat, cold, mechanical stimuli

other skin conditions. However, itch can also be a symptom of C fibers from nociceptors synapse on these inhibitory inter- a number of systemic diseases, including multiple sclerosis, hy- neurons. When activated by a noxious stimulus, the C fibers perparathyroidism, and diabetes mellitus. simultaneously excite the ascending path and block the tonic Th e higher pathways for itch are not as well understood as inhibition ( Fig. 10.11 b). Th is action allows the pain signal from the pathways for pain, but there is an antagonistic interaction the C fi ber to travel unimpeded to the brain. 10 between the two sensations. When something itches, we scratch In the gate control theory of pain modulation, Ab fi bers it, creating a mildly painful sensation that seems to interrupt the carrying sensory information about mechanical stimuli help itch sensation. And many of the opioid painkillers, such as mor- block pain transmission ( Fig. 10.11 c). The Ab fibers synapse phine, relieve pain but in some people they also induce the side on the inhibitory interneurons and enhance the interneuron’s eff ect of itching. inhibitory activity. If simultaneous stimuli reach the inhibitory Pain is a subjective perception, the brain’s interpretation of neuron from the Ab and C fi bers, the integrated response is par- sensory information transmitted along pathways that begin at tial inhibition of the ascending pain pathway so that pain per- nociceptors. Pain is also highly individual and may vary with ceived by the brain is lessened. Th e gate control theory explains a person’s emotional state. Th e discussion here is limited to the why rubbing a bumped elbow or shin lessens your pain: the tac- sensory experience of pain. tile stimulus of rubbing activates Ab fi bers and helps decrease Fast pain, described as sharp and localized, is rapidly the sensation of pain. transmitted to the CNS by small, myelinated Ad f i b e r s . Slow Pain can be felt in skeletal muscles (deep somatic pain) as pain , described as duller and more diff use, is carried on small, well as in the skin. Muscle pain during exercise is associated unmyelinated C fi bers. Th e timing distinction between the two with the onset of anaerobic metabolism and is oft en perceived is most obvious when the stimulus originates far from the CNS, as a burning sensation in the muscle (as in “go for the burn!”). such as when you stub your toe. You first experience a quick Some investigators have suggested that the exercise-induced stabbing sensation (fast pain), followed shortly by a dull throb- metabolite responsible for the burning sensation is K+, k n o w n bing (slow pain). to enhance the pain response. Muscle pain from ischemia (lack Th e ascending pathways for nociception cross the body’s of adequate blood fl ow that reduces oxygen supply) also occurs midline in the spinal cord and ascend to the thalamus and sen- during myocardial infarction (heart attack). sory areas of the cortex (see Fig. 10.8 ). Th e pathways also send Pain in the heart and other internal organs ( visceral pain ) branches to the limbic system and hypothalamus. As a result, is oft en poorly localized and may be felt in areas far removed pain may be accompanied by emotional distress (suff ering) and from the site of the stimulus ( Fig. 10.12 a). For example, the a variety of autonomic reactions, such as nausea, vomiting, or pain of cardiac ischemia may be felt in the neck and down the sweating. left shoulder and arm. Th is referred pain apparently occurs be- Our perception of pain is subject to modulation at several cause multiple primary sensory neurons converge on a single levels in the nervous system. It can be magnifi ed by past experi- ascending tract ( Fig. 10.12 b). According to this model, when ences or suppressed in emergencies when survival depends on painful stimuli arise in visceral receptors, the brain is unable to ignoring injury. In such emergencies, descending pathways that distinguish visceral signals from the more common signals aris- travel through the thalamus inhibit nociceptor neurons in the ing from somatic receptors. As a result, it interprets the pain as spinal cord. Artifi cial stimulation of these inhibitory pathways is coming from the somatic regions rather than the viscera. one of the newer techniques being used to control chronic pain. Chronic pain of one sort or another aff ects millions of peo- Pain can also be suppressed in the dorsal horn of the spi- ple in this country every year. Th is type of pain is oft en much nal cord, before the stimuli are sent to ascending spinal tracts. greater than nociceptor activation would indicate and refl ects Normally, tonically active inhibitory interneurons in the spi- damage to or long-term changes in the nervous system. Chronic nal cord inhibit ascending pathways for pain ( Fig. 10.11 a). pain is a pathological pain and is also called neuropathic pain .

357 Sensory Physiology

THE GATE CONTROL MODEL CLINICAL FOCUS In the gate control model of pain modulation, nonpainful stimuli can diminish the pain signal. Natural Painkillers (a) In absence of input from C fibers, a tonically active inhibitory interneuron suppresses pain pathway. Many drugs we use today for pain relief are deriva- tives of plant or animal molecules. One of the newest painkillers in this group is ziconotide, a synthetic compound related to the poison that South Pacifi c cone snails use to kill fi sh. This drug works by blocking calcium channels on nociceptive neurons. Ziconotide, approved in 2004 for the treatment of severe chronic pain, is highly toxic. To minimize systemic side eff ects, it must be injected directly into the cerebrospinal fl uid surrounding the spinal cord. Ziconotide Inhibitory relieves pain but may also cause hallucinations and other interneuron psychiatric symptoms, so it is a last-resort treatment. Other No signal painkilling drugs from biological sources include aspirin, Slow pain to brain C fiber derived from the bark of willow trees (genus Salix ), and opi- - ate drugs such as morphine and codeine that come from the opium poppy, Papaver somniferum . These drugs have been used in Western and Chinese medicine for centuries, Ascending and even today you can purchase willow bark as an herbal pain pathway remedy. (b) With strong pain, C fiber stops inhibition of the pathway, allowing a strong signal to be sent to the brain.

Noxious stimulus from aspirin to potent opioids such as morphine. Aspirin in- Strong noxious C fiber hibits prostaglandins, decreases infl ammation, and presumably - stimulus to brain slows the transmission of pain signals from the site of injury. + Inhibition Th e opioid drugs act directly on CNS opioid receptors that are stops part of an analgesic system that responds to endogenous opioid molecules. Activation of opioid receptors blocks pain perception by decreasing neurotransmitter release from primary sen- (c) Pain can be modulated by simultaneous somatosensory input. sory neurons and by postsynaptic inhibition of the secondary sensory neurons. Aβ fiber Touch or The endogenous opioids include three families: endor- nonpainful phins, enkephalins, and dynorphins. Enkephalins and dynor- stimulus phins are secreted by neurons associated with pain pathways. Noxious The endogenous opioid β -endorphin is produced from the stimulus Noxious same prohormone as ACTH (adrenocorticotropin) in neuroen- C fiber - + stimulus decreased docrine cells of the hypothalamus. Although opioid drugs are + eff ective at relieving pain, a person taking them for long periods - of time will develop tolerance and need larger and larger doses. As a result, scientists are exploring alternative drugs and Fig. 10.11 strategies for pain relief. Some chronic pain may be caused by sensitization of nociceptive nerve endings near a site of injury when the body releases chemicals in response to the damage. Non-narcotic anti-inflammatory drugs such as aspirin and One of the most common forms of neuropathic pain is diabetic COX2 inhibitors can often relieve pain, but even over-the- neuropathy, which develops as a consequence of chronically el- counter doses may have adverse side eff ects. New research is fo- evated blood glucose concentrations. Scientists do not yet fully cused on blocking receptors in the sensitized nociceptor nerve understand what causes glucose neurotoxicity or neuropathic endings. pain, which makes its treatment diffi cult. For people with severe chronic pain, possible treatments Th e alleviation of pain is of considerable interest to health include electrically stimulating inhibitory pain pathways to the professionals. Analgesic drugs {analgesia, painlessness} range

358 Sensory Physiology

REFERRED PAIN

(a) Pain in internal organs is often sensed on the surface of (b) One theory of referred pain says that nociceptors from several the body, a sensation known as referred pain. locations converge on a single ascending tract in the spinal cord. Pain signals from the skin are more common than pain from internal organs, and the brain associates activation of the pathway with pain in the skin. Adapted from H.L. Fields, Pain (McGraw Hill, 1987).

Skin (usual stimulus)

Heart

Liver and gallbladder Primary sensory 10 neurons

Kidney (uncommon stimulus) Secondary Ascending sensory sensory neuron path to somatosensory cortex of brain

Stomach FIGURE QUESTION Small A man goes to his physician and complains intestine Ureters Appendix of pain that radiates down his left arm. This suggests to the physician that the man may Colon have a problem with what organ?

Fig. 10.12

brain, or in extreme cases, surgically severing sensory nerves at Chemoreception: Smell and Taste the dorsal root. Acupuncture can also be eff ective, although the physiological reason for its eff ectiveness is not clear. Th e lead- The five special senses—smell, taste, hearing, equilibrium, ing theory on how acupuncture works proposes that properly and vision—are concentrated in the head region. Like somatic placed acupuncture needles trigger the release of endorphins by senses, the special senses rely on receptors to transform infor- the brain. mation about the environment into patterns of action potentials that can be interpreted by the brain. Smell and taste are both forms of chemoreception, one of the oldest senses from an evo- lutionary perspective. Unicellular bacteria use chemoreception Concept Check Answers: End of Chapter to sense their environment, and primitive animals without for- malized nervous systems use chemoreception to locate food and 7 . What is the adaptive advantage of a spinal refl ex? mates. It has been hypothesized that chemoreception evolved 8. Rank the speed of signal transmission through the following fiber into chemical synaptic communication in animals. types, from fastest to slowest: (a) small diameter, myelinated fiber; (b) large diameter, myelinated fi ber; (c) small diameter, unmyelinated fi ber. Olfaction Is One of the Oldest Senses 9. Your sense of smell uses phasic receptors. What other receptors (senses) Imagine waking up one morning and discovering a whole adapt to ongoing stimuli? new world around you, a world filled with odors that you never dreamed existed—scents that told you more about your

359 Sensory Physiology

surroundings than you ever imagined from looking at them. ( Fig. 10.13 a). Olfactory sensory neurons have a single dendrite Th is is exactly what happened to a young patient of Dr. Oliver that extends down from the cell body to the surface of the olfac- Sacks (an account is in The Man Who Mistook His Wife for a tory epithelium, and a single axon that extends up to the olfactory Hat and Other Clinical Tales). Or imagine skating along the bulb, located on the underside of the frontal lobe. Olfactory sen- sidewalk without a helmet, only to fall and hit your head. When sory neurons, unlike other neurons in the body, have very short you regain consciousness, the world has lost all odor: no smell lives, with a turnover time of about two months ( Fig. 10.13 c). Stem of grass or perfume or garbage. Even your food has lost much of cells in the basal layer of the olfactory epithelium are continuously its taste, and you now eat only to survive because eating has lost dividing to create new neurons. Th e axon of each newly formed its pleasure. neuron must then fi nd its way to the olfactory bulb and make the We do not realize the essential role that our sense of smell proper synaptic connections. Scientists are studying how these plays in our lives until a head cold or injury robs us of the ability neurons manage to repeat the same connection each time to give to smell. Olfaction allows us to discriminate among thousands us insight into how developing neurons fi nd their targets. of diff erent odors. Even so, our noses are not nearly as sensitive Th e surface of the olfactory epithelium is composed of the as those of many other animals whose survival depends on olfac- knobby terminals of the olfactory sensory neurons, each knob tory cues. Th e olfactory bulb, the extension of the forebrain that sprouting multiple nonmobile cilia that function as dendrites receives input from the primary olfactory neurons, is much bet- ( Fig. 10.13 c). Th e cilia are embedded in a layer of mucus, and ter developed in vertebrates whose survival is more closely linked odorant molecules must fi rst dissolve in and penetrate the mu- to chemical monitoring of their environment ( Fig. 10.13 a). cus before they can bind to an odorant receptor protein. Each The human olfactory system consists of primary sen- odorant receptor is sensitive to a variety of substances. sory neurons ( olfactory sensory neurons ) whose axons form Odorant receptors are G protein–linked membrane recep- the olfactory nerve (cranial nerve I). Th e olfactory nerve syn- tors. Odorant receptor genes form the largest known gene fam- apses with secondary sensory neurons in the olfactory bulb, ily in vertebrates (about 1000 genes, or 3–5% of the genome), which then processes the incoming information ( Fig. 10.13 b). but only about 400 odorant receptor proteins are expressed in Secondary and higher-order neurons project from the olfac- humans. Th e combination of an odorant molecule with its odor- tory bulb through the olfactory tract to the olfactory cortex ant receptor activates a special G protein, Golf, which in turn in- (Fig. 10.13 a). The olfactory tract, unlike most other sensory creases intracellular cAMP. Th e increase in cAMP concentration pathways, bypasses the thalamus. opens cAMP-gated cation channels, depolarizing the cell and This arrangement seems quite simple, but complex pro- triggering a signal that travels along the olfactory sensory neu- cessing takes place in the olfactory bulb before signals pass on ron axon to the olfactory bulb. to the cortex. Some descending modulatory pathways from the What is occurring at the cellular and molecular levels that cortex terminate in the olfactory bulb, and there are reciprocal allows us to discriminate between thousands of different odors? modulatory connections within and between the two branches Current research suggests that each individual olfactory sensory of the olfactory bulb. In addition, olfactory pathways lead to the neuron contains a single type of odorant receptor. Th e axons of cells amygdala and hippocampus, parts of the limbic system involved with the same receptors converge on a few secondary neurons in with emotion and memory. the olfactory bulb, which then can modify the information before The link between smell, memory, and emotion is one sending it on to the olfactory cortex. Th e brain uses information amazing aspect of olfaction. A special cologne or the aroma of from hundreds of olfactory sensory neurons in diff erent combina- food can trigger memories and create a wave of nostalgia for the tions to create the perception of many diff erent smells, just as com- time, place, or people with whom the aroma is associated. In binations of letters create diff erent words. Th is is another example some way that we do not understand, the processing of odors of population coding in the nervous system. through the limbic system creates deeply buried olfactory memories. Particular combinations of olfactory receptors become linked to other patterns of sensory experience so that stimulat- Concept Check Answers: End of Chapter ing one pathway stimulates them all. 10. Create a map or diagram of the olfactory pathway from an olfactory In rodents, an accessory olfactory structure in the nasal sensory neuron to the olfactory cortex. cavity, the vomeronasal organ (VNO), is known to be involved 11. Create a map or diagram that starts with a molecule from the in behavioral responses to sex pheromones. Anatomical stud- environment binding to its odorant receptor in the nose and ends with ies in humans have not provided clear evidence for or against a neurotransmitter release from the primary olfactory neuron. functional VNO, but experiments with compounds believed to act as human pheromones suggest that humans do communi- 12. The dendrites are which part of an olfactory sensory neuron? cate with chemical signals. 13. Are olfactory neurons pseudounipolar, bipolar, or multipolar? Olfactory sensory neurons in humans are concentrated in a 3-cm2 p a t c h o f olfactory epithelium high in the nasal cavity

360 Fig. 10.13 ANATOMY SUMMARY

The Olfactory System

(a) Olfactory Pathways The olfactory epithelium lies high within the nasal cavity, and its olfactory neurons project to the olfactory bulb. Sensory input at the receptors is carried through the olfactory cortex to the cerebral cortex and the limbic system. Cerebral cortex

Limbic system

Olfactory bulb Olfactory tract Olfactory cortex

Cranial Nerve I

(b) The olfactory neurons synapse with secondary Olfactory sensory neurons in the olfactory bulb. neurons in olfactory epithelium Olfactory bulb

Secondary sensory neurons Bone

Olfactory sensory (c) Olfactory neurons in the olfactory epithelium live only about neurons two months. They are replaced by new neurons whose axons Olfactory must find their way to the olfactory bulb. epithelium

Olfactory neuron axons (cranial nerve I) carry information to olfactory bulb. Lamina propria

Basal cell layer includes stem cells that replace olfactory neurons.

Developing olfactory neuron Olfactory sensory neuron FIGURE QUESTION Multiple primary neurons in the epithelium synapse on one secondary neuron in the olfactory Supporting cell bulb. This pattern is an example of what principle?

Olfactory cilia (dendrites) contain odorant receptors.

Mucous layer: Odorant molecules must dissolve in this layer.

361 Sensory Physiology

Taste Is a Combination of Five Basic Sensations shown that there are at least two diff erent types of taste cells. Taste buds contain four morphologically distinct cell types des- Our sense of taste, or gustation, is closely linked to olfaction. ignated I, II, and III, plus basal cells that may be the taste stem Indeed, much of what we call the taste of food is actually the cells. Only the type III taste cells, also called presynaptic cells, aroma, as you know if you have ever had a bad cold. Although synapse with sensory neurons. Th e presynaptic taste cells release smell is sensed by hundreds of receptor types, taste is currently the neurotransmitter serotonin by exocytosis. Presynaptic cells believed to be a combination of fi ve sensations: sweet, sour, salty, respond to sour tastes. bitter, and umami, a taste associated with the amino acid gluta- Th e type II taste cells, or receptor cells, respond to sweet, mate and some nucleotides. Umami, a name derived from the bitter, and umami sensations. Type II cells do not form tradi- Japanese word for “deliciousness,” is a basic taste that enhances tional synapses. Instead they release ATP through gap junction– the fl avor of foods. It is the reason that monosodium glutamate like channels, and the ATP acts both on sensory neurons and (MSG) is used as a food additive in some countries. on neighboring presynaptic cells. Th is communication between Each of the fi ve currently recognized taste sensations is as- neighboring taste cells creates complex interactions. Currently it sociated with an essential body function. Sour taste is triggered + + is not clear which cell type is responsible for responding to salt. by the presence of H and salty by the presence of Na , t w o Some evidence suggests that the glia-like type I, or support cells , ions whose concentrations in body fl uids are closely regulated. may be the salt sensors. Th e other three taste sensations result from organic molecules. Sweet and umami are associated with nutritious food. Bitter taste is recognized by the body as a warning of possibly toxic Taste Transduction Uses Receptors components. If something tastes bitter, our fi rst reaction is oft en to spit it out. and Channels Th e receptors for taste are located primarily on taste buds Th e details of taste cell signal transduction, once thought to be clustered together on the surface of the tongue ( Fig. 10.14 ). relatively straightforward, are more complex than scientists ini- One taste bud is composed of 50–150 taste cells, along with tially thought (Fig. 10.14 b). Th e type II taste cells express mul- support cells and regenerative basal cells. Taste receptors are tiple G protein–coupled receptors. Sweet and umami tastes are also scattered through other regions of the oral cavity, such as associated with T1R receptors. Bitter taste uses about 30 vari- the palate. ants of T2R receptors. Each taste cell is a non-neural polarized epithelial cell In type II taste cells, the receptor proteins are associated tucked down into the epithelium so that only a tiny tip protrudes with a special G protein called gustducin . Gustducin appears to into the oral cavity through a taste pore. In a given bud, tight activate multiple signal transduction pathways. Some pathways + junctions link the apical ends of adjacent cells together, limiting release Ca2 from intracellular stores, while others open cation + movement of molecules between the cells. Th e apical membrane channels and allow Ca2 to enter the cell. Calcium signals then of a taste cell is modifi ed into microvilli to increase the amount initiate ATP release from the type II taste cells. of surface area in contact with the environment ( Fig. 10.14 a). In contrast, salty and sour transduction mechanisms For a substance (tastant ) to be tasted, it must fi rst dissolve both appear to be mediated by ion channels rather than by G in the saliva and mucus of the mouth. Dissolved taste ligands protein–coupled receptors. In the current model for salty taste, + then interact with an apical membrane protein (receptor or Na enters the presynaptic cell through an apical channel, such + channel) on a taste cell (Fig. 10.14 b). Although the details of as the epithelial Na channel (ENaC, pronounced ēē-knack) . signal transduction for the fi ve taste sensations are still contro- Sodium entry depolarizes the taste cell. versial, interaction of a taste ligand with a membrane protein Transduction mechanisms for sour tastes are more con- + initiates a signal transduction cascade that ends with a series of troversial, complicated by the fact that increasing H , the sour + action potentials in the primary sensory neuron. taste signal, also changes pH. There is evidence that H acts Th e mechanisms of taste transduction are a good example on ion channels of the presynaptic cell from both extracellu- of how our models of physiological function must periodically lar and intracellular sides of the membrane. Th e transduction + be revised as new research data are published. For many years mechanisms remain uncertain. Ultimately, H -mediated depo- the widely held view of taste transduction was that an individual larization of the presynaptic cell results in serotonin release. Se- taste cell could sense more than one taste, with cells diff ering in rotonin in turn excites the primary sensory neuron. their sensitivities. However, gustation research using molecular Neurotransmitters (ATP and serotonin) from taste cells biology techniques and knockout mice currently indicates that activate primary gustatory neurons whose axons run through each taste cell is sensitive to only one taste. cranial nerves VII, IX, and X to the medulla, where they syn- In the old model, all taste cells formed synapses with pri- apse. Sensory information then passes through the thalamus to mary sensory neurons (gustatory neurons). Now it has been the gustatory cortex (see Fig. 10.3 ). Central processing of sensory information compares the input from multiple taste cells

362 Fig. 10.14 ESSENTIALS

Taste

(a) Taste buds. Each taste bud is composed of taste cells joined near the apical surface with tight junctions. Taste ligands create Ca2+ signals that release serotonin or ATP.

Sweet Umami Bitter Sour

Tight junction

Type I support cells may Taste buds are located sense salt when Na+ on the dorsal surface enters through Na+ of the tongue. channels.

Taste pore Salt?

(Adapted from Tomchik et al., J Presynaptic Neurosci 27(40): cell (III) 10840–10848, 2007.)

ATP

Serotonin

Receptor cells (type II)

Light micrograph of a taste bud Primary sensory neurons

Sweet, umami, Sour (b) Taste transduction. Each taste cell or bitter ligand + 1 H senses only one type of ligand. Gustducin 1 ? GPCR Presynaptic cells sense sour taste (H+) but it is not clear whether H+ acts on a receptor or enters the cell. Receptor cells with G protein–coupled 2 H+ membrane receptors bind either bitter, Signal 2 sweet, or umami ligands and release transduction 1 ATP as a signal molecule. ? ? Ligands activate the taste cell.

Ca2+ 2 Various intracellular pathways Ca2+ are activated. Ca2+ 3 3

Ca2+ 3 Ca2+ signal in the cytoplasm triggers exocytosis or ATP formation.

4 Neurotransmitter or ATP is ATP released.

4 4 Primary 5 Primary sensory neuron fires gustatory and action potentials are 5 neurons 5 sent to the brain.

363 Sensory Physiology

and interprets the taste sensation based on which populations of The Ear: Hearing neurons are responding most strongly. Signals from the sensory neurons also initiate behavioral responses, such as feeding, and Th e ear is a sense organ that is specialized for two distinct func- feedforward responses that activate the digestive system. tions: hearing and equilibrium. It can be divided into external, Th e sensations we call taste are not all mediated by the tra- middle, and inner sections, with the neurological elements ditional taste receptors. For years physiologists thought fat in the housed in and protected by structures in the inner ear. Th e ves- diet was appealing because of its texture, and food experts use the tibular complex of the inner ear is the primary sensor for equi- phrase “mouth feel” to describe the sensation of eating something librium. Th e remainder of the ear is used for hearing. fatty, like ice cream, that seems to coat the inside of the mouth. The external ear consists of the outer ear, or pinna , and But now it appears that the tongue may have taste receptors for the ear canal ( Fig. 10.15 ). Th e pinna is another example of an long-chain fatty acids, such as oleic acid. important accessory structure to a sensory system, and it var- Research in rodents has identified a membrane receptor ies in shape and location from species to species, depending on called CD36 that lines taste pores and binds fats. Activation of the animals’ survival needs. Th e ear canal is sealed at its internal this receptor helps trigger the feedforward digestive refl exes that end by a thin membranous sheet of tissue called the tympanic prepare the digestive system for a meal. Currently evidence is membrane , or eardrum. lacking for a similar receptor in humans, but “fatty” may turn Th e tympanic membrane separates the external ear from out to be a sixth taste sensation. the middle ear, an air-fi lled cavity that connects with the pharynx Yet other taste sensations are related to somatosensory through the Eustachian tube . Th e Eustachian tube is normally pathways. Nerve endings in the mouth have TRP receptors and collapsed, sealing off the middle ear, but it opens transiently to carry spicy sensations through the trigeminal nerve (CN V). allow middle ear pressure to equilibrate with atmospheric pres- Capsaicin from chili peppers, menthol from mint, and mole- sure during chewing, swallowing, and yawning. Colds or other cules in cinnamon, mustard oil, and many Indian spices activate infections that cause swelling can block the Eustachian tube and these receptors to add to our appreciation of the food we eat. result in fl uid buildup in the middle ear. If bacteria are trapped And what would you say to the idea of taste buds in your in the middle ear fl uid, the ear infection known as otitis media gut? Scientists have known for years that the stomach and in- { oto-, ear + -itis, infl a m m a t i o n + media, middle} results. testines have the ability to sense the composition of a meal and Th ree small bones of the middle ear conduct sound from secrete appropriate hormones and enzymes. Now it appears that the external environment to the inner ear: the malleus {ham- gut chemoreception is being mediated by the same receptors mer}, the incus {anvil}, and the stapes {stirrup}. Th e three bones and signal transduction mechanisms that occur in taste buds are connected to one another with the biological equivalent on the tongue. Studies have found the T1R receptor proteins for of hinges. One end of the malleus is attached to the tympanic sweet and umami tastes as well as the G protein gustducin in membrane, and the stirrup end of the stapes is attached to a thin various cells in rodent and human intestines. membrane that separates the middle ear from the inner ear. An interesting psychological aspect of taste is the phe- Th e inner ear consists of two major sensory structures. Th e nomenon named specifi c hunger . Humans and other animals vestibular apparatus with its semicircular canals is the sensory that are lacking a particular nutrient may develop a craving for transducer for our sense of equilibrium, described in the follow- that substance. Salt appetite, representing a lack of Na + i n t h e ing section. Th e cochlea of the inner ear contains sensory recep- body, has been recognized for years. Hunters have used their tors for hearing. On external view the cochlea is a membranous knowledge of this specific hunger to stake out salt licks tube that lies coiled like a snail shell within a bony cavity. Two because they know that animals will seek them out. Salt appetite membranous disks, the oval window (to which the stapes is at- is directly related to Na + concentration in the body and cannot tached) and the round window, separate the liquid-fi lled co- be assuaged by ingestion of other cations, such as Ca2+ o r K+. chlea from the air-fi lled middle ear. Branches of cranial nerve Other appetites, such as cravings for chocolate, are more dif- VIII, the vestibulocochlear nerve, lead from the inner ear to the fi cult to relate to specifi c nutrient needs and probably refl ect brain. complex mixtures of physical, psychological, environmental, and cultural infl uences. Hearing Is Our Perception of Sound Hearing is our perception of the energy carried by sound waves, Concept Check Answers: End of Chapte r which are pressure waves with alternating peaks of compressed air and valleys in which the air molecules are farther apart 14. With what essential nutrient is the umami taste sensation associated? ( Fig. 10.16 a). Th e classic question about hearing is, “If a tree 15. Map or diagram the neural pathway from a presynaptic taste cell to the falls in the forest with no one to hear, does it make a noise?” gustatory cortex. The physiological answer is no, because noise, like pain, is a perception that results from the brain’s processing of sensory

364 Fig. 10.15 ANATOMY SUMMARY

The Ear

EXTERNAL EAR MIDDLE EAR INNER EAR

The pinna The oval window and the round window separate directs sound the fluid-filled inner ear from the air-filled middle ear. waves into the ear. Malleus Semicircular Oval Incus canals window Nerves

Stapes

Vestibular Cochlea apparatus

Ear canal

Tympanic Round membrane window To pharynx

Eustachian tube

information. A falling tree emits sound waves, but there is no sounds over the frequency range of 20–20,000 Hz, with the noise unless someone or something is present to process and most acute hearing between 1000–3000 Hz. Our hearing is not perceive the wave energy as sound. as acute as that of many other animals, just as our sense of smell Sound is the brain’s interpretation of the frequency, am- is less acute. Bats listen for ultra-high-frequency sound waves plitude, and duration of sound waves that reach our ears. Our (in the kilohertz range) that bounce off objects in the dark. brains translate frequency of sound waves (the number of Elephants and some birds can hear sounds in the infrasound wave peaks that pass a given point each second) into the pitch (very low frequency) range. of a sound. Low-frequency waves are perceived as low-pitched Loudness is our interpretation of sound intensity and is in- sounds, such as the rumble of distant thunder. High-frequency fl uenced by the sensitivity of an individual’s ear. Th e intensity of waves create high-pitched sounds, such as the screech of fi nger- a sound wave is a function of the wave amplitude ( Fig. 10.16 b). nails on a blackboard. Intensity is measured on a logarithmic scale in units called deci- Sound wave frequency (Fig. 10.16 b) is measured in waves bels (dB). Each 10 dB increase represents a 10-fold increase in per second, or hertz (Hz). The average human ear can hear intensity.

365 Sensory Physiology

SOUND WAVES Concept Check A nswers: End of Chapter

(a) Sound waves alternate peaks of compressed air and valleys where 16. What is a kilohertz? the air is less compressed.

Wavelength Sound Transduction Is a Multistep Process Hearing is a complex sense that involves multiple transductions. Energy from sound waves in the air becomes 1 mechanical vibrations, then 2 fl uid waves in the cochlea. Th e fl uid waves open ion channels in hair cells, the sensory receptors for hearing. Ion flow into hair cells creates 3 electrical signals that release 4 neurotransmitter (chemical signal), which in turn Tuning fork triggers 5 action potentials in the primary auditory neurons. These transduction steps are shown in Figure 10.17 . (b) Sound waves are distinguished by their amplitude, measured in Sound waves striking the outer ear are directed down the ear decibels (dB), and frequency, measured in hertz (Hz). canal until they hit the tympanic membrane and cause it to vibrate (fi rst transduction). Th e tympanic membrane vibrations (1) 1 Wavelength are transferred to the malleus, the incus, and the stapes, in that order. Th e arrangement of the three connected middle ear bones creates a “lever” that multiplies the force of the vibration (ampli- fi cation ) so that very little sound energy is lost due to friction. Intensity Amplitude (dB) (dB) If noise levels are so high that there is danger of damage to the inner ear, small muscles in the middle ear can pull on the bones to decrease their movement and thereby dampen sound transmission to some degree. 0 0.25 Time (sec) As the stapes vibrates, it pulls and pushes on the thin tissue of the oval window, to which it is attached. Vibrations at the oval (2) window create waves in the fl uid-fi lled channels of the cochlea (second transduction). As waves move through the cochlea, they push on the fl exible membranes of the cochlear duct and bend sensory hair cells inside the duct. Th e wave energy dis- Intensity Amplitude sipates back into the air of the middle ear at the round window. (dB) (dB)

RUNNING PROBLEM 0 0.25 Time (sec) Anant reports to the otolaryngologist that he never knows when his attacks of dizziness will strike and that they last from 10 minutes to an hour. They often cause him to vomit. FIGURE QUESTIONS He also reports that he has a persistent low buzzing sound in 1. What are the frequencies of the one ear and that he does not seem to hear low tones as well sound waves in graphs (1) and (2) as he could before the attacks started. The buzzing sound in Hz (waves/second)? 2. Which set of sound waves would be (tinnitus) often gets worse during his dizzy attacks. interpreted as having lower pitch? Q2: Subjective tinnitus occurs when an abnormality Fig. 10.16 somewhere along the anatomical pathway for hearing causes the brain to perceive a sound that does not exist Normal conversation has a typical noise level of about outside the auditory system. Starting from the ear canal, 60 dB. Sounds of 80 dB or more can damage the sensitive hear- name the auditory structures in which problems may ing receptors of the ear, resulting in hearing loss. A typical heavy arise. metal rock concert has noise levels around 120 dB, an intensity that puts listeners in immediate danger of damage to their hearing. Th e amount of damage depends on the duration and frequency of the noise as well as its intensity.

366 Sensory Physiology

SOUND TRANSMISSION THROUGH THE EAR

1 Sound waves 2 The sound 3 The stapes is 4 The fluid waves push on 5 Neurotransmitter 6 Energy from the waves strike the wave energy is attached to the the flexible membranes release onto sensory transfers across the tympanic transferred to membrane of the oval of the cochlear duct. Hair neurons creates action cochlear duct into the membrane the three bones window. Vibrations of cells bend and ion potentials that travel tympanic duct and is and become of the middle the oval window channels open, creating an through the cochlear dissipated back into vibrations. ear, which create fluid waves electrical signal that alters nerve to the brain. the middle ear at the vibrate. within the cochlea. neurotransmitter release. round window.

Cochlear nerve Incus Oval Ear canal Malleus Stapes window 5

Vestibular duct 10 (perilymph) 3 Movement Cochlear duct (endolymph) of sound 2 waves 6 1 Tympanic duct (perilymph)

Tympanic Round membrane window Fig. 10.17 Movement of the cochlear duct opens or closes ion channels Th e fl uid in the vestibular and tympanic ducts is similar in on hair cell membranes, creating electrical signals (third transduc- ion composition to plasma and is known as perilymph. Th e co- tion). Th ese electrical signals alter neurotransmitter release (fourth chlear duct is fi lled with endolymph secreted by epithelial cells transduction). Neurotransmitter binding to the primary auditory in the duct. Endolymph is unusual because it is more like intra- neurons initiates action potentials (fifth transduction) that send cellular fl uid than extracellular fl uid in composition, with high coded information about sound through the cochlear branch of the concentrations of K+ and low concentrations of Na +. vestibulocochlear nerve (cranial nerve VIII) and the brain. Th e cochlear duct contains the organ of Corti, composed of hair cell receptors and support cells. Th e organ of Corti sits The Cochlea Is Filled with Fluid on the basilar membrane and is partially covered by the tectorial membrane { tectorium , a cover}, both fl exible tissues that The transduction of wave energy into action potentials takes move in response to fl uid waves passing through the vestibu- place in the cochlea of the inner ear. Uncoiled, the cochlea can lar duct (Fig. 10.18 ). As the waves travel through the cochlea, be seen to be composed of three parallel, fl uid-fi lled channels: they displace basilar and tectorial membranes, creating up-and- (1) the vestibular duct , or scala vestibuli { scala, stairway; ves- down oscillations that bend the hair cells. tibulum, entrance}; (2) the central cochlear duct , or scala media Hair cells, like taste cells, are non-neural receptor cells. { media, middle}; and (3) the tympanic duct, or scala tympani Th e apical surface of each hair cell is modifi ed into 50–100 stiff - { tympanon , drum} ( Fig. 10.18 ). Th e vestibular and tympanic ened cilia known as stereocilia , arranged in ascending height ducts are continuous with each other, and they connect at the ( Fig. 10.19 a). Th e stereocilia of the hair cells are embedded tip of the cochlea through a small opening known as the helico- in the overlying tectorial membrane. If the tectorial membrane trema { helix, a spiral + trema, hole}. Th e cochlear duct is a dead- moves, the underlying cilia do also. end tube, but it connects to the vestibular apparatus through a When hair cells move in response to sound waves, their small opening. stereocilia fl ex, fi rst one way, then the other. Th e stereocilia are

367 Fig. 10.18 ANATOMY SUMMARY Sensory Physiology The Cochlea

Oval Vestibular Cochlear Organ of window Saccule duct duct Corti Cochlea

Uncoiled Helicotrema

Round Tympanic Basilar window duct membrane

Bony cochlear wall

Vestibular duct

Cochlear duct

Tectorial membrane

Organ of Corti

The cochlear nerve transmits action potentials from Basilar Tympanic the primary auditory membrane duct neurons to cochlear nuclei in the medulla, Fluid wave on their way to the auditory cortex.

Cochlear Tectorial duct membrane

Hair cell

Tympanic duct The movement of the tectorial membrane moves the cilia on the hair cells. Basilar membrane Nerve fibers of cochlear nerve

368 Sensory Physiology

SIGNAL TRANSDUCTION IN HAIR CELLS

The stereocilia of hair cells have “trap doors” that close off ion channels. These openings are controlled by protein-bridge tip links connecting adjacent cilia.

(a) At rest: About 10% of the ion (b) Excitation: When the hair cells bend in (c) Inhibition: If the hair cells bend in the channels are open, and a tonic signal one direction, the cell depolarizes, which opposite direction, ion channels close, is sent by the sensory neuron. increases action potential frequency in the cell hyperpolarizes, and sensory the associated sensory neuron. neuron signaling decreases.

+ + + + + Tip link

Stereocilium Some More channels Channels closed. channels open. Less cation entry open Cation entry hyperpolarizes cell. Hair cell depolarizes cell. 10

Primary sensory neuron

Action potentials Action potentials increase No action potentials

Action potentials in primary sensory neuron Time

-30

Release Release

Membrane potential Excitation opens Inhibition closes of hair cell ion channels ion channels Fig. 10.19 attached to each other by protein bridges called tip links . Th e When waves defl ect the tectorial membrane so that cilia tip links act like little springs and are connected to gates that bend toward the tallest members of a bundle, the tip links pop open and close ion channels in the cilia membrane. When the more channels open, so cations (primarily K+ a n d Ca2+ ) enter hair cells and cilia are in a neutral position, about 10% of the ion the cell, which then depolarizes (Fig. 10.19 b). Voltage-gated channels are open, and there is a low tonic level of neurotrans- Ca2+ channels open, neurotransmitter release increases, and mitter released onto the primary sensory neuron. the sensory neuron increases its fi ring rate. When the tectorial

369 Sensory Physiology

membrane pushes the cilia away from the tallest members, the SENSORY CODING FOR PITCH springy tip links relax and all the ion channels close. Cation infl ux slows, the membrane hyperpolarizes, less transmitter is re- (a) The basilar membrane has variable sensitivity to sound leased, and sensory neuron fi ring decreases (Fig. 10.19 c). wave frequency along its length. The vibration pattern of waves reaching the inner ear is Low frequency thus converted into a pattern of action potentials going to the High frequency (low pitch) (high pitch) CNS. Because tectorial membrane vibrations reflect the fre- Basilar membrane quency of the incoming sound wave, the hair cells and sensory neurons must be able to respond to sounds of nearly 20,000 Stiff region near Flexible region round window near helicotrema waves per second, the highest frequency audible by a human ear. (distal end)

(b) The frequency of sound waves determines the displacement Concept Check A nswers: End of Chapter of the basilar membrane. The location of active hair cells creates a code that the brain translates as information about the pitch 17. Normally when cation channels on a cell open, either Na+ o r Ca2+ of sound. enters the cell. Why does K + rather than Na+ enter hair cells when their cation channels open? Eardrum Oval Basilar Helicotrema window membrane

Sounds Are Processed First in the Cochlea Stapes Th e auditory system processes sound waves so that they can be discriminated by location, pitch, and loudness. Localization of 3 100 Hz sound is a complex process that requires sensory input from

both ears coupled with sophisticated computation by the brain m) (see Fig. 10.4 ). In contrast, the initial processing for pitch and 훍 0 loudness takes place in the cochlea of each ear. 0 10 20 30 Coding sound for pitch is primarily a function of the 3 basilar membrane. This membrane is stiff and narrow near its 400 Hz attachment between the round and oval windows but widens and becomes more fl exible near its distal end ( Fig. 10.20 a). High-

frequency waves entering the vestibular duct create maximum 0 displacement of the basilar membrane close to the oval window 0 10 20 30 and consequently are not transmitted very far along the cochlea. Low-frequency waves travel along the length of the basilar mem- 3 1600 Hz

brane and create their maximum displacement near the fl exible Relative motion of basilar membrane ( distal end. This differential response to frequency transforms the 0 temporal aspect of frequency (number of sound waves per sec- 0 10 20 30 ond) into spatial coding for pitch by location along the basilar Distance from oval window (mm) membrane ( Fig. 10.20 b). A good analogy is a piano keyboard, Fig. 10.20 where the location of a key tells you its pitch. Th e spatial coding of the basilar membrane is preserved in the auditory cortex as neurons project from hair cells to corresponding regions in the vestibulocochlear nerve . Primary auditory neurons pro ject from brain. Loudness is coded by the ear in the same way that signal the cochlea to cochlear nuclei in the medulla oblongata ( Fig. strength is coded in somatic receptors. Th e louder the noise, the 10.21 ). Some of these neurons carry information that is pro- more rapidly action potentials fi re in the sensory neuron. cessed into the timing of sound, and others carry information that is processed into the sound quality. Auditory Pathways Project to From the medulla, secondary sensory neurons project to two higher nuclei, one ipsilateral (on the same side of the body) and the Auditory Cortex one contralateral (on the opposite side). Splitting sound signals Once the cochlea transforms sound waves into electrical sig- between two ascending tracts means that each side of the brain nals, sensory neurons transfer this information to the brain. Th e gets information from both ears. Th ese ascending tracts then syn- cochlear (auditory) nerve is a branch of cranial nerve VIII, the apse in nuclei in the midbrain and thalamus before projecting

370 Sensory Physiology

THE AUDITORY PATHWAYS

Sound is processed so that information from each ear goes to both sides of the brain.

Right auditory Left auditory cortex Right Left cortex thalamus thalamus 10

MIDBRAIN

To cerebellum To cerebellum

Right cochlea Left cochlea

MEDULLA Cochlear branch of Cochlear branch of right vestibulocochlear Cochlear nuclei left vestibulocochlear nerve (VIII) nerve (VIII)

Sound waves

Fig. 10.21 to the auditory cortex (see Fig. 10.3 ). Collateral pathways plugged with earwax ( cerumen ), to fl uid in the middle ear from take information to the reticular formation and the cerebellum. an infection, to diseases or trauma that impede vibration of the Th e localization of a sound source is an integrative task that malleus, incus, or stapes. Correction of conductive hearing loss requires simultaneous input from both ears. Unless sound is com- includes microsurgical techniques in which the bones of the ing from directly in front of a person, it will not reach both ears at middle ear can be reconstructed. the same time (see Fig. 10.4 ). Th e brain records the time diff eren- Central hearing loss results either from damage to the neu- tial for sound arriving at the ears and uses complex computation ral pathways between the ear and cerebral cortex or from dam- to create a three-dimensional representation of the sound source. age to the cortex itself, as might occur from a stroke. Th is form of hearing loss is relatively uncommon. Hearing Loss May Result from Mechanical Sensorineural hearing loss arises from damage to the structures of the inner ear, including death of hair cells as a result or Neural Damage of loud noises. The loss of hair cells in mammals is currently Th ere are three forms of hearing loss: conductive, central, and irreversible. Birds and lower vertebrates, however, are able to sensorineural. In conductive hearing loss, sound cannot be regenerate hair cells to replace those that die. This discovery transmitted through either the external ear or the middle ear. has researchers exploring strategies to duplicate the process in Th e causes of conductive hearing loss range from an ear canal mammals, including transplantation of neural stem cells and

371 Sensory Physiology The Ear: Equilibrium BIOTECHNOLOGY Equilibrium is a state of balance, whether the word is used to describe ion concentrations in body fl uids or the position of the Artifi cial Ears body in space. Th e special sense of equilibrium has two compo- One technique used to treat sensorineural hearing nents: a dynamic component that tells us about our movement loss is the cochlear implant. The newest cochlear implants through space, and a static component that tells us if our head have multiple components. Externally, a microphone, tiny is not in its normal upright position. Sensory information from computerized speech processor, and transmitter fi t behind the inner ear and from joint and muscle proprioceptors tells the ear like a conventional hearing aid. The speech processor our brain the location of diff erent body parts in relation to one is a transducer that converts sound into electrical impulses. another and to the environment. Visual information also plays The transmitter converts the processor’s electrical impulses an important role in equilibrium, as you know if you have ever into radio waves and sends these signals to a receiver and gone to one of the 360° movie theaters where the scene tilts sud- 8–24 electrodes, which are surgically placed under the skin. denly to one side and the audience tilts with it! The electrodes take electrical signals directly into the cochlea or to the auditory nerve, bypassing any damaged areas. After Our sense of equilibrium is mediated by hair cells lining surgery, recipients go through therapy so that they can learn the fl uid-fi lled vestibular apparatus of the inner ear. Th ese non- to understand the sounds they hear. Cochlear implants have neural receptors respond to changes in rotational, vertical, and been remarkably successful for many profoundly deaf people, horizontal acceleration and positioning. Th e hair cells function allowing them to hear loud noises and modulate their own just like those of the cochlea, but gravity and acceleration rather voices. In the most successful cases, individuals can even use than sound waves provide the force that moves the stereocilia. the telephone. To learn more about cochlear implants, visit the Vestibular hair cells have a single long cilium called a kinocilium web site of the National Institute for Deafness and Other Com- { kinein, to move} located at one side of the ciliary bundle. Th e munication Disorders ( www.nidcd.nih.gov/health/hearing). kinocilium creates a reference point for the direction of bending. When the cilia bend, tip links between them open and close ion channels. Movement in one direction causes the hair cells to depolarize; with movement in the opposite direction, gene therapy to induce nonsensory cells to differentiate into they hyperpolarize. Th is is similar to what happens in cochlear hair cells. hair cells (see Fig. 10.19 ). Therapy that replaces hair cells would be an important advance. The incidence of hearing loss in younger people is increasing because of prolonged exposure to rock music and The Vestibular Apparatus Provides Information environmental noises. Ninety percent of hearing loss in the about Movement and Position elderly—called presbycusis {presbys, old man + akoustikos, able to be heard}—is sensorineural. Currently the primary Th e vestibular apparatus , also called the membranous labyrinth , treatment for sensorineural hearing loss is the use of hearing is an intricate series of interconnected fl uid-fi lled chambers. (In aids, but amazing results have been obtained with cochlear Greek mythology the labyrinth was a maze that housed a monster implants attached to tiny computers (see Biotechnology box). called the Minotaur.) In humans, the vestibular apparatus con- Hearing is probably our most important social sense. Sui- sists of two saclike otolith organs —the saccule and the utricle — cide rates are higher among deaf people than among those who along with three semicircular canals that connect to the utricle have lost their sight. More than any other sense, hearing con- at their bases ( Fig. 10.22 a). Th e otolith organs tell us about lin- nects us to other people and to the world around us. ear acceleration and head position. Th e three semicircular canals sense rotational acceleration in various directions. The vestibular apparatus, like the cochlear duct, is filled Concept Check Answers: End of Chapter with high- K+, low- Na + endolymph secreted by epithelial cells. 18. Map or diagram the pathways followed by a sound wave entering Like cerebrospinal fluid, endolymph is secreted continuously the ear, starting in the air at the outer ear and ending on the auditory and drains from the inner ear into the venous sinus in the dura cortex. mater of the brain. If endolymph production exceeds the drainage rate, 19. Why is somatosensory information projected to only one hemisphere of the brain but auditory information is projected to both hemispheres? buildup of fluid in the inner ear may increase fluid pressure (Hint: See Figs. 10.4 and 10.8 .) within the vestibular apparatus. Excessive accumulation of endolymph is believed to contribute to Ménière’s disease , a condi- 20. Would a cochlear implant help a person who suffers from nerve tion marked by episodes of dizziness and nausea. If the organ of deafness? From conductive hearing loss? Corti in the cochlear duct is damaged by fl uid pressure within the vestibular apparatus, hearing loss may result.

372 Fig. 10.22 ESSENTIALS

Equilibrium

The vestibular apparatus of the inner ear responds to changes in the body's position in space. The cristae are sensory receptors for rotational acceleration. The maculae are sensory receptors for linear acceleration and head position. (a) Semicircular Canals

SEMICIRCULAR The posterior canal of the The superior canal senses CANALS vestibular apparatus senses the tilt rotation of the head from front of the head toward the right or left to back, such as that which Superior shoulder. occurs when nodding “yes.”

Horizontal Left right Posterior

Cochlea

The horizontal canal senses rotation of the head as it turns left or right, such as that which occurs when shaking the head Cristae within “no.” ampulla Utricle

Saccule (c) Macula Maculae Otoliths are crystals (b) Crista that move in response to gravitational forces. Movement of the endolymph pushes on the Gelatinous otolith gelatinous cupula and activates the hair cells. membrane Hair cells

Nerve fibers Endolymph Cupula

Hair cells Macula Gravity Supporting Head in neutral cells position Nerve

Brush moves Cupula Bone right

Gravity Stationary Endolymph board Head tilted Otolith posteriorly Bristles Hair cells Bone bend left

Direction of rotation of the head

When the head turns right, endolymph pushes the cupula to the left.

373 Sensory Physiology

The Semicircular Canals Sense with them, bending the hair cell cilia and setting off a signal. For example, the maculae are horizontal when the head is in its Rotational Acceleration normal upright position. If the head tips back, gravity displaces Th e three semicircular canals of the vestibular apparatus moni- the otoliths, and the hair cells are activated. tor rotational acceleration. Th ey are oriented at right angles to Th e maculae of the utricle sense forward acceleration or one another, like three planes that come together to form the deceleration as well as head tilt. In contrast, the maculae of the corner of a box (Fig. 10.22 a). Th e horizontal canal monitors rota- saccule are oriented vertically when the head is erect, which tions that we associate with turning, such as an ice skater’s spin makes them sensitive to vertical forces, such as dropping down- or shaking your head left and right to say “no.” Th e posterior ca- ward in an elevator. Th e brain analyzes the pattern of depolar- nal monitors left -to-right rotation, such as the rotation when you ized and hyperpolarized hair cells to compute head position and tilt your head toward your shoulders or perform a cartwheel. Th e direction of movement. superior canal is sensitive to forward and back rotation, such as nodding your head front to back or doing a somersault. Equilibrium Pathways Project Primarily At one end of each canal is an enlarged chamber, the am- to the Cerebellum pulla {bottle}, which contains a sensory structure known as a crista {a crest; plural cristae }. Th e crista consists of hair cells and Vestibular hair cells, like those of the cochlea, are tonically active a gelatinous mass, the cupula {small tub}, that stretches from and release neurotransmitter onto primary sensory neurons of fl oor to ceiling of the ampulla, closing it off (Fig. 10.22 b). Hair the vestibular nerve (a branch of cranial nerve VIII, the vestib- cell cilia are embedded in the cupula. ulocochlear nerve). Th ose sensory neurons either synapse in the How is rotation sensed? As the head turns, the bony skull vestibular nuclei of the medulla or run without synapsing to the and the membranous walls of the labyrinth move, but the fl uid cerebellum, which is the primary site for equilibrium processing within the labyrinth cannot keep up because of inertia (the ten- ( Fig. 10.23 ). Collateral pathways run from the medulla to the cer- dency of a body at rest to remain at rest). In the ampullae, the ebellum or upward through the reticular formation and thalamus. drag of endolymph bends the cupula and its hair cells in the di- There are some poorly defined pathways from the me- rection opposite to the direction in which the head is turning. dulla to the cerebral cortex, but most integration for equilib- For an analogy, think of pulling a paintbrush (a cupula at- rium occurs in the cerebellum. Descending pathways from the tached to the wall of a semicircular canal) through sticky wet vestibular nuclei go to certain motor neurons involved in eye paint (the endolymph) on a board. If you pull the brush to the movement. Th ese pathways help keep the eyes locked on an ob- right, the drag of the paint on the bristles bends them to the ject as the head turns. left (Fig. 10.22 b). In the same way, the inertia of the fluid in the semicircular canal pulls the cupula and the cilia of the hair RUNNING PROBLEM cells to the left when the head turns right. If rotation continues, the moving endolymph finally Although many vestibular disorders can cause the catches up. Th en if head rotation stops suddenly, the fl uid has symptoms Anant is experiencing, two of the most common built up momentum and cannot stop immediately. The fluid are positional vertigo and Ménière’s disease. In positional continues to rotate in the direction of the head rotation, leaving vertigo, calcium crystals normally embedded in the otolith membrane of the maculae become dislodged and fl oat the person with a turning sensation. If the sensation is strong toward the semicircular canals. The primary symptom enough, the person may throw his or her body in the direction of positional vertigo is brief episodes of severe dizziness opposite the direction of rotation in a refl exive attempt to com- brought on by a change in position, such as moving to the pensate for the apparent loss of equilibrium. head-down yoga position called “downward-facing dog.” People with positional vertigo often say they feel dizzy when The Otolith Organs Sense Linear Acceleration they lie down or turn over in bed.

and Head Position Q3: When a person with positional vertigo changes position, Th e two otolith organs, the utricle {utriculus , little bag} and sac- the displaced crystals fl oat toward the semicircular cule {little sac}, are arranged to sense linear forces. Th eir sen- canals. Why would this cause dizziness? sory structures, called maculae , consist of hair cells, a gelatinous Q4: Compare the symptoms of positional vertigo and mass known as the otolith membrane, and calcium carbonate Ménière’s disease. On the basis of Anant’s symptoms, and protein particles called otoliths { oto , ear + lithos, stone}. which condition do you think he has? Th e hair cell cilia are embedded in the otolith membrane, and otoliths bind to matrix proteins on the surface of the membrane ( Fig. 10.22 c). If gravity or acceleration cause the otoliths to slide forward or back, the gelatinous otolith membrane slides

374 Sensory Physiology

EQUILIBRIUM PATHWAYS

Cerebral cortex

Thalamus

Reticular Vestibular branch of formation vestibulocochlear nerve (VIII) Cerebellum Somatic motor neurons Vestibular apparatus Vestibular controlling eye nuclei of movements medulla 10

Fig. 10.23

Accessory structures associated with the eye include six extrinsic Concept Check Answers: End of Chapter eye muscles, skeletal muscles that attach to the outer surface of 21. The stereocilia of hair cells are bathed in endolymph, which has a very the eyeball and control eye movements. Cranial nerves III, IV, + + high concentration of K and a low concentration of Na . When ion and VI innervate these muscles. channels in the stereocilia open, which ions move in which direction to Th e upper and lower eyelids close over the anterior surface of cause depolarization? the eye, and the lacrimal apparatus, a system of glands and ducts, 22. Why does hearing decrease if an ear infection causes fl uid buildup in keeps a continuous fl ow of tears washing across the exposed sur- the middle ear? face so that it remains moist and free of debris. Tear secretion is stimulated by parasympathetic neurons from cranial nerve VII. 23. When dancers perform multiple turns, they try to keep their vision fi xed on a single point (“spotting”). How does spotting keep a dancer from getting dizzy? EXTERNAL ANATOMY OF THE EYE

Lacrimal gland Muscles attached to external surface The Eye and Vision secretes tears. of eye control eye movement. Th e eye is a sensory organ that functions much like a camera. It focuses light on a light-sensitive surface (the retina) using a Upper lens and an aperture or opening (the pupil) whose size can be eyelid adjusted to change the amount of entering light. Vision is the process through which light refl ected from objects in our envi- Sclera ronment is translated into a mental image. Th is process can be Pupil divided into three steps:

1 Light enters the eye, and the lens focuses the light on the retina. Iris 2 Photoreceptors of the retina transduce light energy into an Lower electrical signal. eyelid 3 Neural pathways from retina to brain process electrical signals into visual images. The Skull Protects the Eye

Th e external anatomy of the eye is shown in Figure 10.24 . Like The orbit is a bony cavity Nasolacrimal duct drains sensory elements of the ears, the eyes are protected by a bony that protects the eye. tears into nasal cavity. cavity, the orbit, which is formed by facial bones of the skull. Fig. 10.24

375 Sensory Physiology

Th e pupil is an opening through which light can pass into and exit the eye. Lateral to the optic disk is a small dark spot, the the interior of the eye. Pupil size varies with the contraction and fovea . Th e fovea and a narrow ring of tissue surrounding it, the relaxation of a ring of smooth pupillary muscle. Th e pupil ap- macula , are the regions of the retina with the most acute vision. pears as the black spot inside the colored ring of pigment known Neural pathways for the eyes are illustrated in Figure 10.26 . as the iris. Th e pigments and other components of the iris deter- Th e optic nerves from the eyes go to the optic chiasm in the brain, mine eye color. where some of the fi bers cross to the opposite side. Aft er synapsing Th e eye itself is a hollow sphere divided into two compart- in the lateral geniculate body (lateral geniculate nucleus) of the ments (chambers) separated by a lens ( Fig. 10.25 ). Th e lens , thalamus, the vision neurons of the tract terminate in the occipital suspended by ligaments called zonules, is a transparent disk lobe at the visual cortex. Collateral pathways go from the thala- that focuses light. Th e anterior chamber in front of the lens is mus to the midbrain, where they synapse with eff erent neurons of filled with aqueous humor { humidus, moist}, a low-protein, cranial nerve III that control the diameter of the pupils. plasma-like fl uid secreted by the ciliary epithelium supporting the lens. Behind the lens is a much larger chamber, the vitreous Concept Check A nswers: End of Chapter chamber, fi lled mostly with thevitreous body { vitrum, glass; also called the vitreous humor}, a clear, gelatinous matrix that 24. What functions does the aqueous humor serve? helps maintain the shape of the eyeball. Th e outer wall of the eyeball, the sclera , is composed of connective tissue. Light enters the anterior surface of the eye through the cor- Light Enters the Eye through the Pupil nea, a transparent disk of tissue that is a continuation of the sclera. In the fi rst step of the visual pathway, light from the environ- Aft er passing through the opening of the pupil, light strikes the ment enters the eye. Before it strikes the retina, however, the lens, which has two convex surfaces. Th e cornea and lens together light is modifi ed two ways. First, the amount of light that reaches bend incoming light rays so that they focus on the retina , the photoreceptors is modulated by changes in the size of the pupil. light-sensitive lining of the eye that contains the photoreceptors. Second, the light is focused by changes in the shape of the lens. When viewed through the pupil with an ophthalmoscope Th e human eye functions over a 100,000-fold range of light { ophthalmos, eye}, the retina is seen to be crisscrossed with intensity. Most of this ability comes from the sensitivity of the small arteries and veins that radiate out from one spot, the optic photoreceptors, but the pupils assist by regulating the amount of disk (Fig. 10.25 b). Th e optic disk is the location where neurons light that falls on the retina. In bright sunlight, the pupils narrow of the visual pathway form the optic nerve (cranial nerve II) to about 1.5 mm in diameter when a parasympathetic pathway constricts the circular pupillary muscles. In the dark, the opening of the pupil dilates to 8 mm, a 28-fold increase in pupil area. Dila- CLINICAL FOCUS tion occurs when radial muscles lying perpendicular to the circular muscles contract under the infl uence of sympathetic neurons. Testing pupillary refl exes is a standard part of a neurologi- Glaucoma cal examination. Light hitting the retina in one eye activates the The eye disease glaucoma, characterized by degen- refl ex. Signals travel through the optic nerve to the thalamus, eration of the optic nerve, is the leading cause of blindness then to the midbrain, where eff erent neurons constrict the pu- worldwide. Many people associate glaucoma with increased pils in both eyes (Fig. 10.26 c). This response is known as the intraocular (within the eyeball) pressure, but scientists have consensual reflex and is mediated by parasympathetic fibers discovered that increased pressure is only one risk factor for running through cranial nerve III. the disease. A signifi cant number of people with glaucoma have normal intraocular pressure, and not everyone with elevated pressure develops glaucoma. Many cases of elevated Concept Check Answers: End of Chapter eye pressure are associated with excess aqueous humor, a 25. Use the neural pathways in Figure 10.26 to answer the following fl uid that is secreted by the ciliary epithelium near the lens. questions. Normally the fl uid drains out through the canal of Schlemm (a) Why does shining light into one eye cause pupillary constriction in in the anterior chamber of the eye, but if outfl ow is blocked, both eyes? the aqueous humor accumulates, causing pressure to build (b) If you shine a light in the left eye and get pupillary constriction in up inside the eye. Treatments to decrease intraocular pres- the right eye but not in the left eye, what can you conclude about sure include drugs that inhibit aqueous humor production the aff erent path from the left eye to the brain? About the eff erent and surgery to reopen the canal of Schlemm. Research sug- pathways to the pupils? gests that the optic nerve degeneration in glaucoma may be 26. Parasympathetic fibers constrict the pupils, and sympathetic fibers due to nitric oxide or apoptosis-inducing factors, and studies dilate them. The two autonomic divisions can be said to have in these areas are underway. eff ects on pupil diameter.

376 Fig. 10.25 ANATOMY SUMMARY

The Eye

Optic disk (a) Sagittal section of the eye Central retinal artery and vein

Fovea

Macula: the center of the visual field

Zonules: attach Lens bends (b) View of the rear wall of the eye as seen lens to ciliary light to focus it through the pupil with an ophthalmoscope muscle on the retina.

Optic disk (blind spot): region Canal of Schlemm where optic nerve and blood vessels leave the eye Aqueous humor Central retinal artery and vein Cornea emerge from center of optic disk.

Pupil changes amount of light Optic nerve entering the eye. FIGURE QUESTION Iris Fovea: region of sharpest vision If the fovea is lateral to the optic disk, which eye (left or right) is illustrated in part (b)? Vitreous chamber

Retina: layer that contains photoreceptors Ciliary muscle: contraction alters curvature of the lens. Sclera is connective tissue.

In addition to regulating the amount of light that hits the The Lens Focuses Light on the Retina retina, the pupils create what is known as depth of fi eld. A simple example comes from photography. Imagine a picture Th e physics that describes the behavior and properties of light of a puppy sitting in the foreground amid a fi eld of wildfl ow- is a fi eld known as optics . When light rays pass from air into a ers. If only the puppy and the flowers immediately around medium of diff erent density, such as glass or water, they bend, her are in focus, the picture is said to have a shallow depth or refract . Light entering the eye is refracted twice: fi rst when of fi eld. If the puppy and the wildfl owers all the way back to it passes through the cornea, and again when it passes through the horizon are in focus, the picture has full depth of field. the lens. About two-thirds of the total refraction (bending) oc- Full depth of fi eld is created by constricting the pupil (or the curs at the cornea and the remaining one-third occurs at the diaphragm on a camera) so that only a narrow beam of light lens. Here we consider only the refraction that occurs as light enters the eye. In this way, a greater depth of the image is fo- passes through the lens because the lens is capable of changing cused on the retina. its shape to focus light.

377 Sensory Physiology

PATHWAYS FOR VISION AND THE PUPILLARY REFLEX

(a) Dorsal view (b) Neural pathway for vision, lateral view

Optic tract

Eye Optic chiasm Eye Optic nerve

Optic Optic Optic Lateral geniculate Visual cortex nerve chiasm tract body (thalamus) (occipital lobe)

(c) Collateral pathways leave the thalamus and synapse Eye in the midbrain to control constriction of the pupils. Light Midbrain

Cranial nerve III controls pupillary constriction.

Fig. 10.26

When light passes from one medium into another, the an- When parallel light rays pass through a convex lens, the gle of refraction (how much the light rays bend) is infl uenced single point where the rays converge is called the focal point by two factors: (1) the difference in density of the two media (Fig. 10.27 b). Th e distance from the center of a lens to its focal and (2) the angle at which the light rays meet the surface of point is known as the focal length (or focal distance ) of the lens. the medium into which it is passing. For light passing through For any given lens, the focal length is fi xed. For the focal length the lens of the eye, we assume that the density of the lens is the to change, the shape of the lens must change. same as the density of the air and thus ignore this factor. Th e angle When light from an object passes through the lens of the eye, at which light meets the face of the lens depends on the curva- the focal point and object image must fall precisely on the retina if ture of the lens surface and the direction of the light beam. the object is to be seen in focus. In Figure 10.27 c, parallel light rays Imagine parallel light rays striking the surface of a trans- strike a lens whose surface is relatively fl at. For this lens, the focal parent lens. If the lens surface is perpendicular to the rays, the point falls on the retina. Th e object is therefore in focus. For the light passes through without bending. If the surface is not per- normal human eye, any object that is 20 feet or more from the eye pendicular, however, the light rays bend. Parallel light rays strik- creates parallel light rays and will be in focus when the lens is fl atter. ing a concave lens, such as that shown in Figure 10.27 a, are What happens, though, when an object is closer than 20 refracted into a wider beam. Parallel rays striking a convex lens feet to the lens? In that case, the light rays from the object are bend inward and focus to a point— convex lenses converge light not parallel and strike the lens at an oblique angle that changes waves ( Fig. 10.27 b). You can demonstrate the properties of a the distance from the lens to the object’s image (Fig. 10.27 d). convex lens by using a magnifying glass to focus sunlight onto a Th e focal point now lies behind the retina, and the object image piece of paper or other surface. becomes fuzzy and out of focus.

378 Sensory Physiology

By age 60, many people lose the refl ex completely because the RUNNING PROBLEM lens has lost fl exibility and remains in its fl atter shape for dis- The otolaryngologist strongly suspects that Anant has tance vision. Th e loss of accommodation, presbyopia, is the rea- Ménière’s disease, with excessive endolymph in the vestibular son most people begin to wear reading glasses in their 40s. apparatus and cochlea. Many treatments are available, Two other common vision problems are near-sightedness beginning with simple dietary changes. For now, the and far-sightedness. Near-sightedness, or myopia, occurs when physician suggests that Anant limit his salt intake and take the focal point falls in front of the retina ( Fig. 10.27 j). Far- diuretics, drugs that cause the kidneys to remove excess fl uid sightedness, or hyperopia, occurs when the focal point falls be- from the body. hind the retina (Fig. 10.27 i). Th ese vision problems are caused by abnormally curved or fl attened corneas or by eyeballs that Q5: Why is limiting salt (NaCl) intake suggested as a treatment for Ménière’s disease? (Hint: What is the are too long or too short. Placing a lens with the appropriate relationship between salt, osmolarity, and fl uid volume?) curvature in front of the eye changes the refraction of light entering the eye and corrects the problem. A third common vision problem, astigmatism, is usually caused by a cornea that is not a perfectly shaped dome, resulting in distorted images. To keep a near object in focus, the lens must become more 10 rounded to increase the angle of refraction ( Fig. 10.27 e). Mak- Concept Check Answers: End of Chapter ing a lens more convex shortens its focal length. In this example, 27. If a person’s cornea, which helps focus light, is more rounded than rounding the lens causes light rays to converge on the retina in- normal (has a greater curvature), is this person more likely to be stead of behind it, and the object comes into focus. hyperopic or myopic? ( Hint: See Fig. 10.27 .) Th e process by which the eye adjusts the shape of the lens to keep objects in focus is known as accommodation , and the 28. The relationship between the focal length of a lens (F), the distance between an object and the lens (P), and the distance from the lens to closest distance at which it can focus an object is known as the the object’s image (Q) is expressed as 1 F = 1 P + 1 Q . near point of accommodation. You can demonstrate changing > > > focus with the accommodation refl ex easily by closing one eye (a) If the focal length of a lens does not change but an object moves and holding your hand up about 8 inches in front of your open closer to the lens, what happens to the image distance Q? (b) If an object moves closer to the lens and the image distance Q eye, fi ngers spread apart. must stay the same for the image to fall on the retina, what must Focus your eye on some object in the distance that is visible happen to the focal length F of the lens? For this change in F to between your fingers. Notice that when you do so, your fingers occur, should the lens become fl atter or more rounded? remain visible but out of focus. Your lens is fl attened for distance 29. (a) Explain how convex and concave corrective lenses change the vision, so the focal point for near objects falls behind the retina. refraction of light. Th ose objects appear out of focus. Now shift your gaze to your fi n- (b) Which type of corrective lens should be used for myopia, and why? gers and notice that they come into focus. Th e light rays refl ecting For hyperopia? off your fi ngers have not changed their angle, but your lens has become more rounded, and the light rays now converge on the retina. How can the lens, which is clear and does not have any Phototransduction Occurs at the Retina muscle fi bers in it, change shape? Th e answer lies in the ciliary muscle, a ring of smooth muscle that surrounds the lens and In the second step of the visual pathway, photoreceptors of the is attached to it by the inelastic ligaments called zonules ( Fig. retina convert light energy into electrical signals. Light energy is 10.27 f). If no tension is placed on the lens by the ligaments, the part of the electromagnetic spectrum, which ranges from high- lens assumes its natural rounded shape because of the elasticity energy, very-short-wavelength waves such as X-rays and gamma of its capsule. If the ligaments pull on the lens, it fl attens out and rays to low-energy, lower-frequency microwaves and radio assumes the shape required for distance vision. waves ( Fig. 10.28 ). However, our brains can perceive only a Tension on the ligaments is controlled by the ciliary muscle. small portion of this broad energy spectrum. For humans, vis- When the ciliary muscle is relaxed, the ring is more open and the ible light is limited to electromagnetic energy with waves that 14 lens is pulled into a fl atter shape ( Fig. 10.27 g). When this circular have a frequency of 4.097.5 * 10 cycles per second (hertz, Hz) muscle contracts, the muscle ring gets smaller, releasing tension and a wavelength of 400–750 nanometers (nm). Electromag- on the ligaments so that the lens rounds (Fig. 10.27 h). netic energy is measured in units called photons. Young people can focus on items as close as 8 cm, but the Our unaided eyes see visible light but do not respond to ul- accommodation refl ex diminishes from the age of 10 on. By age traviolet and infrared light, whose wavelengths border the ends of 40, accommodation is only about half of what it was at age 10. our visible light spectrum. On the other hand, the eyes of some other animals can see these wavelengths. For example, bees use ul- traviolet “runways” on fl owers to guide them to pollen and nectar.

379 Fig. 10.27 ESSENTIALS Sensory Physiology Optics of the Eye

Light passing through a curved surface will bend or refract.

(a) A concave lens scatters light rays. (b) A convex lens causes light rays to converge.

Concave lens Convex lens Focal point

Parallel Parallel light rays light rays

Focal length The focal length of the lens is the distance from the center of the lens to the focal point.

For clear vision, the focal point must fall on the retina.

(c) Parallel light rays pass through (d) For close objects, the light rays are no longer (e) To keep an object in focus as it a flattened lens, and the focal parallel. The lens and its focal length have not moves closer, the lens becomes point falls on the retina. changed, but the object is seen out of focus more rounded. because the light beam is not focused on the retina. Focal length Image distance Focal length

Light from distant Lens source

Object Object image

Light from

distant source Lens flattened Lens rounded for distant vision for close vision Focal length Object Image distance (Q) Image distance distance (P) now equals focal length

Focal length of lens (F)

Changes in lens shape are controlled by the ciliary muscle.

(f) The lens is attached to the ciliary (g) When ciliary muscle is relaxed, the (h) When ciliary muscle contracts, it muscle by inelastic ligaments (zonules). ligaments pull on and flatten the lens. releases tension on the ligaments and the lens becomes more rounded.

Ciliary muscle Ciliary Ciliary muscle relaxed muscle contracted

Lens Cornea Lens flattened Cornea Lens rounded Ligaments Iris Ligaments Ligaments pulled tight slacken

380 Phototransduction is the process by which animals convert light energy into electrical signals. In humans, phototransduction takes place when light hits the retina, the sensory organ of the eye ( Fig. 10.29 ). Th e retina develops from the same embry- Common vision defects can be onic tissue as the brain, and (as in the cortex of the brain) neu- corrected with external lenses. rons in the retina are organized into layers. Th ere are fi ve types of

(i) Hyperopia, or far-sightedness, neurons in the retinal layers: photoreceptors, bipolar cells, gan- occurs when the focal point glion cells, amacrine cells, and horizontal cells (Fig. 10.29 f). falls behind the retina. Hyperopia (corrected Backing the photosensitive portion of the human retina is with a convex lens) a dark pigment epithelium layer. Its function is to absorb any light rays that escape the photoreceptors, preventing distracting light from refl ecting inside the eye and distorting the visual image. Th e black color of these epithelial cells comes from granules of the pigment melanin . Photoreceptors are the neurons that convert light energy into electrical signals. Th ere are two main types of photoreceptors, rods and cones, as well as a recently discovered photore- 10 ceptor that is a modifi ed ganglion cell (see Emerging Concepts (j) Myopia, or near-sightedness, Box: Melanopsin). You might expect photoreceptors to be on occurs when the focal point falls in front of the retina. the surface of the retina facing the vitreous chamber, where light Myopia (corrected with will strike them fi rst, but the retinal layers are actually in reverse a concave lens) order. Th e photoreceptors are the bottom layer, with their photosensitive tips against the pigment epithelium. Most light entering the eye must pass through several relatively transparent layers of neurons before striking the photoreceptors. One exception to this organizational pattern occurs in a small region of the retina known as the fovea {pit}. Th is area is free of neurons and blood vessels that would block light recep- tion, so photoreceptors receive light directly, with minimal scat- tering. (Fig. 10.29 d). As noted earlier, the fovea and the macula immediately surrounding it are the areas of most acute vision, and they form the center of the visual fi eld.

THE ELECTROMAGNETIC SPECTRUM

10-5 nm 400 nm Gamma rays 10-3 nm

Wavelength 450 nm X-rays 1 nm 500 nm UV Visible 550 nm light 103 nm

Infrared 600 nm 106 nm

650 nm Micro- waves 109 nm (1 m) 700 nm Energy Radio waves 103 m

Fig. 10.28

381 Fig. 10.29 ANATOMY SUMMARY Sensory Physiology The Retina

(a) Dorsal view of a section of the right eye (b) The projected image is upside down on the retina. Visual processing in the brain reverses the image. Fixation Light Lens Retina Fovea point

Macula Fovea

Optic nerve (d) Light strikes the photoreceptors in the fovea directly because overlying neurons are pushed aside.

Optic nerve

Sclera Light

The choroid layer Fovea contains blood vessels. Cone Rod Pigment epithelium Bipolar neuron Neural cells of retina Ganglion cell Neural cells of retina

(e) Convergence in the retina (f) Retinal photoreceptors are organized into layers.

Bipolar Pigment Amacrine Horizontal Pigment cell Rod epithelium cell cell epithelium To optic nerve

Ganglion cell Light

FIGURE QUESTION How many rods converge on the ganglion cell in (e)? Ganglion Neurons where signals cell Cone (color vision) from rods and cones Bipolar are integrated cell Rod (monochromatic vision) Drawing of photoreceptors in the fovea adapted from E. R. Kandel et al., Principles of Neural Science, 3rd edition. New York: McGraw Hill, 2000.

382 Sensory Physiology

RUNNING PROBLEM EMERGING CONCEPTS Anant’s condition does not improve with the low-salt diet and diuretics, and he continues to suff er from disabling Melanopsin attacks of vertigo with vomiting. In severe cases of Ménière’s Circadian rhythms in mammals are cued by light disease, surgery is sometimes performed when less invasive entering the eyes. For many years, scientists believed treatments have failed. In one surgical procedure for that rods and cones of the retina were the primary the disease, a drain is inserted to relieve pressure in the photoreceptors linked to the suprachiasmatic nucleus endolymph by removing some of the fl uid. If that fails to (SCN), the brain center for circadian rhythms. However, in provide relief, as a last resort the vestibular nerve can be 1999 researchers found that transgenic mice lacking both severed. This surgery is diffi cult to perform, as the vestibular rods and cones still had the ability to respond to changing nerve lies near many other important nerves, including facial light cues, suggesting that an additional photoreceptor nerves and the auditory nerve. Patients who undergo this must exist in the retina. Now scientists believe they have procedure are advised that the surgery can result in deafness found it: a subset of retinal ganglion cells that contain an if the cochlear nerve is inadvertently severed. opsin-like pigment called melanopsin (mRGCs). Axons from these mRGC ganglion cells project to the SCN as well as Q6: Why would severing the vestibular nerve alleviate to brain areas that process visual information. It appears Ménière’s disease? 10 that these newly identifi ed photoreceptors join rods and cones as the light-sensing cells of the mammalian retina, and scientists may have to revise the traditional models of visual processing. To learn more, see C. Sedwick, Melanopsin ganglion cells: a diff erent way of seeing things. PLoS Biol Photoreceptors Transduce Light 8(12): e1001003, 2010 ( www.plosbiology.org ). into Electrical Signals Th ere are two main types of photoreceptors in the eye: rods and cones. Rods function well in low light and are used in night vi- When you look at an object, the lens focuses the object im- sion, when objects are seen in black and white rather than in age on the fovea. For example, in Figure 10.29b, the eye is fo- color. Th ey outnumber cones by a 20:1 ratio, except in the fovea, cused on the green-yellow border of the color bar. Light from which contains only cones. that section of the visual fi eld falls on the fovea and is in sharp Cones are responsible for high-acuity vision and color vi- focus. Notice also that the image falling on the retina is upside sion during the daytime, when light levels are higher. Acuity down. Subsequent visual processing by the brain reverses the means keenness and is derived from the Latin acuere, meaning image again so that we perceive it in the correct orientation. “to sharpen.” Th e fovea, which is the region of sharpest vision, Sensory information about light passes from the photo- has a very high density of cones. receptors to bipolar neurons, then to a layer of ganglion cells Th e two types of photoreceptors have the same basic struc- ( Fig. 10.29 e). Th e axons of ganglion cells form the optic nerve, ture ( Fig. 10.30 ): (1) an outer segment whose tip touches the which leaves the eye at the optic disk. Because the optic disk has pigment epithelium of the retina, (2) an inner segment that no photoreceptors, images projected onto this region cannot be contains the cell nucleus and organelles for ATP and protein seen, creating what is called the eye’s blind spot . synthesis, and (3) a basal segment with a synaptic terminal that releases glutamate onto bipolar cells. In the outer segment, the cell membrane has deep folds Concept Check Answers: End of Chapter that form disk-like layers. Toward the tip of the outer segments 30. Animals that see well in very low light, such as cats and owls, lack in rods, these layers actually separate from the cell membrane a pigment epithelium and instead have a layer called the tapetum and form free-fl oating membrane disks. In the cones, the disks lucidum behind the retina. What property might this layer have that stay attached. would enhance vision in low light? Light-sensitive visual pigments are bound to the disk membranes in outer segments of photoreceptors. Th ese visual 31. How is the diff erence in visual acuity between the fovea and the edge of the visual fi eld similar to the diff erence in touch discrimination between pigments are transducers that convert light energy into a change the fi ngertips and the skin of the arm? in membrane potential. Rods have one type of visual pigment, rhodopsin. Cones have three diff erent pigments that are closely 32. Macular degeneration is the leading cause of blindness in Americans related to rhodopsin. over the age of 55. Impaired function of the macula causes vision loss in which part of the visual fi eld?

383 Sensory Physiology

PHOTORECEPTORS: RODS AND CONES

The dark pigment epithelium absorbs extra light and prevents that light from reflecting back and distorting vision. PIGMENT EPITHELIUM Old disks at tip are phagocytized by pigment epithelial cells.

Melanin granules

OUTER SEGMENT Light transduction takes place in the outer Disks segment of the photoreceptor Disks using visual pigments in membrane disks. Connecting stalks

INNER SEGMENT Mitochondria Location of major organelles and metabolic operations, such as photopigment synthesis and ATP production Rhodopsin molecule

Cone Rods Retinal Opsin

SYNAPTIC TERMINAL Synapses with bipolar cells.

Bipolar cell

LIGHT

Fig. 10.30

Th e visual pigments of cones are excited by diff erent wave- Our brain recognizes the color of an object by interpret- lengths of light, allowing us to see in color. White light is a coming the combination of signals coming to it from the three dif- bination of colors, as demonstrated when you separate white ferent color cones. Th e details of color vision are still not fully light by passing it through a prism. Th e eye contains cones for understood, and there is some controversy about how color is red, green, and blue light. Each cone type is stimulated by a processed in the cerebral cortex. Color-blindness is a condition range of light wavelengths but is most sensitive to a particular in which a person inherits a defect in one or more of the three wavelength ( Fig. 10.31 ). Red, green, and blue are the three types of cones and has diffi culty distinguishing certain colors. primary colors that make the colors of visible light, just as red, Probably the best-known form of color-blindness is red-green, blue, and yellow are the three primary colors that make diff erent in which people have trouble telling red and green apart. colors of paint. The color of any object we are looking at depends on the wavelengths of light refl ected by the object. Green leaves refl ect Concept Check Answers: End of Chapter green light, and bananas refl ect yellow light. White objects refl ect most wavelengths. Black objects absorb most wavelengths, which 33. Why is our vision in the dark in black and white rather than in color? is one reason they heat up in sunlight while white objects stay cool.

384 Sensory Physiology

When a rod is in darkness and rhodopsin is not active, cy- LIGHT ABSORPTION BY VISUAL PIGMENTS clic GMP (cGMP) levels in the rod are high, and both CNG and + 2+ There are three types of cone pigment, each with a K channels are open (Fig. 10.32 1 ). Sodium and Ca i o n characteristic light absorption spectrum. Rods are for infl ux is greater than K+ e ffl ux, so the rod stays depolarized to black and white vision in low light. an average membrane potential of -40 mV (instead of the more Blue Green Red usual - 70 mV). At this slightly depolarized membrane poten- cones Rods cones cones 2+ 100 tial, the voltage-gated Ca channels are open and there is tonic (continuous) release of the neurotransmitter glutamate from the synaptic portion of the rod onto the adjacent bipolar cell. 75 When light activates rhodopsin, a second-messenger cascade is initiated through the G protein transducin (Fig. 10.32 2 ). (Transducin is closely related to gustducin, the G protein found 50 in bitter taste receptors.) The transducin second-messenger cascade decreases the concentration of cGMP, which closes the

Light absorption 25 CNG channels. As a result, cation infl ux slows or stops. (percent of maximum) (percent With decreased cation infl ux and continued K+ e ffl ux, the inside of the rod hyperpolarizes, and glutamate release onto the 10 0 bipolar neurons decreases. Bright light closes all CNG channels and stops all neurotransmitter release. Dimmer light causes a Violet Blue Green Yellow Orange Red response that is graded in proportion to the light intensity. Aft er activation, retinal diff uses out of the rod and is trans- ported into the pigment epithelium. Th ere it reverts to its inactive 400 450 500 550 600 650 700 form before moving back into the rod and being reunited with Wavelength (nm) opsin ( Fig. 10.32 3 ). Th e recovery of rhodopsin from bleaching can take some time and is a major factor in the slow adaptation GRAPH QUESTIONS of the eyes when moving from bright light into the dark. 1. Which pigment absorbs light over the broadest spectrum of wavelengths? 2. Over the narrowest? Concept Check A nswers: End of Chapter 3. Which pigment absorbs the most light at 500 nm? 34. Draw a map or diagram to explain phototransduction. Start with Fig. 10.31 bleaching and end with release of neurotransmitter.

Phototransduction Th e process of phototransduction is similar for rhodopsin (in rods) and the three color pigments (in cones). Signal Processing Begins in the Retina Rhodopsin is composed of two molecules: opsin , a protein em- We now move from the cellular mechanism of light transduc- bedded in the membrane of the rod disks, and retinal , a vitamin tion to the processing of light signals by the retina and brain, A derivative that is the light-absorbing portion of the pigment the third and fi nal step in our vision pathway. Signal process- (see Fig. 10.30 ). In the absence of light, retinal binds snugly into ing in the retina is an excellent example of convergence, in a binding site on the opsin ( Fig. 10.32 ). When activated by as which multiple neurons synapse onto a single postsynaptic cell little as one photon of light, retinal changes shape to a new con- ( Fig. 10.33 a). Depending on location in the retina, as many as fi guration. Th e activated retinal no longer binds to opsin and is 15 to 45 photoreceptors may converge on one bipolar neuron. released from the pigment in the process known as bleaching . Multiple bipolar neurons in turn innervate a single ganglion How does rhodopsin bleaching lead to action poten- cell, so that the information from hundreds of millions of reti- tials traveling through the optical pathway? To understand the nal photoreceptors is condensed down to a mere 1 million axons pathway, we must look at other properties of the rods. Electri- leaving the eye in each optic nerve. Convergence is minimal in cal signals in cells occur as a result of ion movement between the fovea, where some photoreceptors have a 1:1 relationship with the intracellular and extracellular compartments. Rods contain their bipolar neurons, and greatest at the outer edges of the retina. three main types of cation channels: cyclic nucleotide-gated Signal processing in the retina is modulated by input from + + channels (CNG channels) that allow Na a n d Ca2 to enter two additional sets of cells that we will not discuss (Fig. 10.29 f). + + the rod, K c h a n n e l s t h a t a l l o w K to leak out of the rod, and Horizontal cells synapse with photoreceptors and bipolar cells. + voltage-gated Ca2 channels in the synaptic terminal that help Amacrine cells modulate information fl owing between bipolar regulate exocytosis of neurotransmitter. cells and ganglion cells.

385 Sensory Physiology

PHOTOTRANSDUCTION IN RODS

Rods contain the visual pigment rhodopsin. When activated by light, rhodopsin separates into opsin and retinal.

12In darkness, rhodopsin is inactive, Light bleaches rhodopsin. Opsin 3In the recovery phase, retinal cGMP is high, and CNG and K+ decreases cGMP, closes CNG recombines with opsin. channels are open. channels, and hyperpolarizes the cell.

Activated Opsin (bleached Activates Retinal converted to Pigment epithelium cell retinal pigment) transducin inactive form

Disk

Transducin (G protein)

Inactive rhodopsin Cascade (opsin and retinal) cGMP Decreased levels high cGMP Retinal recombines Ca2+ Ca2+ + with opsin to Na CNG channel Na+ CNG channel form rhodopsin. open closes

+ K+ K

Membrane Membrane potential hyperpolarizes in dark = –40mV to –70 mV

Light

Rod FIGURE QUESTION One rod contains about 10,000 CNG channels open in the dark. One photon of light activates one rhodopsin. Each rhodopsin activates 800 transducin. Each transducin cascade removes 6 cGMP. A decrease of 24 cGMP Tonic release of Neurotransmitter release closes one CNG channel. How neurotransmitter decreases in proportion many photons are needed to close onto bipolar neurons to amount of light. all the CNG channels in one rod?

Fig. 10.32

Bipolar Cells Glutamate release from photoreceptors onto bipo- in the dark. When mGluR6 is not activated, the ON bipolar cell lar neurons begins signal processing. Th ere are two types of bipolar depolarizes. OFF bipolar cells have an ionotropic glutamate re- cells, light-on (ON bipolar cells) and light-off (OFF bipolar cells). ceptor that opens ion channels and depolarizes the OFF bipolar ON bipolar cells are activated in the light when glutamate secretion cell in the dark. Bipolar cell signal processing is also modifi ed by by photoreceptors decreases. In the dark, ON bipolar cells are in- input from the horizontal and amacrine cells. hibited by glutamate release. OFF bipolar cells are excited by glutamate release in the dark. In the light, with less glutamate, OFF Ganglion Cells Bipolar cells synapse with ganglion cells, the bipolar cells are inhibited. By using diff erent glutamate receptors, next neurons in the pathway. We know more about ganglion cells one stimulus (light) creates two diff erent responses with a single because they lie on the surface of the retina, where their axons neurotransmitter. are the most accessible to researchers. Extensive studies have Whether glutamate is excitatory or inhibitory depends on been done in which researchers stimulated the retina with care- the type of glutamate receptor on the bipolar neuron. ON bipo- fully placed light and evaluated the response of the ganglion cells. lar cells have a metabotropic glutamate receptor called mGluR6 Each ganglion cell receives information from a partic- that hyperpolarizes the cell when the receptor binds glutamate ular area of the retina. These areas, known as visual fields ,

386 Sensory Physiology

VISUAL FIELDS

(a) Multiple photoreceptors converge on one ganglion cell. Horizontal and amacrine cells influence communication at the rod-bipolar or bipolar-ganglion synapses.

Rod Horizontal cell Amacrine cell

Pigment epithelium

Ganglion cell

Bipolar cell

To optic 10 nerve

(b) A group of adjacent photoreceptors form the visual field for one ganglion cell. This illustration shows an on- center, off-surround field.

Ganglion cells respond most strongly when Bipolar cells are either there is good contrast Visual fields have centers activated or inhibited of light intensity (yellow) and outer by light, depending on between the center surrounds (gray). their type. and the surround.

(c) The retina uses contrast rather Visual field type Field is on-center/off-surround Field is off-center/on-surround than absolute light intensity for better detection of weak stimuli. On-center, off-surround

Ganglion cell is excited Ganglion cell is inhibited Bright light onto center by light in the center by light in the center of the visual field. of the visual field.

Off-center, on-surround Bright light onto surround Ganglion cell is inhibited Ganglion cell is excited by light on the surround by light on the surround Bright light of the visual field. of the visual field. onto surround

Both field types

Diffuse light Ganglion cell responds Ganglion cell responds on both center weakly. weakly. and surround

Fig. 10.33

387 Sensory Physiology

are similar to receptive fi elds in the somatic sensory system. BINOCULAR VISION Th e visual fi eld of a ganglion cell near the fovea is quite small. Only a few photoreceptors are associated with each ganglion The left visual field of each eye is projected to the visual cortex on cell, and so visual acuity is greatest in these areas. At the the right side of the brain, and the right visual field is projected to the left visual cortex. Objects seen by both eyes fall within the edge of the retina, multiple photoreceptors converging onto binocular zone and are perceived in three dimensions. Objects a single ganglion cell results in vision that is not as sharp (Fig. seen with only one eye fall outside the binocular zone and are 10.33 a). perceived in only two dimensions. An analogy for this arrangement is to think of pixels Visual field on your computer screen. Assume that two screens have the Binocular zone is Binocular same number of “photoreceptors,” as indicated by a maxi- where left and zone mal screen resolution of 1280 * 1024 pixels. If screen A has right visual fields one photoreceptor becoming one “ganglion cell” pixel, the ac- overlap. tual screen resolution is 1280 * 1024, and the image is very clear. If eight photoreceptors on screen B converge into one Left Right Monocular zone visual visual ganglion cell pixel, then the actual screen resolution falls to is the portion of field field 160 * 128, resulting in a very blurry and perhaps indistinguish- the visual field associated with able image. only one eye. Visual fi elds of ganglion cells are roughly circular (unlike the irregular shape of somatic sensory receptive fi elds) and are divided into sections: a round center and its doughnut-shaped surround (Fig. 10.33 b). Th is organization allows each ganglion cell to use contrast between the center and its surround to interpret visual information. Strong contrast between the center and surround elicits a strong excitatory response (a series of action potentials) or a strong inhibitory response (no action potentials) Optic chiasm from the ganglion cell. Weak contrast between center and surround gets an intermediate response. Optic nerve Th ere are two types of ganglion cell visual fi elds. In an on- center/off -surround fi eld, the associated ganglion cell responds most strongly when light is brightest in the center of the fi eld (Fig. 10.33 c). If light is brightest in the off -surround region of the fi eld, the on- center/off -surround fi eld ganglion cell is inhib- Optic tract ited and stops fi ring action potentials. Th e reverse happens with off -center/on-surround fi elds . What happens if light is uniform across a visual fi eld? In that case, the ganglion cell responds weakly. Thus, the retina Lateral geniculate body uses contrast rather than absolute light intensity to recognize (thalamus) objects in the environment. One advantage of using contrast is that it allows better detection of weak stimuli. Scientists have now identifi ed multiple types of ganglion cells in the primate retina. Th e two predominant types, which account for 80% of retinal ganglion cells, are M cells and P cells. Large magnocellular ganglion cells, or M cells , are more sensitive to information about movement. Smaller parvocellular ganglion cells, or P cells, are more sensitive to signals that pertain to form and fi ne detail, such as the texture of objects in the visual fi eld. A recently discovered subtype of ganglion cell, the melanopsin Visual cortex retinal ganglion cell , apparently also acts as a photoreceptor. Fig. 10.34 Processing Beyond the Retina Once action potentials leave ganglion cell bodies, they travel along the optic nerves to the fi bers from each eye cross to the other side of the brain for pro- CNS for further processing. As noted earlier, the optic nerves cessing. Figure 10.34 shows how information from the right enter the brain at the optic chiasm. At this point, some nerve side of each eye’s visual fi eld is processed on the left side of the

388 Sensory Physiology brain, and information from the left side of the fi eld is processed optic fi bers synapse onto neurons leading to the visual cortex in on the right side of the brain. the occipital lobe. Th e central portion of the visual fi eld, where left and right Th e lateral geniculate body is organized in layers that cor- sides of each eye’s visual fi eld overlap, is the binocular zone . Th e respond to the diff erent parts of the visual fi eld, which means two eyes have slightly diff erent views of objects in this region, that information from adjacent objects is processed together. and the brain processes and integrates the two views to create This topographical organization is maintained in the visual three-dimensional representations of the objects. Our sense cortex, with the six layers of neurons grouped into vertical col- of depth perception—that is, whether one object is in front of umns. Within each portion of the visual field, information is or behind another—depends on binocular vision. Objects that further sorted by form, color, and movement. fall within the visual fi eld of only one eye are in the monocular Th e cortex merges monocular information from the two eyes zone and are viewed in two dimensions. to give us a binocular view of our surroundings. Information from Once axons leave the optic chiasm, some fi bers project to on/off combinations of ganglion cells is translated into sensitivity to the midbrain, where they participate in control of eye movement line orientation in the simplest pathways, or into color, movement, or coordinate with somatosensory and auditory information for and detailed structure in the most complex. Each of these attributes balance and movement (see Fig. 10.26 ). Most axons, however, of visual stimuli is processed through a separate pathway, creating a project to the lateral geniculate body of the thalamus, where the network whose complexity we are just beginning to unravel. 10

RUNNING PROBLEM CONCLUSION

Ménière’s Disease Anant was told about the surgical options but elected that are available to alleviate Ménière’s disease, do a to continue medical treatment for a little longer. Over the Google search. Now check your understanding of this next two months, his Ménière’s disease gradually resolved. running problem by comparing your answers to those in The cause of Ménière’s disease is still unknown, which the summary table. makes treatment diffi cult. To learn more about treatments

Question Facts Integration and Analysis

1. In which part of the brain is sensory in- The major equilibrium pathways project [Not applicable] formation about equilibrium processed? to the cerebellum. Some information is also processed in the cerebrum.

2. Subjective tinnitus occurs when an The middle ear consists of malleus, incus, Subjective tinnitus could arise from abnormality somewhere along the ana- and stapes, bones that vibrate with a problem with any of the structures tomical pathway for hearing causes the sound. The hearing portion of the inner named. Abnormal bone growth can aff ect brain to perceive a sound that does not ear consists of hair cells in the fl uid-fi lled the middle ear bones. Excessive fl uid exist outside the auditory system. Start- cochlea. The cochlear (auditory) nerve accumulation in the inner ear will aff ect ing from the ear canal, name the auditory leads to the brain. the hair cells. Neural defects may cause structures in which problems may arise. the cochlear nerve to fi re spontaneously, creating the perception of sound.

3. When a person with positional vertigo The ends of the semicircular canals con- If the fl oating crystals displace the cupula, changes position, the displaced crystals tain sensory cristae, each crista consisting the brain will perceive movement that is fl oat toward the semicircular canals. Why of a cupula with embedded hair cells. not matched to sensory information com- would this cause dizziness? Displacement of the cupula creates a sen- ing from the eyes. The result is vertigo, an sation of rotational movement. illusion of movement.

4. Compare the symptoms of positional The primary symptom of positional Anant complains of dizzy attacks typically vertigo and Ménière’s disease. On the vertigo is brief dizziness following a lasting up to an hour that come on with- basis of Anant’s symptoms, which condi- change in position. Ménière’s disease out warning, making it more likely that tion do you think he has? combines vertigo with tinnitus and Anant has Ménière’s disease. hearing loss.

389 Sensory Physiology

RUNNING PROBLEM CONCLUSION (continued)

Question Facts Integration and Analysis

5. Why is limiting salt (NaCl) intake sug- Ménière’s disease is characterized by too Reducing salt intake should also reduce gested as a treatment for Ménière’s much endolymph in the inner ear. Endo- the amount of fl uid in the extracellular disease? lymph is an extracellular fl uid. compartment because the body will re- tain less water. Reduction of ECF volume may decrease fl uid accumulation in the inner ear.

6. Why would severing the vestibular The vestibular nerve transmits informa- Severing the vestibular nerve prevents nerve alleviate Ménière’s disease? tion about balance and rotational move- false information about body rotation ment from the vestibular apparatus to the from reaching the brain, thus alleviating brain. the vertigo of Ménière’s disease.

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Chapter Summary

We all live in the same world, but diff erent animals perceive the world dif- between signal molecules and ion channels or G protein–coupled recep- ferently. Dogs hear sounds we can’t, for instance, and nocturnal animals tors initiate many sensory pathways. Neural and non-neural sensory have better night vision than we do. An animal can perceive only those receptors convert chemical, mechanical, thermal, and light energy into stimuli for which it has sensory receptors. In this chapter, you explored electrical signals that pass along sensory neurons to CNS control centers . sensory receptors in the human body and learned how each type is de- Th e brain processes and fi lters incoming signals, sometimes acting on signed to enable us to perceive diff erent aspects of the world around us. sensory information without that information ever reaching conscious Despite the unique characteristics of each sense, basic patterns awareness. Many of the visceral refl exes you will study are unconscious emerge for sensory transduction and perception. Molecular interactions responses to sensory input.

General Properties of Sensory Systems 5. Th ere are four types of sensory receptors, based on the stimulus to which they are most sensitive: chemoreceptors , mechanoreceptors , 1. Sensory stimuli are divided into the special senses of vision, hear- thermoreceptors , and photoreceptors . ( Tbl. 10.2 ) ing, taste, smell, and equilibrium, and the somatic senses of touch, temperature, pain, itch, and proprioception. 6. Each receptor type has an adequate stimulus , a particular form of energy to which it is most responsive. 2. Sensory pathways begin with a stimulus that is converted by a re- 7. A stimulus that is above threshold creates a graded potential in the ceptor into an electrical potential. receptor. 3. If the stimulus is above threshold, action potentials pass along a 8. Multiple sensory neurons may converge on one secondary neuron sensory neuron to the central nervous system. We become aware of and create a single large receptive fi eld . ( Fig. 10.2 ) some stimuli but are never conscious of others. (Tbl. 10.1 ) 9. Sensory information from the spinal cord projects to the thalamus, 4. Sensory receptors vary from free nerve endings to encapsulated then on to the sensory areas of the cerebral cortex. Olfactory infor- nerve endings to specialized receptor cells. (Fig. 10.1 ) mation does not pass through the thalamus. (Fig. 10.3 )

390 Sensory Physiology

10. Th e central nervous system is able to modify our level of awareness of then fluid waves, chemical signals, and finally action potentials. sensory input. Th e perceptual threshold is the level of stimulus inten- ( Fig. 10.17 ) sity necessary for us to be aware of a particular sensation. 29. The cochlea of the inner ear contains three parallel, fluid-filled 11. Th e modality of a signal and its location are indicated by which sen- ducts. Th e cochlear duct contains the organ of Corti , which con- sory neurons are activated. Th e association of a receptor with a spe- tains hair cell receptors. ( Fig. 10.18 ) cifi c sensation is called labeled line coding . 30. When sound bends hair cell cilia, the hair cell membrane potential 12. Localization of auditory information depends on the timing of re- changes and alters release of neurotransmitter onto sensory neu- ceptor activation in each ear. (Fig. 10.4 ) rons. ( Fig. 10.19 ) 13. Lateral inhibition enhances the contrast between the center of the 31. The initial processing for pitch, loudness, and duration of sound receptive fi eld and the edges of the fi eld. In population coding, the takes place in the cochlea. Localization of sound is a higher function brain uses input from multiple receptors to calculate location and that requires sensory input from both ears and sophisticated com- timing of a stimulus. ( Fig. 10.5 ) putation by the brain. ( Fig. 10.20 , 10.4) 14. Stimulus intensity is coded by the number of receptors activated 32. Th e auditory pathway goes from cochlear nerve to medulla, pons, and by the frequency of their action potentials. (Fig. 10.6 ) midbrain, and thalamus before terminating in the auditory cortex. 15. For tonic receptors, the sensory neuron fires action potentials as Information from both ears goes to both sides of the brain. ( Fig. long as the receptor potential is above threshold. Phasic receptors 10.21 ) respond to a change in stimulus intensity but adapt if the strength of the stimulus remains constant. ( Fig. 10.7 ) The Ear: Equilibrium 10 Somatic Senses 33. Equilibrium is mediated through hair cells in the vestibular apparatus and semicircular canals of the inner ear. Gravity and ac- 16. There are four somatosensory modalities: touch, proprioception, celeration provide the force that moves the cilia. (Fig. 10.22 ) temperature, and nociception. 17. Secondary sensory neurons cross the midline so that one side of the brain processes information from the opposite side of the body. The Eye and Vision Ascending sensory tracts terminate in the somatosensory cortex . 34. Vision is the translation of refl ected light into a mental image. Pho- ( Fig. 10.8 ) toreceptors of the retina transduce light energy into an electrical 18. Touch receptors come in many varieties. Temperature receptors signal that passes to the visual cortex for processing. sense heat and cold. ( Fig. 10.10 ) 35. Th e amount of light entering the eye is altered by changing the size 19. Nociceptors are free nerve endings that respond to chemical, me- of the pupil. chanical, or thermal stimuli. Th eir activation is perceived as pain 36. Light waves are focused by the lens, whose shape is adjusted by con- and itch. tracting or relaxing the ciliary muscle . ( Fig. 10.27 ) 20. Some responses to irritants, such as the withdrawal refl ex, are pro- 37. Light is converted into electrical energy by the photoreceptors of tective spinal refl exes . the retina. Signals pass through bipolar neurons to ganglion cells, 21. Fast pain is transmitted rapidly by small, myelinated fi bers. Slow whose axons form the optic nerve. (Fig. 10.29 ) pain is carried by small, unmyelinated fi bers. Pain may be modu- 38. Th e fovea has the most acute vision because it has the smallest related either by descending pathways from the brain or by gating ceptive fi elds. mechanisms in the spinal cord. ( Fig. 10.11 , Tbl. 10.3 ) 39. Rods are responsible for monochromatic nighttime vision. Cones 22. Referred pain from internal organs occurs when multiple pri- are responsible for high-acuity vision and color vision during the mary sensory neurons converge onto a single ascending tract. daytime. ( Fig. 10.30 ) ( Fig. 10.12 ) 40. Light-sensitive visual pigments in photoreceptors convert light Chemoreception: Smell and Taste energy into a change in membrane potential. The visual pigment in rods is rhodopsin. Cones have three diff erent visual pigments. 23. Chemoreception is divided into the special senses of smell (olfac- ( Fig. 10.31 ) tion ) and taste ( gustation ). 41. Rhodopsin is composed of opsin and retinal. In the absence of light, 24. Olfactory sensory neurons in the nasal cavity are bipolar neurons retinal binds snugly to opsin. (Fig. 10.32 ) whose pathways project directly to the olfactory cortex. (Fig. 10.13 ) 42. When light bleaches rhodopsin, retinal is released and transducin 25. Odorant receptors are G protein–coupled membrane proteins. begins a second-messenger cascade that hyperpolarizes the rod and 26. Taste is a combination of fi ve sensations: sweet, sour, salty, bitter, releases less glutamate onto the bipolar neurons. and umami . 43. Signals pass from photoreceptors through bipolar neurons to 27. Taste cells are non-neural cells with membrane channels or recep- ganglion cells, with modulation by horizontal and amacrine cells. tors that interact with taste ligands. Th is interaction creates an in- ( Fig. 10.33 ) 2+ tracellular Ca signal that ultimately activates the primary sensory 44. Ganglion cells called M cells convey information about movement. neuron. ( Fig. 10.14 ) Ganglionic P cells transmit signals that pertain to the form and texture of objects in the visual fi eld. The Ear: Hearing 45. Information from one side of the visual fi eld is processed on the op- 28. Hearing is our perception of the energy carried by sound waves. posite side of the brain. Objects must be seen by both eyes to appear Sound transduction turns air waves into mechanical vibrations, three-dimensional. ( Fig. 10.34 )

391 Sensory Physiology

Questions

Level One Reviewing Facts and Terms 19. Th e parts of the vestibular apparatus that tell our brain about our movements through space are the , which sense rota- 1. What is the role of the aff erent division of the nervous system? tion, and the organs, which respond to linear forces. 2. D e fi ne proprioception. 20. List the following structures in the sequence in which a beam of 3. What are the common elements of all sensory pathways? light entering the eye will encounter them: (a) aqueous humor, 4. List and briefl y describe the four major types of somatic receptors (b) cornea, (c) lens, (d) pupil, (e) retina. based on the type of stimulus to which they are most sensitive. 21. The three primary colors of vision are , , 5. Th e receptors of each primary sensory neuron pick up information and . White light containing these colors stimulates from a specifi c area, known as the . photoreceptors called . Lack of the ability to distinguish 6. Match the brain area with the sensory information processed there: some colors is called . 22. List six types of cells found in the retina, and briefl y describe their (a) sounds 1. midbrain functions. (b) odors 2. cerebrum (c) visual information 3. medulla Level Two Reviewing Concepts (d) taste 4. cerebellum 23. Compare and contrast the following: (e) equilibrium 5. none of the above (a) the special senses with the somatic senses (b) diff erent types of touch receptors with respect to structure, size, 7. Th e conversion of stimulus energy into a change in membrane po- and location tential is called . Th e form of energy to which a receptor (c) transmission of sharp localized pain with transmission of responds is called its . Th e minimum stimulus required dull and diffuse pain (include the particular fiber types in- to activate a receptor is known as the . volved as well as the presence or absence of myelin in your 8. When a sensory receptor membrane depolarizes (or hyperpolarizes in a discussion) few cases), the change in membrane potential is called the (d) the forms of hearing loss potential. Is this a graded potential or an all-or-none potential? (e) convergence of retinal neurons with convergence of primary so- 9. Explain what is meant by adequate stimulus to a receptor. matic sensory neurons 10. Th e organization of sensory regions in the of the brain 24. Draw three touch receptors having overlapping receptive fields preserves the topographical organization of receptors on the skin, (see Fig. 10.2 ) and number the fields 1–3. Draw a primary and eye, or other regions. However, there are exceptions to this rule. In secondary sensory neuron for each receptor so that they have which two senses does the brain rely on the timing of receptor acti- separate ascending pathways to the cortex. Use the information in vation to determine the location of the initial stimulus? your drawing to answer this question: How many diff erent regions 11. What is lateral inhibition? of the skin can the brain distinguish using input from these three 12. D e fi ne tonic receptors and list some examples. Defi ne phasic recep- receptors? tors and give some examples. Which type adapts? 25. Describe the neural pathways that link pain with emotional distress, 13. Heart pain perceived as coming from the neck and down the left nausea, and vomiting. arm is an example of pain. 26. Trace the neural pathways involved in olfaction. What is Golf? 14. What are the fi ve basic tastes? What is the adaptive signifi cance of 27. Compare the current models of signal transduction in taste buds for each taste sensation? salty/sour ligands and sweet/bitter/umami ligands. 15. The unit of sound wave measurement is , which is a 28. Put the following structures in the order in which a sound wave measure of the frequency of sound waves per second. Th e loudness, would encounter them: (a) pinna, (b) cochlear duct, (c) stapes, or intensity, of a sound is a function of the of the sound (d) ion channels, (e) oval window, (f) hair cells/stereocilia, (g) tym- waves and is measured in . Th e range of hearing for the panic membrane, (h) incus, (i) vestibular duct, (j) malleus average human ear is from to [units], with 29. Sketch the structures and receptors of the vestibular apparatus for the most acute hearing in the range of to equilibrium. Label the components. Briefl y describe how they func- [units]. tion to notify the brain of movement. 16. Which structure of the inner ear codes sound for pitch? Defi ne spa- 30. Explain how accommodation by the eye occurs. What is the loss of tial coding. accommodation called? 17. Loud noises cause action potentials to: (choose all correct answers) 31. List four common visual problems, and explain how they occur. (a) fi re more frequently. 32. Explain how the intensity and duration of a stimulus are coded so (b) have higher amplitudes. that the stimulus can be interpreted by the brain. (Remember, ac- (c) have longer refractory periods. tion potentials are all-or-none phenomena.) 18. Once sound waves have been transformed into electrical signals in 33. Make a table of the special senses. In the fi rst row, write these stim- the cochlea, sensory neurons transfer information to the uli: sound, standing on the deck of a rocking boat, light, a taste, an , from which collaterals then take the information to aroma. In row 2, describe the location of the receptor for each sense. the and . Th e main auditory pathway syn- In row 3, describe the structure or properties of each receptor. In a apses in the and before fi nally projecting fi nal row, name the cranial nerve(s) that convey(s) each sensation to to the in the . the brain.

392 Sensory Physiology

34. Map the following terms related to vision. Add terms if you wish. 38. A clinician shines a light into a patient’s left eye, and neither pupil constricts. Shining the light into the right eye elicits a normal con- Map 1 sensual refl ex. What problem in the refl ex pathway could explain • accommodation refl ex • depth of fi eld • lens these observations? • binocular vision • fi eld of vision • macula An optometrist wishes to examine a patient’s retina. Which of the • blind spot • focal point • optic chiasm 39. following classes of drugs might dilate the pupil? Explain why you • ciliary muscle • fovea • optic disk did or did not select each choice. • cornea • iris • optic nerve • cranial nerve III • lateral geniculate • phototransduction (a) a sympathomimetic [mimicus , imitate] • pupillary refl ex • visual cortex • zonules (b) a muscarinic antagonist • retina • visual fi eld (c) a cholinergic agonist (d) an anticholinesterase Map 2: the retina (e) a nicotinic agonist • amacrine cells • ganglion cells • pigment epithelium 40. Th e iris of the eye has two sets of antagonistic muscles, one for dila- • bipolar cells • horizontal cells • retinal tion and one for constriction. One set of muscles is radial (radiating • bleaching • melanin • rhodopsin from the center of the pupil), and the other set is circular. Draw an • cGMP • melanopsin • rods iris and pupil, and arrange the muscles so that contraction of one set • cones • opsin • transducin causes pupillary constriction and contraction of the other set causes dilation. 41. As people age, their ability to see at night decreases. What changes 10 Level Three Problem Solving in the retina might explain this? 35. You are prodding your blindfolded lab partner’s arm with two needle probes (with her permission). Sometimes she can tell you are using Level Four Quantitative Problems two probes. But when you probe less sensitive areas, she thinks there 42. Th e relationship between focal length (F) of a lens, object distance is just one probe. Which sense are you testing? Which receptors are (P), and image distance or focal point (Q) is 1/F = 1/P + 1/Q. being stimulated? Explain why she sometimes feels only one probe. Assume the distance from lens to retina is 20 mm. 36. Consuming alcohol depresses the nervous system and vestibular ap- (a) For a distant object, P = i n fi nity ( q) and 1/ q = 0. If Pavi sees paratus. In a sobriety check, police offi cers use this information to a distant object in focus, what is the focal length of her lens in determine if an individual is intoxicated. What kinds of tests can meters? you suggest that would show evidence of this inhibition? (b) If the object moves to 1 foot in front of Pavi’s lens and the 37. O ft en, children are brought to medical attention because of speech lens does not change shape, what is the image distance diffi culties. If you were a clinician, which sense would you test fi rst (1 in = 2.54 cm)? What must happen to Pavi’s lens for the closer in such patients, and why? image to come into focus?

Answers

7 . Th e adaptive advantage of a spinal refl ex is a rapid reaction. Answers to Concept Check Questions 8. b, a, c (see Tbl. 10.3 ). 9 . Th ere are many examples, including receptors for taste and touch. 1. Myelinated axons have a faster conduction velocity than unmyelinated axons. 10. Olfactory sensory neuron (primary neuron) → cranial nerve I → 2 . Th e pinna funnels sound into the ear canal. secondary neuron in olfactory bulb → olfactory tract → olfactory cortex in temporal lobe. 3. Muscle length/tension, proprioception = mechanoreception. Pres- sure, inflation, distension = mechanoreception. Osmolarity = 11. If you need help, use Figure 10.13 as the basic pattern for creating mechanoreception. Temperature = thermoreception. Oxygen, glu- this map. cose, pH = chemoreception. 12. The knobby terminals of olfactory sensory neurons function as 4 . K + and Cl - channels in neurons A and C are probably opening dendrites. and causing hyperpolarization. 13. Olfactory neurons are bipolar neurons. 5. Sensory neurons signal intensity of a stimulus by the rate at which 14. Umami is associated with ingestion of the amino acid glutamate. they fi re action potentials. 15. Presynaptic taste cell → primary sensory neuron through cranial 6. Irritant receptors warn the body of danger. If possible, the body nerves VII, IX, or X → medulla (synapse with secondary neuron) → should respond in some way that stops the harmful stimulus. thalamus → gustatory cortex in parietal lobe. Th erefore, it is important that signals continue as long as the stim- 16. A kilohertz is 1000 Hz, which means 1000 waves per second. ulus is present, meaning the receptors should be tonic rather than phasic.

393 Sensory Physiology

17. Endolymph has high [K +] and low [Na+] so the electrochemical 31. In both the retina and skin, the fi nest discrimination occurs in the gradient favors K + movement into the cell. region with the smallest visual or receptive fi elds. 18. Use Figures 10.15, 10.17, and 10.21 to create your map. 32. Damage to the macula, which surrounds the fovea, results in vi- 19. Somatosensory information projects to the hemisphere of the brain sion loss in the central portion of the visual fi eld. Peripheral vision opposite to the side of the body on which the signal originates. Th e remains unaff ected. location of sound is coded by the time a stimulus arrives in each 33. Our dark vision is in black and white because only rods (black and hemisphere, so a signal to both hemispheres is necessary. white vision), not cones (color vision), are sensitive enough to be 20. A cochlear implant would not help people with nerve deafness or stimulated by such low levels of light. conductive hearing loss. It can help only those people with sensori- 34. Use the information in Figures 10.30 and 10.32 to create your neural hearing loss. map. 21. K + entry into hair cells causes depolarization. 22. When fl uid builds up in the middle ear, the eardrum is unable to Answers to Figure and Graph Questions move freely and cannot transmit sound through the bones of the middle ear as effi ciently. Figure 10.3: Th e olfactory and some equilibrium pathways do not 23. When a dancer spots, the endolymph in the ampulla moves with synapse in the thalamus. each head rotation but then stops as the dancer holds the head Figure 10.8: Sensations aff ected would be contralateral pain and still. Th is results in less inertia than if the head were continuously temperature, and ipsilateral proprioception. turning. Figure 10.12: His heart 24. Th e aqueous humor supports the cornea and lens. It also brings nu- Figure 10.13: Multiple neurons synapsing on a single neuron is an trients to and removes wastes from the epithelial layer of the cor- example of convergence. nea, which has no blood supply. Figure 10.16: Graph (1) shows 20 Hz waves (5 waves in the 0.25-sec 25. (a) Th e sensory pathway from one eye diverges to activate motor interval shown means 20 waves in 1 minute). Graph (2) shows 32 pathways for both pupils. (b) Th e aff erent path and its integration Hz waves. Th e waves in (1) have the lower pitch because they have must be functioning because there is an appropriate response on the lower frequency. the right side. Th e motor (eff erent) path to the left eye must not be functioning. Figure 10.25: Th e right eye is shown in this photograph. 26. antagonistic Figure 10.29: Six rods converge on the ganglion cell. 27. A more curved cornea causes light rays to converge more sharply. Figure 10.31 : The pigment in red cones absorbs light over the Th is causes the focal point to fall in front of the retina, so the per- broadest spectrum, and blue cones absorb over the narrowest son will be myopic. range. At 500 nm, the pigments in blue and green cones absorb light equally. 28. (a) Image distance gets longer. (b) Focal length must decrease, * * which is accomplished by the lens becoming more rounded. Figure 10.32 : (10,000 CNG channels 24 cGMP/channel) 1 transducin/6 cGMP * 1 rhodopsin/800 transducin * 1 photon/rho- 29. (a) Convex lenses focus a beam of light, and concave lenses scatter a dopsin = 50 photons needed beam of light passing through them. (b) In myopia, the focal point lies in front of the retina so a concave corrective lens increases the focal length and moves the focal point onto the retina. In hyperopia, the focal point lies behind the retina so a convex corrective lens shortens the focal length. Th is moves the focal point onto the retina. 30. Th e tapetum lucidum refl ects light, which enhances the amount of light hitting the photoreceptors.

394 Sensory Physiology

Answers to Review Questions

Level One Reviewing Facts and Terms two-point discrimination is better. Regions with more convergence have less acute vision or poor two-point discrimination. 1. Carry information from sensory receptors to the CNS. 24. Seven distinct areas: 1, 2, 3, 1+2, 1+3, 2+3, and 1+2+3. 2. The ability to tell where our body is in space and to sense the relative locations of different body parts. 25. Ascending pathways for pain go to the limbic system (emotional distress) and hypothalamus (nausea and vomiting). 3. A sensor and a sensory neuron. Could be one cell or two. 26. Olfactory receptors—olfactory bulb—secondary sensory neuron—higher- 4. Mechanoreceptors—pressure, sound, stretch, etc. Chemoreceptors—specific order neurons—olfactory cortex, with parallel pathways to amygdala and chemicals. Photoreceptors–photons of light. Thermoreceptors—heat and hippocampus. G :G protein of olfactory receptors. cold. olf 27. Bitter, sweet, and umami: membrane receptors on type II receptor cells, with 5. sensory field different G protein–linked receptors and signal transduction pathways for 6. (a) 3, (b) 2, (c) 1, 2, (d) 2, 3, (e) 4 each ligand. Salt ions (Na + ) apparently enter type 1 support cells through 7. transduction; adequate stimulus; threshold ion channels. H+ may enter presynaptic cells through channels or bind to a 8. Receptor potentials are graded potentials. membrane receptor. 9. Adequate stimulus—form of energy to which a receptor is most sensitive. 28. (a), (g), (j), (h), (c), (e), (i), (b), (f), (d) 10. cortex . Exceptions—olfaction and hearing. 29. See Figure 10.22 . 11. Sensory neurons surrounding a sensory field are inhibited, which enhances 30. The lens changes shape due to contraction relaxation of the ciliary muscles. > 10 contrast between the stimulus and surrounding areas. Loss of this reflex—presbyopia. 12. Tonic receptors, such as for heat, adapt slowly and respond to stimuli that 31. Presbyopia—loss of accommodation due to stiffening of the lens with age. need to be constantly monitored. Phasic receptors adapt rapidly and stop Myopia or near-sightedness—longer-than-normal distance between lens responding unless the stimulus changes. An example is smell. and retina; hyperopia or far-sightedness—shorter-than-normal distance. 13. referred Color-blindness—defective cones. 32. Intensity—action potential frequency. Duration—duration of a train of ac- 14. Sweet and umami indicate nutritious foods, and bitter may contain toxins. + + tion potentials. Salty (Na ) and sour (H ) ions are related to body osmolarity and pH, 33. See Table 10.1 and the section for each special sense. respectively. 34. Start with Figure 10.25 and the basic components of vision. Work in details 15. Sound waves per second— hertz (Hz) . Loudness—a function of the wave am- and related terms from the text. plitude and measured in decibels (dB) . Range of hearing: 20–20,000 Hz. Most acute hearing: 1000–3000 Hz . Level Three Problem Solving 16. Basilar membrane. Spatial coding—association of wave frequencies with different areas of the membrane. 35. Testing touch-pressure, mediated through free nerve endings and Merkel receptors. Feeling only one probe means both needles are within the same 17. (a) receptive field. 18. Signals from cochlea to medulla, with collaterals to reticular formation and 36. Walk a straight line, stand on one leg with the eyes closed, count backward cerebellum. Synapses in midbrain and thalamus before projecting to auditory by 3s. cortex in the cerebrum . 37. Test hearing first. If children cannot hear well, they cannot imitate speech. 19. Semicircular canals—rotation; otolith organs —linear forces. 38. Absence of the consensual reflex upon stimulating the left eye suggests damage 20. (b), (a), (d), (c), (e) to the left retina and/or to the left optic nerve. 21. Red, blue, and green; cones ; color-blindness 39. To dilate: a sympathetic agonist (a) or something that blocks muscarinic 22. Rods and cones (photoreceptors), bipolar cells, ganglion cells, horizontal receptors (b). To constrict: a cholinergic agonist (c), a nicotinic agonist (e), cells, and amacrine cells. Photoreceptors transduce light energy. Remaining or an anticholinesterase (d), which prevents breakdown of ACh. cells carry out signal processing. 40. Circular muscles form a ring on the inner part of the iris, surrounding the pupil. When these muscles contract, the pupil gets smaller. The radial muscles extend from the outer edge of the iris to the circular muscles. When Level Two Reviewing Concepts the radial muscles contract, they pull on the relaxed circular muscles and 23. (a) Special senses have receptors localized in the head. Somatic senses have expand the diameter of the pupil (dilation). receptors located all over the body. (b) See Figure 10.10 . (c) Sharp pain— 41. Loss of rods explains loss of night vision. small, myelinated Ad fibers. Dull pain—small, unmyelinated C fibers. (d) Conductive loss: sound cannot be transmitted through the external Quantitative Problems or middle ear. Sensorineural loss: inner ear is damaged. Central hearing Level Four loss: auditory pathways are damaged. (e) Minimal convergence of retinal 42. (a) 0.02 m (b) 1 0.02 m = 1 0.3048 + 1/Q. Q = 21.4 mm, so lens must > > neurons in the fovea results in the sharpest vision. Minimal convergence become rounder to make F smaller. of primary somatic sensory neurons creates smaller receptive fields, and

Photo Credits

CO: Dr. David Furness/Wellcome Images 10.25b: Webvision, John Moran Eye Center, University of Utah 10.14: Todd Derksen 10.27: Gene Chutka/iStockphoto.com.

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