Sensory Systems - Occidental College · Sensory cells respond to stimuli because they possess stimulus-gated ion channels in their membranes. The sensory stimu-lus causes these ion

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55Sensory Systems

Concept Outline

55.1 Animals employ a wide variety of sensoryreceptors.

Categories of Sensory Receptors and Their Actions.Sensory receptors can be classified according to the type ofstimuli to which they can respond.

55.2 Mechanical and chemical receptors sense thebody’s condition.

Detecting Temperature and Pressure. Receptorswithin the skin respond to touch, pressure, pain, heat andcold.Sensing Muscle Contraction and Blood Pressure. Amuscle spindle responds to stretching of the muscle;receptors in arteries monitor changes in blood pressure.Sensing Taste, Smell, and Body Position. Receptorsthat respond to chemicals produce sensations of taste andsmell. Hair cells send nerve impulses when they are bent.

55.3 Auditory receptors detect pressure waves in the air.

The Ears and Hearing. Sound causes vibrations in theear that bend hair cell processes, initiating a nerve impulse.Sonar. Bats orient themselves in space by emitting soundsand detecting the time required for the sounds to bounceoff objects and return to their ears.

55.4 Optic receptors detect light over a broad range ofwavelengths.

Evolution of the Eye. True image-forming eyes evolvedindependently in several phyla.Vertebrate Photoreceptors. Light causes a pigmentmolecule in a rod or cone cell to dissociate; this “bleaching”reaction activates the photoreceptor.Visual Processing in the Vertebrate Retina. Actionpotentials travel from the retina of the eyes to the brain forvisual perception.

55.5 Some vertebrates use heat, electricity, ormagnetism for orientation.

Diversity of Sensory Experiences. Special receptorscan detect heat, electrical currents, and magneticfields.

All input from sensory neurons to the central nervoussystem arrives in the same form, as action potentials

propagated by afferent (inward-conducting) sensory neu-rons. Different sensory neurons project to different brainregions, and so are associated with different sensory modal-ities (figure 55.1). The intensity of the sensation dependson the frequency of action potentials conducted by the sen-sory neuron. A sunset, a symphony, and a searing pain aredistinguished by the brain only in terms of the identity ofthe sensory neuron carrying the action potentials and thefrequency of these impulses. Thus, if the auditory nerve isartificially stimulated, the brain perceives the stimulation assound. But if the optic nerve is artificially stimulated in ex-actly the same manner and degree, the brain perceives aflash of light.

FIGURE 55.1Photoreceptors in the vertebrate eye. Rods, the broad, tubularcells, allow black-and-white vision, while cones, the short, taperedcells, are responsible for color vision. Not all vertebrates haveboth types of receptors.

tems provide only enough information to determine thatan object is present; they call the animal’s attention tothe object but give little or no indication of where it is lo-cated. Other sensory systems provide information aboutthe location of an object, permitting the animal to movetoward it. Still other sensory systems enable the brain toconstruct a three-dimensional image of an object and itssurroundings.

Interoceptors sense stimuli that arise from within thebody. These internal receptors detect stimuli related to

1104 Part XIV Regulating the Animal Body

Categories of Sensory Receptorsand Their ActionsSensory information is conveyed to the CNS and perceivedin a four-step process (figure 55.2): (1) stimulation—a physi-cal stimulus impinges on a sensory neuron or an accessorystructure; (2) transduction—the stimulus energy is used toproduce electrochemical nerve impulses in the dendrites ofthe sensory neuron; (3) transmission—the axon of the sen-sory neuron conducts action potentials along an afferentpathway to the CNS; and (4) interpretation—the brain cre-ates a sensory perception from the electrochemical eventsproduced by afferent stimulation. We actually see (as wellas hear, touch, taste, and smell) with our brains, not withour sense organs.

Sensory receptors differ with respect to the nature ofthe environmental stimulus that best activates their sen-sory dendrites. Broadly speaking, we can recognize threeclasses of environmental stimuli: (1) mechanical forces,which stimulate mechanoreceptors; (2) chemicals,which stimulate chemoreceptors; and (3) electromag-netic and thermal energy, which stimulate a variety of re-ceptors, including the photoreceptors of the eyes(table 55.1).

The simplest sensory receptors are free nerve endings thatrespond to bending or stretching of the sensory neuronmembrane, to changes in temperature, or to chemicals likeoxygen in the extracellular fluid. Other sensory receptorsare more complex, involving the association of the sensoryneurons with specialized epithelial cells.

Sensing the External and Internal Environments

Exteroceptors are receptors that sense stimuli that arisein the external environment. Almost all of a vertebrate’sexterior senses evolved in water before vertebrates in-vaded the land. Consequently, many senses of terrestrialvertebrates emphasize stimuli that travel well in water,using receptors that have been retained in the transitionfrom the sea to the land. Mammalian hearing, for exam-ple, converts an airborne stimulus into a waterborne one,using receptors similar to those that originally evolved inthe water. A few vertebrate sensory systems that functionwell in the water, such as the electrical organs of fish, can-not function in the air and are not found among terrestrialvertebrates. On the other hand, some land-dwellers havesensory systems, such as infrared receptors, that could notfunction in the sea.

Sensory systems can provide several levels of informa-tion about the external environment. Some sensory sys-

55.1 Animals employ a wide variety of sensory receptors.

Stimulus

Transduction of stimulusinto electrochemical impulse

in sensory receptor

Transmission ofaction potential

in sensory neuron

Interpretation of stimulusin central nervous

system

FIGURE 55.2The path of sensory information. Sensory stimuli must betransduced into electrochemical nerve impulses that areconducted to the brain for interpretation.

Table 55.1 Classes of Environmental Stimuli

Mechanical ElectromagneticForces Chemicals Energy

Pressure Taste LightGravity Smell HeatInertia Humidity ElectricitySound MagnetismTouchVibration

muscle length and tension, limb position, pain, bloodchemistry, blood volume and pressure, and body tempera-ture. Many of these receptors are simpler than those thatmonitor the external environment and are believed to bear

a closer resemblance to primitive sensory receptors. In therest of this chapter, we will consider the different types ofinteroceptors and exteroceptors according to the kind ofstimulus each is specialized to detect (table 55.2).

Chapter 55 Sensory Systems 1105

Table 55.2 Sensory Transduction Among the Vertebrates

TransductionStimulus Receptor Location Structure Process

INTEROCEPTION

Temperature

Touch

Vibration

Pain

Muscle stretch

Blood pressure

EXTEROCEPTION

Gravity

Motion

Taste

Smell

Hearing

Vision

Heat

Electricity

Magnetism

Heat receptors andcold receptors

Meissner’s corpuscles,Merkel cellsPacinian corpuscles

Nociceptors

Stretch receptors

Baroreceptors

Statocysts

Cupula

Lateral line organ

Taste bud cells

Olfactory neurons

Organ of Corti

Rod and cone cells

Pit organ

Ampullae of Lorenzini

Unknown

Skin, hypothalamus

Surface of skin

Deep within skin

Throughout body

Within muscles

Arterial branches

Outer chambers ofinner earSemicircular canals of inner earWithin grooves onbody surface of fishMouth; skin of fish

Nasal passages

Cochlea of inner ear

Retina of eye

Face of snake

Within skin of fishes

Unknown

Free nerve ending

Nerve ending within elasticcapsuleNerve ending within elasticcapsuleFree nerve ending

Spiral nerve endings wrappedaround muscle spindleNerve endings over thin part ofarterial wall

Otoliths and hair cells

Collection of hair cells

Collection of hair cells

Chemoreceptors: epithelialcells with microvilliChemoreceptors: ciliated neu-ronsHair cells between basilar and tectorial membranesArray of photosensitive pig-mentsTemperature receptors in twochambers

Closed vesicles withasymmetrical ion channeldistributionUnknown

Temperature change opens/closes ion channels in membraneRapid or extended change inpressure deforms membraneSevere change in pressuredeforms membraneChemicals or changes in pressure or temperatureopen/close ion channels inmembraneStretch of spindle deformsmembraneStretch of arterial walldeforms membrane

Otoliths deform hair cells

Fluid movement deformshair cellsFluid movement deformshair cellsChemicals bind to membranereceptorsChemicals bind to membranereceptorsSound waves in fluid deformmembranesLight initiates process that closes ion channelsReceptors compare temperatures of surface andinterior chambersElectrical field alters ion dis-tribution on membranes

Deflection at magnetic fieldinitiates nerve impulses?

Table 55.2 Sensory Transduction Among the Vertebrates

Sensory Transduction

Sensory cells respond to stimuli because they possess stimulus-gated ion channels in their membranes. The sensory stimu-lus causes these ion channels to open or close, dependingon the sensory system involved. In doing so, a sensorystimulus produces a change in the membrane potential ofthe receptor cell. In most cases, the sensory stimulus pro-duces a depolarization of the receptor cell, analogous tothe excitatory postsynaptic potential (EPSP, described inchapter 54) produced in a postsynaptic cell in response toneurotransmitter. A depolarization that occurs in a sensoryreceptor upon stimulation is referred to as a receptor po-tential (figure 55.3a).

Like an EPSP, a receptor potential is graded: the largerthe sensory stimulus, the greater the degree of depolariza-tion. Receptor potentials also decrease in size (decrement)with distance from their source. This prevents small, irrele-vant stimuli from reaching the cell body of the sensory

neuron. Once a threshold level of depolarization is reached,the receptor potential stimulates the production of actionpotentials that are conducted by a sensory axon into theCNS (figure 55.3b). The greater the sensory stimulus, thegreater the depolarization of the receptor potential and thehigher the frequency of action potentials. There is gener-ally a logarithmic relationship between stimulus intensityand action potential frequency—a sensory stimulus that isten times greater than another stimulus will produce actionpotentials at twice the frequency of the other stimulus.This allows the brain to interpret the incoming signals asindicating a sensory stimulus of a particular strength.

Sensory receptors transduce stimuli in the internal orexternal environment into graded depolarizations,which stimulates the production of action potentials.Sensory receptors may be classified on the basis of thetype of stimulus energy to which they respond.


Na+

Stimulus

Na+

Stimulus-gatedchannels

(a) (b)

Voltage-gatedchannels

Na+

Na+

Na+

Time

Receptor potentialStimulusapplied

20

–10

–40

–70

Vol

tage

(m

V)

Time

Train of action potentialsStimulusapplied

20

–10

–40

–70

Vol

tage

(m

V)

Threshold

FIGURE 55.3Events in sensory transduction. (a) Depolarization of a free nerve ending leads to a receptor potential that spreads by local current flowto the axon. (b) Action potentials are produced in the axon in response to a sufficiently large receptor potential.

Detecting Temperature andPressureWhile the receptors of the skin, called the cutaneous re-ceptors, are classified as interoceptors, they in fact respondto stimuli at the border between the external and internalenvironments. These receptors serve as good examples ofthe specialization of receptor structure and function, re-sponding to heat, cold, pain, touch, and pressure.

The skin contains two populations of thermoreceptors,which are naked dendritic endings of sensory neurons thatare sensitive to changes in temperature. Cold receptors arestimulated by a fall in temperature and inhibited by warm-ing, while warm receptors are stimulated by a rise in temper-ature and inhibited by cooling. Cold receptors are locatedimmediately below the epidermis, while warm receptors arelocated slightly deeper, in the dermis. Thermoreceptors arealso found within the hypothalamus of the brain, wherethey monitor the temperature of the circulating blood andthus provide the CNS with information on the body’s in-ternal (core) temperature.

A stimulus that causes or is about to cause tissue damageis perceived as pain. The receptors that transmit impulsesthat are perceived by the brain as pain are called nocicep-tors. They consist of free nerve endings located through-out the body, especially near surfaces where damage is mostlikely to occur. Different nociceptors may respond to ex-tremes in temperature, very intense mechanical stimula-tion, or specific chemicals in the extracellular fluid, includ-ing some that are released by injured cells. The thresholdsof these sensory cells vary; some nociceptors are sensitiveonly to actual tissue damage, while others respond beforedamage has occurred.

Several types of mechanoreceptors are present in theskin, some in the dermis and others in the underlying sub-cutaneous tissue (figure 55.4). Morphologically special-ized receptors that respond to fine touch are most con-centrated on areas such as the fingertips and face. Theyare used to localize cutaneous stimuli very precisely andcan be either phasic (intermittently activated) or tonic(continuously activated). The phasic receptors includehair follicle receptors and Meissner’s corpuscles, which are pre-sent on body surfaces that do not contain hair, such as thefingers, palms, and nipples. The tonic receptors consist ofRuffini endings in the dermis and touch dome endings(Merkel cells) located near the surface of the skin. Thesereceptors monitor the duration of a touch and the extentto which it is applied.

Deep below the skin in the subcutaneous tissue lie pha-sic, pressure-sensitive receptors called Pacinian corpus-cles. Each of these receptors consists of the end of an af-ferent axon, surrounded by a capsule of alternating layersof connective tissue cells and extracellular fluid. Whensustained pressure is applied to the corpuscle, the elasticcapsule absorbs much of the pressure and the axon ceasesto produce impulses. Pacinian corpuscles thus monitoronly the onset and removal of pressure, as may occur re-peatedly when something that vibrates is placed againstthe skin.

Different cutaneous receptors respond to touch,pressure, heat, cold, and pain. Some of these receptorsare naked dendrites of sensory neurons, while othershave supporting cells that modify the activities of theirsensory dendrites.


55.2 Mechanical and chemical receptors sense the body’s condition.

Meissner's corpuscle

Organ of RuffiniPacinian corpuscle

Hair folliclereceptors

Free nerveending

Merkel cell

FIGURE 55.4Sensory receptors in human skin.Cutaneous receptors may be free nerveendings or sensory dendrites in associationwith other supporting structures.

Sensing Muscle Contraction andBlood PressureMechanoreceptors contain sensory cells with ion channelsthat are sensitive to a mechanical force applied to the mem-brane. These channels open in response to mechanical dis-tortion of the membrane, initiating a depolarization (recep-tor potential) that causes the sensory neuron to generateaction potentials.

Muscle Length and Tension

Buried within the skeletal muscles of all vertebrates exceptthe bony fishes are muscle spindles, sensory stretch recep-tors that lie in parallel with the rest of the fibers in themuscle (figure 55.5). Each spindle consists of several thinmuscle fibers wrapped together and innervated by a sensoryneuron, which becomes activated when the muscle, andtherefore the spindle, is stretched. Muscle spindles, to-gether with other receptors in tendons and joints, areknown as proprioceptors, which are sensory receptors thatprovide information about the relative position or move-ment of the animal’s body parts. The sensory neurons con-duct action potentials into the spinal cord, where theysynapse with somatic motor neurons that innervate themuscle. This pathway constitutes the muscle stretch reflex,including the knee-jerk reflex, previously discussed in chap-ter 54.

When a muscle contracts, it exerts tension on the ten-dons attached to it. The Golgi tendon organs, another type

of proprioceptor, monitor this tension; if it becomes toohigh, they elicit a reflex that inhibits the motor neurons in-nervating the muscle. This reflex helps to ensure that mus-cles do not contract so strongly that they damage the ten-dons to which they are attached.

Blood Pressure

Blood pressure is monitored at two main sites in the body.One is the carotid sinus, an enlargement of the left and rightinternal carotid arteries, which supply blood to the brain.The other is the aortic arch, the portion of the aorta veryclose to its emergence from the heart. The walls of theblood vessels at both sites contain a highly branched net-work of afferent neurons called baroreceptors, which de-tect tension in the walls. When the blood pressure de-creases, the frequency of impulses produced by thebaroreceptors decreases. The CNS responds to this re-duced input by stimulating the sympathetic division of theautonomic nervous system, causing an increase in heart rateand vasoconstriction. Both effects help to raise the bloodpressure, thus maintaining homeostasis. A rise in bloodpressure, conversely, reduces sympathetic activity and stim-ulates the parasympathetic division, slowing the heart andlowering the blood pressure.

Mechanical distortion of the plasma membrane ofmechanoreceptors produces nerve impulses that serveto monitor muscle length from skeletal muscle spindlesand to monitor blood pressure from baroreceptorswithin arteries.


Nerve

Spindle sheath

Skeletal muscleSpecialized muscle fibers(spindle fibers)

Motorneurons

Sensoryneurons

FIGURE 55.5A muscle spindle is a stretch receptor embedded within skeletal muscle. Stretching of the muscle elongates the spindle fibers andstimulates the sensory dendritic endings wrapped around them. This causes the sensory neurons to send impulses to the CNS, where theysynapse with motor neurons.

Sensing Taste, Smell,and Body PositionSome sensory cells, called chemore-ceptors, contain membrane proteinsthat can bind to particular chemicals inthe extracellular fluid. In response tothis chemical interaction, the mem-brane of the sensory neuron becomesdepolarized, leading to the productionof action potentials. Chemoreceptorsare used in the senses of taste and smelland are also important in monitoringthe chemical composition of the bloodand cerebrospinal fluid.

Taste

Taste buds—collections of chemosensi-tive epithelial cells associated with af-ferent neurons—mediate the sense oftaste in vertebrates. In a fish, the tastebuds are scattered over the surface ofthe body. These are the most sensitivevertebrate chemoreceptors known.They are particularly sensitive to aminoacids; a catfish, for example, can distin-guish between two different aminoacids at a concentration of less than 100parts per billion (1 g in 10,000 L ofwater)! The ability to taste the sur-rounding water is very important tobottom-feeding fish, enabling them tosense the presence of food in an often murky environment.

The taste buds of all terrestrial vertebrates are locatedin the epithelium of the tongue and oral cavity, withinraised areas called papillae (figure 55.6). Humans have fourkinds of taste buds—salty, sweet, sour, and bitter. Thesalty taste is produced by the effects of sodium (Na+) andthe sour taste by the effects of hydrogen (H+). Organicmolecules that produce the sweet and bitter tastes, such assugars and quinine, respectively, are varied in structure.Taste buds that respond best to specific tastes are concen-trated in specific regions of the tongue: sweet at the tip,sour at the sides, bitter at the back, and salty over most ofthe tongue’s surface. Our complex perception of taste isthe result of different combinations of impulses in thesensory neurons from these four kinds of taste buds, to-gether with information related to smell. The effect ofsmell on the sense of taste can easily be demonstrated byeating an onion with the nose open and then eating itwith the nose plugged.

Like vertebrates, many arthropods also have tastechemoreceptors. For example, flies, because of their modeof searching for food, have taste receptors in sensory hairslocated on their feet. The sensory hairs contain differentchemoreceptors that are able to detect sugars, salts, and

other molecules (figure 55.7). They can detect a wide vari-ety of tastes by the integration of stimuli from thesechemoreceptors. If they step on potential food, the pro-boscis (the tubular feeding apparatus) extends to feed.


Taste papilla

Taste bud

Tastepore

Supportcell

Nerve fiber

(b)

Receptorcell withmicrovilli

Bitter

(a)

(c)

Sour

Salty

Sweet

(d)

FIGURE 55.6Taste. (a) Human beings have four kinds of taste buds (bitter, sour, salty, and sweet),located on different regions of the tongue. (b) Groups of taste buds are typically organizedin sensory projections called papillae. (c) Individual taste buds are bulb-shaped collectionsof chemosensitive receptors that open out into the mouth through a pore. (d)Photomicrograph of taste buds in papillae.

Signals to brain

Different chemoreceptors

Sensory hair on foot

Pore

Proboscis

FIGURE 55.7Many insects taste with their feet. In the blowfly shown here,chemoreceptors extend into the sensory hairs on the foot. Eachdifferent chemoreceptor detects a different type of food molecule.When the fly steps in a food substance, it can taste the differentfood molecules and extend its proboscis for feeding.

Tastebud

Smell

In terrestrial vertebrates, the sense of smell, or olfaction,involves chemoreceptors located in the upper portion ofthe nasal passages (figure 55.8). These receptors are bipolarneurons whose dendrites end in tassels of cilia that projectinto the nasal mucosa, and whose axon projects directlyinto the cerebral cortex. A terrestrial vertebrate uses itssense of smell in much the same way that a fish uses itssense of taste—to sample the chemical environment aroundit. Because terrestrial vertebrates are surrounded by airrather than water, their sense of smell has become special-ized to detect airborne particles (but these particles mustfirst dissolve in extracellular fluid before they can activatethe olfactory receptors). The sense of smell can be ex-tremely acute in many mammals, so much so that a singleodorant molecule may be all that is needed to excite a givenreceptor.

Although humans can detect only four modalities oftaste, they can discern thousands of different smells. Newresearch suggests that there may be as many as a thousanddifferent genes coding for different receptor proteins forsmell. The particular set of olfactory neurons that respondto a given odor might serve as a “fingerprint” the brain canuse to identify the odor.

Internal Chemoreceptors

Sensory receptors within the body detect a variety ofchemical characteristics of the blood or fluids derivedfrom the blood, including cerebrospinal fluid. Includedamong these receptors are the peripheral chemoreceptors ofthe aortic and carotid bodies, which are sensitive primar-ily to plasma pH, and the central chemoreceptors in themedulla oblongata of the brain, which are sensitive to thepH of cerebrospinal fluid. These receptors were dis-cussed together with the regulation of breathing in chap-ter 53. When the breathing rate is too low, the concen-tration of plasma CO2 increases, producing morecarbonic acid and causing a fall in the blood pH. Thecarbon dioxide can also enter the cerebrospinal fluid andcause a lowering of the pH, thereby stimulating the cen-tral chemoreceptors. This chemoreceptor stimulation in-directly affects the respiratory control center of the brainstem, which increases the breathing rate. The aortic bod-ies can also respond to a lowering of blood oxygen con-centrations, but this effect is normally not significant un-less a person goes to a high altitude.


Olfactory nerve

Nasal passage

Olfactory mucosa Axon

Cilia

Basal cell Support cell

Receptor cell

To olfactorynerve

To olfactorynerve

FIGURE 55.8Smell. Humans detect smells by means of olfactory neurons located in the lining of the nasal passages. The axons of these neuronstransmit impulses directly to the brain via the olfactory nerve. Basal cells regenerate new olfactory neurons to replace dead or damagedcells. Olfactory neurons typically live about one month.

The Lateral Line System

The lateral line system provides fish with a sense of “dis-tant touch,” enabling them to sense objects that reflectpressure waves and low-frequency vibrations. This enablesa fish to detect prey, for example, and to swim in synchronywith the rest of its school. It also enables a blind cave fishto sense its environment by monitoring changes in the pat-terns of water flow past the lateral line receptors. The lat-eral line system is found in amphibian larvae, but is lost atmetamorphosis and is not present in any terrestrial verte-brate. The sense provided by the lateral line system supple-ments the fish’s sense of hearing, which is performed by adifferent sensory structure. The structures and mechanismsinvolved in hearing will be described in a later section.

The lateral line system consists of sensory structureswithin a longitudinal canal in the fish’s skin that extends

along each side of the body and within several canals in thehead (figure 55.9a). The sensory structures are known ashair cells because they have hairlike processes at their sur-face that project into a gelatinous membrane called a cupula(Latin, “little cup”). The hair cells are innervated by sen-sory neurons that transmit impulses to the brain.

Hair cells have several hairlike processes of approxi-mately the same length, called stereocilia, and one longerprocess called a kinocilium (figure 55.9b). Vibrations carriedthrough the fish’s environment produce movements of thecupula, which cause the hairs to bend. When the stereociliabend in the direction of the kinocilium, the associated sen-sory neurons are stimulated and generate a receptor poten-tial. As a result, the frequency of action potentials producedby the sensory neuron is increased. If the stereocilia arebent in the opposite direction, on the other hand, the activ-ity of the sensory neuron is inhibited.


Lateral line

Lateral linescales

Canal Lateral lineorgan

Nerve

Cupula

Cilia

Hair cell

Afferent axons

Opening

Sensory nerves

Stimulation ofsensory neuron

Stereocilia

Inhibition Excitation

Kinocilium(not present inmammalian cochlea)

Hair cell

FIGURE 55.9The lateral line system. (a) This system consists of canalsrunning the length of the fish’s body beneath the surface of theskin. Within these canals are sensory structures containing haircells with cilia that project into a gelatinous cupula. Pressurewaves traveling through the water in the canals deflect the ciliaand depolarize the sensory neurons associated with the hair cells.(b) Hair cells are mechanoreceptors with hairlike cilia that projectinto a gelatinous membrane. The hair cells of the lateral linesystem (and the membranous labyrinth of the vertebrate inner ear)have a number of smaller cilia called stereocilia and one largerkinocilium. When the cilia bend in the direction of thekinocilium, the hair cell releases a chemical transmitter thatdepolarizes the associated sensory neuron. Bending of the cilia inthe opposite direction has an inhibitory effect.(a)

(b)

Gravity and Angular Acceleration

Most invertebrates can orient themselveswith respect to gravity due to a sensorystructure called a statocyst. Statocystsgenerally consist of ciliated hair cellswith the cilia embedded in a gelatinousmembrane containing crystals of calciumcarbonate. These “stones,” or statoliths,increase the mass of the gelatinous mem-brane so that it can bend the cilia whenthe animal’s position changes. If the ani-mal tilts to the right, for example, thestatolith membrane will bend the cilia onthe right side and activate associated sen-sory neurons.

A similar structure is found in themembranous labyrinth of the inner earof vertebrates. The labyrinth is a systemof fluid-filled membranous chambersand tubes that constitute the organs ofequilibrium and hearing in vertebrates.This membranous labyrinth is sur-rounded by bone and perilymph, whichis similar in ionic content to interstitialfluid. Inside, the chambers and tubes arefilled with endolymph fluid, which issimilar in ionic content to intracellularfluid. Though intricate, the entire struc-ture is very small; in a human, it is aboutthe size of a pea.

The receptors for gravity in verte-brates consist of two chambers of themembranous labyrinth called the utricleand saccule (figure 55.10). Within thesestructures are hair cells with stereociliaand a kinocilium, similar to those in thelateral line system of fish. The hairlikeprocesses are embedded within a gelati-nous membrane containing calcium car-bonate crystals; this is known as anotolith membrane, because of its locationin the inner ear (oto is derived from theGreek word for ear). Because the otolithorgan is oriented differently in the utri-cle and saccule, the utricle is more sensi-


Cochlea

Cochlear duct

Cochlear nerveUtricle (horizontalacceleration)

Semicircular canals

Saccule (verticalacceleration)

(a)

Gelatinousmatrix

Otoliths

Haircells

Supportingcells

(b)

FIGURE 55.10The structure of the utricle and saccule. (a) The relative positions of the utricle andsaccule within the membranous labyrinth of the human inner ear. (b) Enlargement of asection of the utricle or saccule showing the otoliths embedded in the gelatinous matrixthat covers the hair cells.

tive to horizontal acceleration (as in a moving car) and thesaccule to vertical acceleration (as in an elevator). In bothcases, the acceleration causes the stereocilia to bend andconsequently produces action potentials in an associatedsensory neuron.

The membranous labyrinth of the utricle and saccule iscontinuous with three semicircular canals, oriented in dif-ferent planes so that angular acceleration in any directioncan be detected (figure 55.11). At the ends of the canals areswollen chambers called ampullae, into which protrude thecilia of another group of hair cells. The tips of the cilia areembedded within a sail-like wedge of gelatinous materialcalled a cupula (similar to the cupula of the fish lateral linesystem) that protrudes into the endolymph fluid of eachsemicircular canal.

When the head rotates, the fluid inside the semicircularcanals pushes against the cupula and causes the cilia tobend. This bending either depolarizes or hyperpolarizesthe hair cells, depending on the direction in which the ciliaare bent. This is similar to the way the lateral line system

works in a fish: if the stereocilia are bent in the direction ofthe kinocilium, a depolarization (receptor potential) is pro-duced, which stimulates the production of action potentialsin associated sensory neurons.

The saccule, utricle, and semicircular canals are collec-tively referred to as the vestibular apparatus. While the sac-cule and utricle provide a sense of linear acceleration, thesemicircular canals provide a sense of angular acceleration.The brain uses information that comes from the vestibularapparatus about the body’s position in space to maintainbalance and equilibrium.

Receptors that sense chemicals originating outside thebody are responsible for the senses of odor, smell, andtaste. Internal chemoreceptors help to monitorchemicals produced within the body and are needed forthe regulation of breathing. Hair cells in the lateral lineorgan of fishes detect water movements, and hair cellsin the vestibular apparatus of terrestrial vertebratesprovide a sense of acceleration.


Semicircular canals

Vestibularnerves

Ampullae

Vestibule

Flow of endolymph

Cupula

Stimulation

Cilia of haircells

Hair cells

Supportingcell

Vestibularnerve

Endolymph

Direction of body movement

FIGURE 55.11The structure of the semicircular canals. (a) The position of the semicircular canals in relation to the rest of the inner ear. (b) Enlargement of a section of one ampulla, showing how hair cell cilia insert into the cupula. (c) Angular acceleration in the plane of thesemicircular canal causes bending of the cupula, thereby stimulating the hair cells.

(a)

(b)

(c)

The Ears and HearingFish detect vibrational pressure wavesin water by means of their lateral linesystem. Terrestrial vertebrates detectsimilar vibrational pressure waves inair by means of similar hair cellmechanoreceptors in the inner ear.Hearing actually works better in waterthan in air because water transmitspressure waves more efficiently. De-spite this limitation, hearing is widelyused by terrestrial vertebrates to mon-itor their environments, communicatewith other members of the samespecies, and to detect possible sourcesof danger (figure 55.12). Auditorystimuli travel farther and morequickly than chemical ones, and audi-tory receptors provide better direc-tional information than do chemore-ceptors. Auditory stimuli alone,however, provide little informationabout distance.

Structure of the Ear

Fish use their lateral line system to de-tect water movements and vibrationsemanating from relatively nearby ob-jects, and their hearing system to detect vibrations thatoriginate from a greater distance. The hearing system offish consists of the otolith organs in the membranouslabyrinth (utricle and saccule) previously described, to-gether with a very small outpouching of the membranouslabyrinth called the lagena. Sound waves travel throughthe body of the fish as easily as through the surroundingwater, as the body is composed primarily of water. There-fore, an object of different density is needed in order forthe sound to be detected. This function is served by theotolith (calcium carbonate crystals) in many fish. In cat-fish, minnows, and suckers, however, this function isserved by an air-filled swim bladder that vibrates with thesound. A chain of small bones, Weberian ossicles, thentransmits the vibrations to the saccule in some of thesefish.

In the ears of terrestrial vertebrates, vibrations in airmay be channeled through an ear canal to the eardrum, ortympanic membrane. These structures are part of the outerear. Vibrations of the tympanic membrane cause movementof three small bones (ossicles)—the malleus (hammer), incus(anvil), and stapes (stirrup)—that are located in a bony cav-ity known as the middle ear (figure 55.13). These middle earossicles are analogous to the Weberian ossicles in fish. The

middle ear is connected to the throat by the Eustachian tube,which equalizes the air pressure between the middle earand the external environment. The “ear popping” you mayhave experienced when flying in an airplane or driving on amountain is caused by pressure equalization between thetwo sides of the eardrum.

The stapes vibrates against a flexible membrane, theoval window, which leads into the inner ear. Because theoval window is smaller in diameter than the tympanicmembrane, vibrations against it produce more force perunit area, transmitted into the inner ear. The inner earconsists of the cochlea (Latin for “snail”), a bony struc-ture containing part of the membranous labyrinth calledthe cochlear duct. The cochlear duct is located in thecenter of the cochlea; the area above the cochlear duct isthe vestibular canal, and the area below is the tympaniccanal. All three chambers are filled with fluid, as previ-ously described. The oval window opens to the uppervestibular canal, so that when the stapes causes it to vi-brate, it produces pressure waves of fluid. These pressurewaves travel down to the tympanic canal, pushing an-other flexible membrane, the round window, that trans-mits the pressure back into the middle ear cavity (see fig-ure 55.13).



FIGURE 55.12Kangaroo rats have specialized ears. Kangaroo rats (Dipodomys) are unique in having anenlarged tympanic membrane (eardrum), a lengthened and freely rotating malleus (earbone), and an increased volume of air-filled chambers in the middle ear. These and otherspecializations result in increased sensitivity to sound, especially to low-frequency sounds.Experiments have shown that the kangaroo rat’s ears are adapted to nocturnal life and allowthem to hear the low-frequency sounds of their predators, such as an owl’s wingbeats or asidewinder rattlesnake’s scales rubbing against the ground. Also, the ears seem to beadapted to the poor sound-carrying quality of dry, desert air.

Transduction in the Cochlea

As the pressure waves produced by vibrations of the ovalwindow are transmitted through the cochlea to the roundwindow, they cause the cochlear duct to vibrate. The bot-tom of the cochlear duct, called the basilar membrane, isquite flexible and vibrates in response to these pressurewaves. The surface of the basilar membrane contains sen-sory hair cells, similar to those of the vestibular apparatusand lateral line system but lacking a kinocilium. The ciliafrom the hair cells project into an overhanging gelatinousmembrane, the tectorial membrane. This sensory apparatus,

consisting of the basilar membrane, hair cells with associ-ated sensory neurons, and tectorial membrane, is known asthe organ of Corti.

As the basilar membrane vibrates, the cilia of the haircells bend in response to the movement of the basilar mem-brane relative to the tectorial membrane. As in the lateralline organs and the vestibular apparatus, the bending ofthese cilia depolarizes the hair cells. The hair cells, in turn,stimulate the production of action potentials in sensoryneurons that project to the brain, where they are inter-preted as sound.


Vestibularcanal

Roundwindow

Tympanicmembrane

Malleus StapesMiddleear

Innerear

Outer ear Semicircularcanals

Auditory nerveto brain

Ovalwindow

Skull

Auditorynerve

To auditorynerveSensory

neurons

Haircells

Organ of Corti

Incus

Bone

Auditorycanal

Cochlearduct

Cochlea

Eustachian tube

(a) (b)

(d)

(c)

Eustachian tube

Pinna

Tympaniccanal

Basilarmembrane

Tectorialmembrane

FIGURE 55.13Structure of the human ear. (a) Sound waves passing through the ear canal produce vibrations of the tympanic membrane, which causemovement of the (b) middle ear ossicles (the malleus, incus, and stapes) against an inner membrane called the oval window. Vibration ofthe oval window sets up pressure waves that (c and d) travel through the fluid in the vestibular and tympanic canals of the cochlea.

Frequency Localization in theCochlea

The basilar membrane of the cochleaconsists of elastic fibers of varyinglength and stiffness, like the strings of amusical instrument, embedded in agelatinous material. At the base of thecochlea (near the oval window), thefibers of the basilar membrane areshort and stiff. At the far end of thecochlea (the apex), the fibers are 5times longer and 100 times more flexi-ble. Therefore, the resonant frequencyof the basilar membrane is higher atthe base than the apex; the base re-sponds to higher pitches, the apex tolower.

When a wave of sound energy en-ters the cochlea from the oval window,it initiates a traveling up-and-downmotion of the basilar membrane.However, this wave imparts most of its energy to that part of the basilar mem-brane with a resonant frequency nearthe frequency of the sound wave, re-sulting in a maximum deflection of thebasilar membrane at that point (figure55.14). As a result, the hair cell depo-larization is greatest in that region,and the afferent axons from that re-gion are stimulated to produce actionpotentials more than those from otherregions. When these action potentialsarrive in the brain, they are inter-preted as representing a sound of thatparticular frequency, or pitch.

The flexibility of the basilar mem-brane limits the frequency range ofhuman hearing to between approxi-mately 20 and 20,000 cycles per sec-ond (hertz) in children. Our ability tohear high-pitched sounds decays pro-gressively throughout middle age.Other vertebrates can detect sounds atfrequencies lower than 20 hertz andmuch higher than 20,000 hertz. Dogs, for example, candetect sounds at 40,000 hertz, enabling them to hearhigh-pitched dog whistles that seem silent to a humanlistener.

Hair cells are also innervated by efferent axons from thebrain, and impulses in those axons can make hair cells lesssensitive. This central control of receptor sensitivity can in-crease an individual’s ability to concentrate on a particularauditory signal (for example, a single voice) in the midst ofbackground noise, which is effectively “tuned out” by theefferent axons.

The middle ear ossicles vibrate in response to soundwaves, creating fluid vibrations within the inner ear.This causes the hair cells to bend, transducing thesound into action potentials. The pitch of a sound isdetermined by which hair cells (and thus which sensoryneurons) are activated by the vibration of the basilarmembrane.


Vestibularcanal

Roundwindow

Tympanicmembrane

Malleus Incus Stapes Ovalwindow

High frequency (22,000Hz)

Medium frequency (2000Hz)

Low frequency (500Hz)

Cochlearduct

Tympaniccanal

ApexBase

Basilarmembrane

FIGURE 55.14Frequency localization in the cochlea. The cochlea is shown unwound, so that thelength of the basilar membrane can be seen. The fibers within the basilar membranevibrate in response to different frequencies of sound, related to the pitch of the sound.Thus, regions of the basilar membrane show maximum vibrations in response to differentsound frequencies. Notice that low-frequency (pitch) sounds vibrate the basilar membranemore toward the apex, while high frequencies cause vibrations more toward the base.

SonarBecause terrestrial vertebrates have two ears located on op-posite sides of the head, the information provided by hear-ing can be used by the CNS to determine direction of asound source with some precision. Sound sources vary instrength, however, and sounds are attenuated (weakened)to varying degrees by the presence of objects in the envi-ronment. For these reasons, auditory sensors do not pro-vide a reliable measure of distance.

A few groups of mammals that live and obtain their foodin dark environments have circumvented the limitations ofdarkness. A bat flying in a completely dark room easilyavoids objects that are placed in its path—even a wire lessthan a millimeter in diameter (figure 55.15). Shrews use asimilar form of “lightless vision” beneath the ground, as dowhales and dolphins beneath the sea. All of these mammalsperceive distance by means of sonar. They emit sounds andthen determine the time it takes these sounds to reach an

object and return to the animal. This process is calledecholocation. A bat, for example, produces clicks that last 2to 3 milliseconds and are repeated several hundred timesper second. The three-dimensional imaging achieved withsuch an auditory sonar system is quite sophisticated.

Being able to “see in the dark” has opened a new ecolog-ical niche to bats, one largely closed to birds because birdsmust rely on vision. There are no truly nocturnal birds;even owls rely on vision to hunt, and do not fly on darknights. Because bats are able to be active and efficient intotal darkness, they are one of the most numerous andwidespread of all orders of mammals.

Some mammals emit sounds and then determine thetime it takes for the sound to return, using the methodof sonar to locate themselves and other objects in atotally dark environment by the characteristics of theecho. Bats are the most adept at this echolocation.


FIGURE 55.15Sonar. As it flies, a bat emits high-frequency “chirps” and listens for the return of the chirps after they are reflected by objects such asmoths. By timing how long it takes for a chirp to return, the bat can locate its prey and catch it even in total darkness.

Evolution of the EyeVision begins with the capture of light energy by pho-toreceptors. Because light travels in a straight line and ar-rives virtually instantaneously, visual information can beused to determine both the direction and the distance ofan object. No other stimulus provides as much detailedinformation.

Many invertebrates have simple visual systems withphotoreceptors clustered in an eyespot. Simple eyespotscan be made sensitive to the direction of a light source bythe addition of a pigment layer which shades one side ofthe eye. Flatworms have a screening pigmented layer onthe inner and back sides of both eyespots allowing stimula-tion of the photoreceptor cells only by light from the frontof the animal (figure 55.16). The flatworm will turn andswim in the direction in which the photoreceptor cells arethe least stimulated. Although an eyespot can perceive thedirection of light, it cannot be used to construct a visualimage. The members of four phyla—annelids, mollusks,arthropods, and chordates—have evolved well-developed,image-forming eyes. True image-forming eyes in thesephyla, though strikingly similar in structure, are believedto have evolved independently (figure 55.17). Interest-ingly, the photoreceptors in all of them use the same light-capturing molecule, suggesting that not many alternativemolecules are able to play this role.

Structure of the Vertebrate Eye

The eye of a human is typical of the vertebrate eye (figure55.18). The “white of the eye” is the sclera, formed oftough connective tissue. Light enters the eye through atransparent cornea, which begins to focus the light. This

occurs because light is refracted (bent) when it travels intoa medium of different density. The colored portion of theeye is the iris; contraction of the iris muscles in bright lightdecreases the size of its opening, the pupil. Light passesthrough the pupil to the lens, a transparent structure thatcompletes the focusing of the light onto the retina at theback of the eye. The lens is attached by the suspensory liga-ment to the ciliary muscles.

The shape of the lens is influenced by the amount oftension in the suspensory ligament, which surrounds the


55.4 Optic receptors detect light over a broad range of wavelengths.

PhotoreceptorsEyespot

Light

Pigment layer

Flatworm will turnaway from light

FIGURE 55.16Simple eyespots in the flatworm. Eyespots will detect thedirection of light because a pigmented layer on one side of theeyespot screens out light coming from the back of the animal.Light is thus the strongest coming from the front of the animal;flatworms will respond by turning away from the light.

Lenses

Lens

LensOpticnerve

Eyemuscles

OpticnerveOptic

nerve

Retinularcell

Retina

Retina

VertebrateMolluskInsect

FIGURE 55.17Eyes in three phyla of animals. Although they are superficially similar, these eyes differ greatly in structure and are not homologous.Each has evolved separately and, despite the apparent structural complexity, has done so from simpler structures.

lens and attaches it to the circular cil-iary muscle. When the ciliary musclecontracts, it puts slack in the suspen-sory ligament and the lens becomesmore rounded and powerful. This is re-quired for close vision; in far vision, theciliary muscles relax, moving away fromthe lens and tightening the suspensoryligament. The lens thus becomes moreflattened and less powerful, keeping theimage focused on the retina. Peoplewho are nearsighted or farsighted donot properly focus the image on theretina (figure 55.19). Interestingly, thelens of an amphibian or a fish does notchange shape; these animals insteadfocus images by moving their lens inand out, just as you would do to focus acamera.

Annelids, mollusks, arthropods,and vertebrates have independentlyevolved image-forming eyes. Thevertebrate eye admits light througha pupil and then focuses this lightby means of an adjustable lens ontothe retina at the back of the eye.


Retina

Optic nerve

Fovea

Vein

ArteryIris

Cornea

Lens

Lens

Ciliary muscle

Suspensoryligament

Suspensoryligament under iris

Sclera

Ciliary muscle

Iris

Cornea

Pupil

FIGURE 55.18Structure of the human eye. The transparent cornea and lens focus light onto the retinaat the back of the eye, which contains the rods and cones. The center of each eye’s visualfield is focused on the fovea. Focusing is accomplished by contraction and relaxation of theciliary muscle, which adjusts the curvature of the lens.

Suspensoryligaments

Iris

Normal distant vision

Normal near vision

Nearsighted

Nearsighted, corrected

Farsighted

Farsighted, corrected

Retina

Lens

FIGURE 55.19Focusing the human eye. (a) In people with normal vision, the image remains focused on the retina in both near and far vision because ofchanges produced in the curvature of the lens. When a person with normal vision stands 20 feet or more from an object, the lens is in itsleast convex form and the image is focused on the retina. (b) In nearsighted people, the image comes to a focus in front of the retina andthe image thus appears blurred. (c) In farsighted people, the focus of the image would be behind the retina because the distance from thelens to the retina is too short.

(a) (b) (c)

Vertebrate Photoreceptors The vertebrate retina contains two kinds of photorecep-tors, called rods and cones (figure 55.20). Rods are respon-sible for black-and-white vision when the illumination isdim, while cones are responsible for high visual acuity(sharpness) and color vision. Humans have about 100 mil-lion rods and 3 million cones in each retina. Most of thecones are located in the central region of the retina knownas the fovea, where the eye forms its sharpest image. Rodsare almost completely absent from the fovea.

Rods and cones have the same basic cellular structure.An inner segment rich in mitochondria contains numerousvesicles filled with neurotransmitter molecules. It is con-nected by a narrow stalk to the outer segment, which ispacked with hundreds of flattened discs stacked on top ofone another. The light-capturing molecules, or photopig-ments, are located on the membranes of these discs.

In rods, the photopigment is called rhodopsin. It con-sists of the protein opsin bound to a molecule of cis-retinal(figure 55.21), which is derived from carotene, a photosyn-thetic pigment in plants. The photopigments of cones,called photopsins, are structurally very similar torhodopsin. Humans have three kinds of cones, each ofwhich possesses a photopsin consisting of cis-retinal boundto a protein with a slightly different amino acid sequence.These differences shift the absorption maximum—the regionof the electromagnetic spectrum that is best absorbed bythe pigment—(figure 55.22). The absorption maximum ofthe cis-retinal in rhodopsin is 500 nanometers (nm); theabsorption maxima of the three kinds of cone photopsins,in contrast, are 455 nm (blue-absorbing), 530 nm (green-absorbing), and 625 nm (red-absorbing). These differencesin the light-absorbing properties of the photopsins are re-sponsible for the different color sensitivities of the threekinds of cones, which are often referred to as simply blue,green, and red cones.

Most vertebrates, particularly those that are diurnal (ac-tive during the day), have color vision, as do many insects.Indeed, honeybees can see light in the near-ultravioletrange, which is invisible to the human eye. Color vision re-quires the presence of more than one photopigment in dif-ferent receptor cells, but not all animals with color visionhave the three-cone system characteristic of humans andother primates. Fish, turtles, and birds, for example, havefour or five kinds of cones; the “extra” cones enable theseanimals to see near-ultraviolet light. Many mammals (suchas squirrels), on the other hand, have only two types ofcones.

The retina is made up of three layers of cells (figure55.23): the layer closest to the external surface of the eye-ball consists of the rods and cones, the next layer containsbipolar cells, and the layer closest to the cavity of the eye iscomposed of ganglion cells. Thus, light must first passthrough the ganglion cells and bipolar cells in order toreach the photoreceptors! The rods and cones synapse

with the bipolar cells, and the bipolar cells synapse withthe ganglion cells, which transmit impulses to the brain viathe optic nerve. The flow of sensory information in theretina is therefore opposite to the path of light through theretina. It should also be noted that the retina contains twoadditional types of neurons, horizontal cells and amacrinecells. Stimulation of horizontal cells by photoreceptors at


Outersegment

Connectingcilium

Innersegment

Mitochondria

Nucleus

Synapticterminal

Rod Cone

Pigmentdiscs

FIGURE 55.20Rods and cones. Thepigment-containing outersegment in each of thesecells is separated from therest of the cell by a partitionthrough which there is onlya narrow passage, theconnective cilium.

All-trans isomer

11-cis isomer

Light

FIGURE 55.21Absorption of light. When light is absorbed by a photopigment,the 11-cis isomer of retinal, the light-capturing portion of thepigment undergoes a change in shape: the linear end of themolecule (at the right in this diagram) rotates about a double bond(indicated here in red). The resulting isomer is referred to as all-trans retinal. This change in retinal’s shape initiates a chain ofevents that leads to hyperpolarization of the photoreceptor.

the center of a spot of light on the retina can inhibit theresponse of photoreceptors peripheral to the center. Thislateral inhibition enhances contrast and sharpens theimage.

Sensory Transduction in Photoreceptors

The transduction of light energy into nerve impulses fol-lows a sequence that is the inverse of the usual way thatsensory stimuli are detected. This is because, in the dark,the photoreceptors release an inhibitory neurotransmitterthat hyperpolarizes the bipolar neurons. Thus inhibited,the bipolar neurons do not release excitatory neurotrans-mitter to the ganglion cells. Light inhibits the photorecep-tors from releasing their inhibitory neurotransmitter, andby this means, stimulates the bipolar cells and thus the gan-glion cells, which transmit action potentials to the brain.

A rod or cone contains many Na+ channels in the plasmamembrane of its outer segment, and in the dark, many ofthese channels are open. As a consequence, Na+ continu-ously diffuses into the outer segment and across the narrowstalk to the inner segment. This flow of Na+ that occurs inthe absence of light is called the dark current, and it causesthe membrane of a photoreceptor to be somewhat depolar-ized in the dark. In the light, the Na+ channels in the outersegment rapidly close, reducing the dark current and caus-ing the photoreceptor to hyperpolarize.

Researchers have discovered that cyclic guanosinemonophosphate (cGMP) is required to keep the Na+ chan-nels open, and that the channels will close if the cGMP isconverted into GMP. How does light cause this conversionand consequent closing of the Na+ channels? When a pho-topigment absorbs light, cis-retinal isomerizes and dissoci-ates from opsin in what is known as the bleaching reaction.As a result of this dissociation, the opsin protein changesshape. Each opsin is associated with over a hundred regula-tory G proteins (see chapters 7 and 54). When the opsinchanges shape, the G proteins dissociate, releasing subunitsthat activate hundreds of molecules of the enzyme phospho-diesterase. This enzyme converts cGMP to GMP, thus clos-ing the Na+ channels at a rate of about 1000 per second andinhibiting the dark current. The absorption of a single pho-ton of light can block the entry of more than a millionsodium ions, thereby causing the photoreceptor tohyperpolarize and release less inhibitory neuro-transmitters. Freed from inhibition, the bipo-

lar cells activate ganglion cells, which transmit action po-tentials to the brain.

Photoreceptor rods and cones contain thephotopigment cis-retinal, which dissociates in responseto light and indirectly activates bipolar neurons andthen ganglion cells.


0

25

50

75

100

400 500 600

Wavelength (nm)

Ligh

t abs

orpt

ion

(per

cent

of m

axim

um)

Green RedBlue

FIGURE 55.22Color vision. The absorption maximum of cis-retinal in therhodopsin of rods is 500 nanometers (nm). However, the “bluecones” have their maximum light absorption at 455 nm; the“green cones” at 530 nm, and the red cones at 625 nm. The brainperceives all other colors from the combined activities of thesethree cones’ systems.

FIGURE 55.23Structure of the retina. Note that the rods andcones are at the rear of the retina, not the front. Lightpasses through four other types of cells in the retina before it reaches the rodsand cones. Once the photoreceptors are activated, they stimulate bipolarcells, which in turn stimulate ganglion cells. The direction of nerve impulsesin the retina is thus opposite to the direction of light.

Light

Axons tooptic nerve

Bipolar cell Choroid

Horizontal cell

Amacrine cell

Rod

Cone

Ganglion cell

Visual Processing in the VertebrateRetinaAction potentials propagated along the axons of ganglioncells are relayed through structures called the lateralgeniculate nuclei of the thalamus and projected to the oc-cipital lobe of the cerebral cortex (figure 55.24). Therethe brain interprets this information as light in a specificregion of the eye’s receptive field. The pattern of activityamong the ganglion cells across the retina encodes apoint-to-point map of the receptive field, allowing theretina and brain to image objects in visual space. In addi-tion, the frequency of impulses in each ganglion cell pro-vides information about the light intensity at each point,while the relative activity of ganglion cells connected(through bipolar cells) with the three types of cones pro-vides color information.

The relationship between receptors, bipolar cells, andganglion cells varies in different parts of the retina. In thefovea, each cone makes a one-to-one connection with abipolar cell, and each bipolar cell synapses, in turn, withone ganglion cell. This point-to-point relationship is re-sponsible for the high acuity of foveal vision. Outside thefovea, many rods can converge on a single bipolar cell,and many bipolar cells can converge on a single ganglioncell. This convergence permits the summation of neuralactivity, making the area of the retina outside of the foveamore sensitive to dim light than the fovea, but at the ex-pense of acuity and color vision. This is why dim objects,such as faint stars at night, are best seen when you don’tlook directly at them. It has been said that we use the pe-riphery of the eye as a detector and the fovea as aninspector.

Color blindness is due to an inherited lack of one ormore types of cones. People with normal color vision aretrichromats; those with only two types of cones are dichro-mats. People with this condition may lack red cones (haveprotanopia), for example, and have difficulty distinguishingred from green. Men are far more likely to be color blindthan women, because the trait for color blindness is carriedon the X chromosome; men have only one X chromosomeper cell, whereas women have two X chromosomes and socan carry the trait in a recessive state.

Binocular Vision

Primates (including humans) and most predators havetwo eyes, one located on each side of the face. Whenboth eyes are trained on the same object, the image thateach sees is slightly different because each eye views theobject from a different angle. This slight displacement ofthe images (an effect called parallax) permits binocularvision, the ability to perceive three-dimensional imagesand to sense depth. Having their eyes facing forwardmaximizes the field of overlap in which this stereoscopicvision occurs.

In contrast, prey animals generally have eyes located tothe sides of the head, preventing binocular vision but en-larging the overall receptive field. Depth perception is lessimportant to prey than detection of potential enemiesfrom any quarter. The eyes of the American Woodcock,for example, are located at exactly opposite sides of itsskull so that it has a 360-degree field of view without turn-ing its head! Most birds have laterally placed eyes and, asan adaptation, have two foveas in each retina. One foveaprovides sharp frontal vision, like the single fovea in theretina of mammals, and the other fovea provides sharperlateral vision.

The axons of ganglion cells transmit action potentials tothe thalamus, which in turn relays visual information tothe occipital lobe of the brain. The fovea provides highvisual acuity, whereas the retina outside the foveaprovides high sensitivity to dim light. Binocular visionwith overlapping visual fields provides depthperception.


Occipital lobe of cerebrum(visual cortex)

Optic nerve

Optic chiasma

Optic tractBrain stem

Lateral geniculatenucleus

Occipital lobeof cerebrum(visual cortex)

Lens

Left eye

Retina

Opticnerve

Optictract

Lateralgeniculate

nucleus

Lens

Right eye

Retina

FIGURE 55.24The pathway of visual information. Action potentials in theoptic nerves are relayed from the retina to the lateral geniculatenuclei, and from there to the visual cortex of the occipital lobes.Notice that the medial fibers of the optic nerves cross to the otherside at the optic chiasm, so that each hemisphere of the cerebrumreceives input from both eyes.

Diversity of Sensory ExperiencesVision is the primary sense used by all vertebrates that livein a light-filled environment, but visible light is by nomeans the only part of the electromagnetic spectrum thatvertebrates use to sense their environment.

Heat

Electromagnetic radiation with wavelengths longer thanthose of visible light is too low in energy to be detected byphotoreceptors. Radiation from this infrared (“below red”)portion of the spectrum is what we normally think of as ra-diant heat. Heat is an extremely poor environmental stimu-lus in water because water has a high thermal capacity andreadily absorbs heat. Air, in contrast, has a low thermal ca-pacity, so heat in air is a potentially useful stimulus. How-ever, the only vertebrates known to have the ability to senseinfrared radiation are the snakes known as pit vipers.

The pit vipers possess a pair of heat-detecting pit organslocated on either side of the head between the eye and thenostril (figure 55.25). The pit organs permit a blindfoldedrattlesnake to accurately strike at a warm, dead rat. Each pitorgan is composed of two chambers separated by a mem-brane. The infrared radiation falls on the membrane andwarms it. Thermal receptors on the membrane are stimu-lated. The nature of the pit organ’s thermal receptor is notknown; it probably consists of temperature-sensitive neu-rons innervating the two chambers. The two pit organs ap-pear to provide stereoscopic information, in much the sameway that two eyes do. Indeed, the information transmittedfrom the pit organs is processed by the visual center of thesnake brain.

Electricity

While air does not readily conduct an electrical current,water is a good conductor. All aquatic animals generateelectrical currents from contractions of their muscles. Anumber of different groups of fishes can detect these elec-trical currents. The electrical fish even have the ability toproduce electrical discharges from specialized electrical or-gans. Electrical fish use these weak discharges to locatetheir prey and mates and to construct a three-dimensionalimage of their environment even in murky water.

The elasmobranchs (sharks, rays, and skates) have elec-troreceptors called the ampullae of Lorenzini. The recep-tor cells are located in sacs that open through jelly-filledcanals to pores on the body surface. The jelly is a verygood conductor, so a negative charge in the opening ofthe canal can depolarize the receptor at the base, causingthe release of neurotransmitter and increased activity ofsensory neurons. This allows sharks, for example, to de-tect the electrical fields generated by the muscle contrac-

tions of their prey. Although the ampullae of Lorenziniwere lost in the evolution of teleost fish (most of the bonyfish), electroreception reappeared in some groups ofteleost fish that use sensory structures analogous to theampullae of Lorenzini. Electroreceptors evolved yet an-other time, independently, in the duck-billed platypus, anegg-laying mammal. The receptors in its bill can detectthe electrical currents created by the contracting musclesof shrimp and fish, enabling the mammal to detect itsprey at night and in muddy water.

Magnetism

Eels, sharks, bees, and many birds appear to navigatealong the magnetic field lines of the earth. Even somebacteria use such forces to orient themselves. Birds keptin blind cages, with no visual cues to guide them, willpeck and attempt to move in the direction in which theywould normally migrate at the appropriate time of theyear. They will not do so, however, if the cage is shieldedfrom magnetic fields by steel. Indeed, if the magnetic fieldof a blind cage is deflected 120° clockwise by an artificialmagnet, a bird that normally orients to the north will ori-ent toward the east-southeast. There has been much spec-ulation about the nature of the magnetic receptors inthese vertebrates, but the mechanism is still very poorlyunderstood.

Pit vipers can locate warm prey by infrared radiation(heat), and many aquatic vertebrates can locate prey andascertain the contours of their environment by means ofelectroreceptors.


55.5 Some vertebrates use heat, electricity, or magnetism for orientation.

InnerchamberMembrane

Outerchamber

Pit

FIGURE 55.25“Seeing” heat. The depression between the nostril and the eye ofthis rattlesnake opens into the pit organ. In the cutaway portion ofthe diagram, you can see that the organ is composed of twochambers separated by a membrane. Snakes known as pit vipershave this unique ability to sense infrared radiation (heat).


Chapter 55 Summary Questions Media Resources

55.1 Animals employ a wide variety of sensory receptors.

• Mechanoreceptors, chemoreceptors, andphotoreceptors are responsive to different categoriesof sensory stimuli; interoceptors and exteroceptorsrespond to stimuli that originate in the internal andexternal environments, respectively.

1. Can you name a sensoryreceptor that does not produce a membrane depolarization?

www.mhhe.com/raven6e www.biocourse.com

• Muscle spindles respond to stretching of the skeletalmuscle.

• The sensory organs of taste are taste buds, scatteredover the surface of a fish’s body but located on thepapillae of the tongue in terrestrial vertebrates.

• Chemoreceptors in the aortic and carotid bodiessense the blood pH and oxygen levels, helping toregulate breathing.

• Hair cells in the membranous labyrinth of the innerear provide a sense of acceleration.

2. What mechanoreceptorsdetect muscle stretch and thetension on a tendon? 3. What structures in thevertebrate ear detect changes inthe body’s position with respectto gravity? What structuresdetect angular motion?

55.2 Mechanical and chemical receptors sense the body’s condition.

• In terrestrial vertebrates, sound waves causevibrations of ear membranes.

• Different pitches of sounds vibrate different regionsof the basilar membrane, and therefore stimulatedifferent hair cells.

• Bats and some other vertebrates use sonar to providea sense of “lightless vision.”

4. How are sound wavestransmitted and amplifiedthrough the middle ear? How is the pitch of the sounddetermined?


• A flexible lens focuses light onto the retina, whichcontains the photoreceptors.

• Light causes the photodissociation of the visualpigment, thereby blocking the dark current andhyperpolarizing the photoreceptor; this inverse effectstops the inhibitory effect of the photoreceptor andthereby activates the bipolar cells.

5. How does focusing in fishesand amphibians differ from thatin other vertebrates? 6. When a photoreceptorabsorbs light, what happens tothe Na+ channels in its outersegment?

55.4 Optic receptors detect light over a broad range of wavelengths.

• The pit organs of snakes allows them to detect theposition and movements of prey. Many aquaticvertebrates can detect electrical currents produced bymuscular contraction. Some vertebrates can orientthemselves using the earth’s magnetic field.

7. Why do rattlesnakes strike amoving lightbulb?8. How do sharks detect theirprey? Why don’t terrestrialvertebrates have this sense?

55.5 Some vertebrates use heat, electricity, or magnetism for orientation.

• Introduction to senseorgans

• Receptors andsensations

• Somatic senses

• Smell• Taste• Sense of balance• Sense of rotational

acceleration

• Sense of taste• Sense of smell• Equilibrium

• Art ActivityHuman ear anatomy

• Hearing

• Art ActivityHuman eye anatomy

• Vision

• Chemorecptors

Sensory Systems - Occidental College · Sensory cells respond to stimuli because they possess stimulus-gated ion channels in their membranes. The sensory stimu-lus causes these ion

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