PowerLecture: Chapter 14 Sensory Systems. Learning Objectives Describe the characteristics of a receptor and list the various types of receptors

PowerLecture:PowerLecture:Chapter 14Chapter 14

Sensory SystemsSensory Systems

Learning ObjectivesLearning Objectives

Describe the characteristics of a receptor Describe the characteristics of a receptor and list the various types of receptors.and list the various types of receptors.

Contrast mechanisms by which the Contrast mechanisms by which the chemical and the somatic senses work.chemical and the somatic senses work.

Understand how the senses of balance and Understand how the senses of balance and hearing function.hearing function.

Describe how the sense of vision has Describe how the sense of vision has evolved through time.evolved through time.

Learning Objectives (cont’d)Learning Objectives (cont’d)

Draw a medial section of the human eyeball Draw a medial section of the human eyeball through the optic nerve, identify each through the optic nerve, identify each structure, and tell the function of each.structure, and tell the function of each.

Identify some common disorders of the eye. Identify some common disorders of the eye.

Impacts/IssuesImpacts/Issues

Private EyesPrivate Eyes

Private EyesPrivate Eyes

Iris scanning is one of the newest Iris scanning is one of the newest security techniques.security techniques.

First, each person’s unique First, each person’s unique arrangement of smooth muscle fibers in arrangement of smooth muscle fibers in the iris of the eye must be recorded in the iris of the eye must be recorded in an electronic database.an electronic database.

Each time the person passes through Each time the person passes through a check point, a small camera looks at the iris a check point, a small camera looks at the iris and compares it with the database. and compares it with the database.

Usually, we use our eyes to see, but in this Usually, we use our eyes to see, but in this new technology, our eyes are seen. new technology, our eyes are seen.

How Would You Vote?How Would You Vote?To conduct an instant in-class survey using a classroom response To conduct an instant in-class survey using a classroom response system, access “JoinIn Clicker Content” from the PowerLecture main system, access “JoinIn Clicker Content” from the PowerLecture main menu. menu.

In which situations should individuals be required to In which situations should individuals be required to submit to iris scanning and registration?submit to iris scanning and registration? a. For any reason; it's not any different than other forms a. For any reason; it's not any different than other forms

of identification.of identification. b. In place of or to enhance government identification, b. In place of or to enhance government identification,

such as a driver's license or passport.such as a driver's license or passport. c. For employment at any company that chooses to c. For employment at any company that chooses to

require it.require it. d. It should never be required, it should only be used as a d. It should never be required, it should only be used as a

voluntary convenience, and then, strictly regulated. voluntary convenience, and then, strictly regulated.

Section 1Section 1

Sensory Receptors and Sensory Receptors and PathwaysPathways

Sensory Receptors and PathwaysSensory Receptors and Pathways

In a In a sensory systemsensory system, a stimulus activates , a stimulus activates a receptor, which transduces (converts) it to a receptor, which transduces (converts) it to an action potential that travels to the brain an action potential that travels to the brain where it triggers sensation or perception.where it triggers sensation or perception.

A A stimulusstimulus is any form of energy that activates is any form of energy that activates receptor endings of a sensory neuron.receptor endings of a sensory neuron.

SensationsSensations are conscious responses to the are conscious responses to the stimuli.stimuli.

PerceptionPerception is an understanding of what is an understanding of what sensations mean.sensations mean.

In-text Fig., p. 250

stimulusenergy

received

stimulus energyconverted to

action potential

brain response(sensation orperception)

Fig. 14.1, p. 251

axon endings of motorneuron terminating onthe same muscle

Stretched muscle stimulatesa stretch receptor (the ending of asensory neuron) that is adjacent to it.

Message travels from stimulatedsensory neuron to motor neuronand interneuron in spinal cord.

interneuron in spinal cordsensory neuron

Message is sent back tothe muscle, also to otherinterneurons in the brain.

motor neuron in spinal cord

muscle spindle

c d

b

e


Six major categories of sensory receptors.Six major categories of sensory receptors. MechanoreceptorsMechanoreceptors detect changes in detect changes in

pressure, position, or acceleration.pressure, position, or acceleration. ThermoreceptorsThermoreceptors detect heat or cold. detect heat or cold. NociceptorsNociceptors (pain receptors) detect tissue (pain receptors) detect tissue

damage.damage. ChemoreceptorsChemoreceptors detect ions or molecules. detect ions or molecules. OsmoreceptorsOsmoreceptors detect changes in solute detect changes in solute

concentration in surrounding fluid.concentration in surrounding fluid. PhotoreceptorsPhotoreceptors detect the energy of visible detect the energy of visible

light.light.

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All action potentials are the same; the brain All action potentials are the same; the brain determines the nature of a given stimulus determines the nature of a given stimulus based on which nerves are signaling, the based on which nerves are signaling, the frequency of the action potentials frequency of the action potentials generated, and the number of axons generated, and the number of axons responding.responding.

Specific sensory areas interpret action Specific sensory areas interpret action potentials in specific ways.potentials in specific ways.

Strong signals make receptors fire action Strong signals make receptors fire action potentials more often and longer.potentials more often and longer.


Stronger stimuli recruit more sensory receptors.Stronger stimuli recruit more sensory receptors.

Sensory adaptationSensory adaptation is the diminishing is the diminishing response to an ongoing stimulus.response to an ongoing stimulus.

Figure 14.1Figure 14.1

Section 2Section 2

Somatic SensationsSomatic Sensations


Somatic Somatic sensationssensations occur when occur when receptor signals receptor signals from body from body surfaces reach surfaces reach the the somatosensory somatosensory cortexcortex in the in the cerebrum.cerebrum.


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Receptors near the body surface sense Receptors near the body surface sense touch, pressure, and more.touch, pressure, and more.

Sensations of touch, pressure, cold, warmth, Sensations of touch, pressure, cold, warmth, and pain are discerned near the body surface and pain are discerned near the body surface by receptors whose numbers vary by body by receptors whose numbers vary by body region.region.

Free nerve endingsFree nerve endings are the simplest receptors. are the simplest receptors.• These are thinly myelinated or unmyelinated These are thinly myelinated or unmyelinated

dendrites of sensory neurons.dendrites of sensory neurons.• One type coils around hair follicles to detect One type coils around hair follicles to detect

movement; another detects chemicals.movement; another detects chemicals.


Encapsulated receptorsEncapsulated receptors are surrounded by a are surrounded by a capsule of epithelial or connective tissue.capsule of epithelial or connective tissue.

• Merkel’s discsMerkel’s discs adapt slowly and adapt slowly and

are important for steady touch.are important for steady touch.• Meissner’s corpusclesMeissner’s corpuscles respond respond

to light touching.to light touching.• Ruffini endingsRuffini endings are sensitive to are sensitive to

steady touch and pressure.steady touch and pressure.• The The Pacinian corpusclesPacinian corpuscles are are

sensitive to deep pressure and sensitive to deep pressure and

vibrations.vibrations.


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Fig. 14.4, p. 253

der

mis Ruffini endings

(pressure, touch)

free nerveendings(pain) hair

Meissner’scorpuscle(light touch)

Merkel’s discs(steady touch)

Merkel’s discs

Ruffiniendings

Meissner’scorpuscle

Paciniancorpuscle(deep pressure,vibrations)hair follicle

receptor(hair displacement)

epid

erm

issu

bcu

tan

eou

sla

yer

Pacinian corpuscle


Mechanoreceptors in skeletal muscle, joints, Mechanoreceptors in skeletal muscle, joints, tendons, ligaments, and skin are responsible tendons, ligaments, and skin are responsible for awareness of the body’s position and of its for awareness of the body’s position and of its limb movements.limb movements.

Pain is the perception of bodily injury.Pain is the perception of bodily injury. PainPain is the perception of injury to some region is the perception of injury to some region

of the body.of the body.


Nociceptors are subpopulations of free nerve Nociceptors are subpopulations of free nerve endings distributed throughout the skin endings distributed throughout the skin ((somatic painsomatic pain) and internal tissues () and internal tissues (visceral visceral painpain).).

• When cells are damaged, they release chemicals When cells are damaged, they release chemicals (bradykinins, histamine, and prostaglandins) to (bradykinins, histamine, and prostaglandins) to activate neighboring pain receptors.activate neighboring pain receptors.

• Pain receptors signal interneurons, which release Pain receptors signal interneurons, which release substance P.substance P.

• Substance P allows for natural opiates called Substance P allows for natural opiates called endorphins and enkephalins to be released to reduce endorphins and enkephalins to be released to reduce pain perception.pain perception.


Referred pain is a matter of perception.Referred pain is a matter of perception. Much visceral pain is Much visceral pain is referred painreferred pain; that is, ; that is,

it is felt at some distance from the real it is felt at some distance from the real stimulation point.stimulation point.

Phantom painPhantom pain is the sensation that amputees is the sensation that amputees feel when they sense the missing part as if it feel when they sense the missing part as if it were still there.were still there.

© 2007 Thomson Higher Education

lungs,diaphragm

heart

liver, gallbladderpancreas

small intestine

ovariescolon

appendix

urinary bladder

kidney

ureter

stomach

Fig. 14.5, p. 253

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Section 3Section 3

Taste and Smell: Taste and Smell:

Chemical SensesChemical Senses

Taste and Smell: Chemical Senses Taste and Smell: Chemical Senses

Taste and smell are chemical senses; they Taste and smell are chemical senses; they begin at chemoreceptors, the signals begin at chemoreceptors, the signals traveling to the brain where they are traveling to the brain where they are perceived, transmitted to the limbic system, perceived, transmitted to the limbic system, and remembered.and remembered.


Gustation is the sense of taste.Gustation is the sense of taste. Sensory organs called Sensory organs called taste budstaste buds hold the hold the

taste receptorstaste receptors. . • Receptors are located on the tongue, roof of the Receptors are located on the tongue, roof of the

mouth, and throat.mouth, and throat.• The five general taste categories are sweet, sour, The five general taste categories are sweet, sour,

salty, bitter, and salty, bitter, and umamiumami.. The flavors of most foods are a combination of The flavors of most foods are a combination of

the five basic tastes plus sensory input from the five basic tastes plus sensory input from olfactory receptors in the nose. olfactory receptors in the nose.

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Fig. 14.6, p. 254

tastebud

tonsil

sweet

saltysour

bitter

sensory nerve

hairlikeendingof tastereceptor

a

db c


Olfaction is the sense of smell.Olfaction is the sense of smell. Olfactory receptorsOlfactory receptors in the olfactory epithelium in the olfactory epithelium

of the nose detect water-soluble or volatile of the nose detect water-soluble or volatile substances—odors.substances—odors.

• The interpretation of smell is done by the olfactory The interpretation of smell is done by the olfactory bulbs located in the brain.bulbs located in the brain.

• Olfaction is one of the most ancient senses, useful in Olfaction is one of the most ancient senses, useful in survival as the receptors respond to molecules from survival as the receptors respond to molecules from food, mates, and predators.food, mates, and predators.

Humans also have a Humans also have a vomeronasal organvomeronasal organ whose receptors can detect pheromones, which whose receptors can detect pheromones, which are signaling molecules with roles in sexual are signaling molecules with roles in sexual attraction.attraction.

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olfactory nerve tract

olfactory bulb

olfactoryreceptorcell body

Fig. 14.7, p. 255

Video: Tongue TiedVideo: Tongue Tied

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Section 4Section 4

A Tasty Morsel of A Tasty Morsel of Sensory ScienceSensory Science

Receptors in taste buds associate the five Receptors in taste buds associate the five main taste categories with particular main taste categories with particular “tastant” molecules that the brain interprets “tastant” molecules that the brain interprets depending on the action potentials that depending on the action potentials that come its way.come its way.

Each taste bud has receptors that can respond Each taste bud has receptors that can respond to tastants of at least two, if not all five, of the to tastants of at least two, if not all five, of the taste classes.taste classes.

Not all taste receptors, however, are equally Not all taste receptors, however, are equally sensitive; bitter receptors tend to be the most sensitive; bitter receptors tend to be the most sensitive.sensitive.

A Tasty Morsel of Sensory Science A Tasty Morsel of Sensory Science

Various tastants commingle together with Various tastants commingle together with odors into what we perceive as flavors.odors into what we perceive as flavors.

A Tasty Morsel of Sensory Science A Tasty Morsel of Sensory Science

Section 5Section 5

Hearing: Detecting Hearing: Detecting Sound WavesSound Waves

Hearing: Detecting Sound Waves Hearing: Detecting Sound Waves

Sounds are waves of compressed air; the Sounds are waves of compressed air; the amplitudeamplitude (loudness) and (loudness) and frequencyfrequency (pitch) of sounds are detected by vibration-(pitch) of sounds are detected by vibration-sensitive mechanoreceptors deep in the sensitive mechanoreceptors deep in the ear.ear.


Am

pli

tud

eA

mp

litu

de

Same loudness,Same loudness,different pitchdifferent pitch

LowLownotenote

HighHighnotenote

Same frequency,Same frequency,different amplitudedifferent amplitude

Frequency perFrequency perunit timeunit time

one cycleone cycle

LoudLoud

SoftSoft

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The ear gathers and sends “sound signals” The ear gathers and sends “sound signals” to the brain.to the brain.

The The outer earouter ear collects sound waves and turns collects sound waves and turns them into vibrations, which are amplified in the them into vibrations, which are amplified in the middle earmiddle ear; vibrations are distinguished in the ; vibrations are distinguished in the

inner earinner ear.. Inner ear structures include Inner ear structures include semicircular semicircular

canalscanals for balance and the for balance and the cochleacochlea where where hearing takes place.hearing takes place.


Sensory hair cells are the key to hearing.Sensory hair cells are the key to hearing. Vibrations are passed from the Vibrations are passed from the tympanic tympanic

membranemembrane to the middle ear bones ( to the middle ear bones (malleusmalleus, , incusincus,, stapesstapes) and on to the ) and on to the oval windowoval window,, stretched across the entrance to the cochlea.stretched across the entrance to the cochlea.

• Sound is amplified because the oval window is Sound is amplified because the oval window is smaller than the tympanic membrane.smaller than the tympanic membrane.

• The cochlea has two compartments in its outer The cochlea has two compartments in its outer chamber (the chamber (the scala vestibuliscala vestibuli and and scala tympaniscala tympani), ), which curl around an inner which curl around an inner cochlear ductcochlear duct; all are ; all are fluid filled.fluid filled.

Fig. 14.9a, p. 256

MIDDLE EAR eardrum, ear bonesOUTER EAR

pinna, auditory canal

Fig. 14.9a, p. 256

INNER EARvestibular apparatus,

cochlea

MIDDLE EAR BONES

stirrup

anvil

hammer

auditory canal

EARDRUM roundwindow

COCHLEA

auditory nerve

Fig. 14.9b, p. 256

OVAL WINDOW (behind stirrup)


eardrumround window

oval window(behind stirrup)

waves of fluid pressurescala vestibuli

scala tympani

Fig. 14.9c, p. 257

waves of air pressure

cochlear duct


• Vibrations of the oval window send pressure waves Vibrations of the oval window send pressure waves through the fluid to the through the fluid to the basilar membranebasilar membrane on the on the floor of the cochlear duct; resting on the membrane floor of the cochlear duct; resting on the membrane is the is the organ of Cortiorgan of Corti, which includes sensory , which includes sensory hair hair cellscells..

• The tips of the hair cells rest against the jellylike The tips of the hair cells rest against the jellylike tectorial membranetectorial membrane; vibrations cause the hair cells ; vibrations cause the hair cells to bend.to bend.

• Bending causes the release of neurotransmitters, Bending causes the release of neurotransmitters, triggering action potentials that travel to the brain.triggering action potentials that travel to the brain.

organ of Corti

cochlear duct

scala tympanisensory neurons (to the auditory nerve)

scala vestibuli

Fig. 14.9d, p. 257


Loudness is determined by the total number of Loudness is determined by the total number of cells that become stimulated; tone or “pitch” cells that become stimulated; tone or “pitch” depends on the frequency of vibration.depends on the frequency of vibration.

The The round windowround window at the far end of the at the far end of the cochlea serves as a release valve for the cochlea serves as a release valve for the pressure waves in the middle ear.pressure waves in the middle ear.

The The eustachian tubeeustachian tube extending from the extending from the middle ear to the throat permits equalization of middle ear to the throat permits equalization of pressures.pressures.

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Section 6Section 6

Balance: Sensing the Balance: Sensing the Body’s Natural PositionBody’s Natural Position

The sense of balance depends on The sense of balance depends on messages from receptors in the eyes, skin, messages from receptors in the eyes, skin, and joints, as well as organs of equilibrium and joints, as well as organs of equilibrium in the inner ear.in the inner ear.

The The vestibular apparatusvestibular apparatus is a closed system is a closed system of fluid-filled sacs and semicircular canals of fluid-filled sacs and semicircular canals inside the ear; the canals are arranged to inside the ear; the canals are arranged to represent the three planes of space.represent the three planes of space.



Animation: Vestibular Apparatus Animation: Vestibular Apparatus and Equilibriumand Equilibrium

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• Rotational receptors are located at the base of each Rotational receptors are located at the base of each semicircular canal; sensory hair cells project into a semicircular canal; sensory hair cells project into a jellylike jellylike cupulacupula..

• Movement of the head causes the hairs to bend Movement of the head causes the hairs to bend within the jelly, generating action potentials.within the jelly, generating action potentials.

• Rotation of the head determines Rotation of the head determines dynamic dynamic equilibriumequilibrium..


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Static equilibriumStatic equilibrium, the head’s position in , the head’s position in space, is monitored by two sacs in the space, is monitored by two sacs in the vestibular apparatus, the vestibular apparatus, the utricleutricle and and sacculesaccule..

• The sacs contain the The sacs contain the otolith organsotolith organs (hair cells) and (hair cells) and otolithsotoliths (ear stones), which detect changes in (ear stones), which detect changes in orientation as well as acceleration and deceleration.orientation as well as acceleration and deceleration.

• Action potentials from different parts of the vestibular Action potentials from different parts of the vestibular apparatus travel to reflex centers in the brainstem.apparatus travel to reflex centers in the brainstem.

• As signals are integrated, the brain orders As signals are integrated, the brain orders compensatory movements necessary to maintain compensatory movements necessary to maintain postural balance.postural balance.


Fig. 14.10, p. 258

nerve

A vestibularapparatus (partof each innerear) consists ofa utricle, asaccule, andthe three canalslabeled here.

vestibular apparatus, asystem of fluid-filled sacsand canals inside the ear

fluid pressure

posteriorcanal

horizontalcanal

superior canal

utricle

saccule

Fig. 14.11, p. 258

cupula

sensoryneuron

haircell

stereocilium otolith

Extreme motion or continuous Extreme motion or continuous overstimulation of the hair cells of the overstimulation of the hair cells of the vestibular apparatus can result in vestibular apparatus can result in motion motion sicknesssickness..


Section 7Section 7

Disorders of the EarDisorders of the Ear

Disorders of the Ear Disorders of the Ear

The hearing apparatus of the ears is sturdy, The hearing apparatus of the ears is sturdy, but it can be damaged by various illnesses but it can be damaged by various illnesses and injuries.and injuries.

Otitis mediaOtitis media, painful inflammation of the middle , painful inflammation of the middle ear, often occurs in children following spread of ear, often occurs in children following spread of a respiratory infection; pus and/or fluid buildup a respiratory infection; pus and/or fluid buildup as a result can cause the eardrum to rupture.as a result can cause the eardrum to rupture.

TinnitusTinnitus, or ringing or buzzing in the ears, can , or ringing or buzzing in the ears, can be triggered by infection, aspirin consumption, be triggered by infection, aspirin consumption, or other, unknown causes.or other, unknown causes.

Disorders of the Ear Disorders of the Ear

DeafnessDeafness is the partial or complete loss of is the partial or complete loss of hearing; deafness may be congenital or due to hearing; deafness may be congenital or due to aging, disease, or environmental causation.aging, disease, or environmental causation.

The loudness of sounds is measured in The loudness of sounds is measured in decibels.decibels.

Quiet conversation occurs at about 50 decibels.Quiet conversation occurs at about 50 decibels. Damage begins when Damage begins when exposed to soundsexposed to sounds

between 75-85 decibels over extended periods.between 75-85 decibels over extended periods. Rock concerts easily reach 130 decibels.Rock concerts easily reach 130 decibels.

Fig. 14.13, p. 259

Outer Hair Cells

scars

Section 8Section 8

Vision: An OverviewVision: An Overview

Vision: An Overview Vision: An Overview

VisionVision is an awareness of the position, is an awareness of the position, shape, brightness, distance, and movement shape, brightness, distance, and movement of visual stimuli as detected by the sensory of visual stimuli as detected by the sensory organs, the organs, the eyeseyes..

The eye is built for photoreception.The eye is built for photoreception. The eye has three layers, sometimes called The eye has three layers, sometimes called

“tunics.”“tunics.”• The outer layer consists of the The outer layer consists of the sclerasclera and and

transparent transparent corneacornea..• The middle layer consists of a The middle layer consists of a choroidchoroid, , ciliary ciliary

bodybody, and , and irisiris..• The inner layer is the The inner layer is the retinaretina..


The sclera (“white” of the eye) protects the eye; The sclera (“white” of the eye) protects the eye; the dark-pigmented choroid underlies the sclera the dark-pigmented choroid underlies the sclera and prevents light from scattering. Most of the and prevents light from scattering. Most of the blood vessels lie in the choroid. blood vessels lie in the choroid.

Behind the cornea is the pigmented iris; the Behind the cornea is the pigmented iris; the hole at the center of the iris is the hole at the center of the iris is the pupilpupil, the , the entrance for light which can be adjusted entrance for light which can be adjusted depending on the level of light present.depending on the level of light present.

• The The lenslens is found behind the iris; the lens is attached is found behind the iris; the lens is attached to the ciliary body, a muscle functioning in the to the ciliary body, a muscle functioning in the focusing of light.focusing of light.

• The lens focuses light onto a layer of photoreceptor The lens focuses light onto a layer of photoreceptor cells in the retina.cells in the retina.


• A clear fluid (A clear fluid (aqueous humoraqueous humor) bathes both sides of ) bathes both sides of the lens; the lens; vitreous humorvitreous humor fills the chamber behind fills the chamber behind the lens.the lens.

The retina is a thin layer of neural tissue at the The retina is a thin layer of neural tissue at the back of the eyeball; axons from some of the back of the eyeball; axons from some of the neurons converge to form the neurons converge to form the optic nerveoptic nerve,, which sends signals to the visual cortex in the which sends signals to the visual cortex in the thalamus.thalamus.

Animation: Eye StructureAnimation: Eye Structure

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Parts of Parts of the Eyethe Eye

Vision: An OverviewVision: An Overview

The curved surface of the cornea bends The curved surface of the cornea bends incoming light so that light rays converge at the incoming light so that light rays converge at the back of the eyeball; images appear “upside back of the eyeball; images appear “upside down and backwards” on the retina but are down and backwards” on the retina but are corrected in the brain.corrected in the brain.

Eye muscle movements fine-tune the focus.Eye muscle movements fine-tune the focus. Because of the bending of the light rays by the Because of the bending of the light rays by the

cornea, cornea, accommodationaccommodation must be made by the must be made by the lens so that the image is in focus on the retina.lens so that the image is in focus on the retina.

Accommodation is performed by the ciliary Accommodation is performed by the ciliary muscles attached to the lens.muscles attached to the lens.


Eye muscle movements fine-tune the focus.Eye muscle movements fine-tune the focus. Because of the bending of the light rays by the Because of the bending of the light rays by the

cornea, cornea, accommodationaccommodation must be made by the must be made by the lens so that the image is in focus on the retina.lens so that the image is in focus on the retina.

Accommodation is performed by the ciliary Accommodation is performed by the ciliary muscles attached to the lens.muscles attached to the lens.

Fig. 14.15a, p. 261

distantobject

muscle relaxed

taut fibers

close object

slack fibers

muscle contracted

Fig. 14.16, p. 261

Accommodation for close objects (lens bulges)

Accommodation for distant objects (lens flattens)

Animation: Visual AccomodationAnimation: Visual Accomodation

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Video: To See AgainVideo: To See Again

From ABC News, Biology in the Headlines, 2005 DVD.From ABC News, Biology in the Headlines, 2005 DVD.

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Section 9Section 9

From Visual Signals From Visual Signals to “Sight”to “Sight”

From Visual Signals to “Sight” From Visual Signals to “Sight”

Rods and cones are the photoreceptors.Rods and cones are the photoreceptors. The retina’s basement layer is pigmented and is The retina’s basement layer is pigmented and is

covered by photoreceptors called covered by photoreceptors called rod cellsrod cells and and cone cellscone cells..

Rod cells are sensitive to dim light and detect Rod cells are sensitive to dim light and detect changes in light intensity; cone cells respond to changes in light intensity; cone cells respond to high-intensity light and contribute to sharp high-intensity light and contribute to sharp daytime vision.daytime vision.

Fig. 14.17a, p. 262

rod cell stacked, pigmented membranes

cone cell

Fig. 14.17b, p. 262


Visual pigments in rods and cones intercept Visual pigments in rods and cones intercept light energy.light energy.


Each rod contains more than a billion molecules Each rod contains more than a billion molecules of of rhodopsinrhodopsin; this pigment can detect and ; this pigment can detect and respond to even a few photons of light, allowing respond to even a few photons of light, allowing us to see in dim light.us to see in dim light.

• Rhodopsin consists of a protein (opsin) and a signal Rhodopsin consists of a protein (opsin) and a signal molecule (molecule (ciscis-retinal) that is derived from vitamin A.-retinal) that is derived from vitamin A.

• Photons of blue-green light stimulate rhodopsin to Photons of blue-green light stimulate rhodopsin to change shape; shape changes alter the distribution change shape; shape changes alter the distribution of ions across the rod cell membrane and slow down of ions across the rod cell membrane and slow down the release of an inhibitory neurotransmitter.the release of an inhibitory neurotransmitter.

• Without the inhibitor, neurons send visual signals to Without the inhibitor, neurons send visual signals to the brain.the brain.


Cone cells have different visual pigments (red, Cone cells have different visual pigments (red, green, or blue); absorption of photons also green, or blue); absorption of photons also prevents release of neurotransmitters, thus prevents release of neurotransmitters, thus allowing signaling to the brain.allowing signaling to the brain.

Visual acuity is Visual acuity is greatest in the greatest in the foveafovea, , a depression located a depression located at the center of the at the center of the retina that is densely retina that is densely packed with packed with photoreceptors.photoreceptors.



The retina processes signals from rods and The retina processes signals from rods and cones.cones.

Signals flow from rods and cones to Signals flow from rods and cones to bipolar bipolar interneuronsinterneurons, and then to , and then to ganglion cellsganglion cells, the , the axons of which form the optic nerves.axons of which form the optic nerves.

Before leaving the retina, signals are Before leaving the retina, signals are dampened or enhanced by dampened or enhanced by horizontal cellshorizontal cells and and amacrine cellsamacrine cells..

Fig. 14.19, p. 263

ganglion cells (axonsget bundled into oneof two optic nerves)

rods

cones

horizontal cells

bipolar cells

amacrine cells

incomingrays oflight

Animation: Organization of Animation: Organization of Cells in the RetinaCells in the Retina

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Receptive fields in the retina.Receptive fields in the retina.• The retina’s surface is organized into “receptive The retina’s surface is organized into “receptive

fields,” areas that influence the activity of individual fields,” areas that influence the activity of individual sensory neurons.sensory neurons.

• Some fields respond to differences in light, others to Some fields respond to differences in light, others to motion, color, or rapid changes in light intensity.motion, color, or rapid changes in light intensity.

Signals move on to the visual cortex.Signals move on to the visual cortex.• The visual field represents the part of the outside The visual field represents the part of the outside

world a person actually sees.world a person actually sees.• The right side of each retina gathers light from the The right side of each retina gathers light from the

left half of the visual field and the left side gathers left half of the visual field and the left side gathers light from the right half of the field.light from the right half of the field.


• The optic nerve from each eye sends signals from The optic nerve from each eye sends signals from the left visual field to the right cerebral hemisphere, the left visual field to the right cerebral hemisphere, and signals from the right visual field to the left and signals from the right visual field to the left hemisphere.hemisphere.

Axons of the optic nerves end in the Axons of the optic nerves end in the lateral lateral geniculate nucleusgeniculate nucleus, from which they proceed , from which they proceed to the brain’s visual cortex, which has several to the brain’s visual cortex, which has several visual fields sensitive to direction, movement, visual fields sensitive to direction, movement, color, and so on; here is where final color, and so on; here is where final interpretation of the signals is made to produce interpretation of the signals is made to produce an organized sense of sight. an organized sense of sight.

lateral geniculate nucleus visual cortexoptic nerve

Fig. 14.20, p. 263

retina

to optic nerve

Animation: Pathway to the Visual CortexAnimation: Pathway to the Visual Cortex

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Section 10Section 10

Disorders of the EyeDisorders of the Eye

Disorders of the Eye Disorders of the Eye

Normal eye function can be disrupted by Normal eye function can be disrupted by disease, injury, inherited abnormalities, and disease, injury, inherited abnormalities, and aging.aging.

Missing cone cells cause color blindness.Missing cone cells cause color blindness. Total color blindness results when an individual Total color blindness results when an individual

has only one of the three kinds of cones.has only one of the three kinds of cones.


Red-green color blindnessRed-green color blindness is the inability to is the inability to distinguish red and green colors in dim light distinguish red and green colors in dim light (and sometimes bright light) due to a lack of red (and sometimes bright light) due to a lack of red and green cone cells.and green cone cells.

Malformed eye parts cause common Malformed eye parts cause common focusing problems.focusing problems.

In In astigmatismastigmatism, one or both corneas have , one or both corneas have uneven curvature and cannot bend light to the uneven curvature and cannot bend light to the same focal point.same focal point.



Nearsightedness Nearsightedness ((myopiamyopia) results ) results when the image when the image is focused in is focused in front of the front of the retina.retina.

Farsightedness Farsightedness ((hyperopiahyperopia) is ) is due to an image due to an image focused behind focused behind the retina.the retina.


Fig. 14.21 (top), p. 264

(focalpoint)

distantobject

(focalpoint)

closeobject

Fig. 14.21 (bottom), p. 264

Animation: Focusing ProblemsAnimation: Focusing Problems

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The eyes are also vulnerable to infections The eyes are also vulnerable to infections and cancer.and cancer.

ConjunctivitisConjunctivitis, inflammation of the membrane , inflammation of the membrane lining the inside of the eyelids and covering the lining the inside of the eyelids and covering the sclera, is among the sclera, is among the

most common reasons most common reasons

for doctor visits in the for doctor visits in the

U.S.U.S.



TrachomaTrachoma, caused by the bacterium , caused by the bacterium responsible for the sexually transmitted disease responsible for the sexually transmitted disease chlamydia, damages both the eyeball and the chlamydia, damages both the eyeball and the conjunctiva, possibly leading to blindness.conjunctiva, possibly leading to blindness.

Herpes infectionHerpes infection of the cornea results from of the cornea results from infection with various herpes simplex viruses infection with various herpes simplex viruses and can also lead to blindness.and can also lead to blindness.

Malignant melanomaMalignant melanoma is eye cancer that is eye cancer that develops in the choroid; develops in the choroid; retinoblastomaretinoblastoma is is cancer of the retina that occurs in infants.cancer of the retina that occurs in infants.


Aging increases the risk of cataracts and Aging increases the risk of cataracts and some other eye disorders.some other eye disorders.

CataractsCataracts, the gradual clouding of the lens , the gradual clouding of the lens associated with aging and diabetes, can associated with aging and diabetes, can completely block light from entering the eye.completely block light from entering the eye.

Macular degenerationMacular degeneration is an age-related is an age-related degeneration of the retina.degeneration of the retina.

GlaucomaGlaucoma results from excess of fluid in the results from excess of fluid in the eyeball, causing pressure on the retina.eyeball, causing pressure on the retina.


Medical technologies can remedy some Medical technologies can remedy some vision problems and treat eye injuries.vision problems and treat eye injuries.

Corneal transplant surgeryCorneal transplant surgery can replace can replace defective corneas with artificial plastic corneas defective corneas with artificial plastic corneas or donor corneas; cataracts may be corrected or donor corneas; cataracts may be corrected in a similar fashion by replacing the lens.in a similar fashion by replacing the lens.

““Lasik” (laser-assisted in situ keratomilieusis) or Lasik” (laser-assisted in situ keratomilieusis) or “lasek” (laser-assisted subepithelial “lasek” (laser-assisted subepithelial keratectomy) surgeries can be used to correct keratectomy) surgeries can be used to correct severe nearsightedness.severe nearsightedness.


Conductive keratoplastyConductive keratoplasty (CK) uses radio (CK) uses radio waves to reshape the cornea.waves to reshape the cornea.

Retinal detachmentRetinal detachment can result from a physical can result from a physical blow to the head; blow to the head; laser coagulationlaser coagulation can be can be used to “reattach” the retina to the underlying used to “reattach” the retina to the underlying choroid.choroid.

Documents

PowerLecture: Chapter 14 Sensory Systems. Learning Objectives Describe the characteristics of a receptor and list the various types of receptors