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Biol 219Lecture17– VisionFall2016 Dr Scott
1
SpecialSenses
TheEyeandVision
ImportantConcepts
• Describe thestructuresoftheeyeand the roleofeachstructure invision.
• Trace thepathway forvisionfromthe retina tothevisual cortex.
• Explainhowphotoreceptorsconvertlightenergyintoactionpotentials.
• Explainsignalprocessing inthe retinaandin thevisual cortex.
AnatomyoftheEye
• Protected inorbitsby thebonesoftheskull
• Upperandlower eyelids
• Lacrimal apparatuswasheswith tears
• Pupilisanopening thatwidensandshrinkswhenpupillarymusclescontract
• Colored ringofpigment isthe iris
©2016PearsonEducation,Inc.
AnatomyoftheEye• Twochambersintheeye
• Infrontofthelensfilledwithaqueoushumorcoveredbycornea
• Behindthelens,larger vitreouschamber filledwithvitreousbody(humor)
• Retinalayerlinesthebackoftheeyeandcontainsphotoreceptors
• Opticdisk(blindspot) islocationwhereneuronsjoinintotheopticnerve
• Opticnervescrossoverintheopticchiasm
• Optictractsendatthevisualcortexintheoccipitallobe
©2016PearsonEducation,Inc.
Biol 219Lecture17– VisionFall2016 Dr Scott
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© 2016 Pearson Education, Inc.
Muscles attached to external surface of eye control eye movement.
Lacrimal glandsecretes tears.
Uppereyelid
Sclera
Pupil
Iris
Lowereyelid
The orbit is a bony cavity that protects the eye.
Nasolacrimal duct drains tears into nasal cavity.
Sagittal Section of the Eye
Zonules: attach lens to ciliary
muscle
Lens bends light to focus it
on the retina.
Optic disk (blind spot): region where optic nerve and blood vessels leave the eye
Central retinal artery and vein emerge from center of optic disk.
Optic nerve
Fovea: regionof sharpest v ision
Retina: layer thatcontains photoreceptors
Vitreous chamber
Sclera is connective tissue.
Iris
Pupil changesamount of light
entering the eye.
Cornea
Aqueous humor
Canal of Schlemm
The Eye Lens shape is adjusted by contraction and relaxation of the ciliary muscle.
Biol 219Lecture17– VisionFall2016 Dr Scott
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TheEyeandVision
• Lightenters theeye
• Focusedon retinabythelens
• Photoreceptors transducelightenergy intoelectrical signal
• Neuralpathwaysprocesselectrical signalsintovisual images
©2016PearsonEducation,Inc.
ThePupil• Lightenterstheeye
• Sizeofthepupilmodulatestheamountoflightthatreachesphotoreceptors
• Shapeoflensfocuses thelight
• Pupillaryreflexisaconsensualreflex• Standardpartofneurologicalexamination
• Constrictinresponsetoparasympatheticfibersinbrightlight
• Dilateinresponsetosympatheticnervoussystemindimlight
©2016PearsonEducation,Inc.
The Visual Pathway – via CNII Figure10.26cPathwaysforvisionandthepupillaryreflex
© 2016 Pearson Education, Inc.
Collateral pathways leavethe thalamus and synapsein the midbrain to controlconstr iction of the pupils.
Opticnerve
Opticchiasm
Optictract
Lateral geniculate body (thalamus)
Visual cortex(occipital lobe)
Eye
Light Midbrain
Cranial nerve III controls pupillary constr iction.
Biol 219Lecture17– VisionFall2016 Dr Scott
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TheLens
• Opticsdescribes lightbehaviorandproperties
• Lightentering eye isrefracted, orbent
• Atthecorneaandlens
• Refractioninfluenced bytheangleatwhichlightmeetsthelens
©2016PearsonEducation,Inc.
Figure10.27abOpticsoftheEye
© 2016 Pearson Education, Inc.
Light passing through a curved surface will bend or refract.
A concave lens scatters light rays.
Concave lens
Paralle llight rays
A convex lens causes light rays to converge.
Convex lens Focal point
Focal length
The focal length of the lens is the distance from the center of the lens to the focal point.
Paralle llight rays
Accommodation
• Processbywhich theeyeadjustslensshape tokeepobjects infocus
• Nearpointofaccommodation istheclosestdistanceatwhich the lenscan focusanobject
• Myopia
• Focalpointfallsinfrontoftheretina
©2016PearsonEducation,Inc.
Accommodation
• Hyperopia
• Focalpointfallsbehindtheretina
• Presbyopia islossofaccommodation
• Astigmatism
• Usuallycausedbyacorneathatisnotaperfectlyshapeddome, resultingindistortedimages
©2016PearsonEducation,Inc.
Biol 219Lecture17– VisionFall2016 Dr Scott
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Focusing on objects at different distances requires changing the shape of the lens:flatter lens for distant objects, more rounded (convex) lens for close objects
AccommodationFigure10.27OpticsoftheEye
© 2016 Pearson Education, Inc.
C ommon vi si on defects can be corrected wi th external l enses.
H yperopi a, or f ar- si ght edness, occurs w hen t he f ocal poi nt f al l s behi nd t he ret i na.
Myopi a, or near- si ght edness, occurs w hen t he f ocal poi nt f al l s i n f ront of t he ret i na.
M yopi a ( correct ed w i t h a concave l ens)
Hyperopi a ( correct ed w i t h a convex l ens)
Phototransduction
• Converts lightenergy intoelectrical signals
• Photoreceptors• Rods andcones
• Modifiedganglioncellscontainmelanopsin torespondtochanginglightcues
• Mostacutevisionoccursatthe foveaandmacula
• Opticdiskhasnophotoreceptorsand iscalled theblindspot
©2016PearsonEducation,Inc.
Figure10.29aTheRetina
© 2016 Pearson Education, Inc.
Dorsal view of a section of the right eye
Fixationpoint
Light Lens Retina Fovea
Macula
Optic nerve
Biol 219Lecture17– VisionFall2016 Dr Scott
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Figure10.29bTheRetina
© 2016 Pearson Education, Inc.
The projected image is upside down on the retina.Visual processing in the brain reverses the image.
Fovea
Figure10.29cTheRetina
© 2016 Pearson Education, Inc.
Optic nerve
Sclera
The choroid layercontains blood vessels.
Pigment epithelium
Neural cells of retina
Axons from the retina exit via the optic nerve.
Figure10.29dTheRetina
© 2016 Pearson Education, Inc.
Light strikes the photoreceptors in the fovea directly because overlying neurons are pushed aside.
Fovea
Light
ConeRodBipolar neuron
Ganglion cell
Neural cells of retina
Pigment epithelium of retina absorbs excess light.
© 2016 Pearson Education, Inc.
Convergence in the retina
To opticnerve
Bipolarcell Rod
Pigmentepithelium
Ganglioncell
How many rods converge on the ganglion cell in (e)?
FIGURE QUESTION
Biol 219Lecture17– VisionFall2016 Dr Scott
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Photoreceptors
• Rodsfunctionwell inlow lightandareused innightvision
• Conesare responsibleforhigh-acuityvisionandcolorvisionduring thedaytime
• Outer, inner,andbasalsegments
©2016PearsonEducation,Inc.
Photoreceptors
• Visualpigmentsconvert lightenergy intoachangeinmembrane potential
• Rods containrhodopsin
• Cones containthreepigmentsprimaryexcitedbyred,green,andbluelight.
• Color-blindness
©2016PearsonEducation,Inc. © 2016 Pearson Education, Inc.
PIGMENTEPITHELIUM
OUTER SEGMENT
Melanin granules
Light transduction takes place in the outer segment of the photoreceptor using visual pigments inmembrane disks.
Location of major organelles and metabolic operations, such as photopigment synthesisand ATP production
INNER SEGMENT
SYNAPTIC TERMINALSynapses withbipolar cells .
Bipolar cell
Disks
Connectingstalks
Mitochondria
Cone Rods
LIGHT
Rhodopsinmolecule
RetinalOpsin
Disks
Old disks at tip arephagocytiz ed bypigment epithelia l cells .
The dark pigment epitheliumabsorbs extra light and preventsthat light from reflecting backand distorting vision.
Biol 219Lecture17– VisionFall2016 Dr Scott
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Figure 10.31Light absorptionbyvisualpigments
© 2016 Pearson Education, Inc.
100
75
50
25
0
Bluecones Rods
Greencones
Redcones
Violet Blue Green Yellow Orange Red
400 450 500 550 600 650 700Wavelength (nm)
Ligh
t ab
sorp
tion
(per
cent
of
max
imum
)
GRAPH QUESTIONS
1. Which pigment absorbs light over the broadest spectrum of wavelengths?
2. Over the narrowest? 3. Which cone pigment absorbs the most
light at 500 nm?© 2016 Pearson Education, Inc.
In darkness, rhodopsin is inactive, cGMP is high, and CNG and K+
channels are open.
Light bleaches rhodopsin. Opsindecreases cGMP, closes CNGchannels, and hyperpolariz es the cell.
In the recovery phase, retinal recombines with opsin.
Pigment epithelium cell Activatedretinal
Opsin (bleachedpigment)
Activatestransducin
Disk
Transducin(G protein)
Inactiverhodopsin
(opsin and retinal)
CNG channelopen
Membrane potentia l in dark = −40mV
K+
Ca2+
Na +
cGMPlevels high
DecreasedcGMP
CNG channelcloses
K+
Ca2+
Na +
Membranehyperpolariz es
to −70 mV
Light
Neurotransmitter releasedecreases in proportion
to amount of light.
Tonic release ofneurotransmitter
onto bipolar neurons
Rod
Retinal converted to inactive form
Cascade
Retinal recombineswith opsin to
form rhodopsin.
FIGURE QUESTIONOne rod contains about 10,000 CNG channels open in the dark. One photon of light activates one rhodopsin. Each rhodopsin activates 800 transducin. Each transducin cascade removes 6 cGMP. A decrease of 24 cGMPcloses one CNG channel. How many photons are needed to close all the CNG channels in one rod?
SignalProcessing
• Photoreceptorcells converge inbipolarneurons
• Multiplebipolarneuronsconverge ontooneganglioncell
• Horizontalcells synapsewithphotoreceptorsandbipolarcells
©2016PearsonEducation,Inc.
SignalProcessing
• Amacrine cellsmodulate informationbetweenbipolarandganglioncells
• Thecentral portionofthevisual field isthebinocular zone
• Processed togive3Dvision
©2016PearsonEducation,Inc.
Biol 219Lecture17– VisionFall2016 Dr Scott
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Figure10.33cVisual fields
© 2016 Pearson Education, Inc.
The retina uses contrast rather than absolute light intensity for better detection of weak stimuli.
Visual Field Type Field Is On-Center/Off-Surround Field Is Off-Center/On-Surround
On-center, off-surround
Bright lightonto center
Bright lightonto surround
Bright lightonto surround
Diffuse lighton both centerand surround
Off-center, on-surround
Both fie ld types
Ganglion cell is excitedby light in the centerof the visual fie ld.
Ganglion cell is inhibitedby light on the surroundof the visual fie ld.
Ganglion cell respondsweakly.
Ganglion cell is inhibitedby light in the centerof the visual fie ld.
Ganglion cell is excitedby light on the surround of the visual fie ld.
Ganglion cell respondsweakly.
Figure10.34Binocularvision
© 2016 Pearson Education, Inc.
Binocular zone iswhere left and right v isual fields overlap.
Monocular zoneis the portion ofthe visual field associated with only one eye.
Leftvisualfie ld
Rightvisualfie ld
Binocularz one
Visual fie ld
Optic chiasm
Optic nerve
Optic tract
Lateralgeniculate body
(thalamus)
Visual cortex
TestYourKnowledgeAnarea ofthe retina thatcontainsonlyconesand isthesiteofsharpestvisionisthe• A)outer segment.
• B)inner segment.
• C)fovea.
• D)opticdisc.
• E)tapetum lucidum.
Biol 219Lecture17– VisionFall2016 Dr Scott
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Anarea ofthe retina thatcontainsonlyconesand isthesiteofsharpestvisionisthe• A)outer segment.
• B)inner segment.
ØC)fovea.
• D)opticdisc.
• E)tapetum lucidum.
Accommodation describesthe focusingoflightonthe retinabychanging• A)theshapeofthe lens.• B)pupillarydiameter.• C)theshapeofthecornea.• D)distanceofthe retina fromthe lens.
Accommodation describesthe focusingoflightonthe retinabychangingØA)theshapeofthe lens.• B)pupillarydiameter.• C)theshapeofthecornea.• D)distanceofthe retina fromthe lens.
Whichofthe followingcells involvedinprocessingvisual informationin theretina synapsewith therods?• A)horizontal cellsonly• B)ganglioncellsonly• C)bipolarcellsonly• D)horizontalandbipolarcellsonly• E)horizontal,ganglion,andbipolar
Biol 219Lecture17– VisionFall2016 Dr Scott
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Whichofthe followingcells involvedinprocessingvisual informationin theretina synapsewith therods?• A)horizontal cellsonly• B)ganglioncellsonly• C)bipolarcellsonly• D)horizontalandbipolarcellsonlyØE)horizontal,ganglion,andbipolar
Photoreceptorssecrete theneurotransmitter• A)acetylcholine.• B)norepinephrine.• C)glutamate.• D)dopamine.
Photoreceptorssecrete theneurotransmitter• A)acetylcholine.• B)norepinephrine.ØC)glutamate.• D)dopamine.