special senses - napa valley college · 2016-10-13 · special senses the eye and vision important...

Biol 219Lecture17– VisionFall2016 Dr Scott

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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



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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.


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TheEyeandVision

• Lightenters theeye

• Focusedon retinabythelens

• Photoreceptors transducelightenergy intoelectrical signal

• Neuralpathwaysprocesselectrical signalsintovisual images


ThePupil• Lightenterstheeye

• Sizeofthepupilmodulatestheamountoflightthatreachesphotoreceptors

• Shapeoflensfocuses thelight

• Pupillaryreflexisaconsensualreflex• Standardpartofneurologicalexamination

• Constrictinresponsetoparasympatheticfibersinbrightlight

• Dilateinresponsetosympatheticnervoussystemindimlight


The Visual Pathway – via CNII Figure10.26cPathwaysforvisionandthepupillaryreflex


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.


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TheLens

• Opticsdescribes lightbehaviorandproperties

• Lightentering eye isrefracted, orbent

• Atthecorneaandlens

• Refractioninfluenced bytheangleatwhichlightmeetsthelens


Figure10.27abOpticsoftheEye


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


Accommodation

• Hyperopia

• Focalpointfallsbehindtheretina

• Presbyopia islossofaccommodation

• Astigmatism

• Usuallycausedbyacorneathatisnotaperfectlyshapeddome, resultingindistortedimages



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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


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


Figure10.29aTheRetina


Dorsal view of a section of the right eye

Fixationpoint

Light Lens Retina Fovea

Macula

Optic nerve


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Figure10.29bTheRetina


The projected image is upside down on the retina.Visual processing in the brain reverses the image.

Fovea

Figure10.29cTheRetina


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


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.


Convergence in the retina

To opticnerve

Bipolarcell Rod

Pigmentepithelium

Ganglioncell

How many rods converge on the ganglion cell in (e)?

FIGURE QUESTION


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Photoreceptors

• Rodsfunctionwell inlow lightandareused innightvision

• Conesare responsibleforhigh-acuityvisionandcolorvisionduring thedaytime

• Outer, inner,andbasalsegments


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.


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Figure 10.31Light absorptionbyvisualpigments


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


SignalProcessing

• Amacrine cellsmodulate informationbetweenbipolarandganglioncells

• Thecentral portionofthevisual field isthebinocular zone

• Processed togive3Dvision



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Figure10.33cVisual fields


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


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.


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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


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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.

special senses - napa valley college · 2016-10-13 · special senses the eye and vision important...

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