Beda Lecture - Vision

8/14/2019 Beda Lecture - Vision

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02/12/10



02/12/10


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Physiology of Vision

Melvin Valera, M.D.



Lecture Points

• The Hardware• Principles of Optics

• Lenses and Refraction

• Optics of the Eye

• Accomodation of the Eye

•

Fluid system of the Eye•



Refraction of Light



Refraction of Light

• Directly perpendicular – nobending, only slowing(shortened wavelengths)

•

• At an angle – slowing and bending



Refraction of Light



Refraction of Light

•

The degree of refraction increases as afunction of the ff:

(1)the ratio of the two refractiveindices of the two transparent

media (relative refraction)•

•(2) the degree of angulation between

the interface and the entering wavefront.



Lenses

•Convex lens



Convex Lens

• perpendicular light rays through thecenter of the lens pass through thelens without being refracted

• light rays striking at the edge are

progressively more angulated

•

• outer rays bend more and more

toward the center; convergence of the rays



Convex Lens

• Half the bending occurs when the raysenter the lens and half as they exit from the opposite side.

•

• A lens with the proper curvature willbend light rays exactly enough sothat all the rays will pass through a

single point, which is called the focalpoint.



Lenses

•Convex lens



•Concave lens

Lenses



Concave lens

• opposite to the effect in the convexlens

• causes the peripheral light rays todiverge away from the light rays

that pass through the center of thelens.

•

• the concave lens diverges light rays,whereas the convex lens convergesfight rays



Spherical vs Cylindrical Lenses




• light rays that pass through the spherical lens are refracted at all edges of the lenstoward the central ray,

• all the rays come to a focal point

•

• cylindrical lenses bend light rays from thetwo sides of the lens but not from the topor the bottom.

•bending occurs in one plane but not the

other.

• parallel light rays are bent to a focal line



• Concave cylindricallenses diverge lightrays in only oneplane

•

• Convex cylindricallenses converge

light rays in oneplane




Cylindrical Lenses

Combination of

two cylindricallenses at rightangles equalsa spherical lens.



Lenses



Lenses

• when rays of light that are already diverging enter a convex lens, thedistance of focus on the other sideof the lens is farther from the lens

than is the focal length of the lens.•

• Degree of refraction is affected by

angulation.•



Lenses



Lenses

• the image is upside down withrespect to the original object,and the two lateral sides of theimage are reversed.

•

• a camera focuses images on thecamera film using this method



The Diopter

• Measurement of the RefractivePower of a Lens

• The more a lens bends light rays, thegreater is its "refractive power."

• measured in terms of diopters

•

• D = 1 / focal length (in meters)



The Diopter

• The refractive power of concavelenses cannot be stated in terms of the focal distance beyond the lensbecause the light rays diverge

• a concave lens that diverges light raysat the same rate that a +1-diopter convex lens converges them is said

to have a dioptric strength of -1.



The Diopter

• concave lenses "neutralize" therefractive power of convex lenses.

• placing a +1-diopter concave lensimmediately in front of a 1-diopter

convex lens results in a lens systemwith zero refractive power .

• In cylindrical lenses, diopters plus

axis must be designated.



Optics of the Eye: Eye as a Camera

•The lens system of the eye is composed of four refractive interfaces:

• (1) the interface between air and the anterior surface of the cornea,

• (2) the interface between the posterior surface of the cornea and the aqueous humor ,

• (3) the interface between the aqueous humor and the anterior surface of the crystalline lens of

• (4) the interface between the posterior surface of the lens and the vitreous humor .



Optics of the Eye



Optics of the Eye

• Diopter is not equivalent to refractive

index!

•

• D = 1/ focal length = refractive power

•

• Refractive index = speed of light in air • speed of light in the substance•

• They are, however, directlyproportional to each other.



Relative refractive power

• About 2/3 of the 59 diopters of refractive power of the eye isprovided not by the crystalline lens but by the anterior surface of the

cornea.• refractive index of the cornea is

markedly different from that of air .




• If the lens were removed from the eye and

then surrounded by air, its refractivepower would be about six times as great.

• the fluids surrounding the lens haverefractive indices not greatly different fromthe refractive index of the lens itself

•

• the smallness of this difference greatly

decreases the amount of light refraction atthe lens interfaces




Source

R f i



Refractive power

• The refractive power of the crystallinelens of the eye can be increasedvoluntarily from 20 D to about 34 Din the young.

• The shape of the lens is changed to amore convex lens.

• This is called accomodation!

M h i f A d ti



Mechanism of Accommodation

M h i f A d ti




• the lens is composed of a strongelastic capsule filled with viscous,proteinaceous, but transparent fluid

• in a relaxed state with no tension on

the capsule, it assumes an almostspherical shape,• due to the elastic retraction of the lens

capsule.

M h i f A d ti




• about 70 suspensory ligaments attach

radially around the lens, pulling the lensedges toward the outer circle of theeyeball, flattening the lens.

• Results from tension from their attachmentsat the anterior border of the choroid andretina.

M h i f A d ti




M h i f A d ti




• The ciliary muscles has 2 separate groups:meridional fibers and circular fibers.

•

• The meridional fibers extend anteriorly from theperipheral ends of the suspensory ligamentsto the corneoscleral junction.

• When these contract, the peripheral insertions of the lens ligaments are pulled forward and to

the middle toward the cornea, releasing tensionon the lens.

M h i f A d ti




• The ciliary muscles has 2 separate groups:meridional fibers and circular fibers.

• The circular fibers are arranged circularly all theway around the ligament attachments.

• When they contract, a sphincter-like actionoccurs, decreasing the diameter of the circle of ligament attachments.

• This also allows the ligaments to pull less on

the lens capsule.

M h i f A d ti




• Contraction of either set of muscle fibers

relaxes the ligaments to the lenscapsule.

• The lens assumes a more spherical shapebecause of the natural elasticity of thelens capsule.

•

• controlled almost entirely by

parasympathetic nerve signalstransmitted to the eye through the thirdcranial nerve from the third nervenucleus in the brain stem.

•

M h i f A d ti




A d ti P bl P b i



Accomodation Problem: Presbyopia

• With aging the lens grows larger and thicker

and becomes far less elastic• due to progressive denaturation of the lens

proteins.

• the ability of the lens to change shape

progressively decreases• accommodation decreases from about 14

diopters in the child to less than 2diopters by ages 45 to 50 years

• essentially zero diopters at age 70 years

• eyes can no longer accommodate for bothnear and far vision - presbyopia





• an older person must wear bifocal glasses with theupper segment normallyfocused for far-seeingand the lower segmentfocused for near-seeing

– reading glasses

Errors of Refraction




• Emmetropia• parallel light rays from distant objects are in

sharp focus on the retina when the ciliarymuscle is completely relaxed – normalvision

•to focus objects at close range, the ciliary

muscles must still contract to provideappropriate degrees of accommodation

•





• Hyperopia (Farsightedness)• parallel light rays are not bent sufficiently

• image focuses behind the retina

• due either to an eyeball that is too short or to a lens system that is too weak

• To overcome this, the ciliary muscle mustcontract to increase the strength of thelens.

•





• Myopia (Nearsightedness)• image focuses in front of the retina

• usually due to too long an eyeball or from a lens system that is too strong

•a myopic person has a definite

limiting "far point“ for clear vision.

•

•





• Myopia (Nearsightedness)

• No natural mechanism exists bywhich the eye can decrease thestrength of its lens to less than

when the ciliary muscle iscompletely relaxed.

•

•

Correction of Refraction Errors



Correction of Refraction Errors

• Use of Glasses (lenses)• in myopia, the excessive refractive power

can be neutralized by placing in front of theeye a concave spherical lens, which willdiverge rays.

•

in hyperopia, abnormal vision can becorrected by adding refractive power usinga convex lens in front of the eye.

• "trial and error"

•





• Astigmatism• refractive error that causes the visual image

in one plane to focus at a different distancefrom that of the plane at right angles

• too great a curvature of the cornea in one of

its planes• like the surface of an egg lying sidewise to

the incoming light

• the degree of curvature in the plane

through the short axis is greater thanthe degree of curvature in the planethrough the long axis





Astigmatism



Astigmatism

• accomodation can never compensatefor astigmatism because thecurvature of the lens changesapproximately equally in both planes

• the two planes can never becorrected at the same time without the help of glasses

Correction of Astigmatism




• to correct for astigmatism, use aspherical lens that corrects thefocus in one of the two planes

• an additional cylindrical lens is used

to correct the error in the remaining plane

•

• both the axis and the strength of therequired cylindrical lens must bedetermined

•





• One method using parallelblack bars

• Spherical lens usually isfound that will causesharp focus of one set of these parallel bars butwill not correct thefuzziness of the set of bars at right angles to thesharp bars

• the examiner tries differentcylindrical lenses untilthe patient sees all thecrossed bars with equalclarity

•

Contact Lenses



Contact Lenses

• the contact lens nullifies almostentirely the refraction that normallyoccurs at the anterior surface of the cornea

• the tears between the contact lensand the cornea have a refractiveindex almost equal to that of thecornea

• the refraction of the contact lenssubstitutes for the cornea's usualrefraction.

Contact Lenses



Contact Lenses

• In keratoconus, the bulging corneacauses such severe abnormality of vision that almost no glasses cancorrect the vision satisfactorily.

• a contact lens neutralizes the cornealrefraction and normal refraction bythe anterior surface of the contactlens is substituted in its place

Contact Lenses



Contact Lenses

• The contact lens has other advantages:• (1) It turns with the eye and gives a

broader field of clear vision thando usual glasses and

• (2) It has little effect on the size of the object that the person seesthrough the lens

• conversely, lenses placed 1centimeter or so in front of theeye affect the size of the image

Pupillary Diameter: Iris



Pupillary Diameter: Iris

•The Iris

• increases the amount of light that entersthe eye during darkness

• decreases the amount of light that enters

the eye in bright light

Pupillary Diameter: Depth of Focus




• The upper lens system has far greater

depth of focus than the bottom lenssystem.

• When a lens system has great depth of focus, the image can be considerably

displaced from the focal plane and still itremains sharp





• The greatest depth of focus occurs

when the pupil is extremely small.

• with a very small aperture, all light rayspass very nearly through the center

of the lens• the central-most rays are always in

focus

Visual Acuity



Visual Acuity

Visual Acuity



Visual Acuity

• the average diameter of cones in the fovea

of the retina is about 1.5 μm• a person can normally distinguish two

separate points if their centers lie 2 μmapart on the retina, slightly greater than

the width of a cone.

• a person with normal acuity looking at two bright pinpoint spots of light 10 metersaway can barely distinguish the spots asseparate entities when they are 1.5 to 2mm apart

•

Clinical Determination of Visual Acuity



Clinical Determination of Visual Acuity

• test chart for testing VA is placed 20 feet

away from the tested person• If the person can see the letters of the size

that a normal person can see at 20 feet,he or she is said to have 20/20 vision

normal vision.

• If the person can see only letters that anormal person see at 200 feet, he or sheis said to have 20/200 vision.

• Comparison of the VA of a test personwith that of a normal person.

•

Depth Perception



Depth Perception

• Determination of the distance of an

object from the eye•

• The optic system normally perceives

distance (depth perception) by threemajor means:• (1) the size of the image of known

objects on the retina• (2) the phenomenon of moving parallax• (3) the phenomenon of stereopsis

•

Depth Perception



Depth Perception

• Determination of Distance by Sizes of

Images of Known Objects•

Depth Perception



p p

• Determination of Distance by Moving

Parallax•

•

•when one’s head is moved

to one side or the other,the images of close-byobjects move rapidlyacross the retinas while

the images of distantobjects remainstationary

Depth Perception



p p

• Determination of Distance by Stereopsis —Binocular Vision

• the 2 eyes are about 2 inches apart, an object thatis 1-2 inches in front of the bridge of the nose

forms an image on the left side of the retina of the left eye but on the right side of the retinaof the right eye

•

• gives a person with two eyes far greater ability

to judge relative distances of nearby objectsthan a person who has only one eye

• virtually useless for distances beyond100 to 200 feet

•

Depth Perception



p p

Fluid System of the Eye



y y

• divided into two portions – the

aqueous and the vitreous humor • the aqueous humor is a freely flowing

fluid

• the vitreous humor is a gelatinousmass• held together by a fine fibrillar network

• composed primarily of greatly

elongated proteoglycan molecules• Substances diffuse slowly in the

vitreous humor, but there is little flowof fluid.

•




y y

• Aqueous humor is continually being formed

and reabsorbed.• formed in the eye at an average rate of 2 to 3

μL/min

• secretion begins with active transport of Na+ ions into the spaces between theepithelial cells.

• the Na+ ions pull Cl- and HCO3 ions along

with them to maintain electrical neutrality

• cause osmosis of water from the blood

capillaries into the epithelial intercellular spaces

• The balance between formation andreabsorption regulates the total volumeand pressure of the intraocular fluid.

•




y y

Outflow of Aqueous Humor



q

• Fluid from the ciliary processes then flows into

the angle between the cornea and the irisand then through a meshwork of trabeculae,finally entering the canal of Schlemm, whichempties into extraocular veins.

• The canal of Schlemm is a thin-walled vein that

extends circumferentially all the way aroundthe eye.• endothelial membrane is so porous that even red

blood cells can pass from the anterior chamber into the canal of Schlemm.

• small veins that lead from the canal of Schlemm to the larger veins of the eyeusually contain only aqueous humor, andthey are called aqueous veins

•




y y



Intraocular Pressure



• The average normal IOP is about 15

mmHg, with a range from 12 to 20mmHg.

• determined mainly by the resistance comingfrom the meshwork of trabeculae

through which the fluid must percolate• These trabeculae have minute openings of only 2 -3

μm.

• The rate of fluid flow into the canal

increases markedly as the pressure rises.• At about 15 mm Hg in the normal eye, the fluid

outflow normally averages 2.5/μl/min and equalsthe inflow of fluid from the ciliary body.

Glaucoma



• one of most common causes of

blindness

• the IOP reaches up to 60-70mmHg

• can cause blindness withindays or even hours.

• Pressures rising above 20-30 mm

Hg can cause loss of visionwhen maintained for longperiods.

•

–

Glaucoma



• axons of the optic nerve are

compressed at the optic disc -ischemia• blocks axonal flow of cytoplasm from

the neuronal cell bodies in the retina

to the extended optic nerve fibersentering the brain

• causes lack of nutrition of the fibers,resulting to death of the fibers

• compression of the retinal artery addsto the neuronal damage by reducingnutrition to the retina

•

•

Glaucoma



Glaucoma



• Usually, the abnormally high pressure

results from increased resistanceto outflow through the trabecular spaces into the canal of Schlemm

– in acute eye inflammation, whiteblood cells and tissue debris canblock these trabecular spaces andcan cause acute increase in IOP

– in chronic conditions, fibrousocclusion of the trabecular spacescauses gradual increase in IOP

Glaucoma



• Angle-closure glaucoma

• Instances when the iridocorneal anglenarrows enough to cause blockade of theoutflow cause drastic increases in IOPwith severe consequences

•

Lecture Points



• The Transformer

• The Retina

• Layers of the retina

• Cells of the retina

• Photochemistry of Vision

• Color Vision

• Contrast

• Light and Dark Adaptation



•

• Receptor and NeuralFunctions of the Retina

Layers of the Retina



• arranged from the outside to the inside:• (1) pigment epithelium• (2) receptor layer – rods and cones

• (3) external limiting membrane• (4) outer nuclear layer • (5) outer plexiform layer • (6) inner nuclear layer • (7) inner plexiform layer • (8) ganglion cell layer • (9) optic fiber layer

• (10) inner limiting membrane•



Pigment Epithelium



• melanin in the pigment layer prevents light

reflection throughout the globe of theeyeball

• this is extremely important for clear vision.

• Like the black coloring inside the bellows of a

camera.• Absence of it would cause diffuse lighting

of the retina rather than the normalcontrast between dark and light spots

required for formation of precise images.



Pigment Epithelium



• also stores large quantities of vitamin

A• an important precursor of the

photosensitive pigments and that

this interchange of vitamin A isimportant for adjustment of the levelof light sensitivity of the receptors.

•

Pigment Epithelium



• Other functions of pigment cells in this layer:

• has tentacle-like processes that extend intothe photoreceptor layer and surround theouter segments of the rods and cones toprevent scatter of light

•

maintain the contact between the pigmentand receptor layers

• phagocytose the ends of the rod’s outer segments which are continuously shed

• reconversion of metabolized

photopigment into a reusable form after itis transported back into the photoreceptors



Fovea



External Limiting Menbrane



• Formed by the connections between

inner segments of rod cells andMϋller cells

• Mϋller cells are glial cells (supporting

cells) that help maintain thegeometry of the retina



Outer Nuclear Layer



• Contains the nuclei of rods and cones

•

• Outer Plexiform Layer

• Contains dendrites and axons of

photoreceptors and retinal interneurons,including bipolar and amacrine cells



Inner Nuclear Layer



• Contains the cell bodies of retinalinterneurons (bipolar cells, horizontal cellsand amacrine cells) and Mϋller cells.

•

•

Inner Plexiform Layer • Contains the dendrites and axons of retinal

interneurons and the ganglion cells



Ganglion Cell Layer andOptic Fiber Layer



• Contain the cell bodies and axons of theganglion cells

• The axons avoid the macula and enter theoptic disc

• The optic fiber layer portions of the ganglion cellaxons are unmyelinated

• This helps permit light to pass through withminimal distortion



Rods and Cones



• in the peripheral retina• rods are 2-5 μm in

diameter

• cones are 5-8 μm indiameter

•

• in the central retina, in thefovea

•the cones are more

slender and only 1.5 μmin diameter

Rods and Cones



• the major functionalsegments of either a rodor a cone:•

(1) the outer segment• (2) the inner segment• (3) the nucleus• (4) the synaptic body

Rods and Cones



• Receptor functions

• The light-sensitive photochemical is foundin the outer segment.

• for the rods, this is rhodopsin

• in the cones, it is one of three "color"

photochemicals, usually called simplycolor pigments

• function almost exactly the same asrhodopsin except for differences in

spectral sensitivity.

Rods and Cones



• The outer segment iscomposed of discs

– an infolded shelf of cellmembrane

– as many as 1000 discs ineach rod or cone

• Both rhodopsin and the color pigments are are incorporatedinto the membranes of the discs inthe form of transmembraneproteins.

• constitute about 40% of the entiremass of the outer segment.

Rods and Cones



• The inner segment contains the usual

cytoplasm of the cell with the usualorganelles.

– particularly important are the mitochondria

– provides the energy for function of the

photoreceptors• The synaptic body connects with the

subsequent horizontal and bipolar cells,that represent the next stages in the

vision chain.•

Rods and Cones



Photochemistry of Vision



• rods and cones contain chemicals

that decompose on exposure tolight

• in the process, nerve fibers leading

from the eye are excited• light-sensitive chemical in the rods is

called rhodopsin

• light-sensitive chemicals in the cones

are called cone pigments or color pigments




• Rhodopsin, or visual purple, constitutes

about 40% of the outer segments of rods.• combination of the protein scotopsin and

the carotenoid pigment retinal (or retinene).

• the retinal is a particular type called 11-cisretinal.

• This cis form is important because only thisform can bind with scotopsin to synthesize

rhodopsin.

•



• the immediate product is bathorhodopsin,

which is a partially split combination of the all-trans retinal and scotopsin.

• bathorhodopsin is extremely unstable anddecays in nanoseconds to

lumirhodopsin.• lumirhodopsin then decays in

microseconds to metarhodopsin I

• metarhodopsin I to metarhodopsin II in amillisecond

• finally, into the completely split products inseconds: scotopsin and all-trans

retinal




• the metarhodopsin II, also called activated

rhodopsin, excites electrical changes inthe rods

• the rods then transmit the visual image intothe central nervous system.




• The re-formation of rhodopsin• The first stage is to reconvert the all-

trans retinal into 11-cis retinal.

• requires metabolic energy and iscatalyzed by the enzyme retinal

isomerase.• 11-cis retinal automatically

recombines with the scotopsin tore-form rhodopsin,

• remains stable until its decompositionis again triggered by absorption of light energy

•




• Role of Vitamin A•

There is a second chemical route by whichall-trans retinal can be converted into 11-cis retinal

• by conversion of the all-trans retinal first intoall-trans retinol, which is one form of vitamin A

• the all-trans retinol is converted into 11-cisretinol under the influence of the enzymeisomerase.

• the 11-cis retinol is interconverted into 11-cis retinal, which combines withscotopsin to form new rhodopsin.




• Role of Vitamin A•

Vitamin A is present both in the cytoplasm of the rods and in the pigment layer

• always available to form new retinalwhen needed.

•

when there is excess retinal in the retina, theexcess is converted back into vitamin A

• reducing the amount of light-sensitivepigment in the retina.

• important in long-term adaptation of the retina to different light intensities

•

Night Blindness



• severe vitamin A deficiency (nyctalopia)•

not enough vitamin A is available to formadequate quantities of retinal

• the amount of light available at night is toolittle to permit adequate vision

• a person with nyctalopia usually had avitamin A-deficient diet for monthsbecause large quantities of vitamin A arenormally stored in the liver

• Once it develops, it may be reversed in lessthan 1 hour by intravenous injection of vitamin A.

•




•The inner segment continuallypumps Na+ outside of the cell,creating a negative potential onthe inside.

•The outer segment of the rod,where the photoreceptor discs arelocated, is very leaky to let in Na+ ions to neutralize much of the

negativity on the inside of theentire cell.



• Excitation of a Rod

• When the rod is exposed to light, theresulting receptor potential causesincreased negativity of the intrarodmembrane potential, which is a state

of hyperpolarization.• it decreases the conductance for Na+

ions in the outer segment of the rod,causing hyperpolarization of the

entire rod membrane




• the greater the amount of light energy

striking the rod, the greater theelectronegativity

• the greater the degree of hyperpolarization

•

• At maximum light intensity, the membranepotential approaches -70 to -80rnillivolts,

•




• Duration of the Receptor Potential

• When a sudden pulse of light strikesthe retina, the transienthyperpolarization that occurs in therods reaches a peak in about 0.3

second and lasts for more than asecond.

• In cones, these changes occur 4x asfast.

• A visual image impinged on the rods of the retina for only 1 millionth of asecond can cause the sensation of seeing the image for longer than asecond.



• The Photo Cascade

• photoreceptors are extremely sensitiveand can amplify the stimulatoryeffects about a millionfold throughthe following mechanism:

•

• 1. The photon activates an electron inthe 11-cis retinal portion of therhodopsin;, leading to the formation of metarhodopsin II




• The Photo Cascade• 2. The activated rhodopsin functions as

an enzyme to activate manymolecules of transducin, a proteinpresent in the membranes of the discs

and cell membrane of the rod.• 3. The activated transducin in turn

activates many more molecules of phosphodiesterase.




• The Photo Cascade• 4. Activated phosphodiesterase

immediately hydrolyzes many moremolecules of cGMP

• the cGMP had been bound with the

Na+ channel protein of the outer membrane in a way that "splints" itin the open state.

• When cGMP is hydrolyzed, the

splinting is removed and causes thesodium channels to close




• The Photo Cascade• 4. Activated phosphodiesterase

immediately hydrolyzes many moremolecules of cGMP

• Several hundred channels close for

each molecule of rhodopsin.• flow of more than a million Na+ ions

is blocked by the channel closurebefore the channel opens again

• Decreased Na+ ion flow excites therod.




• The Photo Cascade•

5. rhodopsin kinase then inactivatesmetarhodopsin II, and the entirecascade reverses back to the normalstate with open Na+ channels.



Photochemistry of Color Vision



• Versus Rods

• the protein portions, the opsins, in thecones are called photopsins

• they are very slightly different from thescotopsin of the rods

• The retinal portion of all the visualpigments is exactly the same in thecones as in the rods.

•

The color-sensitive pigments of thecones are combinations of retinaland photopsins.




• 1 of 3 types of color pigments is

present in each of the differentcones

• This make the cones selectively

sensitive to the different colors:blue, green, and red.• The absorption characteristics of the

pigments in the 3 types of cones

show peak absorbancies at lightwavelengths of 445, 535, and 570 nm.




Color Vision



• Tricolor Mechanism of Color Detection

• the human eye can detect almost allgradations of colors when only red,green, and blue monochromaticlights are appropriately mixed in

different combinations.• The interpretation of color in the

nervous system depends on thestimulus value on each kind of cones

• Stimulus value = percent peakstimulation at optimum wavelength

• Orange = red cone: 99, green cone:42, blue cone: 0

•



Color Vision

f



• Perception of White Light.

• there is no wavelength of lightcorresponding to white

• equal stimulation of all the red,green, and blue cones gives one the

sensation of seeing white

• white is a combination of all thewavelengths of the spectrum

Color Vision Abnormalities

if ith th d i



• if either the red or green cone is

missing, the person cannot use thismechanism for distinguishing thecolors between their respectivewavelengths

• The person is especially unable todistinguish red from green – red-green color blindness.

Color Vision Abnormalities

l f d t



• loss of red cones – protanope

• lacks green cones – deuteranope•

• Red-green color blindness is an X-

linked recessive disorder • Only seen in males

• Females are carriers

Red-Green Color Blindness



Normal

74

Color-blind

21

Red-Blind vs Green-Blind



2 4



02/12/10

•

•

•

•

Neural Function of the Retina

Neurotransmitters

b th th d d th l



• both the rods and the cones release

glutamate at their synapses with thebipolar cells

• amacrine cells secrete at least 8 types of transmitter substances, including GABA,glycine, dopamine, acetylcholine, and indolamine, all as inhibitory transmitters.

• The transmitters of the bipolar, horizontalcells are unclear

• some of the synapses of horizontal cellsinvolve electrical transmission rather than chemical transmission.

Rods and Cones

N l f ti



• Neural functions•

Transforms light energy toelectrochemical energy

• Transmission of most signals isthrough electrotonic conduction,

not by action potentials.• direct flow of electric current in theneuronal cytoplasm from the point of excitation all the way to the outputsynapses

• The ganglion cells are the only retinalneurons that transmit signals bymeans of action potentials, and theysend their signals all the way to thebrain.

•

Rods and Cones

N l f ti



• Neural functions

• electrotonic conduction allows gradedconduction of signal strength andnot all-or-none

• for the rods and cones, the strength of

the hyperpolarizing output signal isdirectly related to the intensity of illumination

Rods and Cones

Ne ral f nctions



• Neural functions

• The neurons and nerve fibers thatconduct the visual signals for conevision are considerably larger thanthose for rod vision

• the signals are conducted to the brain2-5 times as rapidly.

• the circuitries for the two systems areslightly different

Rods and Cones



• Pure Cones• Bipolar cells

• Ganglion cells

•

• Pure rods• Bipolar cells

• Amacrine cells

• Ganglion cells

Horizontal Cells

• Functions for lateral inhibition to enhance



• Functions for lateral inhibition to enhance

visual contrast• The outputs of the horizontal cells are

always inhibitory.• this lateral connection provides the same

phenomenon of lateral inhibition that isimportant in all other sensory systems

• helping to ensure transmission of visualpatterns with proper visual contrast into

the central nervous system

Horizontal Cells



Horizontal Cells

• the centralmost area where the light strikes



• the centralmost area where the light strikes

is excited, whereas an area to the side isinhibited

• transmission through the horizontal cellsputs a stop to this by providing lateral

inhibition in the surrounding area.• allows high visual accuracy in transmitting

contrast borders in the visual image

• amacrine cells probably provide additional

lateral inhibition and further enhancementof visual contrast

Bipolar Cells

• 2 types of bipolar cells:



• 2 types of bipolar cells:•

depolarize when the rods and cones areexcited

• hyperpolarize when the rods and cones areexcited

• Both respond to glutamate released by the rods

and cones, but in different direction of effects.• one receives direct excitation from the rods and

cones

• the other receives its signal indirectly through ahorizontal cell

• Because the horizontal cell is an inhibitorycell, this would reverse the polarity of theelectrical response.

Bipolar Cells

• it provides a second mechanism for lateral



• it provides a second mechanism for lateral

inhibition in addition to the horizontal cell• depolarizing and hyperpolarizing bipolar

cells lie immediately against each other ,• providing a mechanism for separating

contrast borders in the visual image• even when the border lies exactly between

two adjacent photoreceptors

• horizontal cells operate over a much

greater distance

Horizontal Cells



Amacrine Cells

• Some functions include:



• Some functions include:•

4. to respond both when a light is turnedeither on or off , signalling simply a changein illumination irrespective of direction

5.to respond to movement of a spot across theretina in a specific direction –directionally sensitive

•

• amacrine cells are types of interneurons thathelp in the beginning analysis of visual

signals before they ever leave the retina

Ganglion Cells

• Each retina contains about 100



• Each retina contains about 100

million rods and 3 million cones• the number of ganglion cells is only

about 1.6 million.

• an average of 60 rods and 2 conesconverge on each ganglion cell andits subsequent optic nerve fiber

Ganglion Cells



• Central retina• fewer rods and cones converge on each optic

fiber

• the rods and cones both become more slender

•

progressively increases the acuity of visiontoward the central retina

• in the central fovea, there are only slender cones,about 35,000 of them, and no rods.

• the number of optic nerve fibers is almost exactlyequal to the number of cones

Ganglion Cells

• Peripheral retina



• Peripheral retina

• much greater sensitivity to weak light.• because the rods are 30-300 times

more sensitive to light than arecones

• because as many as 200 rodsconverge on a single optic nervefiber in the more peripheral portionsof the retina

• the signals from the rods summate

to give even more intense stimulationof the peripheral ganglion

Ganglion Cells

•



•

• 3 distinct groups of ganglion cells:• W cells

• X cells

•

Y cells•

• Each of these serves a different

function.•

Ganglion Cells



• W cells• 40% of all the ganglion cells

• having a diameter <10 μm

• transmitting signals at the slow rate of only 8m/sec

• receive most of their excitation from rods

• have broad fields in the peripheral retina becausetheir dendrites spread widely in the inner plexiform layer

•

especially sensitive for detecting directionalmovement

• important for much of our crude rod vision under dark conditions.

•

•

Ganglion Cells



• X cells• 55% of the total

• medium diameter, between 10 and 15 μm

• transmit signals at about 14 m/sec

•

have small fields because their dendrites donot spread widely in the retina

• the signals represent rather discrete retinallocations

• the visual image itself is transmitted throughX cells

• receives input from at least one cone andprobably responsible for all color vision

Ganglion Cells



• Y cells• only 5% of the total

• up to 35 μm in diameter

• transmit their signals as fast as or

faster than 50 m/sec.

• have broad dendritic fields covering abroad area of the retina

Ganglion Cells



• Y cells• respond like many of the amacrine cells to

rapid changes in the visual image• either rapid movement or rapid change in light

intensity, sending bursts of signals for only

small fractions of a second.• apprise the central nervous system almost

instantaneously

• without specifying with accuracy thelocation of the event

• just give appropriate clues for moving theeyes toward the exciting vision



02/12/10

Ganglion Cells

• from the ganglion cells, the long fibers of the



from the ganglion cells, the long fibers of the

optic nerve lead into the brain• ganglion cells transmit their signals by

means of action potentials• electrotonic method of conduction is no

longer appropriate because of the distanceinvolved

• even when unstimulated, they still transmitcontinuous impulses at rates varying

between 5-40/sec with the nerve fibersfrom the large ganglion cells firing morerapidly.

• The visual signals in turn are superimposedonto this background ganglion cell firing.

On-off, off-on responses



Responses of ganglion cells to light in(1)an area excited by a spot of light and(2)an area adjacent to the excited spot; the ganglion cells in this area are

inhibited by the mechanism of lateral inhibition.

On-off, off-on responses

• The opposite directions of these responses



pp p

to light are caused by the depolarizing and hyperpolarizing bipolar cells.

• the transient nature of the responsesis probably at least partly generated

by the amacrine cells



Lateral Inhibition – Visual Contrasts

• bright spot of light excites the direct



g p g

pathway through the bipolar cell• Because one of the lateral photoreceptors isin the dark, the affected horizontal cellsare inhibited.

•

this cell loses its inhibitory effect on thebipolar cell, allowing more excitation of thebipolar cell

• where visual contrasts occur, the signals

through the direct and lateral pathwaysaccentuate one another

Lateral Inhibition – VisualContrasts



Light and Dark Adaptation

• the sensitivity of the retina always be



t e se s t ty o t e et a a ays be

adjusted so that the receptorsrespond to the lighter areas but notto the darker areas.

• the eye can change its sensitivity tolight as much as 500,000 to1,000,000 times

• the sensitivity automatically adjusts to

changes in illumination


• If a person has been in bright light for a long



p g g g

time, most of the photochemicals in boththe rods and the cones will have beenreduced to retinal and opsins – lack of rhodopsin.

• the retinal of both the rods and the coneswill have also been converted intovitamin A

• The photosensitive chemicals remaining

in the rods and cones are considerablyreduced.

• The sensitivity of the eye to light iscorrespondingJy reduced.


• If the person remains in darkness for a long



p g

time, the retinal and opsins in the rodsand cones are converted back into thelight-sensitive pigments – surplus of rhodopsin.

• vitamin A is reconverted back into retinalto give still additional light-sensitivepigments

• photosensitive chemicals remaining in the

rods and cones and the sensitivity of theeye to light are considerably increased.


• the sensitivity of the retina is very low on



first entering the darkness• within 1 minute, the sensitivity has alreadyincreased 10-fold

• the retina can respond to light of 1/10 the

previously required intensity• at the end of 20 minutes, the sensitivity has

increased about 6000-fold

• at the end of 40 minutes, about 25,000-fold

Dark Adaptation Curve




• Other Mechanisms:



• change in pupillary size• can cause adaptation of

approximately 30-fold within afraction of a second, because

of changes in the amount of light allowed through thepupillary opening.

•


• Other Mechanisms:



•

neural adaptation • when the light intensity first increases,

the signals transmitted by the bipolar cells, horizontal cells, amacrine cells,and ganglion cells are all intense.

• most of these signals decreaserapidly at different stages of transmission in the neural circuit

• neural and pupillary adaptation, occur in a fraction of a second, versus fulladaptation by the photochemicals,which require many minutes to hours

•

Lecture Points

• The Software



• The Visual Pathway• Dorsal Lateral Geniculate

Nucleus

• Visual Cortices

• Control of Ocular Movements

•



• Central Neurophysiology of Vision



Visual Pathway

• visual nerve signals leave the retinae passing backward



• visual nerve signals leave the retinae, passing backward

through the optic nerves• the optic nerve fibers from the nasal halves of the retinae

cross to the opposite sides at the optic chiasm, joining the fibers from the opposite temporal retinae to

form the optic tracts• optic tract fibers then synapse in the dorsal lateralgeniculate nucleus

• geniculocalcarine fibers pass by way of the opticradiation to the primary visual cortex in the calcarine

area of the occipital lobe

Visual Pathway

• visual fibers pass also to several



p

older areas of the brain:• (1) from the optic tracts to the

suprachiasmatic nucleus of thehypothalamus

•

for controlling circadian rhythms thatsynchronize various physiologic changes of the body with night and day;

• (2) into the pretectal nuclei• for eliciting reflex movements of the eyes to

focus on objects of importance and for activating the pupillary light reflex

Visual Pathway

• visual fibers pass also to several



older areas of the brain:• (3) into the superior colliculus

• for control of rapid directional movementsof the two eyes

• (4) into the ventral lateral geniculatenucleus of the thalamus and then intosurrounding basal regions of the brain

• to help control some of the body's behavioral

functions

Visual Pathway

• the visual pathways can be divided into anld t t th idb i d b f



old system to the midbrain and base of the forebrain and a new system for directtransmission of visual signals into thevisual cortex located in the occipitallobes

• The new system is responsible in humansfor perception of virtually all aspects of visual form, colors, and other

conscious vision.

Dorsal Lateral Geniculate Nucleus

• Functions:



•

1.) relays visual information from theoptic tract to the visual cortex by way of the geniculocalcarine tract.

• There is exact point-to-point transmissionwith a high degree of spatial fidelity

from the retina to the visual cortex(retinotopic map).• the signals from the two eyes are kept

apart in the dorsal lateral geniculatenucleus divided into sex layers:

• Layers II, III, and V (2,3,5)receive signalsfrom the temporal half of the ipsilateralretina

• Layers I, IV, and VI (1,4,6) receive signalsfrom the nasal half of the contralateralretina.




• the nucleus is divided in another way:



•

(1) Layers I and II are calledmagnocellular layers becausethey contain large neurons.

• receive their input almost

entirely from the large type Yganglion cells

• provides a rapidly conductingpathway to the visual cortex

• color blind, transmitting onlyblack and white information,point-to-point transmission ispoor


• the nucleus is divided in another way:



•

(2) Layers III through VI are calledparvocellular layers becausethey contain small to medium-sized neurons.

• receive their input almostentirely from the type Xganglion cells

• transmit color and convey

accurate point-to-point spatialinformation

• only a moderate velocity of conduction

Visual Cortices



Visual Cortices



Primary Visual Cortex

• another name is striate cortex because of its grossly striated appearance



its grossly striated appearance

• coextensive with Brodmann's cortical area17. It is also called visual area 1 or V-1

• Primary visual cortex lies in the calcarine

fissure area • extending forward from the occipital pole on

the medial aspect of each occipital cortex.

• The upper portion of the retina is

represented superiorly and the lower portion inferiorly.




• Signals from the macular area of the retinaterminate near the occipital pole



terminate near the occipital pole,whereas signals from the more peripheral retina terminate in concentric circlesanterior to the pole

• the macula is represented by a large area• retinal fovea transmits its signals into this

region.• the fovea has several hundred times as

much representation in the primary visualcortex as do the most peripheral portions of the retina

•

Secondary Visual Areas

• also called visual association areas,(Brodmann's area 18)



(Brodmann s area 18)

• lie lateral , anterior , superior , and inferior tothe primary visual cortex

•

• Secondary signals are transmitted to theseareas for analysis of visual meanings.

• The importance of all these areas is thatvarious aspects of the visual image are

progressively dissected and analyzed.•

•

Visual Cortex

• The separate neuronal layers in the



LGN remain separated from eachother when they arrive in the primaryvisual cortex.

• The cortical layer is interlaced withstripes of neuronal columns, eachstripe about 0.5 mm wide• signals from one eye enter the

columns of every other stripe,alternating with signals from thesecond eye

Visual Cortex

• The cortex deciphers whether the



two visual images from the twoseparate eyes fit with each other.

• The deciphered information is usedto adjust the directional gaze of

the separate eyes so that they willfuse with each other.

• allows a person to distinguishdistances of objects by themechanism of stereopsis

•

•

Visual Analysis

• Analysis of Contrasts



• The primary visual cortex isconcerned mainly with thecontrasts in the visual scene,rather than with thenoncontrasting areas.

• the areas of maximumexcitation occur along the

sharp borders of the visualpattern

–

Visual Analysis



Visual Analysis

• Analysis of Contrasts



•

equally stimulated adjacent retinalreceptors mutually inhibit oneanother.

• where there is a change from dark to

light or light to dark, mutual inhibitiondoes not occur

• the intensity of stimulation of mostneurons is proportional to the

gradient of contrast• the greater the sharpness of contrast

and the greater the intensitydifference between the light and darkareas, the greater the degree of

Visual Analysis

• Detection of orientation of lines and border • the visual cortex not only detects the



• the visual cortex not only detects theexistence of lines and borders , but it alsodetects the direction of orientation of each line or border

• whether it is vertical or horizontal or

lies at some degree of inclination• These are detected by neuronal cells, called

simple cells.• for each such orientation of a line, specific

simple cells are stimulated• A line oriented in a different direction

excites a different set of cells.

Color Analysis

• detected by means of color contrast, sameas how lines are detected



as how lines are detected

• For instance, a red area is often contrastedagainst a green area, or a blue areaagainst a red, or a green area against a

yellow.• The mechanism of color contrast analysisdepends on the fact that contrastingcolors, called "opponent colors,“ mutually

excite specific neuronal cells.


• Effect of Removing the Primary VisualCortex



Cortex• causes loss of conscious vision, or blindness

• studies demonstrate that such blind peoplecan still react subconsciously to changesin light intensity, to movement in the

visual scene, and even to some grosspatterns of vision

• reactions include turning the eyes,turning the head, and avoidance

• this vision is believed to be subservedby neuronal pathways in thesuperior colliculi and other portionsof the older visual system

•

Fields of Vision: Perimetry

• The field of vision is the visual area seen byan eye at a given instant.



y g

• assessed to diagnose blindness in specificportions of the retinae, the process iscalled perimetry.

•

• the subject is made to look with one eyetoward a central spot directly in front of the eye

• a small dot of light or a small object ismoved back and forth in all areas of thefield of vision

• the person indicates when the spot of lightor object can be seen and when it cannot

•

Perimetry



Perimetry

• a blind spot



a blind spotcaused by lackof rods andcones in theretina over the

optic disc isfound about 15O lateral to thecentral point of vision

Lesions in the Visual Pathway




• Destruction of an entire optic nerve causesblindness of the affected eye.



• Destruction of the optic chiasm causesbitemporal hemianopsia.

• result from tumors of the pituitary gland pressingupward from the sella turcica on the bottom of the optic chiasm.

• Destruction of an optic tract causehomonymous hemianopsia.• Destruction of a part of optic causes

homonymous quadrantanopsia.• Lesions in the visual cortex usually has macular

sparing.• Usually due to thrombosis of the posterior

cerebral artery, which often infarcts theoccipital cortex.

•

•



•

•Control of Eye Movements

Muscular Control

• The eye movements are controlled by threepairs of muscles



p• (1) the medial and lateral recti– move the eyes

from side to side.• (2) the superior and inferior recti – move the

eyes upward or downward.•

(3) the superior and inferior obliques – rotatethe eyeballs to keep the visual fields in theupright position

• each of the three sets of muscles to eacheye is reciprocally innervated so that

one muscle of the pair relaxes whilethe other contracts

Muscular Control



Fixation Movements

• the most important movements of the eyesare those that cause the eyes to "fix" on



ya discrete portion of the field of vision

• Fixation movements are controlled by twoneuronal mechanisms.

1)mechanism that allows the person to movethe eyes voluntarily to find the object tofix one's vision – voluntary fixationmechanism.

2)an involuntary mechanism that holds the

eyes firmly on the object once it has beenfound – involuntary fixationmechanism.

Fixation Movements

• The voluntary fixation movements arecontrolled by a small cortical field located



ybilaterally in the premotor corticalregions of the frontal lobes

• Bilateral dysfunction makes it almostimpossible for a person to "unlock" the

eyes from one point of fixation and thenmove them to another point

• the person has to blink the eyes or put ahand over the eyes for a short time to allow

the eyes to be moved



Fixation Movements

• the involuntary fixation mechanism iscontrolled by the secondary visual areas



y yof the occipital cortex – mainly Brodmann19

• When destroyed bilaterally, one has difficultyor even inability keeping his eyes directed

toward a given fixation point• the posterior cortical eye fields automatically

"lock" the eyes on a given spot of thevisual field, preventing movement of the

image across the retina.• to unlock this fixation, voluntary impulses

must be transmitted from the cortical"voluntary“ eye fields located in the frontalareas

Fixation Movements

• From the visual fixation areas of theoccipital cortex, signals pass to the



g

superior colliculi, before going toreticular areas around the oculomotor nuclei, and then into the motor nucleithemselves.

• This involuntary fixation capability is mostlylost when the superior colliculi aredestroyed.

Superior Colliculi

• a sudden visual disturbance in a lateral areaof the visual field will cause immediate



turning of the eyes in that direction evenafter the visual cortex has been destroyed

• This is mainly due to the function of thesuperior colliculi

• the principal direction of a flash of light in aperipheral retinal field is mapped by thecolliculi

• secondary signals are then transmitted to

the oculomotor nuclei to turn the eyes.

Superior Colliculi

• There is also a retinotopic map in thesuperior colliculi, like in the LGN and in



the primary visual cortex, although withless accuracy.

• also have topological maps of somaticsensations from the body and acoustic

signals from the ears• cause turning of the whole head and even of

the whole body toward the direction of thedisturbance

• other types of disturbances besides visual,

such as strong sounds or even stroking of the side of the body, cause similar turningof the eyes, head, and body

•

Superior Colliculi

• The rapidly conducting Y fibers of the opticnerve from the eyes to the colliculi are



responsible for these rapid turningmovements

• one branch goes to the visual cortex and theother to the superior colliculi

•

• The superior colliculi play a global role inorienting the eyes, head, and body with

respect to external disturbances whether visual, auditory, or somatic.

•



THANK YOU!!

Beda Lecture - Vision

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