Visual Physiology 2

8/7/2019 Visual Physiology 2

http://slidepdf.com/reader/full/visual-physiology-2 1/9

PHYSIOLOGY VISION 2 1

SUBJECT: PHYSIOLOGY

TOPIC: VISION 2

LECTURER: DR. VIC MENDOZA

DATE: MARCH 2011

VISION PHYSIOLOGY 2

Rods vs. Cones–

Rods produce a reliable response to asingle photon of light

–

it takes over a 100 photons toproduce a comparable responsein a cone.

–

Cones adapt better than do rods–

about 200ms for a cone

–

800ms for a rod.

–

Rods synapse onto specific bipolar cells(rod bipolars) that synapse ontoamacrine cells which contact both conebipolars and ganglion cells.

–

Cones go to bipolar cell to RGC[Retinal Ganglion Cells] directly.

–

Rods exhibit convergence-many rods

synapse onto (converge on) a singlebipolar cell, many bipolars onto a singleamacrine cell.

–

cones can be 1-1-1

In the retina… •

There are 91 million rods and 4.5million cones .

•

In most places the density of rodsexceeds that of cones.

•

Changes dramatically in the fovea,central retina (1.2 mm in diameter).

–

Cones increase in density 200fold, become highly packed.

Center of the fovea, calledfoveola, is totally rod free.

–

Gives high visual acuity, whichdecreases rapidly away fromthe fovea.

–

Reason why we are constantlymoving our heads to center oureyes toward what we want to

look at.•

Reason why it is best tosee a dim object bylooking away from it.

Rods Cones

90-120 million 4-6 millionperipheral vision central visionlocate deverywhereexcept fovea

high density in maculaand fovea

sensitive to light less sensitive to lightused in low lightsituations

most normal lightingconditions

one type three tyoes [red, blue,

green]highly convergent non-convergentblack and white color vision

Color Vision•

3 different cone cells. Each have adifferent form of opsin (they have thesame retinal)

•

3 forms of photopigments, each havingdiffererent absorption spectra – blue,

green and red opsin–

10% red cones

–

45% blue cones

–

45% green cones

•

Coloured light will stimulate these 3cells differently - by comparing the



PHYSIOLOGY Vision 2 2

nerve impulses from the 3 kinds of cones the brain can detect any colour

–

Red light stimulates R cones

–

Yellow light stimulates R andG cones equally

–

Cyan light stimulates B and Gcones equally

–

White light stimulates all 3cones equally

•

Called the trichromatic theory of colourvision

The red, green and blue opsin proteins aremade by three different genes. The green and

red genes are on the X chromosome, whichmeans that males have only one copy of thesegenes (i. e. they’re haploid for these genes).

About 8% of males have a defect in one or otherof these genes, leading to red-green colourblindness. Other forms of colour blindness arealso possible, but are much rarer.

Most people can match any color by changingthe intensities of these three colors (RGB).

•

5-6% of males are color blind due tomutations in the red and green opsins.The pigment genes are X-linked andnear each other.

–

Can get hybrid gene or loss of

gene during crossing over inmeiosis. Loss of gene can resultin deficit or absence of eithergreen or red cones.

•

When we look at something the imagefalls on the fovea and we see it in

colour and sharp detail.•

Objects in the periphery of our field of view are not seen in color, or detail .

•

The fovea has high density of cones.

•

Each cone has a synapse with onebipolar cell and one ganglion eachcone sends impulses to the brain aboutits own small area of the retina highvisual acuity

Other types of cells in the Retina...•

Bipolar cells - cell bodies in the innernuclear layer.

•

Gets information from thephotoreceptors in outerplexiform layer and transmits itto ganglion cells and amacrinecells in inner plexiform layer.

Rods and cones use specifictypes of bipolars.

•

Ganglion cells - cell bodies in ganglioncell layer.

•

Output neurons of the retina .Receives information frombipolar and amacrine cells andsend it out through the optic

nerve.•

Horizontal cells - cell bodies in the innernuclear layer

•

Makes multiple contacts withphotoreceptors and bipolarcells [largely responsible forluminance contrast].

•

Amacrine cells - cell bodies in innernuclear layer

•

Makes contact in the innerplexiform layer with bipolarcells and ganglion cells.

•

Several distinct subclasses.

•

Coordinate ganglion cell activity[i.e. motion]

RETINAL CIRCUITRY

Photoreceptors synapse on the dendrites of bipolar cells and horizontal cells in the outerplexiform layer. The horizontal cells make




transverse connections with bipolar cells, andthey receive input from interplexiform cells .

Bipolar cells synapse on the dendrites of ganglion cells and on the processes of amacrine

cells in the inner plexiform layer. Amacrinecells connect with ganglion cells , otheramacrine cells, and interplexiform cells.

RETINAL GANGLION CELLS [RGC]Stephen Kuffler [1950]

•

Measured the action potentials fromspecific RGCs after shining light on the

retina.•

Determined that RGCs have receptivefields . A receptive field can be divided into center and surround .

Ganglion cells come in two types: on-center andoff-center (roughly equal proportions)

•

On center : fires more when the lightthat hits the center is brighter than thatof the surround, and fire less when it isdarker in the center than in thesurround.

•

Off center : fire less when it is brighterin center and fire more when it isdarker in the center.

•

Even in the dark , RGCs are

spontaneously active .•

Receptive fields of RGCs are circular .Smaller in the center and bigger in theperiphery

•

The receptive fields of RGCs overlap sothat multiple RGCs see each point of space.

On-center vs. Off-center RGC Responses

IN SUMMARY,•

For an on center-off surround RGC , a

point of light that fills the entire centerbut not in the surround will givemaximal stimulation (increased actionpotentials)

o

i.e. brighter in center than insurround.

•

A point of light in surround but not inthe center will silence the background .

•

Light that crosses into both will be inthe middle depending on the relativeamounts.

•

Both center and surround illuminated isbasically the same as being in the dark(background levels).

•

Ganglion cells fire depending oncontrast , not by absolute light intensity.

Bipolar Cells•

Do not use action potentials, but usegraded potentials to releasetransmitter.

•

There are two types of bipolar cells-

–

On center: use (metabotropic)glutamate receptors that leadto the closing of Na + channelsand hyperpolarize the cell.

–

Off center uses AMPA receptors (ionotropic) that cause the cellto depolarize in response toglutamate released byphotoreceptors.

Generating Receptive Field Center Responses… •

Light hits cone causeshyperpolarization of cone which leadsto less release of glutamate .




•

Two bipolar cells synapse with cone, anon-center and off-center bipolar cell.

•

On-center bipolars are normallyinhibited by glutamate

•

Less glutamate less

inhibition more release of neurotransmitter increase of on-center RGC firing.

•

Off-center bipolars are normallyactivated by glutamate

•

become hyperpolarized decrease transmitter release decrease firing rate of Off-

center RGC

**Light in the center causes on-center ganglion cells toincrease firing rate and off-center ganglion cells to

decrease their firing rate.**

Generating Antagonistic Surrounds of RGCs… •

Light hitting surround hyperpolarizescones in surround less glutamate tobe released from photoreceptor onto

horizontal cell dendrites.•

Horizontal cells hyperpolarize becauseof less glutamate and decrease theirrate of transmitter release onto the

synaptic terminals back ontophotoreceptors in the center.

•

Horizontal cells normally inhibit cones (using GABA as a neurotransmitter),thus now cones are more depolarizedthan normal releasing more

glutamate than normal(DISINHIBITION).

•

This depolarizes off-centerRGCs, causing them to increasetheir firing rate

•

This also hyperpolarizes on-center RGCs, causing them todecrease their firing rate.

Lateral or Afferent Inhibition•

The inhibition of the response to centralillumination by an increase insurrounding illumination.

•

Activation of a particular neural unit isassociated with inhibition of the activityof nearby units.

•

The inhibition of the center response bythe surround is due to the inhibitory

feedback from one photoreceptor toanother mediated by the horizontalcells.

•

Sharpens the edges of a stimulus andimprove discrimination of touch.

Two Types of Cells in the Retina... •

Large magnocellular ganglion cells, orM-cells, carry information aboutmovement, location, and depthperception.

•

Smaller parvocellular ganglion cells, orP-cells, transmit signals that pertain to

colour, form, and texture of objects inthe visual field.

•

Ganglion cells are further classified by

the properties of the visual field•

i.e. the particular area of retinato which it is linked

•

The area of the retina which connectsto a ganglion cell is known as the visualfield (when activity is recorded via theganglion cell axon).

•

Once action potentials leave these cells,they travel along the optic nerves to theCNS for further processing.

Again…

Magnocellular Cells Parvocellular CellsM Cells P CellsLarger SmallerMovement, location,depth, perception

Color, form, texture




SO…

•

Light falls on photopigment that is

transformed to action potentials thatganglion cells convey to the brain.

•

Phototransduction occurs in rods andcones that have different propertiesthat meet the conflicting demands of sensitivity and acuity.

•

RGCs have a center-surroundarrangement of receptive fields thatmakes them good at contrast detectionand relatively insensitive to background

illumination.

BRAINTerms:

•

Optic disc - all the RGC axons exit theeye at the optic disc (results in a blindspot) and form a big myelinated nervecalled optic nerve (cranial nerve II).

•

Optic chiasm- where the optic nerve from each side cross the midline (decussate), at the base of thehypothalamus and then enters thebrain.

•

Optic radiation- portion of the internalcapsule (connection between thalamusand cortex) containing the axons from

LGN [Lateral Geniculate Nucleus] thatproject to the visual cortex

•

Primary visual c ortex (V1), Brodmann’sarea 17

Central Visual PathwayThe retina (axons of RGCs) projects to multipleareas in the brain. Each area is specialized fordifferent functions.

•

Lateral geniculate nucleus (LGN) -located in the thalamus - receives visualinfo from retina and sends it to thevisual cortex. Most important visual

projection with respect to visualperception.

•

Pretectum - located at midbrain-thalamus boundary . Responsible for

Pupillary Light Reflex .•

Superior colliculus - in midbrain,coordinates head and eye movements.

•

Suprachiasmatic nucleus [inhypothalamus] -involved in day- nightcycles (one of the areas that has gapjunctions).

Afferent projections of Visual System

PUPILLARY LIGHT REFLEX•

Light hits retina, sends out axons(sensory afferents) to both sides of

brain that go to the pretectum.•

Pretectal neurons project to contra andipsilateral Edinger-Westphal nuclei (inmidbrain).

•

Edinger-Westphal projects to the ciliaryganglion (PNS) .

•

Ciliary ganglion projects to the

constrictor muscle in the iris . Shininglight leads to constriction of both

muscles.•

motor efferent – cranial nerve 3– occulomotor nerve

Only one side is shown here. Remember the projections aresymmetrical.




Typical test question : Where is the site of injury if shining a light into the left eye causesthe right eye to constrict but shining light intothe right eye does not cause the right eye toconstrict? Right Optic Tract or Nerve // Retina

of Right Eye

THE SPATIAL RELATIONSHIPS AMONG THE RGCSARE MAINTAINED IN THEIR TARGETS.

•

Referred to as maps

•

Images are inverted and left-rightreversed as they are projected onto theretina.

•

The visual field can be divided intonasal, temporal, inferior (ventral)superior (dorsal). Fixation point is thepart where the fovea aligns.

•

The left half of the visual world isrepresented in the right half of thebrain and vice versa. (*not left eye toright side of brain, but left visual field)

Humans are Binocular… •

There is an overlap in visual fields, suchthat most objects are seen by botheyes.

•

Objects in extreme temporal visualfield are only seen with the ipsilateralnasal retina .

•

Peripheral vision is monocular!

•

Nasal retinal derived axons-(temporalvisual field) cross in the chiasm (contralateral) and temporal retinal axons(nasal visual field) do not cross at thechiasm (ipsilateral).

•

Images in the left visual field projectonto the nasal retina of the left eye andthe temporal retina of the right eye.These go to the same side. Thereforethe left visual field is mapped onto theright side of the brain.

•

The visual map is maintained in V1,central visual field maps to caudal(posterior) and peripheral visual fieldmaps to anterior V1.

Relationships between a visual target, images on the retinas of the two eyes, and the projections of the ganglion cells carryingvisual information about these images.

Refer to picture above:The visual target, an arrow, is in the visual fieldsof both eyes. The visual target in this case is solong that it extends into the monocular

segments of each retina (i.e., one end of thetarget can be seen only by one eye). Theshaded circle at the center of the target showsthe fixation point. The image of the target isreversed on the retinas by the lens system. Theleft half of the visual target is imaged on thenasal retina of the left eye and the temporalretina of the right eye. Thus, the left visualfield is seen by the left nasal retina and theright temporal retina . Similarly, the right half

of the visual target is imaged on and seen bythe left temporal retina and the right nasalretina.

**Images are inverted on the retina.**




Lesions : 1. Right optic nerve; 2. optic chiasm; 3. optic tract;4. Meyer’s loop; 5. cuneus; 6. lingual gyrus; bracket,occipital lobe with macular sparing

As the optic radiation passes caudally, it fansout, and some of the fibers loop forward in thetemporal lobe as Meyer’s loop. The axons inthe Meyer’s loop carry information derivedfrom the lower half of the appropriatehemiretinas . Thus, the axons in Meyer’s lo op

represent the contralateral upper visual field.

The optic radiation ends in the striate cortex, which is located dorsal and ventral to thecalcarine fissure in the occipital lobe. The gyrusdorsal to the calcarine fissure is the cuneus,

which receives information from the upper partof the appropriate hemiretinas, representingthe contralateral lower visual field . The gyrusventral to the calcarine fissure is the lingual

gyrus, which receives information from thelower hemiretinas, representing the

contralateral upper visual field .

Because the spatial relationships in the retinaare maintained in the brain, a careful analysis of

the visual fields of a patient can often indicatewhere brain damage is located.

COLOR VISION•

The eye is sensitive to a narrow band of

wavelengths•

The visible spectrum (397 nm to 723nm)

•

The wavelength

–

The intensity of light necessaryto elicit sensation

–

Determines the hue or chroma

COLOR•

Color is a purely psychologicalphenomenon (subjective).

•

Objects only appear colored becausethey reflect different wavelengths of light from different regions of thevisible spectrum

•

Therefore color is a property of theneural apparatus which detects thereflected light . For an object to appearcolored we need to have the correctphotoreceptors and neurons. Any

differences in our neural apparatus willresult in very different perceptions of

color.•

Color perception arises from the abilityof certain light rays to evoke aparticular pattern of neural responsesin our eye and visual cortex.

**Color perception is not equivalent towavelength because different people mayperceive same wavelengths differently [ex:people who are color-blind].

•

Neural processing of the light that hitsthe retina determines our perceptionof color.




ATTRIBUTES:•

Tone or Hue

–

Wavelength of the lightstimulus

•

Brightness or luminosity

–

Amount of light (intensity of radiation)

•

Saturation or purity

–

White sensation

Primary Colors: RED, YELLOW, BLUESecondary Colors: ORANGE, GREEN, PURPLE

•

Red + Yellow = Orange

•

Yellow + Blue = Green

•

Blue + Red = Purple

Intermediate Colors: created by mixing primaryand secondary colors

•

red-orange

•

yellow-orange

•

yellow-green

•

blue-green

•

blue-purple

•

red-purple

Neutral Colors: BLACK, WHITE, GRAYColor Values:

•

color + white = tint [lightened colors]

•

color + black = shade [darkened colors]

Color Schemes:•

monochromatic [one color scheme]

•

complementary [contrast: opposites inthe color wheel]

•

analogous [3-5 colors that are adjacentto each other in the color wheel]

•

warm [found on the right side of thecolor wheel; colors found in fire andsun]

•

cool [colors on the left side of the colorwheel; colors found on snow and ice]

What color do we see the best?•

Yellow-green at 550 nm

What color do we see the worst?•

Blue at 440 nm

We can perceive color differences of 10 nm atextremes (violet and red) and 2 nm between

blue and yellow

Moreover:•

128 fully saturated hues can bedistinguished.

•

We cannot perceive hue differenceswith less saturated light.

•

Sensitivity to changes in saturation for afixed hue and brightness ranges from16 to 23 depending on hue.

Perception•

Color perception may be difficultbecause:

–

It varies from person to person

–

It is affected by adaptation(stare at a light bulb… don’t)

–

It is affected by surrounding

color.

•

Young-Helmholtz theory

–

Proposed by Young in 1801 andlater modified by Helmholtz

–

Existence of three kinds of cones, each containing a

different photopigment andthat are maximally sensitive toone of the 3 primary colors

–

Sensation of any given colordetermined by the relativefrequency of the impulses fromeach of these cone systems

•

It is in accord with the doctrine of specific nerve energies

•

It explains the positive and negativeafter images

•

Blindness to one color is explained bythe absence of one photochemicalsubstance




Color Blindness John Dalton (1776) •

9% of otherwise normal and healthymen

•

2% of women are color blind to acertain degree

•

According to the Young-Helmholtztheory, color blindness would be due toabnormality in one or more of the 3types of cones

Naming•

Suffix:

–

-anomaly : denotes color

weakness–

-anopia : denotes color blindness

•

Prefix:

–

Prot- : refers to defect of the L(red) cone system

–

Deuter- : refers to defect of the M(green) cone system

–

Tri-: refers to defect of the S(blue) cone system

Trichomats•

With normal color vision (or withprotanomaly, deuteranomaly, ortrianomaly)

•

Have all 3 cone systems though onemay be weak

Dichromats•

With only 2 cones systems

•

Protanopia, deuternopia, or trianopia

Monochromats•

Only one cone system or congenitalabsence of all cone systems

** It is an X-linked, recessive inheritance

Classification of Color Blindness

Trichromats Dichromats Monochromats

Normal

Protanomalo

us

Deuteranom

alous

Protanopia

(most)

Deuteranopia

Tritanopia

(less)

Achromat (total

color blindness)

•

Normal vision is consideredTrichromatic

•

Trichromats who perceive red andgreen differently from normal subject:

•

Protanomalousindividuals are deficientin vision of red

•

Deuteranomalous areones who are deficient

in vision of greenDichromats

•

Are individuals who do not see one of the colors, supposedly because thecorresponding receptor is missing

•

In protanopia , the most commonanomaly

–

There is blindness to red

–

Green is deficiently perceived

–

Blue is seen normally

•

Deuteranopia

–

Blindness to green

–

Red is deficiently perceived

–

Blue is seen normally

•

Tritanopia , there is blindness to blue,but green and red are perceivednormally

–

Is much less frequent

Tests for Color Visions•

Holmgren Test

–

The subject is given a skein of colored wool and told tochoose from skeins of assorted

colors, those of similar hues.–

**skein: coils of yarn

•

Ishihara’s Test

–

(Modification of Stiling’s test).Consists of a series of plates inwhich a digit formed of spots of one color is “hidden” in filed of other colored spots.

•

Matching of Spectral Colors

–

The subject is asked to matchyellow by mixing red and green.The proportions of each colorused are compared with thoseused by normal subjects. This isthe most accurate method.

END OF TRANSCRIPTION

God bless, BATCH 2014!Konti nalang, summer na!!

Documents

Visual Physiology 2