Week 9 Special Senses: The Eye - Collin...

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Collin County Community College

BIOL 2401:

Week 9

Special Senses: The Eye

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VISION

As humans, we rely on Vision more than any otherspecial sense.

The eye itself is surrounded by accessory structures

• Eyelids (palpebrae) that separate the palpebral fissue• Eyelashes• Tarsal glands or Meibomian glands in the eyelids• Lacrimal apparatus

– Lacrimal gland– Lacrimal canuncle with lcrimal canaliculi– Lacrimal sac and naso lacrimal duct

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VISION

Conjuctiva : Epithelium covering the innereyelid and outer surface of the eye

Extends to the edges of the cornea. Thecornea itself is transparent and containsno blood-vessels.

Lacrimal apparatus :Produces, distributesand removes tears.

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VISION: The EYE

The eye is a more or less spherical structure, almost the sizeof a ping pong ball and about 8 grams in weight.

It is located within the eye socket and cushioned by orbital fat.

• Posterior cavity or vitreous chamber : filled with gelatinous liquidcalled vitreous humor or vitreous body

• Anterior cavity located in front of the lens and divided into twochambers ( anterior and posterior chamber). It is filled with aqueoushumor.

The eye-ball is hollow and it can be divided into twochambers according to the location with respect to thelens.

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VISION: The EYE

The eye is a more or less sphericalstructure, almost the size of a pingpong ball and about 8 grams inweight.

It is located within the eye socket andcushioned by orbital fat.

Excessive fat behind the eye causesthe eyes the bulge forward ( such asmay occur due to hyperthyroidism).

Exophthalmos

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VISION: The EYE

The eye-ball has 6 extra-occular (external) muscles thatallows us to move the eye in quick fashion in almost alldirections.

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VISION: The EYE

Cranial nerves that activate these muscles :• A : Abducens nerve (VI)• E : Trochlear nerve (IV)• B,C,D and the medial rectus : Occulomotor nerve (III)

The medial rectus ( not visible)

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VISION: The EYE

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VISION: The EYE

There are basically three main layers (tunics) tothe wall of eye organ

• Outer fibrous tunic• Sclera, cornea, limbus

• Middle vascular tunic• Iris, ciliary body, choroid

• Inner nervous tunic• Retina

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VISION: The EYE

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The EYE: Internal Structures

• The outer layer

• Composed out of the SCLERA and CORNEA

• SCLERA is white part of the eye (the majority ) while thecornea is transparent anterior portion.

• The limbus is the border between these two

Fibrous Tunic

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• The middle layer

• Composed out of the CHOROID, CILIARY BODY andIRIS

• CHOROID is follows the majority of the sclera and isvery vascular. It also contains many melanocytes nearthe border with the sclera.

Vascular Tunic or UVEA

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• Near the anterior portionof the eye, the choroiddevelops into theCILIARY BODY

• It is composed out of theciliary smooth musclesthat extend inwardstowards the lens.

• The lens attaches to theciliary body viasuspensory ligaments.It keeps the lens in frontof the iris and centered.

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• The IRIS extend as a flap of tissue beyond the Cilairybody and provide a central opening for light to enter theeyeball ( the pupil)

• It is composed out of two layers of smooth muscles :radial dilator muscles and circular constrictormuscles

• The iris also contains pigments and blood vessels.


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1 :

2 :

3 :

4 :

5 :

Sclera

Cornea

Iris

Pupil

Lens

6 : Limbus

7 : Ciliary Body

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• Contraction of the two layers of muscles results in pupildilation or pupil constriction.

• It thus functions just like a camera diaphragm, lettingmore or less light inside.


• Circular constrictor contraction is under para-sympatheticinfluence (part of pupillary reflex; makes pupil smaller)

• Radial dilator contraction is under sympathetic influence(makes pupil larger).

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Retina or Neural Tunic

• It is the innermost layer of the eye wall

• Consist out of a pigmented part and a neural part

• Pigmented part is also called the pigmented epitheliallayer

• It has an important function in preventing light frombouncing back

• It also is important in providing biochemical feedback tothe light receptors in the retina

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• The neural part is the actual retina with the lightreceptors

• Rods : do not discriminate color - good for gray shades -highly sensitive to light - good for dim light vision

• Cones : discriminate color - require higher light intensities

• It extends anteriorly only as far as what is called the oraserrata. (light does not hit the inside of the eye anteriorto the ora serrata…. Thus no need for a retina there).

• The retina contains several layers of cells that areimportant in relaying captured light energy to the brain.

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8 : Ora Serrata

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Rods and Cones

Bipolar Cells

Ganglion Cells

Optic Nerve

Synapse with

Synapse with

Their Axons form the

Axons of the ganglion cells leave the eye atthe optic disc ( contains no light receptors; reason for the blind spot)

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Macula lutea is a depression in the retina where no rods occur.Fovea centralis is the center and has highest levels of cones ;provides the sharpest vision.

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The EYE Chambers

• Ciliary body and lens divide the anterior cavity of the eyeinto posterior (vitreous) cavity and anterior cavity

• Anterior cavity further divided– anterior chamber in front of eye– posterior chamber between the iris and the lens

• Aqueous humor circulates within the anterior eye cavity– Made by ciliary body and diffuses through the walls of

the anterior chamber– passes through canal of Schlemm and re-enters

circulation• Vitreous humor fills the posterior cavity.

– Not recycled – permanent fluid

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Blockage of this drainage pathwaymay result in an increase in ocularpressure, resulting in glaucoma !

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The EYE: The Lens

The lens is located Posterior to the cornea and forms theanterior boundary of the posterior cavityThe Lens helps to focus light on the retina by refracting(bending) it as it passes through lens.

The lens is made from slender, elongated cells filled withtransparant proteins called crystallins.Loss of transparency = cataract ( cloudy lens)

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The EYE: The Lens

Hyper-mature cataract

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The EYE: The Lens

Light changes direction when it passes from one medium toanother medium with different density.Most of the refraction occurs when light passes through thecornea.The lens provides the extra refraction adjustments needed tofocus the light onto the retina.If light is not centered on the retina, we end up with blurryvision.

Refraction

Why do things look blurry under water when we open our eyes ?

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The EYE: The Lens

Refraction

Accommodation is the process where the shape (thickness) of the lens isadjusted to keep the focal distance constant

During Accommodation the lens becomes “fatter” when we try to focus ona near-by object and thinner when the object is distant.

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The EYE: The Lens

Accommodation

Accommodation isexecuted via CN III byaction on the ciliary body

• Contraction of the cilary musclescauses relaxation of the ligamentsand bulging of the lens

• Relaxation of the ciliary musclesresults in a pull on the suspensoryligaments , which in turn flattens thelens

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The EYE: The Lens

Accommodation Problems

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VISUAL PHYSIOLOGY

VISUAL physiologyrelates to understandinghow we actually seeimages.

First of all, the imagecreated on the retina isalmost the same aswhen we look through amicroscope : it is upsidedown and backwards

The brain compensatesfor this image reversaland we are never awareof this.

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The rods and the cones are responsible for picking up the information invisible light ( the photons)

VISUAL PHYSIOLOGY

Our rods and comes are sensitive to visible light only, a portion ofthe electromagnetic spectrum with wavelenghts between 400 nm to700 nm.

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VISUAL PHYSIOLOGY

• Rods and cones are elongated specializednerve receptor cells

• There is an Outer segment• embedded into the pigmented epithelial

layer• contains membranous discs with visual

pigments• Narrow stalk connecting outer segment to

inner segment• Inner segments synapse with bipolar cells

Anatomy of Rods and Cones

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VISUAL PHYSIOLOGY

RODS • Functional characteristics

• Very sensitive to dim light

• Best suited for night vision and peripheral vision

• Perceived input is in gray tones only

• Pathways converge, resulting in fuzzy and indistinct images

CONES • Functional characteristics

• Need bright light for activation (have low sensitivity)

• Have one of three pigments that furnish a vividly colored view

• Nonconverging pathways result in detailed, high-resolutionvision

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VISUAL PHYSIOLOGY

• Visual pigments are located in themembranes of the membraneous discspigments

• The visual pigment is called Rhodpsin• Rhodopsin is a molecule made from

• Opsin ( a larger protein)• Retinal ( a smaller visual pigment)

• Retinal is a derivative of Vitamin A• In cones, the Opsin protein is slightly different,

accounting for the color sensitivity of thecones

Visual Pigments

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VISUAL PHYSIOLOGY

PhotoReceptionRod discs

Visual pigmentconsists of

• Retinal• Opsin

Rhodopsin, the visual pigment inrods, is embedded in themembrane that forms discs in theouter segment.

It is made from retinal, theVitamin A derivative, and a largerprotein part, called Opsin

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VISUAL PHYSIOLOGY

PhotoReception

Retinal is derivative from Vitamin A(Retinol)

Retinal has two different configurations• Trans form : molecule has a straight

tail• Cis form : molecule tail has a bend

in itLight energy converts Retinal from the Cis

to Trans form.

The conversion from the Cis to Trans stateis at the basis of photoreception

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11-cis-retinal

Bleaching ofthe pigment:Light absorptionby rhodopsintriggers a rapidseries of stepsin which retinalchanges shape(11-cis to all-trans)and eventuallyreleases fromopsin.

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Rhodopsin

Opsin and

Regenerationof the pigment:Enzymes slowlyconvert all-transretinal to its11-cis form in thepigmentedepithelium;requires ATP.

Dark Light

All-trans-retinal

Oxidation

2H+

2H+

Reduction

Vitamin A

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11-cis-retinal

All-trans-retinal

VISUAL PHYSIOLOGY

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VISUAL PHYSIOLOGY

GTP

cGMP

Guanyl Cyclase

• In photoreceptors, under resting conditions (dark conditions), cGMP ispresent in high concentrations.

• In vision, cGMP is important in that it opens up a chemically regulated Na+

channel. This is similar like cAMP opening up Na+ channels in olfaction.

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VISUAL PHYSIOLOGY

Steps in PhotoReception

STEP 1 : Light energy converts Retinal from the CIS form tothe TRANS form.This now activates the OPSIN part of Rhodopsin

STEP 2 : OPSIN activates the enzyme TRANSDUCIN (G-protein complex)TRANSDUCIN in turn activates a phosphodiesterase.Phosphodiesterase breaks down cyclic GMP (cGMP) levels

THUS, when not stimulated by light, the photoreceptors cells are alwaysdepolarized (around - 40 mV) due to a Na+ ion current ( called the darkcurrent) !

The cells continuously release neurotransmitters at the bipolar synapse !

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VISUAL PHYSIOLOGY

PhotoReception

STEP 3 : Phosphodiesterase thus reduces cGMP levels ; thisreduces the numbers of Na+ channels that are open. Itresults in a hyper-polarization !

STEP 4 : The membrane potential drifts back to around - 70 mV. Thisreduces the release of neurotransmitters at the synapse with thebipolar cells.

Thus, in contrast with what we seen so far, stimulation of the receptorresults in a decrease in released number of N.T.This is a graded response; the higher the intensity of light, the greater thehyperpolarization and the less the amount of N.T. released !

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VISUAL PHYSIOLOGY

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VISUAL PHYSIOLOGY

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Light (photons)activates visual pigment.

Visual pig-ment activates transducin(G protein).

3 Transducin activates phosphodiesterase (PDE).

4 PDE convertscGMP into GMP, causing cGMP levels to fall.

5 As cGMP levelsfall, cGMP-gated cation channels close, resulting in hyperpolarization.

Visualpigment

Light

Transducin(a G protein)

All-trans-retinal

11-cis-retinal

OpencGMP-gatedcation channel

Phosphodiesterase (PDE)

ClosedcGMP-gatedcation channel

VISUAL PHYSIOLOGY

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So, how does the brain receive the information of the light stimuli ?

VISUAL PHYSIOLOGY

• The N.T. released by the photoreceptors are inhibitoryneurotransmitters (glutamate). They thus cause IPSP’s in thebipolar cells ( resulting in nyperpolarixzation)

• The bipolar cells in turn reduce their frequency of stimulation tothe ganglion cells.

• The reduction in N.T. release induced by the light stimuli thusreduces the amount of IPSP’s.

• The removal of inhibition equates with stimulation. It is similar likehaving your foot on the brake and slowly releasing the action ofyour foot !

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VISUAL PHYSIOLOGY

1 cGMP-gated channelsopen, allowing cation influx;the photoreceptordepolarizes. Voltage-gated Ca2+

channels open in synapticterminals.

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Neurotransmitter isreleased continuously.3

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Hyperpolarization closesvoltage-gated Ca2+ channels,inhibiting neurotransmitterrelease.

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No EPSPs occur inganglion cell.6

No action potentials occuralong the optic nerve.7

Neurotransmitter causesIPSPs in bipolar cell;hyperpolarization results.

Na+

Ca2+

Ca2+

Photo-receptorcell (rod)

Bipolarcell

Ganglioncell

In the dark

mV

Dark

- 70 mV

- 40 mV

Due to the open Na+ channel, therod experiences a ‘dark current’that keeps the cell depolarized

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1 cGMP-gated channelsare closed, so cation influxstops; the photoreceptorhyperpolarizes. Voltage-gated Ca2+

channels close in synapticterminals.

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No neurotransmitteris released.3

Lack of IPSPs in bipolarcell results in depolarization.4

Depolarization opensvoltage-gated Ca2+ channels;neurotransmitter is released.

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EPSPs occur in ganglioncell.6

Action potentialspropagate along theoptic nerve.

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Photoreceptorcell (rod)

Bipolarcell

Ganglioncell

Light

Ca2+

In the light

VISUAL PHYSIOLOGY

mV

Dark Light

- 70 mV

- 40 mV

Light response closes Na+channel due to the cGMPbreakdown and the rodexperiences a ‘hyperpolarization’.

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Recovery after stimulation

VISUAL PHYSIOLOGY

After light stimulation, retinal is in the TRANS form and does notspontaneously convert back to the CIS form

Shortly after light stimulation, rhodopsin actually breaks down intoretinal and opsin. (called bleaching effect).

Photoreceptors cannot function with damaged rhodopsin. Thus,before it can become an active molecule again , it needs to be“glued” back together.

This can only occur if retinal is in the CIS position. The occurs in the“dark” and requires enzymes and ATP.

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Recovery after stimulation

VISUAL PHYSIOLOGY

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Dark Adapted State

VISUAL PHYSIOLOGY

When exposed to the dark long enough, all photoreceptors areloaded and ready. Our visual system is then in a highly sensitivestate and receptive to small amounts of light.

Light Adapted State

When moving from a dark area to a bright area, all photoreceptorsbecome bleached and thus reduce the immediate sensitivity to aseries of additional light stimuli.

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Color Vision

VISUAL PHYSIOLOGY

White light is a spectrum of alldifferent colors.

If an object absorbs all color, itappears black.

The color of an object is determined by the wavelength it reflects ( and thusdoes not absorb ) !

A red apple looks red because it absorbs all wavelengths of the visiblespectrum except the wavelengths between 620 and 700 nm (red colors).

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VISUAL PHYSIOLOGY

The cones are sensitive to colored light.

There are three different kinds of cones :• Blue cones have highest sensitivity in blue region• Green cones are most sensitive in green region• Red cones are most sensitive in red region

In a person with normal vision• 16 % Blue cones• 10 % Green cones• 74 % Red cones

Perception of color is due to the relativeintegration of information arriving from each conetype.

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VISUAL PHYSIOLOGY

ColorBlindness is the inability to perceive certaincolors.

It occurs when one or more of the classes ofcones become non functional

The most common type of color blindness idsred-green color blindness; the red cones aremissing and the person cannot differentiatebetween red or green.

The genes for the cones are located on the X chromosome. Thus, colorblindness is more common in males ( 10% of all males) than females(0.67% of al females).

Total colorblindness is rare !

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Visual PathwayThe retina has about

• 130 million rods/cones• 6 million bipolar cells• 1 million ganglion cells

• Most convergence occurs with the rods !• About thousand rods converge on a single ganglion cell.• This ganglion cell ( and thus the rods) monitor a certain portion of the visual

field. Such a cell is called an ‘M’ cell !• Since rods are more effective in dim light, M cells provide information about

the fact that light has arrived in a certain area.

VISUAL PHYSIOLOGY

Thus considerable convergence occurs with respect to signal processing

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Visual Pathway

• The cones shows little convergence• The ratio of cones to bipolar cells is almost 1 : 1 in the fovea• Ganglion cells that monitor cones are called ‘P’ cells• They are active in bright light and provide information about detail, color from

a very specific area• About thousand rods converge on a single ganglion cell.

VISUAL PHYSIOLOGY

Difference between M and P cells can be explained in terms of a computerscreen or photography

• M cells (rods) provide grainy, fuzzy pictures with low resolution, blurry details• P cells (cones) provide high resolution, fine grained, sharp and clear detail

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VISUAL PHYSIOLOGY

Visual Pathway in CNS• Axons from the ganglion cells meet up at

the optic disc and exit the eyeball• They proceed as the OPTIC nerve towards

the diencephalon• The optic nerves cross over at the optic

chiasm and become the optic tracts

• The optic tract proceeds to the LaterateGeniculate Nucleus in the Thalamus

• Here information is passed on via theprojection fibers , called the optic radiation,to the visual cortex of the occipital lobe.

• Collateral fibers at the Geniculate Nucleusconnect with the Superior collicluli, pinealgland, RAS, and other nuclei

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VISUAL PHYSIOLOGY

This partial cross over thus bringstogether that information that comesfrom the same area of the visual field

Not all axons cross over at the opticchiasm.

The axons that come from the left side ofeach eye proceed to the left hemisphere,while those from the right proceed to theright hemisphere.

The Binocular zone is the overlapping zone seen by both eyes,which is important for depth perception.

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VISUAL PHYSIOLOGY

This partial cross over has some interesting effects.

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