Eye q526w

AND PHYSIOLO GY OF THE EYE

 Special VISION 

Sense

Vision: the act of seeing.



Special sense organ is “EYE”



For vision “light” is necessary.

:

Electromagnetic Radiation • Energy in the form of waves that radiates from the sun. • There are many types of electromagnetic radiation : i. gamma rays, ii. x-rays, iii. UV rays, iv. visible light, v. infrared radiation, vi. microwaves, and vii. radio waves.

.This range of electromagnetic radiation is known as the electromagnetic spectrum.

• The distance between two consecutive peaks of an electromagnetic wave is the wavelength. • The eyes are responsible for the detection of visible light. • An object can absorb certain wavelengths of visible light and reflect others; • The object will appear the color of the wavelength that is reflected.



The Eye has 2 major parts :

1.

Optical formation)

..

Req. maintenance of clear media

..

Req. maintenance of normal IOP

2.

Neural system

.

Transmission of image to brain and recognition & interpretation of image.

.

(i.e physiology of vision)

system

(Image

Outline

A. Functional Anatomy of the eye: 1. Accessory structures (External Anatomy of eye) 2. Eye ball structures a. Fibrous Tunic b. Vascular Tunic c. Nervous Tunic 3. Interior of the ball a. Anterior Cavity b. Lens c. Vitreous Chamber 4. Neural pathways : Visual pathway &

External Anatomy of the Eye • The accessory structures of the eye include the : 1. Eyelids, 2. Eyelashes, 3. Eyebrows, 4. Lacrimal apparatus, and 5. Extrinsic eye muscles.

Sagittal section of eye and its accessor

Eye lids or Palpebrae • Movable curtains present in front of eyeball • From superficial to deep, each eyelid

consists of : i.

Skin Layer (epidermis, dermis & subcutaneous tissue)

ii. Orbicularis Oculi iii. Tarsal plate iv. Meibomian Gland

Eye lids or Palpebrae 1. Shade the eyes during sleep, 2. Protect the anterior surface of the globe from dust, debrish,local injury. 3. Aid in regulation of light reaching the eye; 4. In tear film maintenance, by distributing the protective and optically important tear film over the cornea during blinking; and 5. In tear flow, by their pumping action on the conjunctival sac and lacrimal sac.

Eye lashes and Eye brows • Consist of Hair follicles.

• They protect the eye from 1. direct sunlight, 2. dust, 3. perspiration and 4. foreign bodies.

Lacrimal Apparatus

Lacrimal Gland Located in the upper, outer portion of each orbit of the eye. Lobulated exocrine glands secreting tears .

Lacrimal Ducts There are about 6 to 12 in number They dump tears on the surface of the conjunctiva of the upper lid through the palpebral part of the gland

Conjunctiva • The conjunctiva is a mucous membrane lining the eyelids and covering the anterior eyeball up to the edge of the cornea. • Bulbar: covers the sclera • Palpebral: lines the inside of the upper and lower lids

Lid retractors • Responsible for opening the eyelids • levator palpebrae superioris muscle • Lower lid retractor • inferior rectus, extends with the inferior oblique and insert into the lower border of the tarsal plate

Extraocular Muscles • Rectus Muscles • superior rectus • inferior rectus • medial rectus • lateral rectus • Oblique Muscles • Superior oblique • Inferior oblique

Human Eye 

Eyeball is a fluid filled spherical organ with a diameter of 24 mm.



It consists of 3 concentric layers :

1.

The outer layer, or the fibrous coat :-opaque sclera or the “white of the eye” ; the transparent cornea.

2.

The middle layer, or vascular pigmented coat (uveal coat) :- choroid , ciliary body & iris

 

Interior of the eyeball is divided into : 3 spaces : Anterior & posterior chamber & posterior cavity



The crystalline lens is a transparent structure held in place by a circular lens suspensary ligament (zonule).



The zonule is attached to the ciliary body.



The pupil is a black aperture located in the center of the iris of the eye that allows light to enter the retina



A human eyeball is like a simple camera! 

Sclera: outer walls, hard, like a light-tight box.



Cornea and crystalline lens (eye lens): the two lens system.



Retina: at the back of eyeball, like the film.



Iris: like diaphragms or stop in a camera.



Pupil: camera aperture.



Eyelid: lens cover.

Functions of the eye

1.

Sclera : Avascular protective white fibrous coat. - It is covered by conjunctiva ( bulbar)

2.

Cornea

:

Avascular

,transparent

,

more convex curvature - contributes most to refractive power

3. Iris : thin pigmented, circular diaphragm - regulates the amount of light reaching the retina 4. Ciliary body : formation of aqueous humour , role in accommodation. 5.

Choroid : dark brown ,highly vascular

- nutrition to the outer retina ,prevent total internal reflection

Maintenance of clear refractive media of the eye • Structures forming refractive media of eye from anterior to posterior are : 1. Tear film 2. Cornea 3. Aqueous humour 4. Crystalline lens 5. Vitreous humour

Tear Film: •Lipid layer produced by the meibomian gland (oil)(hydrophobic) •Aqueous layer produced by the Lacrimal gland (spreading, control of infectious agents) •The mucous layer produced by microscopic goblet cells in the conjunctiva (coating)

Tear film and functions 

Composition :



98% water ,1.5% NaCl, antibacterial substances like lysozymes, lactoferrin, betalysin



Functions :

1.

Keep cornea & conjunctiva moist

2.

Provides oxygen to cornea

3.

Washes away debris, foreign irritants

Eye Ball Structure: Fibrous Tunic

Cornea and sclera  Form a spherical shell which makes up the outer wall of the eyeball.

• The sclera is : - principally collagenous, - avascular (apart from some vessels on its surface) - relatively acellular. •

The cornea and sclera merge at the corneal edge (the limbus).

Cornea : Avascular ,transparent , more convex curvature - contributes most to refractive power of the eye -It has 5 layers: i.

Epithelium

ii.

Bowman’s membrane

iii.

Stroma

iv.

Descemet’s membrane

v.

Endothelium

Chief functions of the cornea 1. Are protection against invasion of microorganisms into the eye 2. The

transmission

and

focusing

(refraction) of light. 3. Screening ultraviolet sunlight

out (UV)

damaging wavelength

in

Epithelium • A thin, (5-6) layers made up of Stratified Squamous epithelial cells 1. that keeps the stroma dehydrated and 2. to provide nutrients and oxygen to the cornea 3.

resisting the free flow of fluids from the tears

4. acts as a barrier to protect the cornea, and thus

Epithelium • Extremely sensitive to pain • Can regenerate itself if damaged from disorders such as: 1. Recurrent corneal erosion:  characterized by the failure of the cornea's outermost layer of epithelial cells to attach to the underlying basement membrane  2. Epithelial basement membrane dystrophy: the epithelium's basement membrane develops abnormally, causing the epithelial cells to not properly adhere to it.  3. Diabetes mellitus: poor adhesion between epithelial cells and their basement

Bowman’s membrane • smooth, acellular, non-regenerating layer, • Is transparent, • composed of collagen, • cannot regenerate after damage and • form scars as it heals which can lead to vision loss

Stroma • Contributes to 90% of total corneal thickness • Is composed primarily of water (78%) , • collagen (16%), • keratocytes and • Ground substance (glycosaminoglycan) • Giving the cornea its strength, elasticity and form • Fairly dehydrated which contributes greatly to the light-conducting transparency.

Descemet’s membrane •a

thin

but

strong

homogeneous

elastic membrane • Is a protective barrier under the Stroma • Interiorly composed of collagen • made by the endothelial cells that lie below • Can regenerate after injury

Endothelium • Single layer of flattened polygonal cells • Is responsible for regulating fluid and solute transport between the anterior chamber and the stroma • The

endothelial

dependent

pump

mechanism

is

an

resulting

energyin

ion

transported from the stroma to the aqueous humor, • creating an osmotic gradient drawing water out of the stroma

Endothelium • Cells don’t regenerate, • if they are destroyed, a corneal transplant is the only therapy.

Factors responsible for corneal transparency 1.

Anatomical : Avascularity , Absence of pigment Demyelinated nerve supply Regular arrangement , non-keratinization of C.E Lattice theory of Maurice paucity of cells in stroma

2. Relative dehydration of stroma

Eye Ball Structure: Vascular Tunic

Choroid: • Thin brown tissue • Highly vascularized • Provides

nutrients

and

oxygen to the retina • It is opaque making sure no light is scattered from the sclera to the retina.

Ciliary Body •A

thick

tissue

inside the eye • composed

of

ciliary processes muscles • Highly vascularized

and

Ciliary Processes: • Secretes the aqueous humour in the posterior chamber and from it to the anterior chamber • The fluid nourishes and oxygenates the cornea and lens and • then drains into the sclera via Schlemm canal

Aqueous Flow

Ciliary Muscles:

• The set of muscles (meridional, oblique, sphincteric) • that affect the shape of the lens during accommodation.

Iris: • Colored portion of the eye positioned between the cornea and the lens • Smooth muscles : 1. The Sphincter papillae 2. The Dilatator papillae

Interior Of The Eyeball:

Physiology of crystalline lens 

Avascular , highly elastic , Biconvex ,transparent



Diameter 9-10mm & Thickness 3.5mm -5mm



Has following layers

1.

Capsule(tough,

transparent,

semipermeable) 2.

Anterior epithelium

3.

Lens fibers

..

Nucleus

..

Cortex

elastic,





Lens fibers are rich with proteins known as crystallins Responsible for transparency and refractive properties and elasticity

Structure of the crystalline lens

Chemical composition of the lens vs aqueous humour

Factors affecting lens transparency 1.

Avascularity

2.

Semipermeable character of lens capsule

3.

Tightly-packed nature of lens cells,

4.

The arrangement of lens proteins

5.

Same index of refraction in all parts of the lens

6.

Pump mechanism of lens fiber membranes

7.

Auto-oxidation and high concentration of reduced glutathione in the lens

PATHOGENESIS OF CATARACT 

Any opacity in the lens or its capsule is called cataract.



3 basic mechanisms are :

1.

Damage to lens capsule

2.

Increase in lens hydration

3.

Change in the lens fiber protein synthesis

Types of cataract 1.

.

.

.

Cortical senile cataract : Mechanism of loss of transparency ↓ total proteins, A.A & K+ ions with ↑ in Na+ & marked hydration of lens followed by

With ↑ (senility) ↓ active pump mechanism Reversal of Na+/K+ ratio Hydration of fibers

age ↓ oxidative reactions ↓ A. A level ↓ synthesis of proteins

Denaturation of lens proteins Opacification of cortical lens fibers

2. Nuclear senile cataract 

Usual degenerative change of agerelated nuclear sclerosis



Associated with dehydration &



Compaction of the nucleus



Accompanied by ↑ in water insoluble proteins.



Resulting in hard cataract

Anterior cavity • Anterior Chamber located between the cornea and the iris • Posterior chamber located between the lens and the iris • Filled with aqueous humor produced by ciliary processes

Aqueous humor 

It is the clear protein free watery fluid between the cornea and the lens.



Formed

from

plasma

by

diffusion,

ultrafiltration & secretion. 

Vol. in ant. chamber is 0.25 ml & in post. Chamber is 0.06ml



Maintains aqueous pressure about 10-21 mm Hg.

Chemical composition of the lens vs aqueous humour

Mmoles/kg

Aqueous Flow

 Angle of anterior chamber.  Formed by root of iris, anterior-most part of ciliary body, trabecular meshwork and Schwalbe’s line

Increased intraocular pressure leads to Glaucoma

Functions? 

Provides nourishment to the eye lens and cornea. Cannot use the blood vessels:





Will block the light.



Easy for surgical transplant.

Hold the shape of the eyeball and keep the vitreous humor pressurized.

Physiology of Vitreous humor  

The Vitreous Humor (2/3 of eye volume) is the

transparent avascular tissue of

gelatinous consistency fluid between the lens and the retina. 

The space that it fills is called the vitreous body.



Composed of 99% water ,loose collagen fibrils , hyaluronic acid and vitrein

Eye Ball Structure: Nervous Tunic

Retina •

Thin, semitransparent, multilayered sheet of neural tissue

•

Lines

the

inner

aspect of the posterior two thirds of the globe • Terminates

anteriorly

at the ora serrata

Posterior Pole

Layers of the Retina 1. Retinal pigment epithelium

2. Photoreceptor 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. Nerve fiber layer 10. Internal limiting membrane

10 Retina layers & Cells

Physiology Of The Image Formation

Focusing / Image forming mechanism 

The cornea and eye lens form a compound lens system, producing a real inverted image on the retina.



From air to cornea (n=1.37): large bending, the main focusing.



From cornea to eye lens (n=1.42), less focusing power.

 

The eye has a limited depth of field. We cannot see things close and far at the same time.

Optics of Eye. 

Focusing system of eye with refracting structures. (Refractive Indices)     



Cornea.(1.37) Aqueous humour.(1.33) Crystalline lens. (1.42) Vitreous humour. (1.33) These constitute a homocentric system of lenses

So total Diopteric power of eye +60D 

+44D by cornea

Cardinal points of the eye 

Listing and Gauss, while studying refraction by lens combinations, concluded that for a homocentric lenses system, there Exist 3 pairs :

1.

2 principal foci

2.

2 principal points

3.

2 nodal points

.

all situated on the principal axis of the system.

.

When analyzed optically according to Gauss' concept can be resolved into six cardinal

Cardinal points of eye. By Listing & Gauss Schematic Eye. 

By Listing Reduced Eye

For homocentric lens system of eye there are 3 cardinal points

  

Principal foci (2) Principal points (2) Nodal points (2)



Simplified data by choosing single principal point & single nodal point.

Schematic Eye Conceptualizing the optical properties of

human eye in which the curvatures and indices of

refraction of the refracting elements and their intervening distances are listed. Developed

by

Gullstrand,

professor of

ophthalmology in Sweden. Nobel Prize 1911

Schematic eye & reduced eye.

Schematic eye / Reduced eye

Schematic eye

Listing's reduced eye.

Reduced schematic eye Schematic eye can be simplified even further

We can treat eye as it was a single

refracting element Ideal spherical surface separating two media

of refractive indices 1 and 1.33 Known as reduced schematic eye Nodal point Cornea and lens…

Axes and visual angles of the eye Axes of the eye Optical axis Visual axis Fixation axis

Visual angles Angle alpha (ONA) Angle gamma (OCA) Angle kappa (OPA, between optical axis and pupillary line – OP)

Accomodation Emmetropia (optically normal eye) can be defined as a state of refraction, where in the parallel rays of light coming from infinity are focused at the sensitive layer of retina with the accommodation Our eyes have been provided with a unique being at rest. mechanism by which we can even focus the

diverging rays coming from a near object on the retina in a bid to see clearly.

Accommodation 

It is a process by which one can focus the objects at different distances to have a clear vision.



Rather, it is done by changing the focal length of the eyelens.



Ciliary muscles help to change the shape of the lens: accommodation. (The Relaxation theory) 

Muscles relax, long focal length, see objects far way;

Muscles tense, short focal length see

objects close. 

Normal eyes can see 25cm to infinity, however, if

The Relaxation theory. 

Thomas Young & Helmholtz (1885) 

At rest lens compressed in capsule by tension of zonules d/t relaxation of fibres of ciliary muscle.

The Relaxation theory.



Ciliary muscles contract



Ciliary ring shortens.



Choroid move forwards.



Zonules relaxed.



Lens capsule relaxed.



Lens become more spherical



Increases Diopteric power of lens

Ocular changes in accomodation 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Slackening of the zonules Changes in the curvature of lens surface Anterior pole Axial thickness Changes in the tension of lens capsule Lens sinks down Changes within the lens substance Pupillary constriction & convergence of eyes Choroid Ora serrata

Range of accommodation

Near point

Far point Amplitude of

Accommodation 

Near point / Punctum proximum (P)– nearest point at which objects can be seen clearly.



Far point / Punctum remotum (R) – distant point at which objects can be seen clearly.



Range of accommodation – difference between far point & near point.



Amplitude

of

accommodation

-

ANOMALIES OF ACCOMMODATION I.

Presbyopia

:

physiological

a

condition

insufficiency

of of

accommodation leading to a progressive fall in near vision . i. ii.

Causes : Insufficiency of accommodation Paralysis of accommodation (cycloplegia)

iii.

Spasm of accommodation.

Optical aberration of the eye  

The eye, is by no means optically perfect. The lapses from perfection are called aberrations



These are :

1.

Diffraction of light :

.

A bending of light caused by the edge of an aperture or the rim of a lens

.

Actual pattern of a diffracted image point is a series of concentric bright and dark rings

'Airy disc

2.

Spherical aberrations :

It occur owing to the fact that spherical lens refracts peripheral rays more strongly than paraxial rays.

3. Chromatic aberrations It result owing to the fact that the index of refraction of any transparent medium varies with the wavelength of incident

Chromatic aberrations

Errors of Refraction 

Emmetropia (optically normal eye)



Ametropia

1.

Hypermetropia

2.

Myopia

3.

Astigmatism

4.

Presbyopia

Hypermetropia (hyperopia) or long-sightedness 

Refractive state of the eye wherein parallel rays of light coming from infinity are focused

behind

the

retina

accommodation being at rest. 

Mechanism :

1.

Axial

2.

Curvatural

3.

Index

4.

Positional

5.

Absence of lens( aphakia)

with

Characteristic features 1.

Far-sightedness

2.

Near point goes away

3.

Asthenopic symptoms : mild headache , eyeache Feeling of tiredness ,

discomfort in reading 4.

Accommodative convergent squint

Myopia or shortsightedness 

Refractive error in which parallel rays of light coming from infinity are focused in front of the retina when accommodation is at rest.

 1. 2. 3. 4. 5.

Mechansims : Axial Curvatural Positional Index Due to excessive accommodation

Characteristic features 1.

Short sightedness

2.

Far point is a finite point

3.

Asthenopic symptoms

4.

Half shutting of the eyes

.

Optical correction : concave lenses

Astigmatism 

Refractive

error

wherein

the

refraction

varies in the different meridia. 

Consequently, the rays of light entering in the eye cannot converge to a point focus but form focal lines.

Two types : 1. .

2. .

REGULAR ASTIGMATISM when the refractive power changes uniformly from one meridian to another IRREGULAR ASTIGMATISM Refractive power changes irregularly from one meridia to other.

Characteristic features 1.

Defective vision

2.

Blurring of objects

3.

Depending upon the type and degree of astigmatism,

objects

may

proportionately elongated; 4.

Asthenopic symptoms

appear

Optical correction : sphero-cylindrical lenses

Physiology of vision 

Main mechanisms :

1.

Initiation

of

vision

( Phototransduction -photoreceptors) 2.

Processing

&

transmission

of

visual sensation ( image processing cells of Retina & Visual Pathway ) 3.

Visual perceptions ( Visual Cortex & Association areas)

Retina Structure ( Thin transparent membrane) 

Optic disc , Macula lutea (yellow spot) , fovea centralis



Light sensitive layer is made of photo-receptors: rods (120 millions) and cones (7 millions) which absorb the light.



Plexiform Layer: nerve cells that process the signals generated by rods and cones and relay them to the optical nerve.



Choroid : carries major blood vessels to nourish the retina and absorb the light so that it will

Cells present in retina

10 Retina layers & Cells

Rods and Cones 

Covers an area of 5 cm2.



Cones: slow, fine grain, like color film



pigment: Iodopsin



for more precise vision, high visual acuity

  

need strong light. Help to see colors. Highly concentrated in the fovea at the center of the macula lutea



Rods: fast, coarse grain, like black & white film



for peripheral and night vision.



pigment: rhodopsin



Sensitive to dim light.



Mostly distributed away from fovea.



dense at the peripheral of the retina.



responsible for identification of shapes and movement



the discrimination between black and white

Sensitivity 

Cones: 

Need

high

level

of

light

(photopic

condition, day)  

High density, high resolution.

Rods: 

Low level of light (scotopic condition, at night)

 

Adaptation: Changing of retina sensitivity. The existence of these two kinds of input, each

working

maximally

under

different

Structure of Photoreceptor 







It is divided into an outer segment & an inner segment that includes a nuclear region, and a synaptic terminal zone The outer segments are comprised of saccules or membranous disks (contains pigments). The inner segments are rich in mitochondria; This region synthesizes the photosensitive

PHOTORECEPTOR MECHANISM 

PHOTOTRANSDUCTION



Light falling upon the retina causes photochemical changes which in turn trigger a cascade of biochemical reactions that result in generation of electrical changes.



Photochemical changes

1.

Rhodopsin bleaching

2.

Rhodopsin regeneration

.

Visual cycle :

.

Equilibrium between the photodecomposition and regeneration of visual pigments is referred to as visual cycle

Visual cycle

Rhodopsi n regenerti on

Rhodopsi n bleachin g

Biochemical reactions

Biochemical

Depicted is an outer reactions membrane disk in a rod. 

1. Incident photon (hν) is absorbed and activates a rhodopsin by conformational change in the disk membrane to R*.

2. R* makes repeated s with transducin molecules, catalyzing its activation to G* by the release of bound GDP in exchange for cytoplasmic GTP, which expels its β and γ subunits. 3. G* binds inhibitory γ subunits of the phosphodiesterase (PDE) activating its α and β subunits. 4. Activated PDE hydrolyzes cGMP. • Reduced levels of cytosolic cGMP cause cyclic nucleotide gated channels to close preventing

PHOTORECEPTOR POTENTIALS 

Photoreceptors and the electrical responses of most of the other neural elements in the retina are local, graded potentials,



It is only in the ganglion cells that all-ornone action potentials transmitted.



Responses of the rods, cones, and horizontal cells are hyperpolarizing.



Cone receptor potential has a sharp onset and off set.



Rod receptor potential has a sharp onset and slow off set.

IONIC BASIS OF PHOTORECEPTOR POTENTIALS 

Na + channels in the outer segments of the rods and cones are open in the dark, so current flows from the inner to the outer segment



Current also flows to the synaptic ending of the photoreceptor.



The Na , K ATPase in the inner segment maintains ionic equilibrium.

Effect of light on current flow in visual receptors



Release

of

synaptic

transmitter

(glutamate) is steady in the dark 

When light strikes the outer segment, the reactions that are initiated close some of the Na

+

channels,

and

the

result

is

hyperpolarizing receptor potential. 

The

hyperpolarization

reduces

the

release of glutamate, and this generates a signal in the bipolar cells that ultimately leads to action potentials in ganglion cells. 

The action potentials are transmitted to the

Resulting decrease in Ca & Na concentration Activates guanylyl cyclase, which generates more cGMP. It also inhibits the lightactivated phosphodiesterase. Both actions speed recovery, restoring the Na + channels to their open position.

Signal Processing  

Trace the signal through the retina: The retina is a seven-layered structure involved in signal transduction.  Light enters from the GCL side first, and must penetrate all cell types before reaching the rods and cones.  The outer segments of the rods and cones transduce the light and send the signal through the cell bodies of the ONL and out to their axons.



In the OPL photoreceptor axons the

dendrites

horizontal

of

cells.

interneurons

bipolar Horizontal

which

aid

cells

and

cells in

are

signal

processing 

The bipolar cells in the INL process input from photoreceptors and horizontal cells, and transmit the signal to their axons.



In the IPL, bipolar axons ganglion cell dendrites and amacrine cells, another class of interneurons.



The ganglion cells of the GCL send their axons through the NFL to the optic disk to make up the optic nerve. They travel all the way to the lateral geniculate nucleus.

Fovea 

The fovea defines the center of the retina, and is the region of highest visual acuity.



The fovea is directed towards whatever object you wish to study most closely - this sentence, at the moment.



In the fovea there are almost exclusively cones, and they are at their highest density.

Processing Time 



Latency: it takes a bit time for the cells in retina to respond to a flash of light. Persistence of response: the response does not stop at the instant the flash stops.  

1/25 second at low intensity, 1/50 second at high intensity. The persistence allows as to see moving things clearly.



Phenomenon of processing of visual

impulse

is

very

complicated. 

It includes :

1.

Concept of receptive field

2.

Concept of serial processing of image

3.

Concept of parallel processing pathway

Visual Pathway

The

magnocellular

pathway, from layers 1 and 2, carries signals for detection

of

movement, depth, and flicker. The

parvocellular

pathway, from layers 3– 6,

carries

color

signals

vision,

for

texture,

shape, and fine detail

Visual Field Defects

Visual Field Defects

Visual Perception 

It is a complex integration of light sense, form sense, sense of contrast and colour sense.

1. The light sense 

It is awareness of the light.



The minimum brightness required to evoke a sensation of light is called the light minimum.



Visual adaptation



Eye is capable of functioning normally over an exceedingly wide range of illumination by a highly complex phenomenon termed as the Visual adaptation.



Visual adaptation primarily involves :

1.

Dark adaptation (adjustment in dim illumination)

2.

Light adaptation (adjustment to bright illumination).

Dark adaptation 

It is the ability of the eye to adapt itself to decreasing



illumination. This decline in visual threshold is known as dark adaptation.



The time taken to see in dim illumination is called ‘dark adaptation time’.



Mechanisms :

1.

Visual Pigment mechanism

2.

Change in pupillary size

3.

Neural mechanisms

Dark adaptation curve

Delayed dark adaptation occurs in diseases of rods e.g., retinitis pigmentosa and vitamin A deficiency( Nyctalopia).

Light adaptation 

When one es suddenly from a dim to a brightly lighted environment,

the

light

seems

intensely

and

even

uncomfortably bright until 

The eyes adapt to the increased illumination and the visual threshold rises.



The process by means of which retina adapts itself to bright light is called light adaptation.



Mechanisms: same as dark adaptation

Form Sense 

It is the ability to discriminate between the shapes of the objects.



Cones play a major role in this faculty.



Form sense is most acute at the fovea



Visual acuity recorded by Snellen's test chart is a measure of the form sense.

Factors affecting Visual acuity 1.

Optical factors

2.

Retinal factors

3.

Stimulus Factors :

..

Size & distance of object

..

Colour of object

..

Illuminitation , brightness of object

..

Contrast bw object & background

..

length of time for exposure of object

Critical flicker fusion frequency - Intermittent light stimuli are presented on eye ,a sensation of flicker is evoked 3. Sense of contrast 

It is the ability of the eye to perceive slight changes in the luminance between regions which are not separated by definite borders.

Colour sense 

It is the ability of the eye to discriminate between different

colours

excited

by

light

of

different

wavelengths. 

Colour vision is a function of the cones and thus better appreciated in photopic vision.



In dim light (scotopic vision), all colours are seen grey and this phenomenon is called Purkinje shift.

Theory for colour vision 

Trichromatic

theory.

Called

Young-Helmholtz

theory. 

It postulates the existence of 3 kinds of cones, each containing a different photopigment which is maximally sensitive to one of the 3 primary colours viz. red, green and blue.



The sensation of any given colour is determined by the relative frequency of the impulse from each of the three cone systems.



In other words, a given colour consists of

Applied 

Trichomate



Colour blindness

- Congenital : dyschromatopsia

Retinal Pigment Epithelium: • Sheet of melanin-containing epithelial cells lying between the choroid and the neural portion • Which form a single layer extending from the periphery of the optic disc to the Ora Serrata • epithelial cells that assist in the turnover of rods and cones and prevent the scattering of light within the eyeball due to the presence of melanin also works as a barrier between the vascular system of the choroid and the retina.

The neural portion of the retina: • composed of the 9 remaining layers that would convert light into electrical impulses to be transmitted to the thalamus • The neural portion is soft, translucent and purple (due to the presence of Rhodopsin) which becomes opaque and bleached when exposed to light. • Neurons in the retina are classified into different categories

The Ganglion cell layer: • Transmits the visual packets received from the photoreceptors to the brain for further processing • Highly concentrated in the Macula, less in the fovea • Retinal ganglion cells vary significantly in of their size, connections, and responses to visual stimulation but they all share the defining property of having a long axon that extends into the brain. These axons form the optic nerve

The bipolar cell layer: • Radially oriented neurons. Signal couriers between the photoreceptors that react to light stimuli and the ganglion cells, there are two types of bipolar cells Cone bipolar and rod bipolar, receiving information from their respective photoreceptors.

Photoreceptor layer: • Comprised of rod and cons neurons that converts light to receptor potential.

Photoreceptor layer: • The photoreceptors have photopigments that when hit by light goes thru structural change and triggers the initiation of a receptor potential across the nerves. • The cone cells have three photopigments each one interacts at different wavelength (red, green and blue), • The rod cells they only have Rhodopsin which are essential for vision during dimmed light.

Receptor potential initiation in a rod cell • Light isomerizes retinal, which activates Rhodopsin that in turn activates a G protein called transducin which in turn activates the enzyme phosphodiesterase (PDE), this enzyme then detaches cyclic guanosine monophosphate (cGMP) from Na+ channels in the plasma membrane by hydrolyzing cGMP to GMP. The Na+ channels close when cGMP detaches. The membrane’s permeability to Na+ decreases and the rod hyperpolarizes due to the added negativity in neurons. This hyperpolarization decreases the release of the neurotransmitter glutamate into the synaptic cleft between rod and the subsequent bipolar.

ON-OFF mechanism • At dark, Neurotransmitter glutamate is maximally released and the rods are depolarized consequently the ON bipolar gets hyperpolarized and the OFF bipolar depolarized. • In light, Neurotransmitter glutamate is minimal and the rods are hyperpolarized consequently the ON bipolar gets depolarized and the OFF bipolar hyperpolarized. • Depolarizing bipolar cells results in the release of neurotransmitters whereas hyperpolarization stops this release.

Horizontal cells: • Inhibitory interneurons, that help integrate and regulate the input from multiple photoreceptor cells and are responsible for regulating vision under both bright and dim light.

Amacrine cells: • Interneurons in the retina • They are responsible for 70% of input to retinal ganglion cells. The remaining 30% are regulated by the amacrine cells but are handled by the bipolar cells.

Macula • Center of the posterior retina • responsible for fine central vision • has yellow pigment (xantophyll) • histologically empty space tends to the accumulation of extracellular material that cause thickening

Physiology Of The Eye: Physiology of Vision

Alignment of the Eyes: • Binocular vision: The two eyes field of vision overlap and the image coincide creating a single impression • This is done by synced eye movements where both eyes move simultaneously to maintain the overlap of vision.

Fixation • Involves looking stably straight ahead toward an object in space.

Fusion: • Is the power exerted by the eyes to keep the position of the eyes aligned so that the fovea can project the same point in space.

Eye Movement: • Controlled by extraocular muscles • To the left: levoversion To the right: dextroversion Upwards: sursumversion Downwards: deorsumversion