AN ASSINGMENT ONPHYSIOLOGY OF

SENSE ORGAN : EYE

SUBMITTED BY: Sudipta Nag Himel(11/26)

UNDER THE DEPARTMENT OF PHARMACOLOGY, BIOCHEMISTRY & PHYSIOLOGY

An undergraduate student ofChittagong Veterinary and Animal Sciences University

Khulshi, Chittagong-4225.

Eyes are the paired organs that detect light and convert it into electro-chemical impulses in neurons.

EYE : The organ of VISION

Types of eye:Simple eye

is presence in all

chordates Compound eye

is presence in case of

arthopods

Parts of EYE

EYEcomponent mainly

divided into

2parts

• Three layers: – Fibrous outer layer

• Sclera• Cornea

– Vascular middle layer• Choroid, • Ciliary body and • Iris

– The inner nervous layer• Retina • Pupil

• Structures inside the eyeball are the lens, aqueous fluid and vitreous body.

1.EyeballParts of EYE

2.Accessory structures • Conjunctiva

• Eyelid

• Eyelashes

• Lacrimal gland

• Extraocular muscles

Parts of EYE

Sclera“White part of the eye”

Maintaining the shape and form

of the eye ball.

Contains BVs and insertion site

for extrinsic eye muscles.

CorneaThin, transparent epithelium

Allow the free entry of light.

Act as refractive media.

Fibrous outer layer

Iris “Colored part” of the eye which contains BVs, pigment cells, loose connective tissue and pupillary muscle.

Adjusts the pupilRegulates amount of light

Vascular middle layer

Choroid– Pigmented to absorb excess

light– Supplies the retina with

nutrients and oxygen via blood supply

Vascular middle layer

Ciliary BodyRing of smooth muscle around lens

Regulates the shape of the lens for accommodation

Suspensatory Ligaments– Attach the ciliary body to the lens

The retinaThe inner nervous layer

FunctionsIt helps in

Photopic visionScoptic visionMesopic visionSteroscopic visionAcuity of visionColor visionPerception of movement

Retinais a light-sensitive layer of tissue, lining the inner surface of the eye.

The retinaThe inner nervous layer

Cells of retina-I. Photoreceptor cellsII. Bipolar cellsIII.Horizental cellsIV.Amacrine cellsV. Ganglion cells

• The photo receptor cells are 2 types -

(1)The cone cells

(2)The rod cells

Difference betweem CONE cell and ROD cell

Property Cones Rods

Location In the fovea Outside the fovea

Resolution and visual acuity

Large Small

Total number of elements

5 million 120 million

Color sensitivity Yes No

Brightness sensitivity Small, for daylight vision

Large, for night vision

Figure

The retinaThe inner nervous layer

The macula lutea and fovea centralis

The blind spotRetinal blood vessels.

Macula luteaMacula lutea

Vein Vein

Optic disc Optic disc

Artery Artery

Special Areas of the Retina

Fovea centralis (fovea) is a depresion, located in the center of the macula region of the retina.

The macula or macula lutea is an oval-shaped highly

pigmented yellow spot near the center of the retina

The inner nervous layer

Is a hole located in the center of the iris.

It helps to pass the light into the eye.

Pupil

Structures inside the eyeball

Cavity of EYEThe lens and ciliary body

divide the eye into two cavities.The anterior cavityThe posterior cavity

LensClosely-packed concentric

columnar cellsFocuses image on the retina.

Contains aqueous humor which is secreted by ciliary body

Maintains shape of anterior eye.

Constantly produced - leaves eye thru canal that carries it to bloodstream.

Anterior Chamber of Eye

Anterior Chamber

Contains vitreous humor

Maintains shape of posterior eye.

Refracts light rays.

Is not constantly being produced. Loss of vitreous humor may mean loss of eye.

Posterior Chamber of Eye

Vitreous Chamber

Lacrimal gland The lacrimal glands are the sites of tear

production.

Tears function to keep the conjunctiva

and corneal epithelium moist and

wash away foreign material from the

eye.

Accessory structures

Lacrimal apparatusSecretes and drains tears from the eye

Accessory structures

Eyelashes

Eyelashes are short hairs of eye that

may occur in double or triple rows.

They function to protect the eye

from debris.

Lashes may also have

different lengths and

diameters to one another.

Accessory structures Conjunctiva The conjunctiva refers to the lining of the

eye.

It has 2 parts

Palpebral conjunctiva

Bulbar conjunctiva

It helps lubricate the eye by secreting

mucous, and serves as a protective

barrier again microbes.

It contains many goblet cells which

secrete a component of the tears that

bath the eye.

Accessory structures

Eyelid The main function of the eyelid

is to provide the eye with

protection. There are several

types of glands in the eyelids,

including tarsal glands that

produce a sebaceous secretion

that results in an oily surface of

the tear film to prevent the

evaporation of the normal tear

layer.

Accessory structures

Extraocular Muscles

i. Rectus dorsalis

ii. Rectus ventralis

iii. Rectus medialis

iv. Rectus lateralis

v. Obliqus dorsalis

vi.Obliqus ventralis

Functions of Extraocular MusclesIO SR IO/SR SR/IO SR IO

LR MR MR LR

SO IR SO/IR IR/SO IR SO

MR MR

CONVERGENCE

These six muscles responsible for eye movement.

Four rectus muscles control the movement of the eye in

the four cardinal directions:

Up,

Down,

Left and

Right.

The remaining two muscles control the adjustments

involved in counter acting head movement.

Light

Refraction

• Cornea and lens

Accommodation

Photo-pigments

Photo-transduction

Photoreceptor activity

• In the dark

• When exposed to light

Vision Steps of vision

Visual Pathway

Pathway of

LIGHT

Refraction is the phenomenon which makes image formation possible by the eye as well as by cameras and other systems of lenses.

The eye, is optically equivalent to the usual photographic camera.

The lens system of the eye is composed of four refractive interfaces.

Formation of an Image on the Retina.

The lens system of the eye can focus an image on the retina. The image is inverted and reversed with respect to the object. However, the mind perceives objects in the upright position because the brain is trained to consider an inverted image as the normal.

The Eye as a CameraRefraction

AccommodationThe process of changing the

shape of the lens while focusing on an object is termed as accommodation.

At rest focused on distant objects, needs to increase focal power than focus on close objects.

The ciliary muscles contract and release tension in the ligaments and the lens becomes rounder in case of close objects.

There are four types of photopigments, one in the rods and one in

each of three types of cones.

The pigment in the rods is called rhodopsin.

Rhodopsin absorbs all visible wavelengths, so that rods

provide vision only in shades of grey by detecting different

intensities rather than colours.

The three types of cones - red, green and blue

photopsin, can respond selectively to various

wavelengths of light, giving rise to colour vision.

Photopigments

Rhodopsin

๑ The photon is absorbed by rhodopsin. ๑ The light receptor molecule tightly-packed in the

disk membranes

๑ One rod contains 10 billion rhodopsin molecules, which ensures an optimum photon capture rate

Rhodopsin has 2 components Retinal- Vitamin A aldehydeOpsin- single polypeptide

containing 7 transmembrane domains

RHODOPSIN CYCLE

ROLE OF VITAMIN A FOR FORMATION OF RHODOPSIN.

Vitamin A is present both in the cytoplasm of the rods and in the pigment layer of the retina. Therefore, vitamin A is normally always available to form new retinal when needed.

Conversely, when there is excess retinal in the retina, it is converted back into vitamin A, thus reducing the amount of light-sensitive pigment in the retina.

Night blindness.

The retina therefore signals the brain

Light stimulation through an inhibitory response involving a series of

physiological reactions

Action potentials, Only originate in the ganglion cells,

The first neurons in the chain that carry the visual stimuli to the brain.

Vision occurs

This process is called phototransduction.

Phototransduction

ERRORS OF REFRACTION• Astigmatism– defective

curvature of the cornea or lens of the eye.

• Presbyopia– impaired vision of the cornea or lens of the eye, associated with aging.

• Hyperopia– can’t see close objects, rays of light focus behind retina

• Myopia- can’t see far away objects, rays of light focus in front of the retina

COMMON DISORDERS OF THE EYE

(A) Acute bacterial conjunctivitis.

(B) Anisocoria.

(C)Color Blindness Monochromacy

lack of 2 of the 3 types of

cones Dichromacy:

lack of 1 type of normally-function cones

Anomalous trichromacy shift in the normal spectrum

Visual loss

Double vision

Severe eye pain

Foreign body sensation

Some other Symptoms found in a serious

Ocular condition

Internet

ReferencesBooks

Textbook of medical physiology / C. Guyton, John E. Hall.—11th ed

Review of Medical Physiology / William F . Ganong .—22nd ed

Animal Physiology /Eckert & Randall -2nd ed

?Q

UES

TIO

NS