Eye

1 DAWN V TOMY M.Pharm., Asst. Professor, Dept. of Pharmacology, ST.JOSEPH’S COLLEGE OF PHARMACY, CHERTHALA.

Eye

Half the sensory receptors in body is present in the eye. Large part of cerebrum devoted to

process visual information.

Structure consists of the eye ball & accessory structures.

Eye Ball:

Approximately 2.5 cm in diameter made up of 3 layers:

Outer fibrous tunic – consists of cornea and sclera.

Middle vascular tunic – consists of choroid and ciliary body.

Inner retinal layer – consists of 2 neural layer i.e. outer pigmented and inner neural layer.


1. Fibrous tunic: Outermost layer consists of:

• Cornea is a transparent coat over the iris.

– Curved, helps focusing light to retina.

– Non-keratinized, stratified squamous outside.

– Collagen fibers and fibroblasts in the middle.

– Simple squamous in the innerside.

• Sclera is the hard white of eye.

– Gives shape & rigidity to the eye ball formed of dense connective tissue and

collagen fibers.

– Covers entire eye ball except cornea

• Canal of schlemm (scleral venous sinus) is the opening at junction of cornea and

sclera through which the fluid aqueous humor drains.

2. Vascular tunic (uvea): Middle layer consists of 3 parts:

• Choroid is the posterior part, internal to sclera, with blood vessels and melanocytes

forms melanin which gives dark brown color to the eye and also absorb stray light

rays.

• Ciliary body – anterior part of choroid. Dark brown due to melanin.

– Ciliary process is the protrusions on internal surface of ciliary body and its

function is to secrete aqueous humor.

– Zonular fibers and suspensory ligaments are attached to the lens and ciliary

muscles.

– Ciliary muscles are circular band of smooth muscle; contraction and

relaxation affects zonular fibers and thus lens widening and flattening.

• Iris is the colored portion of eye ball between cornea and lens.

– Circular and radial muscles forms the aperture which regulate the amount of

light falling on to the retina.

– Pupil is the hole at the center of the iris.


3. Retina: 3rd and interior layer where optic nerve II end and the visual pathway begin.

– Optic disc or blind spot - where optic nerve exits eye ball visually inactive

area in the retina.

Retina has 2 layers:

• Outer pigmented layer is between choroid and neural layer. Melanin absorbs the

stray light.

• Neural layer is the multilayered outgrowth of brain, process visual data and have 3

layers:

– Photoreceptor cell layer with rods activated at dim light (120 million) and

cones for color vision (blue, red and green cones).

– Bipolar cell layer with bipolar, horizontal and amacrine nerve cells.

– Ganglion cell layer with neurons.

• Macula lutea is the center of posterior portion of retina.

• Central fovea is a depression at macula lutea has the highest area of visual acuity

(sharpness).


Lens – behind the iris and pupil, made of a proteins called crystalline.

Interior of eye ball. Outside lens – anterior cavity - 2 chambers:

Anterior chamber – b/w cornea and iris.

Posterior chamber – behind iris and front of lens – filled with aqueous humor, nourishment

for lens and cornea.

Aqueous humor – produced from ciliary process, reaches posterior chamber between iris and

lens, flow through pupil reaches anterior chamber between pupil and cornea, drains through canal

of schlemn to blood.

Function is nourishment (replaced every 90 min) maintains Intra ocular pressure (16 mm of hg)

by aqueous humor and partly vitreous humor. Gives shape and prevents collapsing.

Vitreous chamber between lens and retina. Vitreous body is a jelly like substance contain water,

collagen, hyaluronic acid, phagocytic cells. Hyloid canal posterior to optic disc drains vitreous

humor.

ACCESSORY STRUCTURES:

Eyelids (palpebrae) – lower palpebra and upper palpebra (more movable).

Function - protection, lubrication.

Muscles - superioris muscle, levator palpebrae.

Palpebral fissure is the space between upper and lower eyelids.

Lateral (near temporal angle) and medial commissure (near nasal bone).

Lacrimal caruncle – small reddish elevation at medial commissure secrets sebaceous oil and

contains sudoriferous sweat glands.

Tarsal plate is thick connective tissue which gives form and support to eyelids.

Tarsal or meibomian glands secretes fluid which keeps eyelids from adhering.

Conjunctiva is a thin mucus membrane above (bulbar) eyeball and inside palpebra (palpebral).

Eye lashes – project from border of eyelid.

Eye brows – arch transversely above upper eyelid gives protection from foreign objects, direct

rays of sun.

Sebaceous ciliary glands secrets lubricating fluid.

Lacrimal apparatus involved in the secretion of tear, flow of tears from lacrimal gland (almond

shaped) through excretory lacrimal duct (6-12 ducts) – superior and inferior lacrimal canal –

lacrimal sac – nasolacrimal duct to nasal cavity. One gland secrets 1 ml/day, facial nerve VII

control the secretion.

Contents of Tear – water, mucus, lysozyme.

Extrinsic eye muscles –

– Rectus – superior, inferior, lateral, medial.

– Oblique – superior, inferior.

Nerves – III, IV, VI.


IMAGE FORMATION:

Reflected light enters the eye by refraction, 75% light refracted by cornea and 25% by lens. Lens

is convex in form and helps in the convergence of light (more convex more convergence and

focusing power). The lens increase or decrease curvature according to the need for distant object

less curvature flat lens relaxing ciliary muscle. Near object more spherical contracting ciliary

muscle. This power of lens to adjust itself according to need is called power of accommodation.

Near point of vision is the minimum distance from eye to be clearly focused i.e.15 cm.

Abnormalities of vision:

Myopia – near sightedness - eyeball too long or thick lens – image in front of retina –

use concave lens.

Hyperopia – hypermetropia - farsightedness - eyeball short or lens thinner –image

behind retina- use convex lens.

Presbyopia - lens loses its elasticity, after age of 40 – near point of vision. Increases –

use bifocal lens.

Astigmatism – cornea or lens has irregular curvature.


PHYSIOLOGY OF VISION:

Photoreceptor layer of retina contains rods & cones with photo pigments.

Rods: Rod shaped, for dim vision, plasma membrane pleats form discs (10000 discs), 1-3 disc

replaced every hour.

Cones: Plasma membrane fold in pleats and cone shaped for color vision.

Photo-pigments:

Glycoprotein – Opsin - 3 opsin in one cone.

Rhodopsin – in rods. Retinal a Vitamin-A derivative, is the light absorbing part. When light reaches

pigments, light is absorbed by pigment and pigment protein undergoes structural changes like

Isomerization, bleaching & regeneration. When light reaches, cis retinal in the opsin isomerizes to

trans-retinal - called isomerization. Trans-retinal separates from opsin to form colorless opsin

called bleaching. Retinal isomerizes convert Trans to cis, and cis gets attached to opsin back called

regeneration, occurs half to 5 minutes and full 30-40 minutes. These chemical changes produce

receptor potential.


1. Increased darkness – glutamate release inhibits bipolar cell layer.

2. Light – no cGMP, no inflow of sodium, no glutamate release and bipolar layer excitation.

• Adaptation – adjustment of visual system to environment.

• Light – quick, dark – slow due to difference in regeneration and bleaching.

VISUAL PATHWAY

• From retina – axons of retinal ganglion cells, exit the eye ball at optic disc form optic nerve.

• Reaches – optic chiasm – crossing over.

– Right side visual field to left side of brain.

– Left side visual field to right side of eye.

– Objects in binocular field in both eyes.


– Temporal half – do not cross – go same side of eye and thalamus.

– Nasal half cross opposite side of the eye and to thalamus.

• Thalamus – crossed and not crossed at lateral geniculate nuclease of thalamus as optic

tract.

• Some to supra chiasma nuclease of hypothalamus – involve in sleep.

• Optic radiations – thalamic neuron form optic radiations project to visual area of cortex

on same side area 17.


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