Biology 1203 - Week 07

macula lutea

central region of the retina responsible for central vision; yellow pigment provides its color

lacrimal ducts

channels that carry tears to the eye

suspensory ligaments

a fibrous membrane that holds the lens of the eye in place

lens II.

a piece of clear material that bends, or refracts light rays passing through it.

Eyeball Anatomy

a) sclera . b) cornea. c) choroid. d) ciliary body . e) iris. f) pupil. g) retina. h) ora serrata. i) macula lutea. j) fovea . k) optic disc. l) optic nerve. m) lens. n) suspensory ligament. o) aqueous humour. p) vitreous humour. q) conjunctiva .

myopia

nearsightedness; difficulty seeing distant objects when light rays fall short of the proper focus on the retina

sclera

tough, fibrous, white outer coat extending from the cornea to the optic nerve

cornea

transparent, anterior part of th eyeball covering the iris, pupil, and anterior chamber that functions to refract light to focus a visual image

choroid

vascular layer beneath the sclera that provides nourishment to the outer portion of the retina

aqueous humour

watery liquid secreted at the ciliary body that fills the anterior and posterior chambers of the eye and provides nourishment for the cornea, iris, and lens

Lens I.

> Completely transparent (and avascular) biconvex flexible structure. > Lies posterior to the iris. > FOCUSES LIGHT RAYS ON THE RETINA- lens changes shape to adjust focus . - NB. cornea is primarily responsible for light refraction. > Suspended in the ciliary body by SUSPENSORY LIGAMENTS. > Divides the eye into anterior and posterior cavities.

Image Formation in the Retina: d) Convergence of the Eyes

> During near vision both eyes rotate MEDICALLY controlled by EXTRINSIC EYE MUSCLE (supplied by CRANIAL NERVES III, IV, VI). > Six extrinsic eye muscles enable the eye to follow moving objects. > Ensures image falls on fovea (sharpest vision).

The Visual Pathway : Processing of Left vs. Right Visual Fields II.

> In other words: Light rays from the RIGHT SIDE OF AN OBJECT are interpreted by the LEFT BRAIN; Light rays from the LEFT SIDE OF AN OBJECT are interpreted by the RIGHT BRAIN. > Note: Nerves from NASAL retina CROSS OVER at optic chiasm, TEMPORAL ones do not.

Accommodation of the lens - summary: Viewing Close Object

> Light rays arriving at cornea are DIVERGENT. > PARASYMPATHETIC activation. > Ciliary muscle CONTRACTS. > Suspensory ligaments loose. > LENS BULGES (more convex). > Lens bends light MORE to form sharp image on retina.

Accommodation of the lens - summary: Viewing Distant Object

> Light rays arriving at cornea are parallel. > SYMPATHETIC activation. > Ciliary muscle relaxes. > Suspensory ligaments tense. > LENS FLATTENS. > Lens has tall thin shape.

Eyeball wall: 3. Retina II

> MACULA LUTEA: centre of the retina (small area). > FOVEA: the small depression in centre of macula, with highest concentration of CONES (photoreceptors); PROVIDES HIGHEST VISUAL ACUITY. > OPTIC DISC: where the ganglion neurons of retina bend posteriorly from the OPTIC NERVES (aka BLIND SPOT because of absence of photoreceptors).

Image Formation in the Retina: c) Constriction of the Pupil.

> Occurs automatically (parasympathetic control) during accommodation for NEAR VISION. > This LIMITS AMOUNT OF LIGHT that passes through the periphery of the bulging lens. > Therefore light passes through the center and thickest part of the lens. > This PREVENTS THE FORMATION OF A BLURRED IMAGE on the retina.

Eyeball wall: Pupil

> Opening in centre of the iris. > Changes in size to alter amount of light entering eye. > Size depends on the contraction of an antagonistic pair of muscles of the iris (CIRCULAR and RADIAL muscles).

Image Formation in the Retina: b) Accommodation of the Lens - what is it?

> Process by which the lens changes shape to focus on a near object; a REFLEX ACTION (autonomic). > The more curved the lens, the greater the refraction of light; reduces focal distance.

Image Formation on the Retina: a) Refraction of the Light Rays

> Refraction : Bending of light rays at the junction of two transparent substances with different densities. > Sites of Refraction in the Eye : Cornea (75%) and Lens (25%). > Cornea : PRINCIPAL REFRACTION medium of eye, this refraction is FIXED. > Lens : VARIABLE REFRACATION to help focus on near or distant objects (accommodation).

iris

A ring of muscle tissue that forms the colored portion of the eye around the pupil and controls the size of the pupil opening

Eyeball cavities (anterior)

Anterior cavity: > Between the cornea and lens. > Filled with AQUEOUS HUMOUR. > Similar in composition to cerebrospinal fluid. > SECRETED THE CILIARY BODY to posterior chamber and drained off through the CANAL OF SCHLEMM into the blood (arrows). Functions: > To MAINTAIN PRESSURE (INTROCULAR PRESSURE) IN THE EYEBALL FOR CLEAR VISION. > To NOURISH LENS AND CORNEA.

fovea

Center of the macula "Point of central focus". Spot on the back of the retina that has more cones that anywhere else. Helps us see fine detail

Anatomy of Eye: Summary of main components and functions

CHART OF EYE ANATOMY

The Retina: Cellular Organization

DIAGRAM CELLULAR ORGANIZATION

The Retina: 2 types of photoreceptors

DIAGRAM OF 2 PHOTORECEPTORS

The Visual Pathway : From the retina to the brain

DIAGRAM OF THE VISUAL PATHWAY

The Visual Pathway : Processing of Left vs. Right Visual Fields I.

DIAGRAM: LEFT v.s RIGHT

Astigmatism

Distortion of curvature of cornea or lens or both. Blurred vision.

Summary: image formation on retina

For the formation of an image in the retina, the following events need to occur: a.) REFRACTION of light rays, mostly by the CORNEA. b.) ACCOMMODATION of the LENS: > nearby vision: ciliary muscles contract, lens bulges. > distant vision: ciliary muscles relax, lens flattens. c.) CONSTRICTION of the PUPIL by contraction of the circular muscles and relaxation of the radial muscles (near objects). d.) CONVERGENCE OF THE EYES by the action of the extrinsic muscles of the eye (near objects).

Eyeball wall: 3. Retina I

Inner layer: retina. > Lines the posterior eyeball. > Functions: - IMAGE FORMATION - converts light to nervous impulses (sent to brain). - STORES VIT. A. > Contains NERVOUS and PIGMENTED layers. > ORA SERRATA: jagged edge of neural layer near ciliary body (optic retina). > Pigmented layer of retina continues over back of ciliary body and iris (ciliary retina).

Cataract

Loss of lens transparency with age. Also exacerbated in diabetes & exposure to sunlight.

Eyeball wall: 2. Vascular Tunic

Middle layer > CHORIOD: Posterior. Dark brown membrane lining sclera; rich blood supply to NOURISH RETINA; pigmented to ABSORB LIGHT RAYS for clearer image. > CILIARY BODY: Anterior. Extends from retina to base of iris; alters the shape of lens for focusing- ACCOMMODATION; secretes AQUEOUS HUMOUR. > IRIS: Coloured part of the eye; ALTERS SIZE OF THE PUPIL (with CIRCULAR and RADIAL muscles) to regulate amount of light entering eye.

Eyeball wall: 1. Fibrous Tunic

Outer layer: thick fibrous avascular tissue. > CORNEA: transparent anterior portion (1/6); main function is to REFRACT light rays and FOCUS them onto retina. > SCLERA: opaque posterior portion (5/6); white of the eye; tough fibrous coat PROTECTS inner parts and GIVES SHAPE to the eye; attachment of EXTRINSIC EYE MUSCLE.

The Retina: Phototransduction cascade : Conversion of light to an electrical impulse

Photodetection -retinal (in rod) absorbs light and changes shape setting off cascade of events that switch on cells of visual pathway.

Eyeball cavities (posterior)

Posterior cavity: > Between the lens and retina. > Filled with VITREOUS HUMOUR (a jelly-like liquid). Functions: > TO PREVENT THE EYEBALL FROM COLLAPSING. > TO HOLD THE RETINA AGAINST THE CHORIOD .

optic disc

Region at the back of the eye where the optic nerve meets the retina. It is the blind spot of the eye because it contains only nerve fibers, no rods or cones, and is thus insensitive to light.

Lacrimal apparatus : Manufacture and draining of tears

Tears = watery solution containing: > Salts. > Mucous. > Lysozyme (bactericidal enzyme). Functions: > Clean and moisten eyes. > Protect against bacterial infections.

pupil

black circular opening in the center of the iris through which light passes as its enters the eye

scotoma

blind spot in vision

Image Formation in the Retina: c) Constriction of the Pupil (for near vision)

diagram

Image Formation on the Retina ; Events

diagram

Image Formation in the Retina: b) Accommodation apparatus - How the lens changes shape?

diagram OF HOW THE LENS CHANGE SHAPE

Anterior and Posterior Cavities of the Eye

diagram of the eye cavities.

hyperopia

farsightedness; difficulty seeing close objects when light rays extend beyond the proper focus on the retina

ora serrata

the serrated junction between the retina and the ciliary body.

lacrimal gland

gland located in the upper outer region above the eyeball that secretes tears

glaucoma

group of diseases of the eye characterized by increased intraocular pressure that results in damage to the optic nerve, producing defects in vision.

retina

innermost layer that perceives and transmits light to the optic nerve

vitreous

jelly-like mass filling the inner chamber between the lens and retina that gives bulk to the eye

conjunctiva

joining together; mucous membrane that lines the eyelids and outer surface of the eyeball

nasolacrimal duct/lacrimal sac

passageway for tears from the lacrimal sac into the nose

conjunctivitis

pink eye; inflammation of the conjunctiva

ciliary body

ring of muscle behind the peripheral iris that controls the power of the lens

lacrimation

secretion of tears

ciliary muscle

smooth muscle portion of the ciliary body, which contracts to assist in near-vision capability