CHAPTER 10

THE EYE AND VISION

EYES- sense organs that provide us with the greatest knowledge of our environment

Light energy Nerve impulses Optic nerve Sight

ACCESSORY ORGANS There are several accessory organs

that house and protect the eye from external hazards

1. Orbital cavities- bony sockets of skull

- surrounded by 7 different bones: Frontal, maxillary, zygomatic, sphenoid, lacrimal, ethmoid, palatine

2. Eye muscles- Intrinsic- found in the eye- Extrinsic- control eye movements* inferior rectus* lateral rectus* medial rectus* superior rectus* inferior oblique* superior oblique

- all of these work to control the position of the eye

3. Lacrimal Gland- responsible for the production of tears

- tears that do not evaporate are drained to nasal cavity by lacrimal ducts

- when tear production increases, lacrimal ducts cannot keep up and tears flow over the bottom lid

4. Eyebrows- protect upper part of orbital cavity

5. Eyelids- controlled by the orbicularis oculi

- are under voluntary control6. Eyelashes- secretions from the tarsal glands

prevent the eyelashes from sticking together

7. Conjunctiva- thin, transparent mucous membrane- Conjunctivitis (pink eye)- caused by irritants or lack of sleep

EYE CHAMBERS

The interior of the eye can be divided into 2 chambers:

1. Posterior cavity- contains gelatinous VITREOUS HUMOR

2. Anterior cavity- subdivided into anterior chamber and posterior chamber

- filled with fluid called AQUEOUS HUMOR

- The shape of the eye is stabilized by both the vitreous humor and the aqueous humor

COMPONENTS OF THE EYE

1. SCLERA- white, opaque membrane that also helps keep the shape of the eye - extrinsic eye muscles insert here

2. CORNEA- transparent and colorless part of the eye through which light waves pass

- contains no blood vessels; its cells obtain nutrients from tears

- often called the “window of the eye”3. UVEA- made up of choroid coat,

ciliary body, and iris- also contains the intrinsic eye

musclesFunctions of the uvea include:- regulating amount of light

entering eye

- secreting and reabsorbing aqueous humor

- controlling shape of lens

*CHOROID- supplies blood vessels to the layers of the eye, especially to retina- contains pigment granules that prevent reflection of light WITHIN the eye (black interior of a camera)*CILIARY BODY- contains one intrinsic muscle (ciliary muscle)

- fibers of this muscle support and modify the shape of the lens*IRIS- attached to ciliary body and is the colored portion of the eye- opening in the center of the iris is the PUPIL

- amount of light entering the pupil is regulated by muscles of the iris

- color of iris determined by number of melanocytes, and the pigmented epithelium on its posterior surface

- when melanocytes are absent, light passes through the iris and bounces off of this epithelium eyes are BLUE

- individuals with increasing number of melanocytes in iris have gray, brown, or black eyes

- no pigment at all- iris appears pink albinos

Why do many newborns have blue eyes?

- It takes several days for melanin to develop in melanocytes

- The amount of melanin is inherited from parents

Components cont.

4. RETINA- innermost layer of eye- completely lines the inner

surface of the eye- contains cells responsible for

converting light into impulses called RODS and CONES

RODS AND CONES IN RETINA

- Rods do not discriminate among colors of light; they enable us to see in dimly-lit rooms, at twilight, or in pale moonlight

- Cones are responsible for color vision; give us sharper, clearer images, but require more light to do this

In the center of the retina is a yellow disk called the MACULA LUTEA

- in the center of the disk is a small depression called the FOVEA CENTRALIS

- this is the region for sharp vision as well as color sensitivity (contains only cones)

- area away from the fovea is the EXTRAFOVEAL REGION

- contains more rods

- when you look directly at an object, its image falls on the fovea centralis

OPTIC DISC- found where optic nerve leaves the eye

- also called BLIND SPOT- no rods or cones

5. LENS- found just behind the pupil and iris

- disc-shaped and somewhat elastic - held in place by a suspensory

ligament- ligament is attached to ciliary

muscle, which controls amount of tension on the lens

THE LENSTHE LENS

STRUCTURE OF LENSSTRUCTURE OF LENS

Lens consists of organized layers of cells Lens consists of organized layers of cells wrapped in a dense fibrous capsulewrapped in a dense fibrous capsule

- capsule is elastic: - capsule is elastic: unless an outside unless an outside force is applied, it will contract and force is applied, it will contract and make the lens sphericalmake the lens spherical

- tension in the suspensory ligaments can - tension in the suspensory ligaments can overpower the elastic capsule and change overpower the elastic capsule and change the shape of the lens into a flattened ovalthe shape of the lens into a flattened oval

Lens is CONVEX- Lens is CONVEX- uniformly curved and uniformly curved and thicker in the middle than at the edgethicker in the middle than at the edge

- parallel rays of light will CONVERGE and - parallel rays of light will CONVERGE and be brought to a single FOCAL POINT be brought to a single FOCAL POINT

REFRACTION AND REFRACTION AND ACCOMMODATIONACCOMMODATION

In order for an image to produce useful In order for an image to produce useful information, it must be in focusinformation, it must be in focus

* meaning that rays of light from an object * meaning that rays of light from an object strike the retina in a precisely ordered manner strike the retina in a precisely ordered manner to form a miniature image of the objectto form a miniature image of the object

Focusing occurs in 2 steps:Focusing occurs in 2 steps:

- - through the corneathrough the cornea

- - through the lensthrough the lens

REFRACTION- REFRACTION- bending of light when it passes bending of light when it passes from one medium to another with a different from one medium to another with a different densitydensity

- in the eye, the greatest amount of refraction - in the eye, the greatest amount of refraction occurs when light passes from the air into the occurs when light passes from the air into the corneacornea

- the lens provides the extra refraction needed to - the lens provides the extra refraction needed to focus light rays from an object toward a specific focus light rays from an object toward a specific FOCAL POINT on the retinaFOCAL POINT on the retina

FOCAL DISTANCE- FOCAL DISTANCE- distance between the center of distance between the center of the lens and the focal pointthe lens and the focal point

- determined by - determined by

* distance of object from the lens (closer the object, * distance of object from the lens (closer the object, the longer the focal distance)the longer the focal distance)

* * shape of lens (rounder the lens, the shorter the shape of lens (rounder the lens, the shorter the focal distance)focal distance)

- the lens changes shape to keep the focal distance - the lens changes shape to keep the focal distance constantconstant

ACCOMMODATION- ACCOMMODATION- process of focusing process of focusing an image on the retina by changing an image on the retina by changing the shape of the lensthe shape of the lens

- lens either becomes rounder or flattens - lens either becomes rounder or flattens

When you view a nearby object, ciliary When you view a nearby object, ciliary muscles contract, reducing tension in the muscles contract, reducing tension in the suspensory ligaments and making the suspensory ligaments and making the lens rounderlens rounder

When you view a distant object, ciliary When you view a distant object, ciliary muscles relax, increasing tension in the muscles relax, increasing tension in the suspensory ligaments and making the suspensory ligaments and making the lens flatlens flat

IMAGE FORMATIONIMAGE FORMATION

The image of an object that arrives on the The image of an object that arrives on the retina is:retina is:

- - a miniature of the originala miniature of the original

- - upside down and backwardupside down and backward

Our brain compensates for the reversal Our brain compensates for the reversal without our conscious awarenesswithout our conscious awareness

VISUAL ACUITYVISUAL ACUITY

VISUAL ACUITY- clarity of visionVISUAL ACUITY- clarity of vision

- - rated on basis of the sight of a “normal” rated on basis of the sight of a “normal” personperson

- 20/20 vision means that a person can see - 20/20 vision means that a person can see details at a distance of 20 feet as clearly as a details at a distance of 20 feet as clearly as a “normal” individual would“normal” individual would

- 20/15 is better than average- at 20 feet, this - 20/15 is better than average- at 20 feet, this person is able to see details that a normal person is able to see details that a normal person could only see at 15 feetperson could only see at 15 feet

- 20/30 is below average- this person can - 20/30 is below average- this person can see details at 20 feet that a “normal” see details at 20 feet that a “normal” person could see at 30 feetperson could see at 30 feet

- - when visual acuity falls below 20/200, when visual acuity falls below 20/200, the individual is considered to be the individual is considered to be legally blindlegally blind

ACCOMMODATION ACCOMMODATION PROBLEMSPROBLEMS

A normal eye condition is called A normal eye condition is called EMMETROPIA- when ciliary muscles are EMMETROPIA- when ciliary muscles are relaxed and the lens is flattened, a distant relaxed and the lens is flattened, a distant image will be focused on the retinaimage will be focused on the retina

IRREGULARITIES:IRREGULARITIES:

1. ASTIGMATISM- 1. ASTIGMATISM- irregular shape of lens irregular shape of lens or cornea; can usually be corrected or cornea; can usually be corrected with glasses or contactswith glasses or contacts

2. MYOPIA (nearsightedness)2. MYOPIA (nearsightedness)

- eyeball is too deep; image of a distant object - eyeball is too deep; image of a distant object will form in FRONT of the retina, picture will will form in FRONT of the retina, picture will be blurrybe blurry

- - these individuals can see objects that are these individuals can see objects that are closeclose

- can be corrected by placing a CONCAVE lens - can be corrected by placing a CONCAVE lens in front of the eye- in front of the eye- rays of light diverge (go rays of light diverge (go away from each other)away from each other)

3. HYPEROPIA (farsightedness)3. HYPEROPIA (farsightedness)

- - eyeball is too shallow; lens cannot provide eyeball is too shallow; lens cannot provide enough refractionenough refraction

- these individuals can see objects at a distance- these individuals can see objects at a distance

- - can be corrected by placing a convex lens can be corrected by placing a convex lens (converging) in front of eye(converging) in front of eye

- older individuals become farsighted as their - older individuals become farsighted as their lenses lose elasticity lenses lose elasticity PRESBYOPIAPRESBYOPIA

VISUAL PHYSIOLOGYVISUAL PHYSIOLOGY

PHOTORECEPTORSPHOTORECEPTORS

Rods and cones are called Rods and cones are called PHOTORECEPTORS because they detect PHOTORECEPTORS because they detect PHOTONS- PHOTONS- the basic units of visible lightthe basic units of visible light

- light is a form of energy, which is radiated in - light is a form of energy, which is radiated in waves and described in terms of waves and described in terms of WAVELENGTHSWAVELENGTHS

- - we can see wavelengths that make up the we can see wavelengths that make up the visible spectrumvisible spectrum

WHAT IS WHAT IS

ROY G BIV?ROY G BIV?

- - an acronym for the spectrum of visible light: an acronym for the spectrum of visible light: Red, Orange, Yellow, Green, Blue, Indigo, Red, Orange, Yellow, Green, Blue, Indigo, VioletViolet

- color depends on the wavelength of light- color depends on the wavelength of light

- - longer wavelengths = less energylonger wavelengths = less energy

MORE ABOUT RODS AND MORE ABOUT RODS AND CONESCONES

Rods provide the CNS with information about the Rods provide the CNS with information about the presence or absencepresence or absence of photons, not about of photons, not about wavelengthwavelength

- - so, they do not discriminate among colorso, they do not discriminate among color

Cones do provide information about the wavelength Cones do provide information about the wavelength of photonsof photons

- they are less sensitive than rods, so function only - they are less sensitive than rods, so function only in bright lightin bright light

- - there are 3 types of cones: blue, green, redthere are 3 types of cones: blue, green, red

COLORBLINDNESS- inability to distinguish certain COLORBLINDNESS- inability to distinguish certain colorscolors

- - occurs because one or more classes of cones occurs because one or more classes of cones are absent or do not functionare absent or do not function

- in most common condition, red cones are missing, - in most common condition, red cones are missing, and individual cannot distinguish red light from and individual cannot distinguish red light from green lightgreen light

- colorblindness is common in men, but very rare in - colorblindness is common in men, but very rare in womenwomen

TestTest

STRUCTURE OF A STRUCTURE OF A PHOTORECEPTORPHOTORECEPTOR

The outer segment of a rod or cone contains The outer segment of a rod or cone contains hundreds to thousands of flattened hundreds to thousands of flattened membranous discsmembranous discs

- - names “rods” and “cones” refer to the names “rods” and “cones” refer to the shape of the outer segmentshape of the outer segment

The inner segment synapses with other cells The inner segment synapses with other cells and releases neurotransmittersand releases neurotransmitters

Rods and conesRods and cones

The discs of the outer segment in both rods and The discs of the outer segment in both rods and cones contain special organic compounds cones contain special organic compounds called VISUAL PIGMENTScalled VISUAL PIGMENTS

- the absorption of photos by visual pigments is - the absorption of photos by visual pigments is the first step in the process of photoreception- the first step in the process of photoreception- the detection of lightthe detection of light

- these pigments are derivatives of the - these pigments are derivatives of the compound RHODOPSIN- compound RHODOPSIN- consists of the consists of the protein opsin bound to the pigment retinalprotein opsin bound to the pigment retinal

PHOTORECEPTIONPHOTORECEPTION

Photoreception begins with a photon strikes a Photoreception begins with a photon strikes a rhodopsin molecule in the outer segment of rhodopsin molecule in the outer segment of a rod or conea rod or cone

- shortly after this, the rhodopsin molecule - shortly after this, the rhodopsin molecule begins to break down into retinal and opsin begins to break down into retinal and opsin called BLEACHINGcalled BLEACHING

- the rhodopsin must then be regenerated - the rhodopsin must then be regenerated from the retinal and opsinfrom the retinal and opsin

- this process takes some time- this process takes some time

- - bleaching contributes to the lingering bleaching contributes to the lingering visual impression that you have after a visual impression that you have after a camera flash goes offcamera flash goes off

- after this exposure to light, a photoreceptor - after this exposure to light, a photoreceptor cannot respond to further stimulation until its cannot respond to further stimulation until its rhodopsin molecules have been rhodopsin molecules have been regenerated (so you see the “ghost” image)regenerated (so you see the “ghost” image)