THE EYE AND VISION

EYESsense organs that provide us with the greatest knowledge of our environment
Light energy  Nerve impulses 
Optic nerve  Sight

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
- Intrinsicfound in the eye
- Extrinsiccontrol 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
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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. Eyebrowsprotect upper part of orbital cavity
5. Eyelids
- controlled by the orbicularis oculi

are under voluntary control
6. 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

The interior of the eye can be divided into 2 chambers:
1. Posterior cavitycontains gelatinous VITREOUS HUMOR
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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

1. SCLERAwhite, opaque membrane that also helps keep the shape of the eye
- extrinsic eye muscles insert here
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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. UVEAmade up of choroid coat, ciliary body, and iris
- also contains the intrinsic eye muscles
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Functions of the uvea include: regulating amount of light entering eye

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- secreting and reabsorbing aqueous humor controlling shape of lens

*CHOROIDsupplies 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 BODYcontains one intrinsic muscle (ciliary muscle)

- fibers of this muscle support and modify the shape of the lens
*IRISattached 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

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- individuals with increasing number of melanocytes in iris have gray, brown, or black eyes no pigment at all- iris appears pink  albinos

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- It takes several days for melanin to develop in melanocytes
The amount of melanin is inherited from parents

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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 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
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- 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
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OPTIC DISC- found where optic nerve leaves the eye also called BLIND SPOT- no rods or cones

5. LENSfound just behind the pupil and iris
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- 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

Lens consists of organized layers of cells wrapped in a dense fibrous capsule
- capsule is elastic: unless an outside force is applied, it will contract and make the lens spherical
- tension in the suspensory ligaments can overpower the elastic capsule and change the shape of the lens into a flattened oval

Lens is CONVEXuniformly curved and thicker in the middle than at the edge
- parallel rays of light will CONVERGE and be brought to a single FOCAL POINT

In order for an image to produce useful information, it must be in focus
* meaning that rays of light from an object strike the retina in a precisely ordered manner to form a miniature image of the object
Focusing occurs in 2 steps:
through the cornea
through the lens

REFRACTIONbending of light when it passes from one medium to another with a different density
- in the eye, the greatest amount of refraction occurs when light passes from the air into the cornea
- the lens provides the extra refraction needed to focus light rays from an object toward a specific FOCAL POINT on the retina

FOCAL DISTANCEdistance between the center of the lens and the focal point
- determined by
* distance of object from the lens (closer the object, the longer the focal distance)
* shape of lens (rounder the lens, the shorter the focal distance)
- the lens changes shape to keep the focal distance constant

ACCOMMODATIONprocess of focusing an image on the retina by changing the shape of the lens
- lens either becomes rounder or flattens
When you view a nearby object, ciliary muscles contract, reducing tension in the suspensory ligaments and making the lens rounder

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

The image of an object that arrives on the retina is:
a miniature of the original
upside down and backward
Our brain compensates for the reversal without our conscious awareness

VISUAL ACUITY- clarity of vision
rated on basis of the sight of a
“normal” person
- 20/20 vision means that a person can see details at a distance of 20 feet as clearly as a “normal” individual would
- 20/15 is better than average- at 20 feet, this person is able to see details that a normal person could only see at 15 feet

- 20/30 is below average- this person can see details at 20 feet that a “normal” person could see at 30 feet
when visual acuity falls below 20/200, the individual is considered to be legally blind

A normal eye condition is called
EMMETROPIA- when ciliary muscles are relaxed and the lens is flattened, a distant image will be focused on the retina
IRREGULARITIES:
1. ASTIGMATISMirregular shape of lens or cornea; can usually be corrected with glasses or contacts

2. MYOPIA (nearsightedness)
- eyeball is too deep; image of a distant object will form in FRONT of the retina, picture will be blurry
these individuals can see objects that are close
- can be corrected by placing a CONCAVE lens in front of the eyerays of light diverge (go away from each other)

3. HYPEROPIA (farsightedness)
eyeball is too shallow; lens cannot provide enough refraction
- these individuals can see objects at a distance
can be corrected by placing a convex lens (converging) in front of eye
- older individuals become farsighted as their lenses lose elasticity  PRESBYOPIA

Rods and cones are called
PHOTORECEPTORS because they detect
PHOTONSthe basic units of visible light
- light is a form of energy, which is radiated in waves and described in terms of
WAVELENGTHS
we can see wavelengths that make up the visible spectrum

WHAT IS
ROY G BIV?
an acronym for the spectrum of visible light: Red, Orange, Yellow, Green, Blue,
Indigo, Violet
- color depends on the wavelength of light
longer wavelengths = less energy

Rods provide the CNS with information about the presence or absence of photons, not about wavelength
so, they do not discriminate among color
Cones do provide information about the wavelength of photons
- they are less sensitive than rods, so function only in bright light
there are 3 types of cones: blue, green, red

COLORBLINDNESS- inability to distinguish certain colors
occurs because one or more classes of cones are absent or do not function
- in most common condition, red cones are missing, and individual cannot distinguish red light from green light
- colorblindness is common in men, but very rare in women
Test

The outer segment of a rod or cone contains hundreds to thousands of flattened membranous discs
names “rods” and “cones” refer to the shape of the outer segment
The inner segment synapses with other cells and releases neurotransmitters
Rods and cones

The discs of the outer segment in both rods and cones contain special organic compounds called VISUAL PIGMENTS
- the absorption of photos by visual pigments is the first step in the process of photoreceptionthe detection of light
- these pigments are derivatives of the compound RHODOPSINconsists of the protein opsin bound to the pigment retinal

Photoreception begins with a photon strikes a rhodopsin molecule in the outer segment of a rod or cone
- shortly after this, the rhodopsin molecule begins to break down into retinal and opsin
 called BLEACHING
- the rhodopsin must then be regenerated from the retinal and opsin

- this process takes some time
bleaching contributes to the lingering visual impression that you have after a camera flash goes off
- after this exposure to light, a photoreceptor cannot respond to further stimulation until its rhodopsin molecules have been regenerated (so you see the “ghost” image)