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T H E E Y E

Biology 3201 Nervous system #8

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Eye Diagram (see Fig. 12.19, p. 410)

Major Parts of the Eye

Conjunctiva: is a clear membrane covering the white of the eye (sclera)

Iris: the muscle that adjusts the pupil to regulate the amount of light that enters the eye.

Pupil: the aperture in the middle of the iris of the eye. The size of the aperture can be adjusted to control the amount of light

Lens: a transparent, bi-convex body situated behind the iris of the eye to focus an image on the retina

Retina: the innermost layer of the eye; contains rods and cones, bipolar cells and ganglion cells

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Parts of the Eye

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Major Parts of the Eye

Sclera: the thick, white outer layer that gives the eye its shape

Cornea: the clear part of the sclera at the front of the eye

Choroid layer: the middle layer of the eye, which absorbs light and prevents internal reflection. This layer forms the iris at the front of the eye

Rods: photoreceptors in the eye; more sensitive to light than cones, but unable to distinguish color

Cones: color receptors in the eye (red, green, blue)

Rods and Cones

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Major Parts of the Eye

Macula: is a small area in the retina that provides our most central, acute vision.

Small depressed area in centre of macula directly in line with centre of cornea & lens is called the FOVEA.

Fovea is the region of keenest vision.

Light-sensitive cones are concentrated in fovea.

optic nerve: conducts information received from rods and cones to the brain for interpretation.

Blind spot: an area on the retina where there are no rods or cones present; locate where blood vessels enter the eye

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Retinal Layers

Composed of three layers:

1. Ganglion cell layer

2. Bipolar cell layer

3. Rod and cone cell layer

Retinal Layers

rods and cones - actual photoreceptors

ganglion cells - transmit to the brain; the axons of these ganglion cells make up the optic nerve.

bipolar cells - process input from photoreceptors

and transmit the signal to ganglion cells.

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How the eye works

As light enters the eye, the pupil will dilate if there isn’t enough light or it will constrict if there’s too much. As well, the shape of the lens changes depending on how far away the object is.

Accommodation: in the eye, adjustment that the ciliary body makes to the shape of the lens to focus on objects at varying distances

- When the object is far away, the lens is flattened

- When the object is close, the lens is rounded

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How the eye works

Light enters the eye through the pupil. As it does, light rays become bent at the cornea and the lens in such a way that an inverted and reversed image of the object focuses on the retina.

Information from this image is captured by rods and cones, which transmit their info to bipolar cells and then ganglion cells (optic nerve). Cones transmit information to a single bipolar cell, but require more light to become stimulated. As a result, cones see more detail and are best suited for lighted situations (daytime). Rods, however, are very sensitive to light and cannot distinguish color. As well, many rods connect to a single bipolar cell (up to 100 rods per bipolar cell). This causes images to be blurry. As a result, rods are best suited to situations where there isn’t much light and details are not important.

Disorders of the Visual System

1. Myopia (nearsightedness) eyeball is too long & retina is too far from lens

Light rays converge at point in front of the retina, & are again diverging (separating) when they reach retina, resulting in blurred image

Concave lenses correct nearsightedness by causing light rays to converge farther back

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Disorders of the Visual System

2. Hypermetropia (Hyperopia) -farsightedness eyeball that is too short & retina is too close to the lens.

Light rays strike the retina before they have converged, again resulting in a blurred image.

Convex lenses correct farsightedness by causing light rays to converge farther forward.

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Disorders of the Visual System

3. Astigmatism Unequal curvature of lens

or cornea

Light rays passing through another area of the lens are not focused on the retina properly

To correct this lenses must be ground unequally to compensate for the unequal curvature of the lens or cornea

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Disorders of the Visual System

4. Glaucoma in a normal eye fluid flows continuously in and out of the

chamber and nourishes nearby tissues.

fluid leaves the chamber at the open angle where the cornea and iris meet.

fluid flows through a spongy meshwork, like a drain, and leaves the eye.

in Glaucoma, fluid passes too slowly through the meshwork drain.

As the fluid builds up, the pressure inside the eye rises to a level that may damage the optic nerve.

Glaucoma

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Disorders of the Visual System

5. cataracts- cloudy or opaque areas on the lens of the eye that increases in size over time and can lead to blindness if not medically treatment

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What is 20/20 vision?

20/20 vision is a term used to express normal visual acuity (the clarity or sharpness of vision) measured at a distance of 20 feet. If you have 20/20 vision, you can see clearly at 20 feet what should normally be seen at that distance. If you have 20/100 vision, it means you must be as close as 20 feet to see what a person with normal vision can see at 100 feet.

Treatments of Eye Disorders

(1) Corrective lenses – glasses, contact lenses (see. Fig 12.22, p. 414) with near-sightedness, the image focuses in front of the retina.

This can be fixed using a concave lens

with far-sightedness, the image focuses behind the retina. This can be fixed using convex lenses

astigmatisms are unique and may require combinations of convex and/or concave lenses to bring images into focus on the retina

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Treatments of Eye Disorders

(2) Corneal transplant Corneas come from organ donors; no need to match blood

types

Recovery long; most patients do well though

Recurrence of disease unusual

Treatments of Eye Disorders

(3) Laser surgery – two types Photorefractive keratectomy (PRK): non-invasive, simple

procedure

LASIK surgery: more complex, some surgery required (corneal)

Both surgeries may diminish eyesight

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PRK (Photo-Refractive Keratectomy

the surgeon uses a laser beam, guided by a computer, to vaporize tiny amounts of tissue on the surface of the cornea. The procedure removes just enough tissue to reshape the cornea in a way that corrects vision.

LASIK

LASIK stands for Laser-Assisted In Situ Keratomileusis and is a procedure that permanently changes the shape of the cornea, using an laser.

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A knife, called a microkeratome, is used to cut a flap in the cornea. A hinge is left at one end of this flap. The flap is folded back revealing the stroma, the middlesection of the cornea. Pulses from a computer-controlled laser vaporize a portion of the stroma and the flap is replaced.