SENSES AND THE BRAIN

Sight

The Eye and Vision

70 percent of all sensory receptors are in the eyes

Each eye has over a million nerve fibers

Cranial nerve – optic nerve

Takes most time to develop

Protection for the eye

Most of the eye is enclosed in a bony orbit

A cushion of fat surrounds most of the eye

Only 1/6 of the eye is visible

Vision is your ability to see.

Most

people

blink

every

2-10

seconds.

Vision involves the eye and the brain.

The eye gathers pictures and sends them to the brain.

approx. 1 inch diameter

Iris • Colored, contractile portion of the eye

Iris Pupil

Pupil • An opening in the center of iris

• by contraction and dilatation, regulates the entrance of light into the eye

Ciliary Body Located on each

side of the lens Muscles

responsible for adjusting the shape of the lens to view objects near and far

Suspensory Ligaments Hold the lens in place Assist in adjusting the shape of the lens for proper focusing of the eye

Secretes aqueous humor

Structures of the Eye, cont.

Lens Colorless, biconvex, flexible structure that

focuses images on the retina

Structures of the Eye

Sclera White portion of the

eye Maintains the shape

of the eyeball Encases and protects

the eyeball

Thinnest over the anterior surface, thickest at the back of the eye, near opening for optic nerve

Cornea

Continuous with anterior portion of sclera

Transparent, nonvascular layer covering the colored part of the eye

Helps focus light entering the eye

Anterior Chamber Located in front of the lens

Filled with clear, watery fluid called

aqueous humor

Posterior Chamber Located behind the lens

filled with clear gelatinous fluid –

vitreous humor

Retina nerve cell layer

Changes the energy of the light rays into nerve impulses Transmits nerve impulses via optic nerve to brain

rods and cones

Rods are responsible for vision in dim light and for peripheral vision

Cones responsible for visualizing colors, central vision, and vision in bright light

Retina (continued)

Macula Lutea

Oval, yellowish spot near the center of the retina

Fovea Centralis

Small depression located in the macula

Sharpest image is obtained when image focuses directly on fovea

Optic Nerve

Receives impulses from retina and transmits them to the brain Images are then

interpreted as vision

Optic Disc

Contains no rods or cones Known as the “blind spot” of the eye

Center of optic disc serves as point of entry for artery that supplies retina

choroid

Vascular middle layer of the eye Just beneath the sclera

Contains extensive capillaries that provide blood supply and nutrients to the eye

Structures of the Eye

(Front View) 17

Conjunctiva Thin mucous membrane layer that lines inner

surface of the eye and inner part of eyelids

Lacrimal duct

Located at upper outer edge of each eye

Produces tears

Lacrimal gland

Located at inner edge of eye Tears drain from the eye through this duct

Lacrimal Apparatus

About 1 ml of tears produced per day. Spread over eye by blinking. Contains bactericidal enzyme called lysozyme. 19

Eyelids, Eyelashes & Eyebrows

Eyelashes & eyebrows help protect from foreign objects, perspiration & sunlight

Sebaceous glands are found at base of eyelashes (sty)

5/6 of eyeball inside orbit & protected

Eyelids Cover the eyeball and keep surface of eyeball lubricated and protected from dust and debris through blinking

Extraocular Muscles

Six muscles that insert on the exterior surface of the

eyeball

23

the mechanics of sight

how we “see”

Vision

Vision requires four mechanisms:

Coordination of external eye muscles so that both eyes move together

Correct amount of light admitted by pupil

Correct focus of light upon retina by lens

Optic nerve transmitting sensory images to brain

How we “see”

Eye acts much like a camera Lens of eye adjusts to bring object into

clear focus Pupil of eye constricts to allow less light to

enter in bright setting or dilates to allow more light to enter in darker setting

Through bending of light rays, image reaches retina

Image is transmitted to brain for interpretation

Visual Pathway

Photoreceptors of the

retina

Optic nerve

Optic nerve crosses at

the optic chiasma

Figure 8.11

Convergence of the Eyes 28

Binocular vision in humans has both eyes looking at

the same object

As you look at an object close to your face, both

eyeballs must turn inward.

convergence

How We See

Light rays pass through:

Cornea

Pupil

Aqueous humor

Lens

Vitreous humor

Then strike retina

Stimulating rods and cones

Near Point of Vision 30

Near point is the closest distance from the eye an

object can be & still be in clear focus

4 inches in a young adult

8 inches in a 40 year old

31 inches in a 60 to 80 year old

Major Processes of Image Formation 31

Refraction of light by cornea & lens

light rays must fall upon the retina

Accommodation of the lens changing shape of lens so that light is

focused

Constriction of the pupil less light enters the eye

Constriction of the Pupil 32

Prevents light rays from entering the eye through

the edge of the lens

Sharpens vision by preventing blurry edges

Protects retina very excessively bright light

Refraction Process of bending of light rays

as they pass through the various structures of the eye to produce a clear image on the retina

In the eye, light is refracted by the surfaces of the cornea and the lens

Lens Accommodation

Light must be focused to

a point on the retina for

optimal vision

The eye is set for

distance vision

(over 20 ft away)

The lens must change

shape to focus for closer

objects

Upside-down image forms on retina

Optic nerve transmits this image to brain

Brain turns upside-down image into right-side

up image

Rods & Cones--Photoreceptors

Rods----rod shaped

shades of gray in dim light

120 million rod cells

discriminates shapes &

movements

distributed along periphery

Cones----cone shaped

sharp, color vision

6 million

Cone Sensitivity

There are three types

of cones

Different cones are

sensitive to different

wavelengths

Color blindness is the

result of lack of one

cone type

Pathway of Nerve Signal in Retina Light penetrates retina

Rods & cones transduce

light into action potentials

Rods & cones excite

bipolar cells

Bipolars excite ganglion

cells

Axons of ganglion cells

form optic nerve leaving

the eyeball (blind spot)

To thalamus & then the

primary visual cortex 39

Color Blindness & Night Blindness 40

Color blindness

inability to distinguish between certain colors

absence of certain cone photopigments

red-green color blind person can not tell red from green

Night blindness (nyctalopia)

difficulty seeing in low light

inability to make normal amount of rhodopsin

possibly due to deficiency of vitamin A

Processing of Image Data in the Brain 44

Visual information in optic

nerve travels to

occipital lobe for vision

midbrain for controlling pupil

size & coordination of head and

eye movements

hypothalamus to establish sleep

patterns based upon circadian

rhythms of light and darkness

Filling in the “blanks”

optical illusions

An optical illusion is characterized by visually perceived images that are deceptive or misleading. The information gathered by the eye is processed by the brain to give a perception that does not tally with the stimulus source

- literal optical illusions - create images that are different from the objects that make them, - physiological illusions that are the effects on the eyes and brain of excessive stimulation of a specific type - brightness, tilt, color, movement, and - cognitive illusions where the eye and brain make unconscious inferences.

There are three main types of illusions

example

Testing How You “See”

What is your

visual acuity?

How well do

you distinguish

color?

Are you color

blind?

Click here to view a video on using a Snellen chart.

Snellen Chart Video

Myopia (near-sightedness)

People with near-sightedness cannot see clearly at distance.

Medical Terminology: A Living Language, Fourth Edition Bonnie F. Fremgen and Suzanne S. Frucht

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458

All rights reserved.

Figure 13.10 – Myopia (nearsightedness). In the uncorrected top figure, the image comes into focus in front of the lens, making the image on the

retina blurry. The bottom image shows how a biconcave lens corrects this condition.

Hyperopia (farsightedness)

People with far-sightedness cannot see clearly up close

Medical Terminology: A Living Language, Fourth Edition Bonnie F. Fremgen and Suzanne S. Frucht

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458

All rights reserved.

Figure 13.9 – Hyperopia (farsightedness). In the uncorrected top figure, the image would come into focus behind the retina, making the image on the retina blurry. The bottom image shows how a biconvex lens corrects this

condition.

Correction for Refraction Problems

Emmetropic eye (normal) can refract light from 20 ft away

Myopia (nearsighted) eyeball is too long from front to

back

glasses concave

Hypermetropic (farsighted) eyeball is too short

glasses convex (coke-bottle)

Astigmatism corneal surface wavy

parts of image out of focus

Eyeball Pathology

achromatopsia unable to perceive one or more

colors; color blindness

monochromatism unable to perceive one specific color

amblyopia loss of vision not as a result of eye

pathology; commonly called lazy eye

corneal abrasion scraping injury to cornea

Eyeball Pathology

astigmatism blurred vision due to uneven cornea; light

rays do not focus sharply on retina

hyperopia

image comes into focus behind retina;

can see clearly at a distance but not up

close; also called far sightedness

myopia

image comes into focus in front of retina;

can see clearly up close but not at a

distance; also called nearsightedness

Eyeball Pathology

cataract damage to lens causing it to become

cloudy

glaucoma chronic increase in intraocular pressure;

results in atrophy of optic nerve

macular

degeneration

deterioration of macula lutea area of

retina

Eyeball Pathology

retinal

detachment

separation of retina from choroid layer;

damages blood vessels and nerves

causing blindness

retinitis

pigmentosa

progressive disease in which retina

becomes hard and pigmented, then

atrophies

retinoblastoma malignant eye tumor occurring in young

children

Experts in action

Eye floater laser surgery

Cataract removal and IOL implantation

Modern technique

procedure

Figurer 13.13 – LASIK surgery. The cornea has been lifted in order to reshape it. (Chris Barry/Phototake NYC)

procedure

surgery

Corneal transplant

Triple surgery

Transplant surgery

Macular Degeneration Video

New technology

Conjunctiva Pathology

pterygium hypertrophied conjunctival tissue in inner

corner of eye

trachoma chronic bacterial infection of conjunctiva

Medical Terminology: A Living Language, Fourth Edition Bonnie F. Fremgen and Suzanne S. Frucht

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458

All rights reserved.

Figure 13.4 – Photograph of an infant with strabismus. The left eye is turned inward, called esotropia.

(Bart's Medical Library/Phototake NYC)

Eye Muscle Pathology

strabismus eye muscle weakness resulting in eyes

looking in different directions at same time

esotropia

(ST)

type of strabismus with inward turning of

eye; also called cross-eyed

exotropia

(XT)

type of strabismus with outward turning of

eye; also called wall-eyed

Brain-related Vision Pathology

hemianopia loss of vision in half of visual field; often

result of a stroke

nystagmus jerky involuntary eye movements;

indicator of brain injury

Eye Examination Tests

color vision

tests

use of multicolored charts to determine

ability to recognize colors

fluorescein

angiography

injection of fluorescein dye into

bloodstream to observe blood flow within

eye

fluorescein

staining

applying fluorescein eye drops to cornea

to look for corneal abrasions

Eye Examination Tests

keratometry measures curvature of cornea

ophthalmoscopy examination of interior of eye

refractive error

test

vision test for defect in ability of eye to

focus image on retina; tests for

hyperopia and myopia

slit lamp

microscopy examining posterior surface of cornea

Eye Examination Tests

Snellen chart used for testing distance vision

tonometry measures intraocular pressure

visual acuity

(VA) measures sharpness of vision