Transcript
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
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
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