Transcript Sense Of Vision

Sense of Vision
Introduction
• The human eye is the organ which gives us the
sense of sight and allowing us to observe and
learn more about the surrounding world than we
do with any of the other four senses. We use our
eyes in almost every activity we perform,
whether reading, working, watching television or
writing etc.
• The eye is able to detect bright light or dim light,
but it cannot sense objects when light is absent.
Cont…
• 70 percent of all sensory receptors are
in the eyes
• The vision is a photoreceptor sense
Anatomy of EYE
• The adult eye is a sphere about 1 inch (2.5 cm) in
diameter.
• Only 1/6th of the eye’s anterior surface can normally
be seen.
• Protection for the eye
 The rest of the eye is enclosed in a bony orbit.
 A cushion of fat surrounds most of the eye.
 It has two basic structures
 Eyeball
 Accessory structures
Structure of the Eyeball
Pupil
• Pupil is a central opening of the iris
• Regulates the amount of light entering the eye during:
 Close vision and bright light – pupils constrict
 Distant vision and dim light – pupils dilate
 Changes in emotional state – pupils dilate when
the subject matter is appealing or requires
problem-solving skills
Pupil Dilation and Constriction
Lens
• Biconvex crystal-like structure
• Held in place by a suspensory ligament attached
to the ciliary body
• Light passing through a convex lens is bent so
that the rays converge to a focal point
• When a convex lens forms an image, the image
is upside down and reversed right to left
Retina
• Extends anteriorly only to the ciliary body.
• Contains receptor cells (photoreceptors)
 Rods
 Cones
• Signals leave the retina toward the brain
through the optic nerve.
• No photoreceptor cells are at the optic
disk, or blind spot
Retina
Visual Receptors
Rods
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Hundreds of times more sensitive to light than cones
Provide vision in poor light.
Produce colorless (black and white) vision.
Nerve fibers converge so impulses produce more
general outlines.
 Concentration of rods increases in areas away from
fovea centralis.
That means that your peripheral vision is better in
the dark than your direct vision.
Visual Receptors
Cones
 Provide vision in good light.
 Produce colored and sharp vision
 Nerve fibers do not converge as much so impulses
produce more detailed images.
 Concentration of cones greatest in fovea centralis.
 Concentration of cones decreases in areas away from
fovea centralis.
 Fovea centralis – area of the retina with
only cones found laterally to each blind spot,
this is the area of greatest visual acuity, or
point of sharpest vision
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
Visual Pigments
Rods contain rhodopsin (visual purple)
 Light causes rhodopsin to change shape and release
opsin which acts as an enzyme in further reactions.
 Net result is hyperpolarization directly proportional
to intensity of light stimulus.
 Rhodopsin replenished in dim light.
 In dim light, a rhodopsin-replenished eye is said to be
dark-adapted (can see in dark).
Visual Pigments
Cones contain iodopsins
 A group of pigments sensitive to light waves of
different frequencies.
Eye Cavities
• The internal space of the
eye is subdivided by the
lens into two separate
cavities.
 Anterior cavity
 Posterior cavity
Cont…
• The anterior cavity is the
space anterior to the lens
and posterior to the cornea
• The iris of the eye subdivides
the anterior cavity further
into two chambers.
 anterior chamber is
between the iris and
cornea
 posterior chamber is
between the lens and the
Cont…
• Posterior cavity is
posterior to the lens and
anterior to the retina.
• Transparent, gelatinous
vitreous body which
completely fills the space
between the lens and the
retina.
Physiology of EYE
• Light waves from an object enter the eye first through the
cornea, The light then proceed to the pupil
• Fluctuations in incoming light change the size of the
pupil. When the light entering the eye is bright enough,
the pupil will constrict due to the pupillary light response.
Cont…
• The light continues through the vitreous humor
then, ideally, back to a clear focus on the retina,
behind the vitreous. The small central area of the
retina is the macula, which provides the best vision
of any location in the retina.
• Within the layers of the retina, light impulses are
changed into electrical signals. Then they are sent
through the optic nerve, along the visual pathway, to
the occipital cortex . Here, the electrical signals are
interpreted or “seen” by the brain as a visual image.
• Actually, we do not “see” with our eyes but,
rather, with our brains. Our eyes merely are the
beginnings of the visual process
It is not that easy …!
• To see every thing clearly three processes must talk
place
The Refraction or bending of light by the lens and
cornea.
The change in shape of the lens Accommodation.
The Constriction or narrowing of the pupil
Refraction
• Images focused on the retina are inverted
(upside down). They also undergo light to left
reversal.
• 75% of the refraction occurs at the cornea.
• 25% occurs at the lens, which also changes the
focus to view either distant or near objects.
Refraction Abnormality
• Myopia or nearsighted
is results from a lens that is
too strong or a long eyeball,
when the image focuses in
front of the retina,
corrected by concave a lens.
• Hyperopia or farsighted
is results from a lens that is
too lazy or a too short
eyeball, when the image
focuses behind the retina,
corrected by convex a lens.
Accommodations
• When the eye is focusing on a close object, the
lens becomes more curved, causing greater
refraction of the light rays.
• This increase in the curvature of the lens is called
Accommodation.
Near & Far Accommodation
• When viewing distant objects, the ciliary muscle
is relaxed and the lens is flatter because the
taught zonular fibers are stretching it in all
directions.
• When viewing close objects, the ciliary muscle
contracts, which pulls the ciliary processes
towards the lens. This releases tension on the
lens and zonular fibers. The lens is elastic and
then becomes more spherical.
Constriction of the Pupil
• Constriction of the pupil is a narrowing of the
diameter of the hole through which light enters
the eye due to contraction of the circular
muscles of the iris.
• This occurs automatically during accommodation
to prevent light from entering at the periphery of
the lens.
• The pupil also constricts in bright light.
Visual Pathway
1. Axons of all retinal ganglion cells in one eye exit the
eyeball at the optic disc and form the optic nerve on
that side.
2. At the optic chiasm, axons from the temporal half
of each retina do not cross but continue directly to
the lateral geniculate nucleus of the thalamus on the
same side.
3. Axons from the nasal half of each retina cross the
optic chiasm and continue to the opposite
hypothalamus.
Visual Pathway
4. Each optic tract consists of crossed and uncrossed
axons that project from the optic chiasm to the
thalamus on one side.
5. Axon collateral extend to the midbrain to govern
pupil constriction and to the hypothalamus to
govern patterns of sleep and other circadian
rhythms relevant to light and darkness.
6. Axons of thalamic neurons form the optic radiations
and project to the primary visual area of the cortex
on the same side.
• Optic nerve. Cutting the optic nerve causes blindness in the ipsilateral (same
side) eye. Thus, cutting the left optic nerve causes blindness in the left eye. All
sensory information coming from that eye is lost because the cut occurs before
any fibers cross at the optic chiasm.
• Optic chiasm. Cutting the optic chiasm causes heteronymous (both eyes)
bitemporal (both temporal visual fields) hemianopia. In other words, all
information is lost from fibers that cross. Thus, information from the temporal
visual fields from both eyes is lost because these fibers cross at the optic
chiasm.
• Optic tract. Cutting the optic tract causes homonymous contralateral
hemianopia. As shown in the figure, cutting the left optic tract results in loss of
the temporal visual field from the right eye (crossed) and loss of the nasal
visual field from the left eye (uncrossed).
• Geniculocalcarine tract. Cutting the geniculocalcarine tract causes
homonymous contralateral hemianopia with macular sparing (the visual field
from the macula is intact). Macular sparing occurs because lesions of the visual
cortex do not destroy all neurons that represent the macula.
Visual cortex
• The visual cortex is the largest system in the
human brain and is responsible for processing
the visual image. It lies at the occipital lobe of
the brain above the cerebellum. The region that
receives information directly from the LGN is
called the primary visual cortex, (also called V1
and striate cortex). Visual information then flows
through secondary visual areas, These areas
include V2, V3, V4 and area V5/MT.
Visual cortex
How we can test our sense
of vision ?
A. Test of visual acuity
• A clear focused image requires adaptation on the
focal length of the optic lens by alteration of its
curvature to suit the varying of distance
• It is a measure of the smallest retinal images
which can be appreciated
• By using a Snellen’s chart we can recode the
visual acuity for distance and for near
Cont…
• A visual acuity test is a measure of how well you see
or the sharpness and clarity of your vision. Your eye
doctor will ask you to read letters on a chart while
standing 20 feet away. The smallest letters you are
able to read will be recorded as your acuity.
• Your visual acuity may be written as 20/20 or (6/6)
if your vision is normal. If your vision is reduced, it
might be recorded as less than 20/20, such as
20/100.
Cont…
• If you have 20/100 vision, it means that you must
be as close as 20 feet to see what a person with
normal vision can see at 100 feet. Someone with
20/60 vision would need to move up to 20 feet
away to read what a person with normal vision
could read from 60 feet away
B. Test of visual filed
• The normal visual filed extends 160 degree
horizontally and 130 degree vertically
60
> 90
60
60
70
>90
60
70
Cont…
• Sit directly facing the pt about 1 meter away
• The pt field of vision is compared with that of the
examiner having a normal field of vision
• First examine both eye together , then You
should test each eye separately for the
Peripheral visual filed
• To test the central visual filed we use a red hatpin
to one eye qualitatively at a time
C. Ocular Alignment
To test the Extraocular muscles functions ( check cranial
nerves 3,4,6)
1. Stand or sit 3 to 6 feet in front of the patient.
2. Ask the patient to follow your finger with their eyes without
moving their head.
3. Check gaze in the six cardinal directions using an “H” pattern
of motion (i.e. move your finger from the center out (as if
you were checking visual fields and then up and down,
drawing a big “H”)
4. Record whether your volunteer can follow your fingers in all
directions and whether the pursuit was smooth
D. Test of Color Vision
• Red and Green color vision can be assessed using
Ishihara test plate.
• These coloured spots forming numbers which
the patient reads out
E. Pupillary Examination
• Examine the pupil for the shape and symmetry
• Ask the pt to fix the eye on a distant point
straight ahead
• Bring a bright torchlight from the side to shine on
the pupil
• Look for constriction of that pupil ( direct light
reflex) and for the constriction of the opposite
pupil ( consensual light reflex )
Cont…
Cont…
• With the pt vision fixed on a distant point ,
present an object about 15 cm in front of the eye
and ask the pt to focus on it .
• look for pupil constriction and convergence
( Accommodation reflex)