Transcript 43 Physiology of visual analyzer

Physiology of visual
analyzer
The eyes mediate sight
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Function
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Sensory organ for sight
Detects light and converts it into neural responses
that the brain interprets
EYE
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External Eye Structures
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The eyes are complex sense organs that
have evolved from primitive lightsensitive spots on the surface of
invertebrates.
Within its protective casing, each eye
has a layer of receptors, a lens system
that focuses light on these receptors, and
a system of nerves that conducts
impulses from the receptors to the brain.
Lacrimal Apparatus
Visual Pathways & Fields
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Objects reflect light
Rays refracted by cornea, aqueous humor,
lens, vitreous body and onto retina.
Light stimulus is changed to nerve impulses,
travel thru optic nerve to visual cortex in
occipital lobe
Image on retina is upside down & reversed.
At the optic chiasm retinal fibers cross over.
Right side of brain looks at left side of world.
Parts of the Eye
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Sclera: a tough white layer of connective
tissue that covers all of the eyeball except
the cornea.
 Conjunctiva: external cover of the
sclera — keeps the eye moist.
Cornea: transparent covering of the front
of the eye.
 Allows for the passage of light into the
eye and functions as a fixed lens.
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Choroid: thin, pigmented layer lining the
interior surface of the sclera.
 Prevents light rays from scattering and
distorting the image.
 Anteriorly it forms the iris.
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The iris regulates the size of the pupil.
Retina: lines the interior surface of the
choroid.
 Contains photoreceptors.
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Except at the optic disk (where the optic
nerve attaches).
Focusing Light
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The lens and ciliary body divide the eye into two
cavities.
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The anterior cavity is filled with aqueous
humor produced by the ciliary body.
The posterior cavity is filled with vitreous
humor.
The lens, the aqueous humor, and the vitreous
humor all play a role in focusing light onto the
retina.
** Internal Anatomy:
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Outer layer: sclera it’s
a protective white
covering, cornea bending
light rays to focused on
inner retina- cornea
sensitive to touch(corneal
reflex)-trigeminal
nerve(CN V) carries
afferent sensation into
brain, facial nerve(CN
VII) carries efferent
messages that stimulates
the blink.
The Pupil is an Aperture
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Pupil
 Opening in the center
of the eyeball
 Bounded by the Iris
 The iris controls
the size of the
pupil
 Opening through
which light enters the
eye
Pupil
Iris
Iris and Pupil
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Iris = flat, round, regular, even color
bilaterally.
Pupils = PERRLA
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Resting size norm = 3-5mm
5% population have pupils of 2 diff. Sizes
called Anisocoria
** Visual Reflexes:
# papillary light reflex: is a normal
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constriction of pupil when light shines on
retina( a direct reflex & a consensual reflex).
Mechanism: light →retina → optic
Nerve(II)__ efferent or sensory → midbrain
→ CN III (oculomotor) _ afferent(motor) →
constriction of iris muscles for both eyes.
Direct reflex → same eye _Consensual
reflex → opposite eye.
Lenses of the Eye
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Cornea
Crystalline Lens
Primary function = focus the image on
the back of the retina
Focus
Refraction
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Bending of the path of a light wave as it
passes across the boundary separating
two media
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Cause:
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Change in the speed of the light wave
No change in speed = no refraction!
Material A (fast)
Material B (slow)
Focusing point sources of light.
(a) When diverging light rays enter a dense medium at an angle to its
convex
surface, refraction bends them inward.
(b) Refraction of light by the lens system. For simplicity, refraction is
shown only
at the corneal surface (site of greatest refraction) although it also occurs
in the lens and elsewhere. Incoming light from
a (above) and b (below) is bent in opposite directions, resulting in b'
being above a' on the retina.
Emmetropia
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Emmetropia the refractive condition in focus on
fovea that parallel lights enter eye through refractive
system without accommodation
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Punctum remotum of emmetropia: infinite
Accomodation
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Viewing Distant Objects
Ciliary muscles relaxed
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Lens assumes a flatter
(skinnier) shape
Cornea is not pushed out =
less curvature
C-L system has a long
focal length
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Low refractive Power
Viewing Nearby Objects
 Ciliary muscles contract
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Squeeze the lens into a
more convex (fat) shape
Pushes cornea bulge out
further = greater
curvature
C-L system has a
short focal length
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High refractive power
Accommodation
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Accommodation
the capability that eyes change refractive
condition in order to acquire clear near sight
ACCOMMODATION
Accommodation
Helmhotz accommodation mechanism
See near
Ciliary muscle contract
Elastic deformation of lens
Lens zonule relax
accommodation
Ante-&post-lens zonule relax
See near
Ciliary muscle contract
equatorial lens zonule tension
Peripheral lens flatten
accommodation
Central lens project
Accommodation
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Accommodation = diopter for far diopter for
near
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Range of accommodation distance of far
point — distance of near point
Myopia
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Myopia: the refractive condition that parallel lights enter
eye through refractive system and focus before fovea on
static refraction
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Punctum remotum of myopia: a point before eye
Hyperopia
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Hyperopia the refractive condition that parallel lights enter
eye through refractive system and focus after fovea on
static refraction
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Punctum remotum of myopia: a point after eye
Hyperopia
Myopia and correction
Hyperopia and correction
RETINA
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The retina extends anteriorly
almost to the ciliary body. It
is organized in 10 layers and
contains the rods and cones,
which are the visual
receptors, plus four types of
neurons: bipolar cells,
ganglion cells, horizontal
cells, and
amacrine cells
Neural components of the
extrafoveal portion of the
retina.
C, cone;
R, rod;
MB, RB, and FB, midget,
rod, and flat bipolar cells;
DG and MG, diffuse and
midget ganglion cells;
H, horizontal cells;
A, amacrine cells.
Vision
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Rhodopsin (retinal + opsin) is the visual
pigment of rods.
The absorption of light by rhodopsin initiates a
signal-transduction pathway
Receptor potential is hyperpolization .
Fig. 49.13
Vision
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Rods and Cones
synapse with
nuerons called
bipolar cells
Bipolar cells
synapse with
galgion cells of
optic nerve
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Near the posterior pole of the
eye is a yellowish pigmented
spot, the macula lutea. This
marks the location of the
fovea centralis, a thinnedout, rod-free portion of the
retina that is present in
humans and other primates. In
it, the
cones are densely packed, and
each synapses to a single
bipolar cell, which, in turn,
synapses on a single ganglion
cell, providing a direct pathway
to the brain. There are very
few overlying cells and no
blood vessels. Consequently,
the fovea is the point where
visual acuity is greatest
RECEPTORS
Visual Acuity
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Visual acuity is the degree to which the
details and contours of objects are
perceived, and it is usually defined in
terms of the shortest distance by which
two lines can be separated and still be
perceived as two lines. Clinically,
visual acuity is often determined by the
use of the familiar Snellen letter charts
viewed at a distance of 20 ft (6m).