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7/29/2019 Physiology of the Eye (2)
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Physiology of The eye
Part Two
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Eye movement Lateral rectusabduction Medial rectusadduction
Superior rectus
elevation & adduction+intorsion Inferior rectusdepression & adduction
+extorsion Superior obliquedepression & abduction
+intorsion Inferior obliqueelevation & abduction +
extorsion
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4 Types of eye movements
Saccades : sudden jerky movements,occur as the gaze shifts from oneobject to another
Smooth pursuit movements : trackingmovements of the eyes as they followmoving objects
Vestibular movements : adjustmentsthat occur in response to stimuliinitiated in the semicircular canals,maintain visual fixation as the head
moves
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4 Types of eye movements Convergence movements : bring the
visual axes toward each other as
attention is focused on objects near theobserver
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StrabismusAbnormalities of the coordinating
mechanisms
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StrabismusVisual axes no longer are maintained in
a position that keeps the visual images
on corresponding retinal points Treatments : surgical shortening of
some of the eye muscles, eye muscles
training exercise, using glasses withprisms lenses
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Strabismus When visual images chronically fall on
noncorresponding points in the 2 retinas
in children under 6, one is eventuallysupressed [supression scotoma] anddiplopia disappears. This is a corticalphenomenon,and doesnt develop in
adultsimportant to treat before onevisual image supressed, because if itpersist, there will be permanent loss ofvisual acuity
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Pupil Circular muscles contract in response
of bright light & when viewing nearby
objects pupil constrict [miosis] Radial muscles contract in response
of dim light & when viewing far away
objects pupil dilates [midriasis]
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Pupil Sympathetic stimulation causes
midriasis
Parasympathetic stimulation causesmiosis
Our eyes may not only be a window to
the outside, but a mirror to the insideas well
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Color vision
Young-Helmholtz theory :
There are 3 kinds of cones, each
containing a different photopigmentand maximally sensitive to one of the 3primary colors, with the sensation ofany given color determined by therelative frequency of impulses fromeach of these cone system
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Rods and Cones Both have dendrites with an inner segment
and an outer segment. Photosensitivepigments of the photoreceptors are containedin discs of the outer segments
Several rods synapse with a single bipolarneuron enables them to respond to lowerlight intensities but lower acuity
One cone synapse with a single bipolarneuron provides for high acuity and verysharp visual imaging
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Rods and Cones The bipolar neurons synapse with the
ganglion neurons of the inner zone of
the retina. Several bipolar neuronssynapse on one ganglion neuron. Theaxons of the ganglion neurons turn
outward at the area of the optic discand join to form the optic nerve
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Stimulation of the rods
When there is no light present, retinal isin the cis-form and combined with the
scotopsin, forming rhodopsin rodsdepolarized
Light causes the rods hyperpolarized
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Activation of rodhopsin activates
transducintransducin binds GTP
activates PDE that catalyzes cGMP to 5-cGMP. [cGMP in the cytoplasm of
photoreceptors maintain sodium
channels in the open position]
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Stimulation of the cones
The opsin of the cones are calledphotopsins. Each photopsin is sensitive
to a different range of wavelengths Chemical events in the excitation of
cones are similar to those in rods
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Light adaptation : if one passes suddenly from adim to a brightly lighted environment, the lightseems intensely and even uncomfortably brightuntil the eyes adapt to the increased illuminationand the visual threshold rises [5 minutes]
The presence of bright light causes thebreakdown of the visual pigments of the rodsand the cones into retinal and opsin. The retinalis converted to vit A. Prolonged exposure to
bright light decrease the consentration of visualpigments in rods and cones. The result is areduced sensitivity of the eye to light
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Refraction
It is estimated that about 80% of the lightrefraction for vision occurs at the air-cornea
interface Less refraction takes place upon the cornea
aqueous humor interface
Aqueous humor-lens interface and the lens-vitreous humor interface provide additionalrefraction that can be altered by changing theshape of the lens
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Refraction
Far point of vision : the distance from anobject to the eye that results in focusing on
the fovea centralis without accomodation [6m]
Near point of vision : the distance from anobject to the eye that results in focusing on
the fovea centralis with maximalaccomodation [10 cm]. Under control ofparasympathetic nerves to the cilliary muscles
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Eye convergence
Human use single binocular vision in viewingan object [ both eyes focus on the same
object. The light rays that enter the eyes arefocused onto corresponding points of the tworetinas
Convergence : medial movement of the eyes
by extrinsic muscles to facilitate thealignment of both of the eyes to the objectbeing viewed
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Clinical highlight
Conjunctivitis : an inflammation of theconjunctiva resulting in reddened eyes
Scleritis, ceratitis, retinitis, uveitis Watery eyes : accumulation of tears at
the medial canthus because intranasal
openings of the nasolacrimal ducts areblocked, as experienced during a coldor allergic reaction
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Clinical highlight
Glaucoma : increased intraocularpressure causes damage to the retina
and the optic nerve. The cause ofincreased pressure is a build up of theaqueous humor, which presses the lensinward. Treated by : drugs that inhibit
the secretion of aqueous humor orenhance its outflow, or by surgery thatpromotes its drainage
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Clinical highlight
Myopia, hyperopia, astigmatism
Strabismus : uncoordinated extrinsic
eye muscles
Diplopia : double vision
Nyctalopia [night blindness] : disruptionof rod function, caused by avitaminosis
A
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Clinical highlight
Color-blind : the most common form ofcolor-blindness is red-green color
blindness
SEE YOU
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SEE YOU
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