physiology of the eye (2)

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

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physiology of the eye (2)

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