physiology ....final material .... cns .... vision

8/3/2019 physiology ....final material .... CNS .... Vision

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VISION


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External eye anatomy


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Internal eye anatomy

2. Choroid, highlypigmented, many bloodvessels that nourishthe retina

3. Retina, outer pigmented

layer and an innernervous-tissue layer (rods&cones)

1. Sclera: C.T


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Internal eye anatomy


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The Iris & pupil

q Iris: Not all light enters the eye Pigmented color flecks, lines code

q Pupil: Sympathetic vs.

parasympathetic


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Light energy is a form of electromagneticradiation that travelsin wavelike fashion

q Light energy varies in wavelength (e.G., Red vs. Blue)and in intensity (e.G.,Intense red vs. Light red);that is, the amplitude, orheight, of the wave


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Electromagnetic spectrum

F


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Focus ng o verg nglight rays

q Refraction: bending of a light ray as it strikes new medium (e.g.,cornea, lens) at an angle

cornea,lens


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Refraction


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.surfaces


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Focusing of distant and nearsources of light

q The rays from a distant (far) lightsource (more than 20 feet from theeye) are parallel by the time therays reach the eye

q To focus both a distant and a nearlight source in the same distance(the distance between the lens and

retina), a stronger lens must beused for the near source

q A long distance is required for alens of a given strength to bend thediverging rays from a near lightsource into focus


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Sharp vs. blurred images


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Accommodation

q The ability to adjust the strength ofthe lens

q The strength of the lens depends onits shape, which in turn is regulated

by the ciliary muscle


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Mechanism ofaccommodation


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Far vision and near vision in thenormal eye


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Refraction problems andcorrections


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Retinal layers


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View of the retina seenthrough an ophthalmoscope


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The Fovea

q Fovea: The point of most distinct vision Located in the exact center of the

retina, The bipolar and ganglion cell layers are

pulled aside Light strikes the photoreceptors directly Consists of cones only

q Macula lutea: The area immediately surrounding

the foveahas a high concentrationof cones

Fairly high acuity

Ph t t


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Photoreceptors


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Phototransduction

q Photopigment:

1. Opsin2. Retinene:

. derivative ofvitamin A

. light-absorbing


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Phototransduction


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Rods vs. Cones


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Dark adaptation

v Breakdown of photopigments during exposure tosunlight tremendously decreases photoreceptorsensitivity.

q In the dark, the photopigments broken downduring light exposure are gradually regenerated

q Sensitivity of the eyes gradually increases so theycan begin to see in the darkened surroundings


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Light adaptation

q Initially, very sensitive to the dazzling light. Withlittle contrast between lighter and darker parts,the entire image appears bleached

q As some of the photopigments are rapidly broken

down by the intense light, the sensitivity of theeyes decreases and normal contrasts can onceagain be detected


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Color perception

q The pigments in various objectsselectively absorb particularwavelengths of light transmitted tothem from light-emitting sources,

and the unabsorbed wavelengthsare reflected from the objectssurfaces.

q An object perceived as greenabsorbs the red and bluewavelengths of light and reflectsthe green wavelengths, which canbe absorbed by the photopigment inthe eyes green cones, therebyactivating them


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Color vision

q Color vision is the capacity of anorganism or machine to distinguishobjects based on the wavelengths (or

frequencies) of the light they reflect,emit, or transmit.


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Color Vision

q There are three types ofcones, each one of themresponding best todifferent wavelengths(short, middle, and long)

& less to others. Theircombined responsesgenerate color vision.

q Color vision, depends onthe three cone typesvarious ratios ofstimulation in response to

different wavelengths

Th i l th


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The visual pathway

Electroretinography


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Electroretinography(ERG)

q A test in which the electricalpotentials generated by the retinaof the eye are measured when theretina is stimulated by light.

q An electrode is placed on thecornea to measure the electricalresponses to light of various cell

types in the retina, including thephotoreceptors (rods & cones) andthe ganglion cells


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ERG

q If a flash ERG is performed on a dark-adapted eye, the responseis primarily from the rod system.

q Flash ERGs performed on a light adapted, eye will reflect theactivity of the cone system


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ERG


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The END


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Abbreviated ERG. A test in which theelectrical potentials generated by theretina of the eye are measured when

the retina is stimulated by light.

In an ERG, an electrode is placed onthe cornea at the front of the eye.

The electrode measures the electricalresponse of the rods and cones, the

visual cells in the retina at the back


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What is electroretinography?

Electroretinography (ERG) is an eye test used to detect abnormalfunction of the retina (the light-detecting portion of the eye).Specifically, in this test, the light-sensitive cells of the eye, the rodsand cones, and their connecting ganglion cells in the retina are

examined. During the test, an electrode is placed on the cornea (atthe front of the eye) to measure the electrical responses to light ofthe cells that sense light in the retina at the back of the eye. Thesecells are called the rods and cones.

How is an ERG done?

The patient assumes a comfortable position (lying down or sittingup). Usually the patient's eyes are dilated beforehand with standarddilating eye drops. Anesthetic drops are then placed in the eyes,causing them to become numb. The eyelids are then propped openwith a speculum, and an electrode is gently placed on each eye witha device very similar to a contact lens. An additional electrode is

placed on the skin to provide a ground for the very faint electrical
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f i i d i i i


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far vision and near vision inthe nearsightedness

Farsource

nearsource

f i i d i i i


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far vision and near vision inthe farsightedness

Farsource

nearsource

physiology ....final material .... cns .... vision

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