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Visual sensation

lec#41

dr. fasalmohammad

date : 27/4/2011

As usual a quick review of the last lec ……..

The requirement for vision:

1-Reflection of the light in four medium of the eye :

a-The cornea

b-The ant cavity which consist of ant and post chamber contain an aqueous fluid

c-The lens

d-The post cavity

  • We can replace them by one lens has a power of 6 diopter .thecornea contribute in 35%-40% of it’s power , that’s why it’s have an imp in transplantation . the lens contribute by 15% . and the other medium have the rest .

2-Accommodation which mean changing the power of the lens by changing it’s convexity.

  • More convex , less focal length , more power ( and this is what happen in near vision )
  • Less convex , more focal length , less power ( that’s happen in far vision )

How is that??????

By the stimulation of parasympathetic in near vision , the ciliary contract , they come closer to each other , the suspensory ligament lax ( less tension on the capsule ) , so the lens ‘ll take more convex shape ( almost spherical ).

In far vision , the inhibition of parasympathetic the ciliary body relax and get away from each other ,it’s ‘ll tight the suspensory ligament , make the lens flatten ( less convex ) , less power .

  • The near point of vision :- is the minimum distance between two point that eye can distinct them . it’s changeable in people .

3-contraction and dilation of the pupil to control the amount of the light inter the eye .

  • In near vision by the parasympathetic from oculomoternerve , the circular muscle gonna contract , reducing the amount of light interning the eye ( myosis ).
  • In far vision by the inhibition of para and activation the sympatheticthe radial muscle gonna dilate increasing the amount of light interning (Mydriasis ) .

That’s why in yoga , they told them to look far away to relax muscles 

Note : in clinic test sometime they use parasympathetic inhibitor(anticholinergic drug ) like atropine , to dilate the eye .

4-the converge of the two image in the cortex because we have a binocular vision which mean that we have two pictures from two retina reach the cortex, and there they ‘ll lap on each other to create one image .

-if there an interference with the converging we gonna see more than one image this is called ( diplopia ).

  • Presbyopia : ( weakness of the eye ) by the age after 40 or 50 , the lens become less accommodating due to denaturation of protein in the lens and the lens become more fixed mean even if there is an order to contract or to relax it won’t respond in good way . if the denaturation become more extensive it will lead to cataract(اعتام العين او الماء البيضاء )

Error of refraction

*Which include near sightedness and far sightedness .due to defect in the distance between the lens and retina .

Very imp to notice that when I have 6/6 or 6/12 vision this is an acuity of vision , it is totally differs from the error of refraction , although both of them treat by the same way . oky

  • Normal vision is called Emmetropia
  • When the distance between the lens and retina is less than normal , the image gonna form behind the retina this is called hyperopic or hypermetropia ( far sightedness ) .
  • When the distance is more than normal the image is forming in front of the retina .called (myopia or near or short sightedness) .

*Treatment :

*In far sightednessthe eye can be corrected by accommodateit self , how ??? we u look from short distance the muscle contract to make the lens convex to get the pic on the retina , but this exhaust the eye , soooooo we give them a glasses with convex lenses .

In short sightedness the eye can’t accommodateit self ,,, soooo u need to glasses with concave lens ( divergen to bring the pic back on the retina ) .

  • Astigmatism (انحراف الرؤية )

As we know the cornea is formed by many cylindrical lenses , and each one of them has an focal line , and all the focal line must meet at one focal point .

If one of the line is abnormal and doesn’t meet with other this will case astigmatism .treatment by using cylindrical lens .

Visual acuity test

  • Normally the distance between the lens and the retina is 17 mm .
  • Visual acuity test is like two point discrimination ,to do that we need to stimulate two receptor , if one is hit no discrimination gonna happen .
  • In the center fovea ( thesit of sharp vision ) , it’s contain cons and the diameter of each is 1.5 micro m , so to hit two cons the distance between two cones has to be at least is 2 micro m .
  • Now if I got light from the two end of capital C at a distance of 10 m , the light from the two point gonna hit 2 receptors , that ‘s gonna form an angel between the light beam when they converge at the lens . by mathematics it’s gonna be about 62 sec
  • Not : we know the angels total is 360 degree , each degree is 60 min , each min is 60 sec . mashi :PPPPP
  • But in practical usually , we can see the light from two point , if the angel is one min ,,, sooo to see the c the angel need to be one min , if we gonna see it as o
  • in the test they ask u what is the direction of the opening for the c, because it’s indication about the eye condition
  • The principle whenever you go at any distance the angel stay one min , to have the discrimination
  • Now what do they mean by 6/6 or by 6/12 ?????
  • If a person with normal vision and we put him at 6 m distance ( either in 3 m width room with a mirror to get the 6 m or it can be done the computer ) anyway if he can see the c from this distance and discriminate the two end then we say that his vision is 6/6 .
  • If a person with a weak vision like 6/12 , it’s mean that the normal person can see that two point at 12 m distance but the abnormalperson see it at shorter distance which is 6 m , now why is that ????
  • Because the receptor are far from each other , so we create a large angel than 1 min to hit the two receptor . because the receptor density in that person is less since some of them is degenerate , sooo the two point decrement is worst
  • We fix it by concave lens to diverge the light and make it hit the two receptor at any distance since the receptor far from each other .

This is the introduction for the next lec

Visual receptor

  • Retina is the sensitive part of the eye . it’s consist of :

1- rods : responsible for night and dark vision , has less kind of receptor because there aren’t responsible for color vision

2-cons : responsible for day light and color vision , has may kind receptor for colors since we have a wide color spectrum ( 400-700) we have 3 main receptor for colors:

a- blue cons

b- green cons

c- red cons

  • For the other colors , there stimulus gonna make a mixing in the 3 receptor to create the color that we see

the light hit the central fovea for sharp vision

layers of retina: ( from back to ant toward the post cavity )

  • all the layers are pulled aside to have the best hit of the light on the central fovea .

first one ,pigmented layer : sometime it consider as part of retina and sometime from choroid ,actually it’s develop from choroid layer .it’s the site of vitamin A2 storage.

it’s dark in color to absorb the extra light . if it’s lighter in color that will case reflation of the light at many part of the retina , forming many stimulus at the same time , which ‘ll lead to hussy pic ( tooo bad :P).

in albino people : they have agnatic defect that case less melanocyte activity , making the pigmented layer lighter in color , sooo there vision is weak than other

done by : Farah A .Omari

to my special Alooshaty , just stay strong <3 <3

w 2keed : hanenhamouda,hadeel msam7 , hadeeltareq , sama7 , salam , , nour , osama , zaid , firas 2l momani :PPP…

wshalat 2li ma 2lo shela

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