The Physiology of Visionpart 2

Defects of image forming• 1- Hyperopia ( farsightedness) :- Is a defect in which the eye-ball is shorter

than normal.- Parallel rays are focused behind the retina ,

so the image is formed behind the retina.- Sustained accommodation partially

compensates for the defect , but it may lead to strabismus because of muscle fatigue.

- It is corrected using convex lenses.

2- Myopia (nearsightedness) :- The anteroposterior axis of the eyeball is too

long .- It is mainly a genetic disorder ( look at my

family !!! )- The image is formed in front of the retina.- Corrected using concave lenses.

• 3- Astigmatism:- Curvature of the cornea is not uniform.- Some light rays are refracted to other spots

making this part of the image blurry.- Corrected using cylindrical lenses.

Photoreceptor mechanisms• Light acts on photosensitive compounds in the

rods & cones of the retina , triggering action potentials.

• This is mainly due to the chemical changes that occur in these photosensitive compounds.

• Receptor potentials of photoreceptors ( & most other neural elements) are local & graded .

• Only ganglion cells produce all-or-none potentials

Photoreceptor mechanisms• Rods , cones & horizontal cells are

hyperpolarizing.• Bipolar cells maybe either hyper- or hypo-

polarizing.• Amacrine cells produce depolarizing

potentials that act as generator potentials for propagated spikes in ganglion cells.

• Cones have a sharp onset & offset of action potentials

• Rods have a sharp onset & a slow offset.

Ionic events1- In the dark :• Na+ channels are open.• Na+ flows from the inner segment to the

outer segment & the synaptic end of the receptor .

• Na+ - K+ pump maintains the equilibrium of this state.

• Neurotransmitter release is steady .

2- when light strikes the outer segment : - Chemical reactions occur near the sodium

channels & close them , leading to hyperpolarization of the membrane .

- Hyperpolarization decreases neurotransmitter release.

- The decrease in neurotransmitter release triggers a signal in the bipolar cells.

- Bipolar cells generate action potentials in ganglion cells .

Photosensitive compounds• Are found in rods & cones.• Mainly opsin ( a protein ) & retinine1 ( a form

of vitamin A ).

• Rhodopsin :- Made up of retinine + scotopsin.- Also called visual purple.- Found in the membranes of rods.- Has a peak sensitivity to light at a wavelength

of 505 nm.

• In the dark , rhodopsin’s retinine is in the 11-cis form.

• When light strikes it , it is transformed to an all-trans form.

• Metarhodopsin II is formed & leads to closure of Na+ channels by decreasing cGMP levels in the cell.

• This causes hyperpolarization , leading to decreased release of neurotransmitters & triggering an action potential.

• After turning into the all-trans conformation , retinine is separated from scotopsine.

• Some of it is converted back to 11-cis & reassociates with scotopsin ( recycling ).

• Some retinine is synthesized de novo from vitamin A.

Light

Change in photopigment

Metarodhopsin II

Activation of transducin ( g-protein)

Activation of phosphdiestrases

Decreased cGMP

Closure of NA channels

Hyperpolarization , decreased release of NTs

Action potential.

What happens.• When retinine is converted to its all-trans

form , it dissociates from scotopsin.• Scotopsin then activates transducin ( g-

protein).• Transducin’s alpha-subunit activates cyclic

GMP phosphodiestrase .• The phosphodiestrase converts cGMP to 5’-

GMP.• This causes closure of the sodium channel ,

because cGMP is what keeps them open.

Image formation• Is a 3-stage process :1- the image is formed on the retina’s

photoreceptors.2- it is changed to a second image in the

bipolar cells.3- then it is changed into a third image in the

ganglion cells.- The third image is altered by the horizontal

amacrine cells , then it reaches the occipital visual cortex.

Color vision• Red , green & blue are the primary colors.• Other colors are produced by mixing them.

• Young – helmoholtz theory :- Postulates that humans possess 3 types of cones

, each containing a different photopigment.- Each photopigment is maximally sensitive to one

of the three primary colors.- The sensation of any given color is determined

by the relative frequency of the impulses from each of the 3 cone systems .

Color blindness• Ishihara charts are the most common

method of diagnosing color blindness.• Terminology :1) -anomaly : means weakness.2) -anopia : means blindness.3) Prot- : is red.4) Deuter- : is green.5) Trit- : is blue.

• Normal people are trichromats , they can see the three primary colors clearly.

• Dichromats have only two cone systems , they may have protanopia , deuteranopia , or tritanopia.

• Monochromats have only one system ( extremely rare ).

• Color blindness is mainly inherited , but can be caused by a lesion in V8 ( the part of the visual cortex that is responsible for color vision).

• V8 lesions cause achromatopsia ( loss of color vision).• In Caucasians , 8% of the males & 0.4% of the females

inherit color blindness.

Dark adaptation.• When an individual leaves a bright lighted space to a

dim lighted one , his retinas become more sensitive to light.

• This phenomena is known as dark adaptation.• In contrast , leaving a dark area to a bright one causes

light adaptation , which only requires 5 minutes. • Dark adaptation reaches its maximum in 20 minutes.• It has two components :1- adaptation of the cones : rapid ( 5 mins.) but small in

magnitude.2- adaptation of the rods : slower ( 15-20) , with great

magnitude.