The Physiology of Vision part 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 events 1- 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.