eye physiology

Question 1

adaptation to obscurity.

Answer 1

If an individual who passed much time in a very illuminated environment is placed in a place weakly illuminated, his retina becomes progressively netly more sensitive to light.
This decrease in visual threshold

Question 2

which photoreceptor is responsible for rapid adaptation of the eye?

Answer 2

Cones are responsible for a rapid adaptation but with weak amplitude, the rods ensure a slower but wider adaptation.

Question 3

Neural adaptation

Answer 3

involving neurons in the different steps of the visual chain in the retina itself, i.e. when light intensity increases, the intensity of signals transmitted by bipolar cells, horizontal cells, amacrine cells and ganglion cells are all very intense. However, the intensities of all those signals decrease in course of time.

Question 4

EFFECTS OF VITAMIN DEFICIENCIES ON VISION

Answer 4

When the total quantity of vitamin A in the blood becomes strongly reduced, quantities of vitamin A, retinene and rhodopsin in the rods, as well as color photosensitive substances in the cones, are all depressed, thereby decreasing the sensitivity of cones and rods.

Question 5

FUSION OF INTERMITTENT LIGHTS BY THE RETINA

Answer 5

1. A light instantaneous flash excites visual receptors during 1/15 to 1/10 of a second and because of the persistance of excitation, light flashes that follow each other rapidly fuse and give the appearance to be continuous. . (Cf. Film: 24 images/second; TVscreen: 60 images/second).
2. critical fusion frequency

Question 6

NEURAL PATHWAYS

Answer 6

1. The axons of the ganglion cells pass caudally in the optic nerve and optic tract to end in the lateral geniculate body in the thalamus.
2. The fibers from each nasal hemiretina decussate in the optic chiasm.
3. In the geniculate body, the fibers from the nasal half of one retina and the temporal half of the other synapse on the cells whose axons form the geniculocalcane tract.
4. This tract passes to the occipital lobe of the cerebral cortex.
5. The primary visual receiving area (primary visual cortex, Brodmann’s area 17) is located principally on the sides of the calcarine fissure.
6. The primary visual receiving area (primary visual cortex, Brodmann’s area 17) is located principally on the sides of the calcarine fissure.
7. Some ganglion cell axons pass from the lateral geniculate nucleus to the pretectal region of the midbrain and the superior colliculus, where they form connections that mediate pupillary reflexes and eye movements.
8. The frontal cortex is also concerned with eye movement, especially its refinement
9. The bilateral frontal fields in this part of the cortex are concerned with the control of saccades, and an area just anterior to these fields is concerned with vergence and near response
10. Other axons pass directly from the optic chiasm to the suprachiasmatic nulcei in the hypothalamus, where they form connections that synchronize a variety of endocrine and other circadian rhythms with the light- dark cycle.
11. The fibers from the nasal half of each retina decussate in the optic chiasm so that the fibers in the optic tracts are those from the temporal half of one retina and nasal half of the other.
12. A lesion that interrupts one optic nerve causes blindness in that eye.
13. A lesion in one optic tract causes blindness in half of the visual field, and is called homonymous (same side of both visual fields)
14. Hemianopia (half blindness). Lesions affecting the optic chiasm destroy fibers from nasal hemiretinas and produce a heteronymous (opposite sides of visual fields) hemianopia.
15. Occipital lesions may spare the fibers from the macula because of the separation in the brain of these fibers from the other’s subserving vision.