PHYSIOLOGY OF PHYSIOLOGY OF VISIONVISION

Structure of visual analyser

• Receptor part – rods & cones of the retina

• Conducting pathways- optic nerve, optic chiasm,optic tracts,lateral geniculate body

• Visual cortex – occipital lobe around calcarine fissure

Refractive mediums of the eye

• Cornea (40-43 D)• Lens (19-33 D)• Vitreous body (0 D)• Light conduction & refraction• Refractive power of the eye is 59-

73 D

Construction of image on the retina of the reduced

eye• Real• Reduced• Turned upside down

Emmetropia

Hypermetropia

Hypermetropia corrected with convex lens

Myopia

Myopia corrected with concave lens

Physiological abnormalities of

refraction• Spherical abberation• Chromatic abberation• Astigmatism

Pupillary reflex• Pupil constriction under the

influence of light• Midbrain reflex • Parasympathetic nucleus of

n.oculomotorius

Accomodation reflex• Changing the curvature of

crystaline lens for better refraction at close vision

• Common pathways with pupillary reflex

Flat lensRelaxed ciliary muscle

contractedZinnon ligament

contracted ciliary muscle

Convex lens

Eye at rest

Zinnon ligamentRelaxed Zinnon ligament

Accomodation reflex

Close & distant points of clear vision

• Close point of clear vision- min distance at which the object is clearly seen at max tension of accomodation mechanism (10 cm)

• Distant point of clear vision- min distance at which the object is clearly seen at max relaxation of accomodation mechanism (infinity)

Acuity of vision• Is determined by the least distance

between two points that the eye can distinguish

• Normal eye can distinguish 2 points subtended by an angle of 60 seconds

• Measured by Snellen’s types

Field of vision• Aggregate number of points seen

simultaneously when the eye is fixed steadily on one point

• The extent of the field of vision is measured by perimeter

Receptor department• Rods -110-125 mln• Cones -6-7 mln

Conducting pathways