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1. Physiology of Vision
Dr Om Prakash Yadav

2. Learning Objectives
At the end of the lecture, the students should be able to,
• explain the optical mechanism and its aberration and corrections.
• explain the functions of the different types of photoreceptors.
• discuss the generation of impulses and transmission of impulses in visual pathways
and abnormalities.
• discuss visual field, perimetry and its interpretation.
• discuss visual acuity, the use of the Snellen chart and its interpretation.
• explain light and dark adaptation.
• explain colour blindness and its test.
• discuss pupillary, accommodation and corneal reflexes and their clinical significances

3. Anatomy Of The Eye

4. Refractive Power of a Lens
• Ability to bend the parallel rays of the
light
• Proportional to the curvature of a lens.
• Measured in diopters.
• The refractive power of a convex lens
is equal to 1 meter divided by its focal
length.
• Placing a 1-diopter concave lens
immediately in front of a 1-diopter
convex lens results in a lens system
with zero refractive power

5. The “Reduced” Eye
• If all the refractive surfaces of the
eye are algebraically added together
and then considered to be one single
lens, represented as a “reduced eye.”
• Total refractive power of the eye is
around 60 diopters.
• About two thirds is contributed by
the cornea.
• The total refractive power of the
internal lens of the eye is only 20
diopters.

6. Errors of Refraction
• Emmetropia: Normal vision
Hyperopia (farsightedness)
• The parallel rays of light are brought to a
focus behind the retina.
• Due to shorter eyeballs or weaker lens
system.
• Corrected with convex lenses.
Myopia (Nearsightedness)
• When the ciliary muscle is completely
relaxed, the light rays are focused in
front of the retina.
• Usually due to too long eyeball.
• Corrected with concave lens.

7. Astigmatism
• The curvature of the cornea is not uniform.
• When the curvature in one plane is
different from that in others, light rays in
that plane are refracted to a different
focus, so that part of the retinal image is
blurred.
• Can be corrected with cylindrical lenses
placed in such a way that they equalize the
refraction in all planes

8. Visual acuity
• Defined as the resolving power of the eyes.
• Smallest gap by which two lines can be
separated.
• Can be tested using Snellen’s chart
• Visual acuity is recorded using formula;
V = d/D
Where V is visual acuity, d is distance at which
letters are read and D is distance at which
letters normally should be read.

9. Accommodation
• Change in refractive power of the lens by
changing its curvature.
• Tension of suspensory ligaments causes
lens to remain flat.
• Contraction of cilliary muscle relaxes the
suspensory ligaments.
• Stimulation of the parasympathetic nerves
contracts ciliary muscle fibers.
• Total loss of accommodation is called
Presbyopia.
• Correction: Bifocal lens

10. Layers of Retina
• The rods and cones synapse
with bipolar cells, and the
bipolar cells synapse with
ganglion cells.
• Horizontal cells form
synapses with the
photoreceptors and the
bipolar cells in the outer
plexiform layer.
• Amacrine cells are located
in the plexiform layer, where
they synapse with bipolar
cells and ganglion cells.

11. Bind Spot and Foveal Region
• Since there are no visual receptors over the disk, this area of the
retina does not respond to light and is known as the blind spot.
• Yellowish pigmented spot called the macula.
• The central fovea, only 0.3 millimeters in diameter, is composed
almost entirely of cones.
• The blood vessels, ganglion cells, inner nuclear layer of cells, and
plexiform layers are all displaced to one side

12. Pigment Layer of the Retina
• The black pigment melanin in the pigment layer prevents light
reflection throughout the globe of the eyeball.
• Recycles the visual pigment molecules and stores large
quantities of vitamin A.
• Deficient in albinism.

13. Rods and Cones
• The major functional segments of
either a rod or cone are: (1) the
outer segment, (2) the inner
segment, (3) the nucleus, and (4)
the synaptic body.
• Many membrane infoldings called
discs are present in the outer
membrane.
• The light-sensitive photochemicals
(rhodopsin) are present in the disc
as transmembrane proteins.
• Approx. 6 million cones and 120
million rods in each human.

14. Properties of Rods and Cones
• The rods are extremely sensitive to light and are the receptors for night vision (scotopic
vision).
• The scotopic visual apparatus is incapable of resolving the details and boundaries of objects
or determining their color.
• The cones have a much higher threshold, but the cone system has a much greater acuity
and is the system responsible for vision in bright light (photopic vision) and for color vision

15. Rhodopsin-retinal Visual Cycle
• Rhodopsin is a combination of protein scotopsin
and carotenoid pigment retinal in cis form.
• When light energy is absorbed by rhodopsin,
retinal is converted into trans form.
• Rhodopsin decomposes and splits into scotopsin
and all trans retinal within a fraction of second.
• Retinal isomerase catalyzes reconversion of all-
trans-retinal into 11-cis retinal which
automatically recombines with the scotopsin to
re-form rhodopsin.
• Without vitamin A, the amounts of retinal and
rhodopsin that can be formed are severely
depressed – Night blindness

16. Excitation of the Rod
• Under normal dark
conditions, there is
reduced electronegativity
inside the membrane of
the rod about −40
millivolts
• When the rod is exposed
to light, the resulting
receptor potential is
hyperpolarizing.

17. Excitation of the Rod
• Light is absorbed by the rhodopsin,
causing photoactivation of the
electrons in the retinal portion
• The activated rhodopsin stimulates a
G protein called transducin, which
then activates cGMP
phosphodiesterase, an enzyme that
catalyzes the breakdown of cGMP to
5 -cGMP
′
• The reduction in cGMP closes the
cGMP-gated sodium channels and
reduces the inward sodium current

18. Color Vision
• Only one of three types of color (blue,
green or red) pigments is present in
each cone.
• Blue, green and red color pigments
show peak absorbencies at light
wavelengths of 445, 535, and 570
nanometers, respectively.
• Human eye can detect almost all
gradations of colors when only red,
green, and blue monochromatic lights
are appropriately mixed in different
combinations.

19. Color Blindness
• Defect in the perception of color.
• X linked recessive.
1. Trichromats
• The person is less sensitive to one of the primary colors.
• Protanomaly (weakness for red color), Deuteranomaly (weakness for green color)
2. Dichromats
• The person perceives 2 primary colors.
• Protanopia (red blindness), deuteranopia (green blindness) and tritanopia (blue blindness)
3. Monochromats
• The person perceives only one primary color.

20. Light and dark adaptation
• If a person has been in bright light for hours, large portions of the
photochemicals will have been reduced to retinal and opsins.
• Much of the retinal will have been converted into vitamin A.
• The sensitivity of the eye to light is reduced. This is called Light
Adaption.
• Conversely, if a person remains in darkness for a long time, the retinal
and opsins in the rods and cones are converted back into the light-
sensitive pigments and vitamin A is converted back into retinal to
increase light-sensitive pigments. This process is called dark
adaptation

21. Pupillary Light Reflex
• When light is directed into one eye, the pupil
constricts (direct light response). The pupil of the
other eye also constricts (consensual light
response).
• Some of the optic nerve fibers leave the optic tract
near the lateral geniculate bodies and enter the
midbrain via superior colliculus and terminate in
the pretectal nucleus.
• From this nucleus, nerve fibers project to the
ipsilateral and contralateral Edinger–Westphal
nuclei that contain preganglionic parasympathetic
neurons within the oculomotor nerve to constrict
the pupil.
• Argyll Robertson pupil: pupil fails to respond to light
but does respond to accommodation

22. Corneal Reflex
• When the sclerocorneal junction is touched with cotton wool, there is
reflex blinking of the eyes.
• Used to test the integrity of trigeminal nerve.

23. Rod and Cone Visual Pathway
• Direct pathway from Fovea (1) cones, (2)
bipolar cells (3) ganglion cells.
• Direct Pathway for rod vision (1) rods, (2)
bipolar cells, (3) amacrine cells (4) ganglion
cells.
• Both the rods and the cones release
glutamate.
• The only retinal neurons that always transmit
visual signals by means of action potentials are
the ganglion cells
• Otherwise, all the retinal neurons conduct
their visual signals by electrotonic conduction

24. Visual Pathways
• The visual nerve signals leave the
retinas through the optic nerves.
• At the optic chiasm, fibers from the
nasal halves of the retinas cross to the
opposite sides.
• The fibers of each optic tract synapse
in the dorsal lateral geniculate nucleus
of the thalamus
• From there, geniculocalcarine fibers
pass by way of the optic radiation to
the primary visual cortex

25. • Visual fibers also pass to several older areas of the brain
(1) to the suprachiasmatic nucleus of the hypothalamus
(2) into the pretectal nuclei in the midbrain
(3) into the superior colliculus
(4) into the ventral lateral geniculate nucleus of the thalamus

26. Effect Of Lesions In The Optic Pathways
• Optic nerve- Blindness of one eye
• Optic tract- Homonymous hemianopia
• Optic chiasm- heteronymous hemianopia
• LGB – homonymous hemianopia
• Inferior quadrantic hemianopia occurs in
lesions of parietal lobe (containing
superior fibres of optic radiations).
• Superior quadrantic hemianopia (pie in
the sky) occur in lesions of the temporal
lobe (containing inferior fibres of optic
radiations).

27. Field of Vision
• The portion of the external world visible
to the eye when gaze is fixed at particular
point is called field of vision.
• The visual field of both eyes overlap in
their medial part to form area of binocular
vision.
• Normal field of vision; temporal 1000
,
inferior 750
, superior 600
, nasal 600
• Can be measured using perimeter.