The eye

Physiology Of Vision – Dr.Chintan
Important: Please refer standard textbook of PHYSIOLOGY for further reading…
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The EYE
-Dr. Chintan

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INDEX
1. Visual Pathway & Lesion ………………………………………………………………….03
2. Accommodation reflex …………………………...………………………………………..08
3. Refractive errors ……………………………………………………………………………..10
4. Color blindness ……………………………………………………......................................12
5. Dark Adaptation ……………………………………………………………………………...13
6. Light Reflex …………………………………………………………………………………….15
7. Night blindness ……………………………………………………………………………….15
8. Layers of retina ……………………………………………………………………….………16

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Q-1. Visual Pathway & Lesion
Retina
↓
Optic nerve (Nasal fiber carry temporal half field of vision, temporal fiber
carry nasal half of vision)
↓
Optic chiasm (Nasal fiber cross to opposite side, temporal fiber same side)
↓
Optic tract
↓
LGB
↓
Optic Radiation
↓
Visual cortex (Primary visual cortex 17, Visual association area 18, 19, Frontal
Eye Field 8)
- (1) from the optic tracts to the suprachiasmatic nucleus of the
hypothalamus, to control circadian rhythms that synchronize various
physiologic changes of the body with night and day;
- (2) into the pretectal nuclei in the midbrain, to elicit reflex movements
of the eyes to focus on objects of importance and to activate the
pupillary light reflex;
- (3) into the superior colliculus, to control rapid directional
movements of the two eyes

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LGB:
- Layers II, III, and V receive signals from the lateral half-temporal fibers
of the ipsilateral retina,
- layers I, IV, and VI receive signals from the medial half-nasal fibers of
the retina of the opposite eye
- Layers I and II - magnocellular layers - large neurons – input large type
Y retinal ganglion cells - rapidly conducting pathway - color blind,
transmitting only black and white information.
- Layers III through VI - parvocellular layers - large numbers of small to
medium sized neurons - type X retinal ganglion cells that transmit
color - moderate velocity of conduction

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Visual Cortex (6 Layers):

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Lesions of Visual Pathway:

Page7

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Q-2. Accommodation reflex
- In children, the refractive power of the lens of the eye can be increased
voluntarily from 20 diopters to about 34 diopters; this in an
“accommodation” of 14 diopters
- The shape of the lens is changed from that of a moderately convex lens
to that of a very convex lens
- In a young person, the lens is composed of a strong elastic capsule filled
with viscous, proteinaceous, transparent fluid
- When the lens is in a relaxed state with no tension on its capsule, it
assumes an almost spherical shape, owing mainly to the elastic
retraction of the lens capsule

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- Suspensory ligaments are constantly tensed by their attachments at
the anterior border of the choroid and retina.
- The tension on the ligaments causes the lens to remain relatively flat
under normal conditions of the eye
- Ciliary muscle - meridional fibers and circular fibers
- when they contract - releasing the ligaments’ tension on the lens
- The ciliary muscle is controlled almost entirely by parasympathetic
nerve signals transmitted to the eye through the third cranial nerve
from the third nerve nucleus in the brain stem
- Presbyopia - lens grows larger and thicker and becomes far less elastic,
partly because of progressive denaturation of the lens proteins – weak
ciliary muscle
- The power of accommodation decreases from about 14 diopters in a
child to less than 2 diopters by the time a person reaches 45 to 50
years; it then decreases to essentially 0 diopters at age 70 years
- no longer accommodate for both near and far vision - bifocal glasses
- Accommodation reflex
- Convergence of eyeballs, pupillary constriction, ant. Surface of lens
becomes more convex
- Pathway: Rods & Cones of retina – optic nerve – optic chiasm – optic
tract – LGB – optic radiation – visual cortex area 17 – frontal eye field
area 8 – Edinger Westphal nucleus (3rd nerve nucleus) – pre ganglionic
fiber of oculomotor nerve - ciliary ganglion – post ganglionic fiber
through short ciliary nerves – sphincter pupillae muscle of iris
(constriction), ciliary muscle contraction (lens becomes more convex) &
contraction of medial rectus muscle (convergence)

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Q-3. Refractive errors
Normal (Emmetropia) – parallel rays of light focus on retina
Hypermetropia (Long sightedness) – rays focus behind retina – near vision
defective – corrected by spherical biconvex (converging +) lens
Myopia (Short sightedness) – rays focus in front of retina – distant vision
defective - corrected by spherical biconcave (diverging -) lens
CauseCa Myopiapia Hypermetropia
Axial Eyeball length more Eyeball length less
Curvatural Lens, Cornea more convex Lens, Cornea more flatter
Positional Lens anterior Lens posterior
Index Refractive index more Refractive index less
Missel. Spasm of accommodation Aphakia

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Astigmatism – irregular shaped cornea like keratoconus or eyeball – different
planes, rays will focus differently & form line focus & not point focus –
corrected by cylindrical lens

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Q-4. Color blindness
- Normal vision – trichromatic
- Abnormal – dichromatic, monochromatic
- Triatanomaly – defective blue Colour
- Protanomaly - defective red Colour
- Deutranomaly - defective green Colour
- Triatanopia – complete blindness for blue
- Protanopia - complete blindness for red
- Deutranopia - complete blindness for green
- Tested by,
- Ishihara chart, edridge green lantern test, Holmgren wool test

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- Most common – red green - genetic disorder that occurs almost
exclusively in males
- Color blindness almost never occurs in females because at least one of
the two X chromosomes almost always has a normal gene for each
type of cone.
- Because the male has only one X chromosome, a missing gene can lead
to color blindness
- Because the X chromosome in the male is always inherited from the
mother - color blindness is passed from mother to son, and the mother
is said to be a color blindness carrier
Q-5. Dark Adaptation
- 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.
- vitamin A is converted back into retinal to give still more light-
sensitive pigments,
- the final limit being determined by the amount of opsins in the rods
and cones to combine with the retinal - This is called dark adaptation
- the sensitivity of the retina is very low on first entering the darkness,
but within 1 minute, the sensitivity has already increased 10-fold
- At the end of 20 minutes, the sensitivity has increased about 6000-fold,
and at the end of 40 minutes, about 25,000-fold
- all the chemical events of vision, including adaptation, occur about four
times as rapidly in cones as in rods

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- Despite rapid adaptation, the cones cease adapting after only a few
minutes, while the slowly adapting rods continue to adapt for many
minutes and even hours
- change in pupillary size
- Entering dark – mydriasis
- Adaptation 30-fold within a fraction of a second
- Neural Adaptation

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Q-6. Light Reflex
- When light is thrown on an eye – there is constriction of pupil in same eye –
direct light reflex
- There is constriction of pupil in opposite eye also – indirect (consensual)
light reflex
- Pathway: Rods & Cones of retina – optic nerve – optic chiasm – optic tract –
tectum of midbrain - Edinger Westphal nucleus (3rd nerve nucleus) of both
side – pre ganglionic fiber of oculomotor nerve - ciliary ganglion – post
ganglionic fiber through short ciliary nerves – sphincter pupillae muscle of iris
– constriction of pupil in both eyes
Q-7. Night blindness

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- Nyctalopia
- Night blindness occurs in any person with severe vitamin A deficiency -
the amounts of retinal and rhodopsin that can be formed are severely
depressed.
- The amount of light available at night is too little to permit adequate
vision
- For night blindness to occur, a person usually must remain on a vitamin
A–deficient diet for months, because large quantities of vitamin A are
normally stored in the liver
- Once night blindness develops, it can sometimes be reversed in less
than 1 hour by IV injection of vitamin A
Q-8. Layers of retina
1. Pigmented layer – melanin – prevent refraction of light
2. Layers of rods & cones – outer segment of rods & cones
3. External limiting membrane – Muller cells
4. Outer nuclear layer – nuclei of rods & cones
5. Outer plexiform – synaptic connection between rods, cones & horizontal,
bipolar cells
6. Inner nuclear layer – nuclei of horizontal, bipolar, amacrine cells
7. Inner plexiform – synaptic connection between bipolar, amacrine cells &
ganglion cells
8. Ganglionic cell layer - ganglion cells
9. Nerve fiber layer – optic nerve fibers
10. Internal limiting membrane – Muller cells

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The eye

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