Physiology of the eye

This ppt file belongs to Mr. Yonas Akalu one of my best instructors ...


Vision is by far the most used of the five senses and is one of the primary means that we use to gather information from our surroundings. More than 75% of the information we receive about the world around us consists of visual information.

The eye is often compared to a camera. Each gathers light and then transforms that light into a "picture." Both also have lenses to focus the incoming light. Just as a camera focuses light onto the film to create a picture, the eye focuses light onto a specialized layer of cells, called the retina.
The human eye is an organ which reacts to light and pressure. As a sense organ, the mammalian eye allows vision. Human eyes help to provide a three dimensional, moving image, normally coloured in daylight. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can differentiate between about 10 million colors[1] and is possibly capable of detecting a single photon.

1. By Yonas A.(HO, MSc)
PHYSIOLOGY OF EYE

2. 2
Outline
1.Functional structures of the eye
2.Image-forming mechanism
3.Visual pathway
4.Abnormalities of vision

3. Physiology of eye
3
Eye
Essentially an optical camera
 Equipped with a lens
Within its protective casing, each eye has a layer of receptors,
 A lens system that focuses light on these receptors, and
A system of nerves that conducts impulses from the receptors to the
brain.
The eyes are complex sense organs that
have evolved from primitive light-
sensitive spots.

4. Physiological Significance of Eye
Create mental image of external world
Perception of : location, size , shape, color and texture of
objects
If object moving: speed and direction
 An image is formed on the retina by the refractive surfaces of
the eye.
The light energy is transduced into electrical signal by the rods
and the cones ( Photoreceptors).
The information needed to create the mental image is encoded by
the neurons within the retina.
The information is used by the visual cortex to create the visual
perception described as“ seeing”
4

5. Functional
anatomy
Functional structures of the eye

6. Functional
anatomy

7. Anatomy of the human
eye with its functional
structures
Eye lids
Lacrimal apparatus
Cornea
Intraocular fluids
Lens
7
Transverse section section of human eye
Functional structures of the eye cont’d…

8. Eyelids
8
The eye has lids to close it off
from environment.
The process of closing lids
occur as result of:
relaxation of levator palpebrae
muscle supp. by CN III
contraction of orbicularis
oculi muscle supplied by CN
VII

9. Lacrimal Apparatus
9
Lacrimal Gland: secretes tears
continuously.
Located -upper and outer corner of orbit

The lacrimal apparatus consists of a tear-
secreting gland and a series of ducts that
carry tears into nasal cavity.
Tears exit lacrimal gland through tiny
tubules and flow downward and
medially across the eye.
Superior and inferior canaliculi
collects tears  into lacrimal sac
located in groove of lacrimal bone 
nasolacrimal duct which empties into
nasal cavity

11. Lacrimal apparatus cont’d…
11
Normal secretion of tear is less than 1ml/day.
Tear has PH value of 7.4
Iso-osmotic with blood plasma.
Function
- Moistens and lubricates surface of the eye
- mechanical function
- has lysosome action, an anti-bacterial action
- cleans and nourishes cornea
- maintain corneal surface moist
- lacrimal fluid distributed evenly by reflex blinking
Blinking also helps to keep the cornea moist.

12. 14

13. 15
The eye is moved within the orbit
by six extraocular muscles.
Four rectus muscles control eye
ball movement in perpendicular
axes.
Two oblique muscle →
transitional rotation of eye
Three cranial nerves are
concerned with eyeball
movements
1.Oculomotor: superior rectus,
medial rectus, inferior rectus &
inferior Oblique
2.Trochlear: superior Oblique
3.Abducens: lateral rectus
Extraocular muscle

14. emedicine.medscape.com/article/1189759-overview
IO SR IO/SR SR/IO SR IO
LR MR MR LR
SO IR SO/IR IR/SO IR SO
MR
MR
CONVERGENCE

15. The Eye ball
The eye is fluid filled sphere enclosed
by 3 layers of tissues
1.External layer
Sclera
fibrous,tough,protective outer coat
Gives shape and protection
cornea
2. Middle layer - consists
Choroid
Ciliary body and
Iris (front)
3. Inner layer -consists
Photoreceptors (cons & rods)
nervous tissue
fovea
retinal artery
17

16. The Eye ball cont’d…
2. Middle layer
Choroid:
have rich capillary bed (for nourishing photoreceptors of the
eye)
blood supply for entire eye is provided by choroid
high conc. of light absorbing pigment melanin.
Ciliary body (muscle):
Extension from choroid in front of the eye.
Encircles the lens, consists of muscular component important
for adjusting refractive power of the lens
18

17. 2. Middle layer cont’d…
Ciliary processes :
vascular component produces fluid (aqueous humor)
that fills the front of the eye
Iris: colored part.
regulate entry of light into the eye
Pupil:
central opening of the iris.
size controlled neuronally by 2 sets of muscles with
opposing actions, which allow the size of the pupil to
be adjusted
19

18. 20
1. Aqueous humor
Is a freely flowing & transparent fluid.
Found b/n cornea & lens
Produced by ciliary epithelium by
active secretion.
By ciliary process in anterior
chamber(region between lens and iris)
It supplies cornea and lens with
nutrients e.g. glucose, amino acid and
Removes waste material.
Aqueous humor is continually being
formed and reabsorbed.
Intraocular fluids compartments

19. 1. Aqueous humor cont’d
21
Regulate intra-ocular
pressure =15mmHg
It is normally drained
through a network of
trabeculae into the canal
of Schlemm, a venous
channel at the junction
between the iris and the
cornea (corneo-scleral
junction).

20. 22

21. 1. Aqueous humor cont’d…
23
Intra-ocular pressure is constant at all times b/c there is a
balance between formation and reabsorption of aqueous
humor
Obstruction of this outlet leads to increased intraocular
pressure and causes Glaucoma.
One cause of increased pressure is decreased permeability
through the trabeculae (open-angle glaucoma), and
another is forward movement of the iris, obliterating the
angle (angle-closure glaucoma).

22. 2. Vitreous humor (vitreous body)
24
Occupies the space b/n lens & retina
Is also transparent jelly-like fluid
80% volume of eye
shape of eyeball
Contain phagocytic cells that
remove blood and debris that might
otherwise interfere with light
transmission
Produced by retinal glial cells and
ciliary epithelial cells.
Provides and maintain spherical
shape of the eye

23. 25
Cornea is about 11mm in diameter
Thickness of 1mm
Bulges forward
Has no blood vessels
It obtain oxygen supply through diffusion
from air and surrounding structure.
Transparent window of the eye
Composed of connective tissue with a
thin layer of epithelium and collagenous
fibers
Supplied by trigeminal nerve
Function
-light refraction (42 diopter)
-protect against harmful agents eg. foreign
bodies, dust, etc.
Cornea

24. Corneal reflexCorneal reflex
26
It is a protective reflex shutting of the eye by its lids
on touching the cornea
Components of Corneal reflex
Receptor: corneal receptor.
Afferent nerve: ophthalmic division of trigeminal
nerve→ trigeminal ganglion →trigeminal nucleus in
pons.
Center: superior colliculus
Efferent: Facial nerve
Effecter: orbicularis oculi muscle
Result: shutting of the eyelids.

25. 27
is a common condition in which the curvature of the cornea is
not uniform.
too great a curvature of the cornea in one plane of the
eye.
Blurred vision, usually caused by an uneven (non spherical)
contour of the cornea.
Corrected with prescription glasses (cylindrical) lens
Astigmatism

26. 28
Avascular, biconvex
structure
Enclosed in elastic capsule
Lies directly behind the iris
and pupil
Lens is a flexible
transparent body with a
naturally rounded shape
This transparent structure
(crystalline lens) held in
place by a circular lens
ligament (zonule).
lens

27. Lens cont’d…
29
Zonule is attached to the thickened anterior part of the
choroid, the ciliary body.
Ciliary body contains circular muscle fibers and
longitudinal muscle fibers that attach near the
corneoscleral junction.
Function of lens
Provides an important refractive media of eye (29
Diopter)
unlike the cornea, the refractive power of the lens is under
physiologic control.
Enables the eye to see near objects (accommodation)
Protects retina from harmful effects of ultraviolet
raysentering the eye.

28. Lens cont’d…
Transparency: as an individual ages, the lens
may develops opacities called cataracts.
Normal vision can be restored by surgically
removing the opaque lens and replacing it with a
plastic lens.
30

30. Iris
32
In front of the lens is the pigmented and opaque iris.
It is the colorful part of the eye w/c is connected with the
choroid
The iris contains circular and radial muscle fibers that
constrict and dilate the pupil to regulate entry of light to the
retina.
It is a circular pigmented structure
Circular (constrictor) muscles
act to decrease the pupil size
under parasympathetic control.
Radial (dilator) muscles
act to increase the pupil size
under sympathetic control.

31. Pupillary light ReflexesPupillary light Reflexes
33
When light is directed into one eye, the pupil constricts
(pupillary light reflex).
The pupil of the other eye also constricts (consensual light
reflex).
It is initiated by visual photoreceptors (cones & rods)
Impulses relayed by bipolar cells → ganglion cells of retina →
discharged to optic nerve.
Partial crossing of optic nerve occurs at optic chiasma with
subsequent formation of optic tract.
The optic nerve fibers that carry the impulses initiating these
pupillary responses leave the optic nerves near the lateral
geniculate bodies.
On each side, they enter the midbrain via the brachium of the
superior colliculus and terminate in the pretectal
nucleus.

32. Pupillary light Reflexes cont’d…Pupillary light Reflexes cont’d…
34
From this nucleus, the axons of pretectal nucleus
project to the ipsilateral Edinger–Westphal nucleus
and the contralateral Edinger–Westphal nucleus.
The fibers from this nucleus proceed as the oculomotor
nerve till reaching the orbit.
The preganglionic fibers relay in the ciliary ganglion
The postganglionic fibers (short ciliary nerves) pass
from this ganglion to the ciliary body.
This pathway is dorsal to the pathway for the near
response.
Effectors: circular muscle in iris
Result: pupillary constriction

33. The pupil allows light to enter the posterior
segment of the eye.
The iris constricts or dilates to adjust size of the pupil.

34. Abnormality of pupil
Argyll Robertson pupil =
If no pupillary light response
The pupil is small in dim light
Does not constrict further when eye is exposed to bright
light
Occur example in neuro- syphilis
36

35. THE IMAGE-FORMING
MECHANISM
37
The eyes convert energy in the visible spectrum into
action potentials in the optic nerve.
The wavelengths of visible light range from
approximately 397 nm to 723 nm.
The images of objects in the environment are focused
on the retina.
The light rays striking the retina generate potentials in
the rods and cones.
 Impulses initiated in the retina are conducted to the
cerebral cortex, where they produce the sensation of
vision.

36. Accommodation
38
It is the process by which the curvature of lens is increased
to see near object
The problem of bringing diverging rays from close objects to
a focus on the retina can be solved either by increasing
the distance between the lens and the retina or by
increasing the curvature or refractive power of
the lens.


37. Accommodation reflex
39
The ability of the lens to adjust its shape to facilitate focusing.
distant objects= thinner, less convex
Close objects= lens thickens
when one looks near object, the curvature of lens increases
so the image of object falls on photoreceptors.
parasympathetic stimulation to ciliary muscle
↑convexity of lens
↑refractive power
 accommodation for near vision
Sympathetic stimulation to ciliary muscle

↓
curvature of lens

↓
refractive power
 Accommodation for far vision

38. Near Point Response
40
When the gaze is directed at a near object, the ciliary
muscle contracts → relaxes the lens ligaments and lens
become more convex shape.
Accommodation is an active process, requiring muscular
effort, and can therefore be tiring.
The ciliary muscle is one of the most used muscles in the
body.
The degree to which the lens curvature can be increased
is, of course limited, and light rays from an object very
near the individual cannot be brought to a focus on the
retina even with the greatest of effort.
The nearest point to the eye at which an object can be
brought into clear focus by accommodation is called the
near point of vision.

39. 41
The Near Response
In addition to accommodation, the visual axes converge and
the pupil constricts when an individual looks at a near object.
This three-part response—accommodation, convergence of
the visual axes, and pupillary constriction—is called the near
response.
Mechanism of accommodation

40. The Near Response cont’d..
42
The near point recedes throughout life,
slowly at first and then rapidly with
advancing age.
Lens become less elastic due to denature
of protein, w/c decrease at which the
curvature of the lens can be increased,
resulting loss of accommodation.
By the time a normal individual reaches age
40–45, the loss of accommodation is
usually sufficient to make reading and close
work difficult. (Inability to focus near
objects)
This condition (age related problem) is
presbyopia, can be corrected by wearing
glasses with convex lenses (bifocal lens).

41. Cataracts
Occurs mainly in old peoples
Is the opacity of lens that reduces the transparent nature of
lens.
Causes
↑↑glucose
Trauma (radiation)
Degeneration and coagulation of lens
proteins with deposition of ca-salts
Treatment:
surgical removal of lens
43

42. Principles of Optics (Refraction of light)
44
Light rays travel through air at a velocity of about 300,000
km/sec, but they travel much slower through transparent solids
and liquids.
Light rays are bent (refracted) when they pass from a medium
of one density into a medium of a different density.
Parallel light rays striking a biconvex lens are refracted
to a point (principal focus) behind the lens.
The principal focus is on a line passing through the
centers of curvature of the lens, the principal axis.
The distance between the lens and the principal focus
is the principal focal distance.
Biconcave lenses cause light rays to diverge.

43. Refraction
45

44. 46
Refractive index
Is measurement of refractive power of lens
The refractive index of transparent substance is the ratio of the
velocity of light in air to that of substance.
The refractive index of air is 1.00
Refractive power
 is the capacity of lens to bend light rays.
Biconvex or converging lens (+ve) causes light rays to bend
toward the center of lens and form an image
Biconcave or diverging lens(-ve) causes light rays to bend away
from the centre of lens . It can’t form image.

45. 47
The greater the curvature of a
lens, the greater its refractive
power.
The refractive power of a lens
is conveniently measured in
diopters, the number of
diopters being the reciprocal
of the focal length in meters.
For example, a lens with a
principal focal distance of 0.25
m has a refractive power of
1/0.25, or 4 diopters.
The human eye has a
refractive power of
approximately 60 diopters at
rest.

46. 48
Errors of refraction
Refractive error (ametropia) in which light rays
come to a point focus either
Behind the retina (hyperopia) or
In front of retina (myopia) and
Emmetropia (Normal Vision)
Normal condition of the eyes;
eyes with no refractive defects

47. 49
Myopia- nearsightedness
it is result of too long axial
length for refractive power of
the eye. In this case the focal
point is in front of retina.
then distance objects can’t be
focused on retina.
The object can be seen clearly if
it is moved closer to eye.
Myopia is said to be genetic in
origin.
can be corrected with concave
lens

48. 50
when an axial length is too short
for refractive power of the eye, then the
distant objects can’t be focused clearly
because the focal point is in back of
retina.
Sustained accommodation, even when
viewing distant objects, can partially
compensate for the defect, but the
prolonged muscular effort is tiring and
may cause headaches and blurring of
vision.
can be corrected with convex lens.
Hypermetropia -farsightedness

49. •In hyperopia, the eyeball is
too short and light rays come to
a focus behind the retina.
•A biconvex lens corrects this
by adding to the refractive
power of the lens of the eye.
•In myopia, the eyeball is too
long and light rays focus in front
of the retina.
•Placing a biconcave lens in
front of the eye causes the
light rays to diverge slightly
before striking the eye, so that
they are brought to a focus on
the retina.
Correction of Errors of refraction
51

50. 52
Is tool of vision
Site of image formation
The retina is the light-
sensitive portion of the
eye that contains
 cones, which are
responsible for color
vision, and
 rods, which are mainly
responsible for black
and white vision and
vision in the dark.
Retina

51. 53
Layers of the Retina
are arranged in layers from
the outside to the inside as
follows:
1.Pigmented layer
2.Rods and cones layer
3.Outer limiting layer
4.Outer nuclear layer
containing the cell bodies of
the rods and cones
5.Outer plexiform layer
6.Inner nuclear layer
7.Inner plexiform layer
8.Ganglionic layer
9.Optic nerve fibers layer
10.Inner limiting membrane.
Figure. Layers of retina.
Structural and Function Elements of the Retina

52. 54
Retina consists of functional layers
1.Pigmented layer
2.cones and rods
3.Bipolar cells
4.Ganglion cell layer
5.Amacrine cell layer
6.Horizontal cell layer
1. Pigmented layer
•The receptor layer of the retina rests on the pigment epithelium
next to the choroid.
•The pigment epithelium absorbs light rays, preventing the
reflection of rays back through the retina.
•Such reflection would produce blurring of the visual images.
Layers of the Retina cont’d…

53. Layers of the Retina cont’d…
2. cones and rods
55
They are types of photoreceptors differ in :
1. Shape
2. Range of operation
3. Distribution
4. Visual function

54. Layers of the Retina cont’d…
Cones & rods --- cont’d
56
The outer segment of the cone
is conical in shape.
The rods are narrower and
longer than the cones, but this
is not always the case.
Rods are 2-5 µm in diameter,
Cones are 5-8 µm in diameter
Outer portion contain
photosensitive pigment
Inner pigment concerned with
metabolic reaction

55. Cones & rods --- cont’d
57
Photopigment consists of
protein (opsin) &
chromphore called
retinine1(aldehyde of
vit.A)
Opsin photopigments
consists
Rod photopigment
(rodopsin )
 3 cone photopigment

57. Function of cones & rods
59
I. Color vision
only cones involved in color vision
II. Sensitivity to light
Cones are sensitive to high light
intensity
Cones are responsible for day
(photopic) vision
III. visual acuity
Cones have high level of visual acuity

58. CONES RODS
1. Involved in color vision 1. Black & white vision
2. Responsible for day
(photopic) light
2. Responsible for night
(scotopic) light
3. High visual acuity 3. Low visual acuity
4. Lower in number 4. higher in number
5. Conc. In fovea 5. Throughout in retina but
not in fovea
6. Cone shaped 6. Rod shaped
7. Short outer segment 7. Longer outer segment
8. Synapse with bipolar cells 1:1 8. Many to one
Difference b/n rods & cones
60

59. 61

60. Visual acuity
62
Visual acuity is the spatial
resolving capacity of the visual
system.
It is the ability of eyes to
distinguish between two
points.
The maximum visual acuity is
in the fovea centralis.
Clinical test is dark letter on
light background (Snellen Snellen
letter chartsletter charts)

61. Retinal layers --- cont’d
3. Bipolar cells
Transmit impulse from
photoreceptors to ganglion
cell layer
4. Ganglion cell layer
Are 2
nd
order neurons
Their axons made of optic
nerve
5. Amacrine cells
Interconnect bipolar cells
6. Horizontal cells
Interconnect photoreceptors
release inhibitory NTS
NTS: Ach, GABA, DA, 5-HT,
sub.p
63

63. Blind spot
Blind spot
The space where the
ganglion cells leave
the back of the retina
is lacking
Photoreceptors. This
is the blind spot.
Blind spot

64. Retinal detachment
detachment of the rest of
the retina from the
pigment epithelium can
lead to hyperpolarization,
however, attachment can
be accomplished by laser
surgery

65. 67
The light-sensitive chemical in the rods is
called rhodopsin
The light-sensitive chemicals in the cones,
called cone pigments or color pigments, have
compositions only slightly different from that
of rhodopsin.
The outer segment of the rod has a
concentration of light-sensitive pigment called
rhodopsin, or visual purple, which is a
combination of the protein scotopsin and the
carotenoid pigment retinal ("retinene").
The retinal is a particular type called 11-cis
retinal. This cis form of retinal is important
because only this form can bind with scotopsin
to synthesize rhodopsin.
Photochemistry of Vision

66. Night Blindness (nyctalopia)
Night blindness occurs in any person with severe vitamin A
deficiency.
The simple reason for this is that without vitamin A, the amounts
of retinal and rhodopsin that can be formed are severely depressed.
This condition is called night blindness because the amount of
light available at night is too little to permit adequate vision in
vitamin A–deficient persons.
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 and can be made
available to the eyes.
Once night blindness develops, it can sometimes be reversed in
less than 1 hour by intravenous injection of vitamin A
68

67. Ionic Basis of Photoreceptor Potentials
(Mechanism of phototransduction)
A.Darkness:
 Rods and cones are depolarized in the dark.
 Their resting membrane potential is low (approximately
- 40mV)
1. The low resting membrane potential results from the high Na
+

conductance of the outer segment.
i. Na
+
channels are maintained in the open state by cyclic guanosine
monophospate (cGMP), which is synthesized from guanosine
triphosphate (cGTP) by guanylate cyclase.
When cGMP binds to the Na
+
channel, the channel opens.
That is, in this case, cGMP, acts by activating the channel directly,
not by activating a protein kinase.
Therefore Na
+
channels in the outer segments of the rods and cones
are open in the dark, so current flows from the inner to the outer
segment.

68. Cont-----
ii.The numerous mitochondria in the inner
segment provide the large quantities of ATP
required to maintain the high Na
+
-K
+
pump
activity.
This large flow of current into the cell through
the outer segment and out of the cell through
the inner segment is called the dark current.
 

71. Cont----
Light:
When light strikes the outer segment, the light
will be absorbed by rhodopsin which leads to a
reactions that are initiated to close some of the Na
+

channels by hydrolysis of cGMP and resulting in
a hyperpolarization of receptor potential of a cell.
The hyperpolarization reduces the release of
synaptic transmitter, and this generates a signal in
the bipolar cells that ultimately leads to action
potentials in ganglion cells.
The action potentials are transmitted to the brain.

74. Fig. Effect of light on current flow in visual receptors

75. Summary of the effect of light
77
When light strikes
photosensitive pigments,
rhodopsin is converted to active
form (metarhodopsin-II)
↓
metarhodopsin-II activates
phosphodiesterase w/h convert cGMP
into 5’GMP & cGMP
↓
↓
closure of Na channel
↓
hyperpolarization of rods
(↓synaptic transmission)
↓
depolarization of B-cells
↓
depolarization of ganglion-
cells
↓
nerve impulse (optic nerve)

76. Summary:
Depolarization:
•The membrane potential of photoreceptors is
depolarized in the dark due to sustained opening of
cyclic guanosine monophosphate (cGMP)-dependent
cation channels.
Hyperpolarization:
•The absorption of light by the visual pigment
rhodopsin stimulates the G protein transducin to
increase cGMP phosphodiesterase activity. cGMP is
broken down to guanosine monophosphate (GMP),
•which causes cation channels to close and results in a
hyperpolarizing receptor potential.

77. Retinitis Pigmentosa
Heterogeneous group of hereditary eye
disorders
characterized by progressive vision loss
due to a gradual degeneration of
photoreceptors

78. Retinal adaptation
80
Is the ability of retina to adjust to d/t light intensities
Dark Adaptation
Is the mechanism of adjustment as one is transferred from light to dark.
The following changes occur during the process of dark adaptation
Pupillary dilation
Formation of more photosensitive pigments
↑ in retinal sensitivity to light
Visual acuity decrease
The adjustment require half an hour
Light Adaptation
when a person come from a dark place into bright light, the eyes
temporarily blind. Vision returns after a while.
Light adaptation occur faster than dark adaptation (3-5min)
Changes (reverse of dark adaptation)
 Pupillary constriction
↓ photosensitive pigments
↓ retinal sensitivity to light
In dark, the eyes resynthesize the photopigments & become more
sensitive to light. This is called dark adaptation.

79. Distribution of rods and cones
Density of rods much greater than cones
rods = 90million
cones = 4.5 million
Distribution:
Fovea: -cones only
-one-to-one relationship with bipolar cells
and retinal ganglion cells

83. Cones less sensitive than rods to light.
Cones provide color vision and greater visual acuity.
High light intensity bleaches out the rods, and color vision
with high acuity is provided by cones.
Trichromatic theory of color vision:
According to the region of visual spectrum absorbed.
- 3 types of cones:
Erythrolable(red cones)- red sensitive(570nm)
Chlorable(green cones)- green sensitive(535nm)
Cyanoable(blue cones)- blue sensitive(445nm)
Cones and Color Vision

84. Each type of cone contains
retinene associated with
photopsins.
Photopsin protein is unique
for each of the 3 cone
pigment.
The absorption characteristics of
the pigments in the three types
of cones show peak
absorbencies at light
wavelengths of 570, 535 and
445 nanometers for red, green &
blue cones respectively.
Cones and Color Vision (continued)

85. Color blindness
87
Color blind person fails to detect
one or two colors, due to absence
of particular type of cone
photopigments.
Color blindness is genetically
inherited and seen more in males
than in females.
Color blindness is tested using
Ishihara chartsIshihara charts.

86. Color blindness
88
Patients with color blindness can be
classified in to:
Protonopes- lack red pigment
 Deuteranopes- lack green
pigment
 Tritanopes- lack blue pigment
 Monochromats- no cone
photopigment unable to see any
color
green
blue
red

87. 89
oThe visual field of one eye is
that part of a sphere on which
all objects seen by that eye
when fixed on a point in a
space
oVisual field is a conical space
diverging distally from each
eye.
Visual Field

88. Neural Pathways
The rods and cones, synapse with bipolar cells, and the
bipolar cells synapse with ganglion cells.
Amacrine cells connect ganglion cells to one another in the
inner plexiform layer via processes of varying length and
patterns.
The axons of the ganglion cells converge and leave the eye as
the optic nerve.
At the optic chiasm, the optic nerve fibers from the nasal
halves of the retinas cross to the opposite sides, where they
join the fibers from the opposite temporal retinas to form the
optic tracts.
The fibers of each optic tract then synapse in the dorsal
lateral geniculate nucleus of the thalamus
90

90. Neural Pathways cont’d….
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 geniculocalcarine tract w/c pass to
the primary visual cortex (Area 17) in the occipital lobe.
Visual fibers also pass to several older areas of the brain:
1. axons pass directly from the optic chiasm to the
suprachiasmatic nuclei in the hypothalamus, where they
form connections that synchronize a variety of endocrine and
other circadian rhythms with the light–dark cycle .
2. into the pretectal nuclei in the midbrain, to elicit reflex
movements of the eyes to focus on objects and to activate
the pupillary light reflex;
92

91. Neural Pathways cont’d….
3. into the superior colliculus, to control rapid directional movements
of the two eyes; and
4. into the ventral lateral geniculate nucleus of the thalamus and
surrounding basal regions of the brain, presumably to help control
some of the body’s behavioral functions.
Thus, the visual pathways can be divided roughly into
 An old system to the midbrain and base of the forebrain and
A new system for direct transmission of visual signals into the visual
cortex located in the occipital lobes.
In human beings, the new system is responsible for perception of
virtually all aspects of visual form, colors, and other conscious
vision.

93

92. Neural Pathways cont’d….
Function of the Dorsal Lateral Geniculate Nucleus of the
Thalamus
The optic nerve fibers of the new visual system terminate in
the dorsal lateral geniculate nucleus, located at the dorsal
end of the thalamus and also called simply the lateral
geniculate body.
The lateral geniculate nucleus serves two principal functions:
First, it relays visual information from the optic tract to the
visual cortex by way of the optic radiation (also called the
geniculocalcarine tract). The signals from the two eyes are
kept apart in the lateral geniculate nucleus.
94

93. Neural Pathways cont’d….
The second major function of the lateral geniculate nucleus is to
“gate” the transmission of signals to the visual cortex that is, to
control how much of the signal is allowed to pass to the cortex.
This nucleus receives gating control signals from two major
sources:
1. From the primary visual cortex via corticofugal fibers, and
2. Reticular areas of the mesencephalon.
 It is assumed that both of these gating circuits help highlight the
visual information that is allowed to pass.
95

94. Neural Pathways cont’d….
Primary Visual Cortex (Area 17)
The primary visual cortex lies in the calcarine fissure
area of each occipital cortex.
This area is the terminus of direct visual signals from the
eyes.
The upper portion of the retina is represented superiorly
and the lower portion inferiorly.
Based on retinal area, the fovea has several hundred times
as much representation in the primary visual cortex as do
the most peripheral portions of the retina (responsible for
the highest degree of visual acuity).
Area 17 is responsible for the conscious identification of
visual sensation and coordination of eye movements.
96

95. Neural Pathways cont’d….
Secondary Visual Areas of the Cortex
visual association areas, lie lateral, anterior, superior, and
to the primary visual cortex.
•Secondary visual cortex (area 18 & 19) which are where
virtually all signals from the primary visual cortex pass next.
Responsible for
Analysis of visual meanings
Localization of objects in relation to the position of the observer
Integration of the two halves of the visual field by means of corpus
callosum
Interpretation of visual impulses into meaningful written words

97

96. Neural Pathways cont’d….
The frontal cortex is also concerned with eye movement,
and especially its refinement.
The bilateral frontal eye fields(area 8) in this part of the cortex:
- Located above the Broca´s area
- Connected to the visual center in the occipital lobe
- Controls mov´t of eye & eyelid
Lesion in this area results in fixation (locking) of the eye on
specific objects
98

97. 99
1.Light strikes retinal photoreceptors
2.Photoreceptors stimulate 1
st
order neuron
(bipolar cells)
3.Which stimulate 2
nd
order neuron (ganglion
cells)
4.Optic nerves undergoes partial decussation at
optic chiasma.
5.The optic nerve fiber transmitting visual
impulse from the nasal half of each retina cross
opp. Side.
6.Optic nerve tract fiber terminate in the
following sites
- pretectal nucleus in MB
- sup. Colliculus in MB
- lateral geniculate body in thalamus
7. Cells of LGB are 3
rd
order neuron & they are
called optic radiation .
8.Their axon radiate through internal capsule to
occipital lobe of c.cortex.
- area 17
- area 18 & 19
Summery of Neural pathway of vision

98. 101
The Central Retinal Artery
The nutrient & blood supply for the
internal layers of the retina is derived
from the central retinal artery, which
enters the eyeball through the center of
the optic nerve and then divides to
supply the entire inside retinal surface.
Choroids
The outermost layer of the retina is
adherent to the choroids, which is also
a highly vascular tissue lying between
the retina and the sclera.
The outer layers of the retina,
especially the outer segments of the
rods and cones, depend mainly on
diffusion from the choroid blood
vessels for their nutrition, especially for
their oxygen.
choroid is a capillary bed
Supplies photoreceptors
Blood Supply of the Retina

99. Blood Supply of the Retina cont’d…
The optic nerve leaves the eye and the retinal blood vessels
enter, which is visible through the ophthalmoscope as optic
disk.
There are no visual receptors over the disk, and consequently
this spot is blind (the blind spot).
The arteries, arterioles, and veins in the superficial layers of
the retina near its vitreous surface can be seen through the
ophthalmoscope.
Ophthalmoscopic examination of this area has great value in
the diagnosis and evaluation of diabetes mellitus,
hypertension, and other diseases that affect blood vessels.
102

100. Electroretinography (ERG)
103
Electroretinography measures the electrical
responses of various cell types in the retina,
including the photoreceptors (rods and cones),
inner retinal cells (bipolar and amacrine cells), and
the ganglion cells.
Electrodes are usually placed on the cornea and the
skin near the eye.
During a recording, the patient's eyes are exposed
to standardized stimuli
Turning on light elicit a , b and c waves

101. 104
•The electroretinogram (ERG)
is used for the diagnosis of
various retinal diseases
•Atrophy of the retina and
choroid
•Congenital stationary night
blindness
•Cone dystrophy
•Toxic retinopathies
•Autoimmune retinopathies
•Retinal detachment
•Diabetic retinopathy
•Central retinal vein
occlusion
•Assessment of retinal
function after trauma,

102. Effect of lesion in visual pathway
105
1.Right optic nerve lesion:
Blindness of right eye
Loss of direct pupillary light reflex
2. Lesion in optic chiasma:
Destruction of fibers from both nasal halves of retina
Heteronymous hemianopia (half blindness of opp. Sides of
visual fields)
3. Lesion in optic tract: Blindness in half of visual field
4. Bilateral lesion in area 18 & 19:
Visual agnosia
- the objects are seen in their form and color by means
of area 17, but their nature, use, value can’t be
perceived & the patient lose significance of written
words (visual aphasia)

103. Different eye disorder
106
IritisIritis
Inflammation of the iris
White blood cells are shed into
the anterior chamber of the eye
in the aqueous humor.
These cells can accumulate and
cause adhesions between the iris
and the lens.
Treatments
Antibiotics and steroids

104. AmblyopiaAmblyopia
107
Amblyopia is the
medical term for poor
development of vision
in one eye.
 [ambly- (dull) + -opia
(vision)]
Amblyopia is often
referred to as "lazy
eye."
The eye is anatomically
normal, but visual
acuity is reduced even
with glasses

105. ConjunctivitisConjunctivitis
108
Inflammation of the conjunctiva
Cause
1.Viruses
2.Bacteria
3.irritating substances
(shampoo, dirt, smoke, pool
chlorine)
4.sexually transmitted diseases
(STDs) or allergens
(substances that cause
allergies) can all conjunctivitis.

106. 109
ExophthalmosesExophthalmoses
Abnormal protrusion of the eyes
Associated with hyperthyroidism and Grave’s disease.
In the case of Graves Disease, the displacement of the eye is due to
abnormal connective tissue deposition in the orbit and extraocular
muscles
If untreated, exophthalmos can causes the eye lids to fail to close
during sleep leading to corneal damage.
The process that it causing the displacement of the eye may also
compress the optic nerve or ophthalmic artery leading to blindness

107. BlepharitisBlepharitis
110
Inflammation of the eyelid margins

108. The end