Visual physiology

1. THE PHYSIOLOGY OF VISION
Dr. EZE Ejike Daniel, PhD
Tel: 0782975042
Watsap : +2348036254165
JANUARY, 2023

2. THE PHYSIOLOGY OF VISION
1. Physiologic anatomy of the human eye
• Principles of optics
• The lenses
• The refractive media of the eye
2. The outer layer of the eye
• The cornea
• The sclera
3. The middle layer of the eye
• The choroid
• The iris
• The ciliary body

3. 4. The inner layer of the eye (The retina)
• Structure of the retina
• The rods and cones
• The neural functions of the retina
• Photo transduction (mechanism of signal transmission in the retina)
• Retinal changes on exposure to light
5. Dark adaptation
6. Light adaptation
7. The visual acuity
• Factors affecting the visual acuity
8. Color vision
• The mechanism of color vision’
• Color blindness and types
• Tests for color vision

4. 9. The visual pathway
10. The visual fields, binocular vision and eye movements
11. The aqueous humour and intraocular pressure
12. Pathophysiology
• The Argyll Robertson pupil
• Glaucoma
- Definition
- Types
- Causes and Treatment
13. Refractive errors of the eye
- Types and corrections
a) Hypermetropia
b) Myopia
c) Astigmatism
d) Presbyopia

5. The Eyes and Vision
• Vision is important for:
1. Identification of objects
2. Learning through written speech
3. Maintenance of equilibrium

6. Structure of the Human Eye
• Adult human eye is nearly globular
(spherical) in shape, about 1 inch in
diameter
• Consist of THREE important layers
viz:
1. Outer fibrous or protective layer:
• Posterior 5/6 of this layer is opaque
and is called the sclera
• Anterior 1/6 is transparent and is called
the cornea (through which light rays
enter the eye).
• Consists of the following parts:

7. The Cornea
• Constitutes anterior 1/6 of the outer layer of the
eyeball.
• 0.5 to 1mm thick and its diameter is about
11mm.
• Performs the following functions
i. Allows entry of light into the eye due to its
transparency
ii. Most important refractive medium in the eye,
since it acts as a convex lens having a
refractive power of 39-43 diopters.
iii. Its sharp curvature leads to formation of sharp
retinal images.
iv. Permeable to isotonic fluids. A property which
allows various kinds of eye drops to get into
the eye easily.
v. Protects the delicate inner structures of the eye
(with the sclera).
vi. Protects the eye through absorption of a
considerable amount of the ultraviolet rays
(with the lens).

8. Note:
• Cornea is always covered by a thin film of
tears secreted by the lacrimal glands which
gives the eye its characteristic luster
(brightness)
• Contains extremely sensitive non-
myelinated nerve endings which transmit
pain as well as other sensations from the
cornea.
• Cornea is protected by:
i. Eyelids and the lid usually reflexes which
leads to blinking
ii. Pre-corneal film of tears which dilutes and
washes irritant substances and contain
also an antibacterial enzyme called the
lysosome.
iii. Corneal reflex.

9. Structure of eyeball
Cornea Reflex
• Touching the cornea of one eye by a foreign body
(e.g. a cotton) causes blinking at both eyes.
• It is a superficial reflex (refer to CNS).
• Afferent impulses are transmitted from the cornea
via the ophthalmic division of the trigeminal (5th
cranial) nerve → pons where, they stimulate the
facial (7th cranial) nuclei at both sides.
• Efferent impulses are then transported by the
facial nerves → contraction of the orbicularis
oculi muscles at both sides resulting in bilateral
blinking.
• Reflex is primarily protective.
• Commonly tested clinically to check:
1. Integrity of the 5th nerve
2. Depth of anesthesia.

10. Corneal Nutrition and Metabolism
• Normally cornea contains no blood
vessels (completely an avascular
structure) in order to maintain its
transparency.
• Glucose is the main source of energy.
• Receives its oxygen and nutrient supply
from its own lymph vessels and the
aqueous humor as well as from the fluid
filtered from the blood capillaries at the
limbus.
• Contains ascorbic acid and glutathione
which play an important role in the
oxidative processes that take place in the
cornea.

11. Causes of Corneal Transparency
i. Structure:
• Regular arrangement of the corneal
connective tissue lamellae is one of the
important factors for production of corneal
transparency.
ii. Avascularity (Main cause of maintaining
corneal transparency)
Note:
• Vascularization of the cornea often occurs as
a result of deficiency of vitamin B 2
(riboflavin) and may lead to loss of its
transparency.
• Deficiency of vitamin A causes
xerophthalmia (dry eye) which may also
lead to marked impairment of corneal
transparency.

12. iii. Absence of myelin sheath in the corneal nerve
fibers.
iv. Relative corneal dehydration:
• Transparency of the cornea also depends on its
water content.
• In order to maintain the transparency of the
cornea, it should be kept relatively dehydrated
since its excessive hydration leads to corneal
cloudiness.
Note:
• Riboflavin is essential for formation of the
flavoprotein enzyme which is concerned with
respiration of avascular structures (e.g. the cornea
and lens).
• Accordingly its deficiency leads to corneal
hypoxia which is followed by its vascularization
as a compensatory effect.

13. Structure of eyeball
The sclera
• Constitutes the posterior 5/6 of the outer layer of
the eye.
• Formed by hard fibrous tissue and is covered
anteriorly by the conjunctival membrane.
• Whitish in adults and bluish in infants and young
children.
• Color can be changed in disease (becomes
yellowish in jaundice).
• Opaque i.e. it does not transmit light rays due to
marked irregularity of its connective tissue-the
lamellae.
• Performs the following functions:
1. Protects the delicate inner eye structures
2. Gives attachment to the external ocular muscles
(which move the eyeball in various directions)

14. 2. Middle Vascular Layer of the Eye (The Uveal tract)
• Called the choroid and rich in blood vessels. It consists
of the of the following:
i. Choroid posteriorly
ii. Iris anteriorly: Is a colored partition in front of the
crystalline lens that has a central round aperture
(opening) called the pupil.
iii. Ciliary body in between
Functions of choroid
i. Vascular layer which provides blood supply to the eye.
ii. Pressure inside its vessels maintains the intraocular
pressure.
iii. Rich in melanin which forms a dark coating for the
interior of the eyeball. This is important because it
prevents reflection of light rays inside the eye which
increases its optical efficiency.
iv. Give attachment to the ciliary muscle

15. Functions of the ciliary body
i. Contains the ciliary muscle and the ciliary
processes.
ii. Ciliary muscle is essential for
accommodation
iii. Ciliary processes secrete the aqueous humor
into the posterior chamber and also give
attachment to the suspensory ligament of the
lens.

16. The iris
• Circular diaphragm-like opaque structure
that lies anterior to the crystalline lens.
• Has a central round aperture or opening
called the pupil, the diameter of which is
greatly variable
• Contains the pigment that gives the eye its
characteristic color particularly in its
posterior layer.
• Contains the following two types of smooth
muscles which control the size of the pupil:
a) Circular smooth muscle fibers which
constitute the constrictor pupillae muscle.
b) Radial smooth muscle fibers, which
constitute the dilator pupillae muscle.

17. Functions of the iris
1. Regulates the entry of light rays into the eye through the pupil.
2. Prevents both spherical and chromatic aberrations by cutting out most of the
peripheral rays falling on the eye.
3. Protects the retina by preventing excessive entry of UV rays into the eye.
4. Pupillary reflexes are important in the diagnosis of many nervous lesions and
diseases.

18. Dilation and constriction of the pupil:
1. In dim light, the radially arranged smooth muscle fibers are stimulated to contract by sympathetic
neurons (stimulation) thus, dilating the pupil.
2.In bright light, the circularly arranged smooth muscle fibers are stimulated to contract by
parasympathetic (stimulation) neurons thus, constricting the pupil.

19. 3. Inner Nervous layer of the Eye (The Retina)
• Retina constitutes the light sensitive part of the
eye containing the photosensitive receptors (Rods
& Cones).
• It extends anteriorly and ends just behind the
ciliary body.
• In its posterior part there is a small area called the
macula lutea where vision is most acute & the
central part of this area is called fovea centralis.
• Point at which the optic nerve leaves the retina is
called the optic disk or blind spot (because it
contains no photo receptors).
• Behind the iris there is a biconvex crystalline lens
suspended to the ciliary body by the suspensory
ligament (lens ligament or zonule).
• Lens divides the eye ball into two compartments:
posterior & anterior compartments

20. i. Posterior compartment: Occupied by
a gelatinous mass known as the
vitreous humor
ii. Anterior compartment: Filled with a
fluid called the aqueous humor-and is
divided by the iris into two chambers
called the anterior and posterior
chambers.
• At the circular corneo-sceleral junction
(Limbus) is an important canal which
drains the aqueous humor known as the
Canal of Schlemm
• Thin membrane called the conjunctiva
line the inner surfaces of the eyelids,
then it is reflected on the anterior
surface of the eyeball and at the limbus
it continues with the superficial layers of
the cornea.

21. Fovea centralis:
1.When the eyes “track” an object, the image is cast upon the fovea centralis of the retina.
2. The fovea is literally a “pit” formed by parting of the neural layers.
3. In this region, light thus falls directly on the photoreceptors (cones)

22. Layers of the retina: Because the retina is inverted,
light must pass through various layers of nerve cells
before reaching the photoreceptors (rods and cones)
Layers of the Retina
• Retina is a nervous structure that forms the
inner layer of the eye.
• Consists of :
a) a single-cell-thick pigmented epithelium,
b) photoreceptor neurons (rods and cones)
and
c) layers of other neurons.
• Neural layers of the retina are a forward
extension of the brain, hence, in this sense,
the optic nerve can be considered a tract
• Because the retina is an extension of the
brain, the neural layers face outward,
toward the incoming light.
• Light, therefore, must pass through several
neural layers before striking the
photoreceptors.

23. Layers of the retina: Because the retina is inverted,
light must pass through various layers of nerve cells
before reaching the photoreceptors (rods and cones)
Layers of the Retina Cont’d….
• Photoreceptors synapse with other
neurons, so that synaptic activity
flows outward in the retina.
• Ganglion cells are outer layers of
neurons that contribute axons to the
optic nerve. Ganglion cells receive
synaptic input from bipolar cells,
• Bipolar cells in turn receive input
from rods and cones
(photoreceptors).
• In addition to the flow of
information from photoreceptors to
bipolar cells to ganglion cells
• Neurons called horizontal cells
synapse with bipolar cells.
• And neurons called amacrine cells
synapse with several ganglion cells.

24. Structure of Rods and Cones
• Photoreceptors of the retina contain
specific photosensitive pigments
which are chemically changed on
exposure to light and such changes
initiate signals (action potentials) in
the optic nerve fibers that are
transmitted to the visual cortex of the
brain.

25. The Rods
• In each human retina there are about
12 million rods which are present
mostly at the periphery of the retina.
• Few are in the central part of the retina
and totally absent in the fovea
centralis.
• Photochemical pigment of rods is
called rhodopsin which is extremely
sensitive to dim or faint light.
• Peripheral vision perceived by rods is
responsible for night vision (scotopic
vision) in which the details of objects
and their colors cannot be perceived
clearly.

26. • Each rod and cone consists of an inner and an outer segment.
• Each outer segment contains hundreds of flattened membranous sacs or discs, with the photo pigment
molecules required for vision.
• Each retinal pigment epithelial cell is in contact with 50 to 100 photoreceptor outer segments
• Microvilli project from the pigment epithelial cells toward the photoreceptors, aiding interactions
between pigment epithelium and photoreceptors.

27. The Cones
• In each human retina there are about 6 million cones which are present mostly at the central part of the retina and
few in the peripheral parts of the retina.
• The fovea centralis contain only cones.
• There are 3 types of cones which contain 3 different types of photochemical pigments responsible for color vision.
• Cones are stimulated only by bright light so they are much less sensitive to light than rods.
• Concerned with day vision (photopic vision) in which the details of objects and their colors are clearly perceived.
• Central vision perceived by cones in the fovea allows for acute or sharp vision as well as color vision.
• Cones are not acting at night.

28. • Pigment epithelium of the retina has many functions
that are important for vision:
1. Contains large epithelial cells that are rich in
melanin.
2. Absorbs light rays that are not absorbed by the
photoreceptors, thus preventing reflection of light
rays by retina which would lead to blurring of vision
as a result of stimulation of a large number of
photoreceptors at the same time (as occurs in
albinos).
3. Essential for acute vision and stores considerable
amounts of vitamin A which plays an important role
in retinal function.
4. Delivery of nutrients from the blood to the
photoreceptors
5. Conversion of visual pigment from the
photoreceptors into its active form, which is recycled
back to the photoreceptors in a process called the
visual cycle of retinal

29. Differences between rods and cones

30. Structure of the Eyeball

31. Visual Process
Chemical basis of visual process
• Photosensitive pigments present in rods and cones are concerned with the chemical basis of visual
process.
• The chemical reactions involved in these pigments lead to the development of electrical activity in
retina and eventual generation of impulses (action potentials),which are transmitted via optic nerve.
Rhodopsin
• Photosensitive pigment of rod cells
• Made up of :
i. Opsin (present in rhodopsin as Scotopsin)
ii. Chromophore (present in the rod cells as Retinal)
• Retinal (aldehyde of vitamin A) derived from food sources and not synthesized in the body.
• Retinal is present in the form of 11-cis retinal called Retinine 1 (Present in human eyes).
• Significance of 11-cis form of retinal is that, it is only in this form it is able combines with Opsin to re-
synthesize rhodopsin.
• Retinine 2 is present in the eyes of some animals

32. Photodissociation of rhodopsin:
(a) The photopigment rhodopsin consists of the protein opsin combined with 11–cis-retinal
(retinene).
(b) Upon exposure to light, the 11–cis-retinal is converted to a different form, called all-trans,
and dissociates from the opsin.
This photochemical reaction induces changes in ionic permeability that ultimately result in
stimulation of ganglion cells in the retina

33. Photochemical Changes in Rhodopsin – Wald Visual Cycle/Effect of Light on Rhodopsin
• In the dark, the rods appear in red because of rhodopsin.
• During exposure to light, rhodopsin is bleached and the color becomes yellow. This is because when rhodopsin
absorbs the light that falls on retina rhodopsin dissociates into: (a) Pigment retinal and (b) Protein called opsin
• Retinal exist in two possible configurations: (a) All- trans form (More stable) and (b) 11- cis form (Usually
combine with opsin)
• In response to absorbed light energy, 11- cis -retinal is converted to all- trans isomer (causing it to dissociate from
the opsin).
• This dissociation reaction in response to light initiates changes in the ionic permeability of the rod plasma
membrane and results in the production of nerve impulses in the ganglion cells.
Photochemical changes and re-synthesis of rhodopsin (Wald visual cycle)
NADH2 = Reduced nicotinamide adenine dinucleotide

34. Photochemical changes and re-synthesis
of rhodopsin (Wald visual cycle)
NADH2 = Reduced nicotinamide
adenine dinucleotide
Re-synthesis of Rhodopsin
1. All-trans retinal derived from metarhodopsin II is
converted to 11-cis retinal (by the enzyme retinal
isomerase).
2. 11-cis retinal immediately combines with
Scotopsin to form rhodopsin.
3. All-trans retinol is converted to the 11-cis retinol
(by the activity of enzyme retinol isomerase).
4. 11-cis retinol is converted to 11-cis retinal, which
combines with Scotopsin to form rhodopsin.
5. All trans retinol is also reconverted into all-trans
retinal.
6. Rhodopsin can be re-synthesized directly from all
trans retinol (vitamin A) in the presence of
Nicotinamide adenine dinucleotide (NADH2).
7. Synthesis of rhodopsin from 11-cis retinal is
faster than from 11-cis retinol (vitamin A)

35. Photo transduction
• Process by which light energy is converted into receptor potential in visual receptors.
• Resting membrane potential (RMP) in sensory receptor cells is usually between –70 and –90 mV.
• But in visual receptors during darkness RMP is about –40 mV because of influx of sodium ions.
• In dark, Na+ ions are pumped out of inner segments of rod cell to ECF, but these Na+ ions leak back
into the rod cells through membrane of outer segment and reduce the electronegativity inside rod cell.
• Therefore, Na+ ions influx maintains a decreased negative potential up to –40 mV.
• It is this potential that is called a dark current.
(1) In the dark, Na+ enters the photoreceptors, producing a
dark current that causes a partial depolarization

36. • Influx of Na+ ions into outer segment of rod cell occurs mainly because of c-GMP present in the
cytoplasm of cell;
• c-GMP always keeps the sodium channels opened.
• Closure of sodium channels occurs when there is reduction in c-GMP.
• The concentration of sodium ions inside the rod cell is also regulated by Na+ – K + ions pump.
• When light falls on retina, rhodopsin is stimulated leading to development of receptor potential in the
rod cells
1) In the dark, Na+ enters the photoreceptors, producing a
dark current that causes a partial depolarization

37. Light stops the dark current in photoreceptors
• In light:
(1) 11-cis-retinal is converted into all-trans-retinal.
(2) This causes G-proteins associated with the opsin to dissociate.
(3) α - subunit of G-proteins binds to and activates phosphodiesterase, which converts c-GMP into GMP
thereby reducing the concentration of c-GMP
(4) As a result of reduced c-GMP concentration, the Na+ channels close, stopping the dark current and
hyperpolarizing the membrane of photoreceptors

38. Effects of light on the retina:
(a) In the dark
1. The continuous dark current depolarizes
the photoreceptors and causes them to
release inhibitory neurotransmitter at their
synapses with bipolar cells.

39. (b) In the light
(2) cGMP declines due to its conversion to GMP,
stopping the dark current and hyperpolarizing the
photoreceptors.
(3) As a result the release of inhibitory
neurotransmitter is stopped because they are
inhibited in the light,
(4) Bipolar cells release excitatory
neurotransmitter at their synapses with ganglion
cells so that the ganglion cell axons are
stimulated to produce action potentials which are
transmitted to brain so that light that can be
perceived

40. Note:
• Process of receptor potential in
visual receptors is unique in nature
in the following ways:
1. When other sensory receptors are
excited, the electrical response is in
the form of depolarization
(receptor potential).
2. But, in visual receptors, the
electrical response is in the form of
hyperpolarization
3. Hyperpolarization in visual
receptor cells reduces the release
of synaptic neurotransmitter
leading to the development of
response in bipolar cells and
ganglionic cells so that, the action
potentials are transmitted to
cerebral cortex via optic pathway.

41. Phototransduction cascade: cGMP = Cyclic guanosine monophosphate.

42. Photosensitive Pigments in Cones
• Photosensitive pigment in cone cells is of three types, namely:
1. Porphyropsin
2. Iodopsin
3. Cyanopsin
• Only one of these pigments is present in each cone.
• Photopigment in cone cell also is a conjugated protein made up of :
1. Photopsin
2. Chromophore (Retinal)
• Each type of cone pigment is sensitive to a particular light
• Various processes involved in phototransduction in cone cells are similar to those in rod cells.

43. Dark Adaptation
• Transition of the retina from the light-adapted to the dark-adapted
state by which the person is able to see the objects in dim light.
• If a person enters a darkroom from a bright-lighted room, he is
blind for some time, i.e. he cannot see any object. But after
sometime his eyes get adapted and he starts seeing the objects
slowly. Maximum duration for dark adaptation is about 20 minutes.
Causes : are due to the following changes in eyeball:
1. Increased sensitivity of rods as a result of re-synthesis of
rhodopsin.
• Time required for dark adaptation is partly determined by the time
for re-synthesis of rhodopsin.
• In bright light, most of the pigment molecules (rhodopsin) in rod
cells are bleached (broken down).
• But in dim light, it requires some time for regeneration of certain
amount of rhodopsin, which is necessary for optimal rod function.
• Dark adaptation occurs in cones also.
2. Dilatation of pupil during dark adaptation allows more and more
light to enter the eye

44. Visual pathway
• Nervous pathway that transmits impulses from
retina to visual center in cerebral cortex.
In binocular vision:
• It is the ability to maintain visual focus on an
object with both eyes, creating a single visual
image.
• Light rays from temporal or outer half of visual
field fall upon the nasal or inner part of
corresponding retina.
• Light rays from nasal or inner half of visual
field fall upon the temporal part of retina.
Note:
• Lack of binocular vision is normal in infants.
• Adults without binocular vision experience
distortions in depth of perception and visual
measurement of distance.

45. Visual pathway
Visual Receptors (photoreceptors)
• Are the rods and cones present in the retina of eye. Fibers from visual receptors synapse with dendrites of bipolar
cells of inner nuclear layer of the retina.
First order neurons : Are bipolar cells n the retina. Their axons synapse with dendrites of ganglionic cells.
Second order neurons Are the ganglionic cells in ganglionic cell layer of retina. Their axons form optic nerve.
Optic nerve leaves the eye and terminates in lateral geniculate body.
Third order neurons : Lateral geniculate body. Fibers arising from here, reach the visual cortex.
Layers of the Retina:
Because the retina is
inverted, light must pass
through various layers of
nerve cells before reaching
the photoreceptors (rods &
cones)

47. Visual pathway
Course of Visual Pathway
• Visual pathway consists of (6) six
components:
1. Optic nerve
2. Optic chiasma
3. Optic tract
4. Lateral geniculate body
5. Optic radiation
6. Visual cortex.

48. Visual Pathways
1. Optic Nerve
• Formed by the axons of ganglionic cells|
• Leaves the eye through optic disk.
• Fibers from temporal part of retina are in lateral part of the nerve & they carry the impulses from nasal half
of visual field of same eye.
• Fibers from nasal part of retina are in medial part of the nerve & they carry the impulses from temporal half
of visual field of same eye.

49. 2. Optic chiasma
• Nasal fibers of each optic nerve
cross the midline and join the
uncrossed lateral fibers of opposite
side, to form the optic tract.
• This area of crossing of the optic
nerve fibers is called optic chiasma.

50. Visual Pathways
3. Optic Tract
• Formed by uncrossed fibers of optic nerve on the
same side and crossed fibers of optic nerve from
the opposite side.
• Fibers of optic tract run backward, outward and
towards the cerebral peduncle from where the
peduncle fibers of optic tract reach the lateral
geniculate body in thalamus.
• Due to crossing of medial (nasal) fibers in optic
chiasma, the left optic tract carries impulses from
temporal part of left retina and nasal part of right
retina, i.e. it is responsible for vision in nasal half
of left visual field and temporal half of right visual
field.
• The right optic tract contains fibers from nasal half
of left retina and temporal half of right retina.
• It is responsible for vision in temporal half of left
visual field and nasal half of right visual field

51. Visual Pathways
4. Lateral Geniculate Body
• Fibers of optic tract terminate in lateral geniculate body,
which forms the subcortical center for visual sensation.
• From lateral geniculate body, the optic radiation arises
and relay to the visual cortex.
• Some of the fibers from optic tract do not synapse in
lateral geniculate body, but pass through it and
terminate in one of the following centers:
i. Superior colliculus: Concerned with reflex movements
of eyeballs and head, in response to optic stimulus
ii. Pretectal nucleus: Concerned with light reflexes
iii. Supraoptic nucleus of hypothalamus: Concerned with
the retinal control of pituitary in animals. But in
human, it does not play any important role.
5. Optic Radiation
• Fibers from lateral geniculate body pass through
internal capsule and form optic radiation.
• Optic radiation ends in visual cortex

52. Visual Pathways
6. Visual Cortex
• Primary cortical center for vision is
located on the medial surface of occipital
lobe.
• Areas of Visual Cortex and their
Function
i. Primary visual area (area 17), concerned
with the perception of visual impulses
ii. Secondary visual area or visual
association area (area 18), concerned
with the interpretation of visual
impulses
iii. Occipital eye field (area 19), concerned
with the movement of eyes.

53. Types of Hemianopia
Applied physiology
Effects of lesion at different levels of visual pathway
• Injury to any part of visual pathway causes visual
defect.
• Nature of defect depends on the fibers involved,
location and extent of injury.
• Loss of vision in one visual field is called anopia.
• Loss of vision in one half of visual field is called
hemianopia.
• Hemianopia is classified into two types:
1. Homonymous hemianopia:
• Loss of vision in the same halves of both the visual
fields. Examples:
• Right Homonymous Hemianopia (Loss of vision in
right half of visual field of both eyes).
• Left Homonymous Hemianopia (Loss of vision in
left half of visual field of both eyes).

54. Types of Hemianopia
2. Heteronymous hemianopia:
• Loss of vision in opposite halves of
visual field.
• Examples:
• Binasal heteronymous hemianopia (Loss
of vision in right half of left visual field
and left half of right visual field (nasal
half of both visual fields).
• Bitemporal heteronymous hemianopia
(Loss of sight in left side of left visual
field and right side of right visual field
(temporal half of both visual fields).

56. Effects of lesions of optic pathway: Dark shade in
circles indicates blindness
Effects of lesions of optic pathway:
A. Lesion of left optic nerve - Total blindness
of left eye
B. Lesion of right optic nerve - Total blindness
of right eye
C. Lesion of lateral fibers in left side of optic
chiasma - Left nasal hemianopia
D. Lesion of lateral fibers in right side of optic
chiasma - Right nasal hemianopia
C + D. Lesion of lateral fibers in both sides of
optic chiasma - Binasal hemianopia
E. Lesion of medial fibers in optic chiasma -
Bitemporal hemianopia
F. Lesion of left optic radiation - Right
homonymous hemianopia
G. Lesion of right optic radiation - Left
homonymous hemianopia

57. Applied physiology
Lens is:
1. Normally completely clear, due to its very unique structure.
2. Composed of about a thousand layers of cells aligned in parallel and joined together tightly,
so that gaps don’t form as the shape of the lens changes.
3. Transparent because:
a) Avascular;
b) Its cell organelles have been destroyed in a controlled process that stops before the cells
die;
c) Cell cytoplasm is filled with proteins called crystallin and because of this structure, every
region of the lens normally has the same refractive index.
• However, damage from ultraviolet light, dehydration, or oxidation may cause the crystallin
proteins to change shape and aggregate to produce the cloudy patches in a person’s visual
field known as cataracts.

58. Cataract
• Disease of the crystalline lens in which it loses its transparency (becoming opaque and whitish)
as a result of denaturation of its proteins.
• Degenerative condition of the crystalline lens that occurs due to one of the following causes:
i. Exposure to UV rays: leads to denaturation of the lens proteins followed by their coagulation
i.e. forming opaque areas.
ii. Exposure to high temperatures or X— rays radiations: produce similar effects as those
produced by the UV rays.
iii. Diabetes mellitus: Cataract occurs secondary to disturbances of the glucose metabolism in the
lens.
iv. Old age (Senile cataract): Occurs as a result of decreased glutathione content in the lens.
Treatment
i. Cataract is treated by surgical removal of the opaque lens and supplying the patient with an
artificial convex lenses of a suitable power in the form of either glasses, contact or implanted
lenses.

59. Glaucoma
• Glaucoma is a group of eye conditions that damage the optic nerve, the health of which
is vital for good vision.
• This damage is often caused by an abnormally high pressure in the eye.
• Glaucoma is one of the leading causes of blindness for people over the age of 60.

60. Types of Glaucoma
1. Open-angle glaucoma or wide-angle glaucoma
• Here the angle formed by the iris and cornea is
unobstructed, so that aqueous humor can reach
the canal of Schlemm, however, the aqueous
humor does not drain properly.
• Most common type of glaucoma.
• Here the drain structure in your eye (called the
trabecular meshwork) looks fine, but fluid
doesn’t flow out like it should.
2. Angle-closure glaucoma or narrow-angle
glaucoma
• Here the angle formed by the iris and cornea is
narrowed or obstructed e.g. by a tumor or
inflammation.
• The eye doesn’t drain like it should because the
drain space between the iris and cornea becomes
too narrow & this cause a sudden buildup of
pressure in the eye.

61. Refraction
• Light that passes from a medium of one
density into a medium of a different
density is refracted, or bent.
• Degree of refraction depends on the
comparative densities of the two media
involved as indicated by their refractive
indices as follows:
1. Refractive index of air is set at 1.00;
2. Refractive index of the cornea, by
comparison, is 1.38
3. Refractive index of the aqueous humor
1.33
4. And that of the lens is 1.40,
respectively.

62. • Because the greatest difference in
refractive index occurs at the air-cornea
interface, the light is refracted most at the
cornea.
• Degree of refraction also depends on the
curvature of the interface between two
media.
• Curvature of the cornea is constant, but
the curvature of the lens can vary.
• Therefore, the refractive property of the
lens can thus provide fine control for
focusing light on the retina.
• As a result of light refraction, the image
formed on the retina is upside down and
right to left

63. The image is inverted on the retina:
1. Refraction of light, which causes the image to be inverted,
occurs to the greatest degree at the air-cornea interface.
2. Changes in the curvature of the lens, however, provide the
required fine focusing adjustments.

64. Color Vision
• Human eye can recognize about 150 different colors in
the visible spectrum. Discrimination and appreciation
of colors depend upon the ability of receptors in retina.
Visible spectrum
• When sunlight or white light is passed through a glass
prism, it is separated into different colors.
• And the series of colored light produced by the prism is
called the visible spectrum
• Colors that form the spectrum are called spectral colors
Spectral colors
• Red, orange, yellow, green, blue, indigo and violet
(ROYGBIV )
• In the spectrum, colors occupy the position according
to their wavelengths
• Red has got the maximum wavelength of about 8,000 Å
• Violet has got the minimum wavelength of about 3,000
Å.

65. • Light rays longer than red are called infrared
rays.
• Rays shorter than violet are called ultraviolet
rays
• Refraction of spectral colors by the prism also
depends on wavelengths
• Red is refracted less and violet is refracted more.
• So, longer the light rays, lesser is the refraction
by the prism.
Extra-spectral Colors
• Colors other than those present in visible
spectrum
• These colors are formed by the combination of
two or more spectral colors
• For example, purple is the combination of
VIOLET and RED
• Pink is the combination of RED and WHITE.

66. Primary colors
• Colors which when combined together produce the white
• Include: Red, Green and Blue
• These three colors in equal proportion give white.
Complementary colors
• Are pair of two colors, which produce white when mixed or combined in proper proportion.
• Examples are red and greenish blue; orange and cyan blue; yellow and indigo blue; violet and greenish
yellow, purple and green.

67. Theories of Color Vision
• Many theories are available to explain the mechanism of perception of color by eyes
• However, most of the theories are not accepted universally
• Following are the five theories, which are recognized:
1. Thomas Young Trichromatic Theory
• According to this theory, the retina has three types of cones.
• Each one possesses its own photosensitive substance.
• Each cone gives response to one of the primary colors – red, green and blue.
• Different color sensations are produced by the stimulation of various combinations of these
three types of cones.
• For sensation of white light, all the three types of cones are stimulated equally
2. Helmholtz Trichromatic Theory
3. Granit Dominator Modulator Theory
4. Hartridge polychromatic theory
5. Hering’s theory of opposite colors

68. Colour blindness
• Failure to appreciate one or more colors. This usually happens between greens and reds, and
occasionally blues
• Common in 8% of males and 0.4% of females.
• Inherited sex-linked recessive character
• Can also be acquired in conditions such as ocular diseases or injury or disease of retina.
• The Term Color blind’ does not mean that objects are seen only in black and white.
• It means that there are many types and degrees of color blindness
• Most appropriate term for color blindness is deficiency of color vision.

69. Causes For Acquired Color Blindness
1. Trauma: Injury to eye due to accidents or strokes results in color blindness.
2. Chronic Diseases
• Glaucoma and degeneration of macula of eye
• Retinitis, sickle cell anemia, leukemia, diabetes, liver diseases, Parkinson disease
3. Drugs : Antibiotics, antihypertensive drugs, anti-tuberculosis drugs
4. Toxins
• Fertilizers, carbon monoxide, carbon disulfide & chemicals with high lead content.
5. Alcoholism
• Chronic alcoholism results in color blindness.
6. Aging
• Color blindness can occur after 60 years of age due to various changes in eye.

70. Classification of Color Blindness
• Based on Young-Helmholtz trichromatic theory, color blindness is classified into three types :
1. Monochromatism
• Means total inability to perceive color i.e. total color blindness. Its very rare.
• Persons with monochromatism are called monochromats.
• Retina of monochromats is totally insensitive to color and they see the whole spectrum in only
black, white and different shades of grey.
• Monochromatism is divided into two types:
a) Rod monochromatism
• Condition in which cones are functionless and the vision depends purely on rods.
• That is the individuals are totally color blind.
• Dazzled by light but definitely are not blind during daylight.
b) Cone monochromatism
• Condition in which vision depends upon one single type of cone.

71. 2. Dichromatism
• Subject can appreciate only two colors because the receptors for third color are defective.
• Classified into three groups:
a) Protanopia
• Caused by the defect in receptor of first primary color i.e. red
• Red color cannot be appreciated
• Individual use blue and green to match the colors.
b) Deuteranopia
• Caused by the defect in receptor of second primary color, i.e. green.
• Individuals use blue and red colors and they cannot appreciate green color.
C) Tritanopia
• Caused by the defect in receptor of third primary color, i.e. blue.
• Individuals use red and green colors and they cannot appreciate blue color.

72. 3. Trichromatism
• Intensity of one of the primary colors cannot be appreciated correctly though the affected
persons are able to perceive all the three colors.
• Classified into three types:
a) Protanomaly
• Perception for red is weak & so to appreciate red color, the person requires more intensity of red
than a normal person.
b) Deuteranomaly
• Perception for green color is weak.
a) Tritanomaly
• There is weak perception for blue color
Tests for Color Blindness
1. By using Ishihara color charts
2. By using Holmgren colored wool
3. By using Edridge-Green lantern

73. Divisions of visual field
Field of Vision
• Part of the external world seen by one eye, when it is fixed in one direction is called field of
vision or visual field of that eye.
• Or it’s the entire area that can be seen when the eye is directed forward plus that which is seen
with peripheral vision.

74. Binocular vision
• Vision in which both the eyes are used together, so that a portion of external world is seen by the eyes
together.
• In human and some animals, eyeballs are placed in front of the head so that visual fields of both the eyes
overlap and because of this, a portion of the external world is seen by both eyes.
• Humans have binocular vision, which means that there is overlap of a portion of the visual world
perceived by each eye.
• The binocularity of human vision requires that the position of the eyes to be carefully controlled such
that the same part of the visual field falls on corresponding parts of the retina of each eye.

75. Monocular vision
• Vision in which each eye is used separately.
• It is when there is vision in one eye only, meaning you will lose part of your field of view and may have difficulty
with depth perception.
• In some animals like dog, rabbit and horse, the eyeballs are present at the sides of head, so, the visual fields of
both eyes overlap to a very small extent and because of this, different portion of the external world is seen by each
eye.
• In zoology, a monocular vision is a type of vision found mainly in animals with eyes placed on opposite sides of
their head, such as fish, rabbits, and birds of prey.
• Most preys have monocular vision.

76. • Visual field can be tested to measure the extent and distribution of the field of vision.
• Test may be done by a number of methods including what are termed confrontation, tangent
screen exam and automated perimetry.
• Many diseases can adversely affect the visual field including:
1. Glaucoma,
2. Strokes,
3. High blood pressure, (hypertension),
4. Diabetes mellitus,
5. multiple sclerosis,
6. Overactivity of the thyroid gland (hyperthyroidism).
• Medications can also affect the visual field, some of which include:
1. Antimalarial medicines chloroquine (ATABRINE)
2. Hydroxychloroquine (PLAQUENIL).

77. Purpose of Visual Field Test
• Is a subjective measure of:
a) Central
b) Peripheral vision or side vision
• Help your ophthalmologist diagnose problems with your eyes, optic nerve or brain,
including:
a) Loss of vision
b) Glaucoma
c) Disorders of your retina (layer of cells that lines the back of your eye)
d) Other neurologic conditions, including brain tumors, multiple sclerosis, and stroke

78. Normal Visual Field
• An island of vision measuring:
1. 90 degrees temporally to central Fixation,
2. 50 degrees superiorly and nasally,
3. 60 degrees inferiorly.
Causes of Visual Field Loss
1. Damage to the retina or optic nerve of the eye include
 glaucoma
 coloboma (Defect – when there is a hole in one of the structures of the eye e.g. iris, retina
choroid e.t.c.)
 A Toxoplasma infection (Caused by a parasite Toxoplasma gondii)
 Tumors,
Age-related macular degeneration

79. OCULAR MUSCLES
• Muscles of the eyeball are of two types:
A. Intrinsic muscles
B. Extrinsic muscles.
A. Intrinsic Muscles
• Formed by smooth muscle fibers and are controlled by autonomic nerves.
• They include:
1. Constrictor papillae
2. Dilator papillae
3. Ciliary muscle: Contraction of ciliary muscle increases the anterior curvature of lens
during accommodation.

80. Extrinsic muscles of eyeball:
Numbers in parenthesis indicate the cranial nerve
supplying the muscle
B. Extrinsic Muscles
• Formed by skeletal muscle fibers and are
controlled by the somatic nerves.
• Eyeball moves within the orbit by six
extrinsic skeletal muscles (Four straight
muscles (rectus) and two oblique
muscles.
• Extrinsic muscles are:
1. Superior rectus
2. Inferior rectus
3. Medial or internal rectus
4. Lateral or external rectus
5. Superior oblique
6. Inferior oblique

81. Innervation of Ocular Muscles
1. Innervation of Intrinsic Muscles
• By both sympathetic and parasympathetic divisions of autonomic nervous system
Parasympathetic nerve fibers
• Innervate the ciliary muscle and constrictor pupillae.
• Stimulation causes contraction of ciliary muscle and constrictor pupillae.
Sympathetic nerve fibers
• Innervate the ciliary muscle and dilator pupillae
• Stimulation of sympathetic nerve fibers causes relaxation of ciliary muscle and contraction of dilator
pupillae.
2. Innervation of Extrinsic Muscles
• By somatic motor nerve fibers which reach the ocular muscles via three cranial nerves:
a) Oculomotor (third) nerve supplies - Superior rectus, Inferior rectus, Medial rectus (internal rectus) and
Inferior oblique.
b) Trochlear (fourth) nerve supplies - Supplies the superior oblique
c) Abducent (sixth) nerve - Supplies the lateral rectus (external rectus)

82. Movements of right eye: MR = Medial rectus, SO = Superior oblique,
LR = Lateral rectus, IO = Inferior oblique, SR = Superior rectus, IR =
Inferior rectus
OCULAR MOVEMENTS
• Eyeball moves within the
orbital socket in any of the three
primary axes namely:
1. Vertical,
2. Transverse
3. Anteroposterior axis

83. Simultaneous Movements of Both Eyeballs
1. Conjugate Movement
• Movement of both eyeballs in the same direction.
• Visual axes of both eyes remain parallel and it is due to contraction of medial rectus of one eye
and lateral rectus of the other eye.
2. Dis-jugate Movement
• Movement of both eyeballs in opposite direction.
• It is of two types:
a) Convergence:
• Movement of both eyeballs towards nose.
• Due to simultaneous contraction of medial rectus and simultaneous relaxation of lateral rectus of
both eyes.
• Visual axes move close to each other. of medial rectus and simultaneous relaxation of lateral
rectus of both eyes.
• Visual axes move close to each other and occurs during accommodation.

84. b) Divergence
• Movement of both eyeballs towards temporal side.
• Due to the simultaneous contraction of lateral rectus and simultaneous relaxation of medial
rectus of both eyes.
• Visual axes of the eyes move away from each other.
3. Pursuit Movement
• Movement of eyeballs along with object, when eyeballs follow a moving object.
4. Saccadic Movement
• Quick jerky movement of both eyeballs when the fixation of eyes (gaze) is shifted from one
object to another object. It is also called optokinetic movement.

85. Pupillary Movements
• Pupillary movements are controlled by pupillary reflexes
Pupillary reflexes
• Alter the size of pupil.
• Classified into three types:
1. Light reflex
• Reflex in which pupil constricts when light is flashed into the eyes and is of two types:
a) Direct light reflex
• Reflex in which there is constriction of pupil in an eye when light is thrown into that eye
b) Indirect light reflex.
• Involves constriction of pupil in both eyes when light is thrown into one eye.
• If light is flashed into one eye, the constriction of pupil occurs in the opposite eye, even
though no light rays falls on that eye.
• Also called consensual light reflex

86. Pathway for light reflex
1. When light falls on the eye, the visual receptors are
stimulated.
2. Afferent impulses from the receptors pass through the optic
nerve, optic chiasma and optic tract.
3. At the midbrain level, few fibers get separated from optic
tract and synapse on the neurons of pretectal nucleus, which
lies close to the Superior colliculus.
4. Pretectal nucleus of midbrain forms the center for light
reflexes.
5. Efferent impulses from this nucleus are carried by short
fibers to Edinger-Westphal nucleus (parasympathetic
nucleus) of oculomotor nerve (3rd cranial nerve).
6. From Edinger-Westphal nucleus, preganglionic fibers pass
through oculomotor nerve and reach the ciliary ganglion.
7. Postganglionic fibers arising from ciliary ganglion pass
through short ciliary nerves and reach the eyeball.
8. These fibers cause contraction of constrictor pupillae muscle
of iris
9. Reason for consensual light reflex is that, some of the fibers
from pretectal nucleus of one side cross to the opposite side
and end on opposite Edinger- Westphal nucleus.

87. 2. Cilio-spinal reflex
• Dilatation of pupil in eyes caused by painful stimulation of skin over the neck.
• Due to the contraction of dilator pupillae muscle.
• Sensory impulses pass through cutaneous afferent nerve.
• Center is in first thoracic spinal segment.
• Efferent impulses pass through sympathetic fibers and reach dilator pupillae.

88. Accommodation
• Adjustment of eye to see either near or distant objects clearly.
• Process by which light rays from near objects or distant objects are brought to a focus on sensitive part of retina.
• Achieved by various adjustments made in the eyeball.
1. Changes in the shape of the lens that permit accommodation
(a) Lens is flattened for distant vision when the ciliary muscle fibers are relaxed and the suspensory ligament is
stretched
(b) Lens is more spherical for close-up vision when the ciliary muscle fibers are contracted and the suspensory
ligament is relaxed.

89. 2. Other Adjustments in eyeball during accommodation
- Increase in anterior curvature of the lens,
- Two more adjustments are made in the eyeball during
accommodation for near vision.
a. Convergence of both eyeballs:
- Necessary to bring the retinal images on to the
corresponding points
b. Constriction of pupil: Necessary to:
- Increase the visual acuity by reducing lateral chromatic
and spherical aberrations
- Reduce the quantity of light entering eye
- Increase the depth of focus through more central part of
lens as its convexity is increased
Chromatic aberration of a single lens causes different
wavelengths of light to have differing focal lengths
1. Top is a depiction of a lens without spherical
aberration: All incoming rays are focused in the focal
point.
2. Bottom depicts a lens with spherical which produces
spherical aberration: The different rays do not meet
after the lens in one focal point. The further the rays
are from the optical axis`

90. 3. Accommodation reflex
• Reflex action
• When a person looks at a near object after seeing a far object, three adjustments are made in
the eyeballs:
1. Convergence of the eyeballs due to contraction of the medial recti
2. Constriction of the pupil due to the contraction of constrictor pupillae of iris
3. Increase in the anterior curvature of the lens due to contraction of the ciliary muscle.
• It involves both skeletal muscle (medial recti) and smooth muscle (ciliary muscle and
sphincter pupillae).
• During accommodation, all the adjustments are carried out simultaneously.
• Although is a reflex action, it can be controlled by willpower to a certain extent.

91. Pathway for Accommodation Reflex
A. Afferent Pathway
• Visual impulses from retina → Optic nerve → Optic
chiasma, → Optic tract → Lateral geniculate body
→ Optic radiation to visual cortex (area 17) of
occipital lobe → Frontal eye field
B. Center
• Frontal eye field (area 8) in the frontal lobe of
cerebral cortex
C. Efferent Pathway
1. Efferent fibers to ciliary muscle and sphincter
pupillae fibers
• Frontal eye field → Edinger-Westphal nucleus of 3rd
cranial nerve → Ciliary ganglion → Short ciliary
nerves →Ciliary muscle → Constrictor pupillae.
2. Efferent fibers to medial rectus
• Some of the fibers from frontal eye field→ Somatic
motor nucleus of oculomotor nerve→ Medial rectus.

92. APPLIED PHYSIOLOGY
1. Argyll Robertson Pupil
• Clinical condition in which the light reflex is lost but the accommodation reflex is present
• Common in tertiary syphilis (one of the STDs)
• Occurs because of lesion in Edinger-Westphal nucleus, diabetes and alcoholic neuropathy
2. Horner’s syndrome
• Caused by damage to cervical sympathetic nerve
• Also called Bernard-Horner syndrome or Oculo-sympathetic palsy
• Symptoms include:
i. Ptosis (drooping of upper eyelid which covers the eye) due to paralysis of superior tarsal
muscle (responsible for the widening of palpebral fissure)
ii. Swelling of lower eyelid
iii. Miosis (abnormal pupillo-constriction) due to paralysis of dilator pupillae muscle
iv. Enophthalmos (sinking of eyeball into its cavity)
v. Absence of sweating on affected side of the face.

93. 3. Presbyopia
• Characterized by progressive diminished ability of eyes to focus on near objects with age.
• Due to the gradual reduction in the amplitude of accommodation.
• Progresses as the age advances (presbyos = old; ops = eye). Starts developing after middle age.
• Distant vision is unaffected. Its only the near vision is affected
• Near point is away from eye because the anterior curvature of lens does not increase during
near vision.
• So, the light rays from near objects are not brought to focus on retina.
Causes of presbyopia
1. Decreased elasticity of lens is because of the physical changes in lens and its capsule during
old age and so the anterior curvature is not increased during near vision.
2. Decreased convergence of eyeballs due to the concomitant weakness of ocular muscles in old
age.
Correction of presbyopia
• Corrected by using biconvex lens

94. Acuity of Vision
• Ability of eye to determine the precise shape and details of the object
• Refers to the sharpness of vision.
Test for Visual Acuity (VA)
• VA is use to teste for distant vision as well as near vision.
• If there is any difficulty in seeing the distant object or the near object, the defect is referred
to as an error of refraction.
1. Snellen chart: Used to test the acuity of vision for distant vision in the diagnosis of
refractive errors of the eye.
2. Jaeger chart: Used to test the visual acuity for near vision.

95. Factors Affecting Visual Acuity (VA)
1. Age:
• VA most sharp in children and decreases markedly with age
2. Refractive power of the eye:
• VA is decreased if there are errors in the refractive system of the eye e.g. due to myopia or
hypermetropia or astigmatism (commonest cause of reduction of the VA)
3. The time of Exposure
• A light flash of very short duration may not be perceived by the eye
4. Degree of illumination of the test chart and contrast between the letters and the
background
5. Diseases of the eyes
• VA is decreased in various diseases of the eye: e.g. keratitis (corneal inflammation), Iridocytitis
(inflammation of the iris), cataract, glaucoma and retinitis pigmentosa (VA is decreased).

96. Night blindness
• Loss of vision when light in the environment becomes dim.
• Also called defective dim light (scotopic) vision
Causes of Night Blindness
1. Deficiency of vitamin A, which is essential for the function of rods.
• Deficiency of vitamin A occurs because of following causes:
i. Decreased intake of diets containing vitamin A
ii. Decreased absorption of vitamin A from intestine.
Consequences of Deficiency of vitamin A
i. Defective cone function.
ii. Anatomical changes in rods and cones and finally the degeneration of other retinal layers
occurs.
Treatment/Correction
• Retinal function can be restored, only if treatment is given with vitamin A before the visual
receptors start degenerating.

97. Refractive Errors of the eye and Correction
(a) In a normal eye, parallel rays of light are
brought to a focus on the retina by refraction in
the cornea and lens.
(b) If the eye is too long, as in myopia the focus
is in front of the retina.
This can be corrected by a concave (-Ve or
diverging) lens
(c) If the eye is too short, as in hyperopia, the
focus is behind the retina.
Corrected by a convex(+Ve or converging)
lens.
(d) In astigmatism, light refraction is uneven
because of irregularities in the shape of the
cornea or lens
This is corrected by cylindrical glass lens