Physiology of eye

Functional anatomy of eye
Human eyeball is globe shaped with a diameter of about 24mm.
The eye ball is situated in a bony cavity known as orbital cavity or eye
socket whose function is protection of the eye.
The eyelids protect the eyeball from foreign particles coming in contact
with its surface and cutoff the light during sleep.
The conjunctiva is a thin mucous membrane,which covers the exposed
part of the eye.
The lacrimal gland is located in the outer and upper part of upper
eyelid.it secretes tears which keep the eye moist.tears also contain
bactericidal enzymes like lysozyme.

The wall of the eyeball is composed of 3 layers
1.outer layer(tunica externa or tunica fibrosa),which includes cornea and
sclera.The outer layer preserves the shape of eyeball .the posterior 56 of
this coat is opaque and is called the sclera.The anterior 16 is transparent
and avascular and is known as cornea.
2.Middle layer or ( tunica media or tunica vasculosa) surrounds the eyeball
completely except for a small opening in front known as the pupil.This layer
comprises 3 structures
1. Choroid: thin vascular layer of eyeball situated between sclera and
retina.,which forms iris and cilliary body anteriorly.
2. cilliary body is the thickened anterior part of middle layer of eye situated
between choroid and iris.
3. Iris is the thin coloured curtain like structure of eyeball.it has a circular
opening in the center called pupil.the diameter of the pupil is regulated
by circular and radial muscles of iris

3.inner layer or tunica interna or tunica nervosa or retina
Retina is a delicate light sensitive membrane that forms the innermost layer of
eyeball.retina has the receptors of vision.
Retina is made up of 10 layers
Layer of pigment epithelium
Layer of rods and cones
External limiting membrane.
Outer nuclear layer
Outer plexiform layer
Inner nuclear layer
Inner plexiform layer
Ganglion cell body
Layer of nerve fibers
Internal limiting membrane

Fundus oculi fundus is the posterior part of interior of eyeball.the fundus
has 2 structures
1.optic disk(blind spot) which contains all the layers of retina except rods
and cones.therefore it is insensitive to light.
2.macula lutea(yellow spot) which is situated little lateral to the optic disk
of retina.it has fovea centralis in its center.the fovea is the region of the
most acute vision.when one looks at an object,the eyeballs are directed
towards the object.so that the image of that object falls on the fovea of
each eye and the person can see the object very clearly.it is known as
foveal vision.
Intraocular fluid
Two types of fluids are present in the eye
Vitreous humor
Aqueous humor

Vitreous humor: fluid present behind the lens in the space between
the lens and retina.it helps to maintain the shape of the eyeball.
Aqueous humor: fluid present between the lens and cornea.this
space is divided into anterior and posterior chambers by iris.both
the chambers communicates with each other through pupil.
Functions of aqueous humor:
Maintains shape of the eyeball
Maintains the intraocular pressure
Provides nutrients,oxygen, and electrolytes to the avascular
structures like lens and cornea.
Removes metabolic end products from lens and cornea.

Formationcirculation and drainage of
aqueous humor

Intraocular pressure:it is the measure of fluid pressure in the eye
exerted by aqueous humor.the normal range is between 12 and 20
mm Hg.it is measured by tonometer.
Lens:lens is biconvex and transparent.it is enclosed in the lens
capsule and kept in place by the suspensory ligament.
Ocular muscles:2 types
Intrinsic muscles(constrictor pupillae,dilator pupillae and cilliary muscle)
Formed by smooth muscle fibers and are controlled by autonomic nerve
Extrinsic muscles(superior rectus,inferior rectus,medial or internal
rectus,lateral or external rectus,superior oblique and inferior oblique)
Formed by skeletal muscles fibers and are controlled by the somatic
nerves

Ocular Movements
Movements in vertical axis
1.abduction or lateral movement or outward movement
2.Adduction or medial movement or inward movement
Movements in transverse axis
1.elevation or upward movement
2.depression or downward movement.

Movements in anteroposterior axis(torsion or wheel movements)
1.extorsion-eye ball is rotated in such a way that the cornea turns
upward and outward direction
2.intorsion-eyeball is rotated so that,the cornea moves in
downward and inward direction.
Simultaneous movements of both eyeballs
1.conjugate movement—movement of both eyeballs in the same
direction
2.disjugate movement– movement of both eyeballs in opposite
direction.2 types:
1.convergence-movement of both eyeball towards nose.
2.divergence- movement of both eyeballs towards the temporal side.

3.persuit movement-movement of eyeballs along with object when
eyeballs are fixed on a moving object.
4.saccadic movement(orthokinetic movement)- it is the quick jerky
movement of both eyeballs when the fixation of eyes is shifted from
one object to another object

Applied physiology
Glaucoma
It is a group of diseases characterized by increased intraocular
pressure which causes damage of optic nerve resulting in
blindness.when intraocular pressure increases about 60-70 mm
hg,glaucoma occurs.
Types
1.primary open angle glaucoma:no visible obstruction in the
drainage system.still the intraocular pressure increases causing
damage to optic nerve.

2.primary angle –closure glaucoma
It is characterized by visible obstruction of drainage system for aqueous
humor.
Cause;
Mainly due to blockage in the drainage system of aqueous humor.secondary
causes include diabetes,inflammation,or injury to eyes and excess use of
drugs such as corticosteroid.
Symptoms
Heaviness around eyeball,headache,reduction in visual acuity and visual field
,nausea,coloured rings around bulb light etc
Treatment;
eyedrops or medicine alone or in combination with laser treatment or surgery.

cataract
Cataract is the opacity or cloudiness in the natural lens of
eyes.cataract develops in old age after 55-60 years.
Cause
Eye injury,alcoholism,family history,unprotected exposure to
sunlight,disesaes such as diabetes,hypocalcemia,previous eye surgery.
Symptoms
Glare,painless blurred vision,diplopia in affected eye.fading of colours
Treatment :surgery

Visual receptors
The retina contains the visual receptors which are also called
light sensitive receptors,photoreceptors or electromagnetic
receptors.
The visual receptors are the rods and cones.
The distribution of photoreceptors in the retina is non
uniform,having greatest concentration of cones in the fovea.
Optic disk is devoid of photoreceptors.

Structure of rods and cones.
The rods and cones have 3 parts.
1.outer segment
2.inner segment that includes a nuclear region
3.synaptic body.

The outer segment consists of folds of cell membrane(discs)
arranged in a pile.
There are as many as 1000 discs in each rod or cone.
The photosensitive pigment that reacts to light is present in
the outer segment.
Pigment of rod- rhodopsin
Pigment of cones - iodopsin

Inner segment contain cytoplasm with cytoplasmic organelles
with large number of mitochondria which provide energy for
the function of the photoreceptors.inner segment contains
nuclear region.
The synaptic body contains synaptic vesicles which stores the
neurotransmitter glutamate.
The synaptic terminal synapses with the bipolar cells and
horizontal cells.

Nerve cells in retina
1.bipolar cells
2.ganglion cells
3 .horizontal cells
4.amacrine cells.

Functions of rods and cones
Rods
The rods are responsible for the dim light vision or night vision
or scotopic vision.Rods donot take part in visual acuity or
colour of the objects.the vision by rod is black,white or in the
combination of black and white namely gray.therefore ,the
coloured objects appear faded or grayish in night.

Cones
Cone cells are called receptors of bright light vision or photopic
vision or day light vision.
The cones are also responsible for visual acuity of vision and
the color vision.

Photochemistry of vision(chemical
basis of visual process)
Photosensitive pigments present in rods and cones are
concerned with chemical basis of visual process.
The chemical reactions involved in these pigments lead to the
development of electrical activity in retina and generation of
impulses(action potentials ) which are transmitted through optic
nerve.the photochemical changes in the visual receptors cells
are called wald’s visual cycle (rhodopsin- retinene visual cycle)

Visual pigments
Rods: rhodospsin or visual purple
Cones : iodopsin.(cyanolabe,chlorolabe,erythrolabe)
When rhodopsin absorbs the light that falls on retina,it is split into
retinine and protein called scotopsin through various photochemical
reactions.stages are:
1.rhodopsin is decomposed into bathorhodopsin that is very unstable.
2.bathorhodopsin is converted into lumirhodospin.
3.lumirhodopsin is converted into metarhodopsin I

4.metarhodopsin I is changed to metarhodopsin II
5.Metarhodopsin II is split into scotopsin and all-transretinal.
6.the all-transretinal is converted into all –transretinol by the
enzyme dehydrogenase in the presence of NADH2.
Metarhodopsin is usually called the activated rhodopsin since it
is responsible for the development of receptor potential in rod
cells.

Resynthesis of rhodopsin:
First the all transretinal derived from metarhodopsin II is
converted into 11-cis retinal by the enzyme retinal isomerase.
11-cis retinal immediately combines with scotopsin to form
rhodopsin.
The all-transretinol is converted into 11-cis retinal,which
combines with scotopsin to form rhodopsin.

Night blindness
Night blindness is defined as the lose of vision when light in
the environment becomes dim.
Causes
Deficiency of vitamin A.(essential for the function of rod)
Due to diet containing less amount of vitamin A or decreased
absorption of vitamin A from the intestine.

Visual acuity
It is the ability of the eye to determine the precise shape and
details of the object.
It is also defined as the ability to recognize the separateness of 2
objects placed together.
Cones of the retina are responsible for the acuity of vision
Tests for visual acuity.
Acuity of vision is tested for distant vision and near vision.
If any difficulty in seeing the distant object or near object,the defect
is known as error of refraction.

Distant vision:
Snellen’s chart is used to test the acuity of vision for distant
vision
Near vision:
Jaeger’s chart is used to test the visual acuity for near vision.

Visual field
The total area of the external world that can be seen with
one eye when it is fixed straight on an object is the visual
field.the other eye should be kept closed.

Dark adaptation
Dark adaptation is the process by which the person is able to
see the objects in dim light.
If a person enters a dim lighted room from a bright lighted
area,he is blind for sometime.
After sometimes,his eyes get adapted and he starts seeing the
objects slowly.
The maximum duration for dark adaptation is about 20 mins

Causes
Increased sensitivity of rods as a result of resynthesis of
rhodopsin
Dilatation of pupil.

Light adaptation
Light adaptation is the process in which eyes get adapted to
increased illumination.
The maximum period for light adaptation is about 5 minutes
Cause
Reduced sensitivity of rods
Constriction of pupil.

Visual optics
When light passes through the eye refraction occurs at different
interfaces.there is bending of light rays when it passes from air to
cornea,cornea to aqueous humor,aqueous humor to lens and lens to
vitrous humor.
Far point:
The farthest point from the eye at which an object can be focused
clearly on the retina is called far point.
Near point:
the minimum distance at which the eye can see objects clearly is called
near point.

Normaly by the age of 20 years,it is about 10 cm.at the age of 50
years,near point is about 50 cm from the eye.
 accommodation
Accommodation is the adjustment of the eye to see either near or distant
objects clearly.it is the process by which light rays from near objects or
distant objects are brought to a focus on the sensitive part of retina.it is
achieved by various adjustments made in the eyeball.
Light rays from distant objects are approximately parallel and are less
refracted while getting focused on retina.
Light rays from near objects are divergent.so to be focused on retina,these
light rays should be refracted or converged to a greater extent by
increasing the curvature of lens,convergence of eyeballs and constriction of
pupil which occurs on viewing a near object is called near reflex or
accommodation reflex.

Errors of refraction
Emmetropia
The eye with normal refractive power (14 Dioptres)is called
emmetropic eye and the condition is called emmetropia.
Ametropia
Any deviation in the refractive power from normal condition resulting
in inadequate focusing on retina is called ametropia.
Types :
1.Myopia
2.hypermetropia

Myopia or short sightedness
Eye defect characterized by the inability to focus on distant object.
Cause :increase in anteroposterior diameter of the eyeball.
Correction: biconcave lens
Hypermetropia or long sightedness :
Eye defect characterized by the inability to focus on near object.
Cause ;decrease in anteroposterior diameter of the eyeball.
Correction:biconvex lens
Anisometropia:
It is the condition in which the 2 eyes have unequal refractive
power.
Correction: biconcave or biconvex for each eye as required.

Presbyopia :
It is a condition characterized by progressive diminished ability
of eyes to focus on near objects with age.
Cause:loss of elasticity of lens and weakness of ocular muscles
due to old age.
Correction :biconvex lens.

Astigmatism
It is a condition in which the light rays are not brought to a
sharp point upon retina.
Cause:due to lack of uniformity of curvature of cornea or the
lens in different planes.
Correction :cyclindrical lens

Color vision:
The 3 primary colors are red,blue,and green.
These colors when added in appropriate proportion can produce
the sensation of other colors.
When the 3 primary colors are mixed in equal proportion,it
gives the sensation of white.

Young-Helmholtz trichromatic theory
of color vision
According to this theory,there are 3 types of cones namely,red
cone,green cone, and blue cone.
These cones contains the visual pigments erythrolabe,chlorolabe,
and cyanolabe .these pigments are collectively called iodopsins
The cone for each color is stimulated by different wavelengths of
light-red cone by light of wave length 650nm,green by 530 nm,
and blue cone by 440 nm
Differential stimulation of these cones produces other colors.

Color blindness
Color blindness is the failure to appreciate one or more colors.
Color blindness is inherited as an x-linked recessive character..it can also be
be acquired.
Causes for acquired color blindness:
Trauma,chronic diseases such as glaucoma,diabetes,liver diseases,Parkinson’s
disease or Alzheimer’s disease.
Drugs like antibiotics,antihypertensives,antiTB drugs.etc
Toxins such as fertilizers,carbon monoxide.etc
Alcoholism
Aging (after 60 years)

classification
1.monochromatism:
Its a condition characterized by total inability to perceive color.it is
also called total color blindness or achromatopsia.
2.dichromatism:
Color blindness in which the subject can appreciate only 2
colors.persons with this defect are called dichromats.
Red blind individuals are called protanopes.
Green –blind individuals are called deuteranopes
Blue - blind individuals are called tritanopes.

3.trichromatism
In this case,the affected person can see all the 3 colors but
have weakness for one color.
Types-protanomaly –red cone weak
Deuteranomaly-green cone weak
Tritanomaly-blue cone weak.

Tests for color blindness
By using ishihara’s color charts
By using colored wool
By using edridge-green lantern

Lesions of visual pathway:
the loss of vision in one visual field is known as anopia.
Losss of vision in one half of visual field is called hemianopia.
2 types:
Homonymous hemianopia
Loss of vision in the same halves of both the visual field.
Heteronymous hemianopia.
Loss of vision in opposite halves of visual field.

Binasal heteronymous hemianopia means loss of vision in nasal
half of both visual fields.
Bitemporal heteronymous hemianopia means loss of vision in
temporal half of both visual field.
Right homonymous hemianopia means loss of vision in right half
of visual field of both eyes
Left homonymous hemianopia means loss of vision in left half
of visual field of both eyes

Pupillary light reflexes (light reflex)
It is the reflex in which the pupil constricts when light is
flashed into the eyes.
2 types
1.direct reflex
2.indirect light reflex.

1.direct light reflex (direct pupillary light reflex)
It is the reflex in which there is constriction of pupil in an eye
when light is thrown into that eye.
2.indirect light reflex(consensual light reflex)
It is the reflex that involves constriction of pupil in both eyes
when light is thrown into one eye.

Pathway for light reflex:
when light falls on the eye,the visual receptors are
stimulated.The afferent impulses from the receptors pass
through the optic nerve,optic chiasma,and optic tract.
At the midbrain level,few fibers get separated from the optic
tract and synapse on the neurons of pretectal nucleus.which lies
close to the superior colliculus.the pretectal nucleus of midbrain
forms the center for light reflexes.
The efferent impulses from this nucleus are carried by short
fibers to edinger-westphal nucleus of oculomotor nerve.

From there,the preganglionic fibers pass through oculomotor
nerve and reach the cillary ganglion.
The post ganglionic fibers arising from the ciliary ganglion pass
through the short ciliary nerves and reach the eyeball.
These fibers cause contraction of constrictor pupillae muscle of
iris.

Visual pathway
Areas of visual cortex and their function
1.primary visual area(area 17)-concerned with perception of
visual impulses
2.visual association area(area 18) -concerned with interpretation
of visual impulses
3.occipital eye field (area 19)-concerned with movement of eye.

Macular sparing:
If there is a small lesion in visual cortex,there will be macular
sparing.
Macular sparing is the retention of macular function in spite of
losses in the adjacent visual field.
This is because macula of retina has a larger area of
representation in the visual cortex.

Corneal transplants:
Defective cornea is removed and a donor cornea of similar
diameter is transplanted

Physiology of eye

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