VISION

ANATOMY

Eyelids

o “Palpebrae”

o Upper eyelid, more movable than the lower eyelid.

o Palpebral fissure

Space between upper and lower eyelids.

Exposes eyeball.

o Lacrimal Caruncle

1. Reddish elevation at the medial commissure.

2. Has sebaceous and sudoriferous glands.

o Commissures

1. Medial commissure

Broader.

Near nasal bone.

2. Lateral commissure

Narrower.

Closer to temporal bone.

o Conjunctiva

Thin, protective mucous membrane.

2 divisions:

Bulbar Conjunctiva

Anterior surface of eyeball.

o SORE EYES - Dilation and congestion of

blood vessesl on this area.

Palpebral Conjunctiva

Inner aspect of eyelids.

o Tarsal Plate

Fold of connective tissue.

Gives form and support to the eyelids.

Embeded in it:

Tarsal or Meibomian Glands

Modified sebaceous gland

Keeps eyelids from adhering to each

other

- CHALAZION - infection of the tarsal

gland

o Eyelashes and Eyebrows

o Sebaceous ciliary glands (Glands of Zeis)

Release fluid at the base of the hair follicles.

STYE - infection on this gland.

o Eyelashes

Project from border of the eyelid.

o Eyebrows

Arch transversely above upper eyelid.

Protects eyeballs from foreign object.

o Lacrimal Apparatus

o Group of structures that produces and drains lacrimal

fluid or tears.

o Lacrimal Glands

Produces 1mL lacrimal fluid/ day

Shape and size of an almond

Secrete lacrimal fluid.

o Excretory lacrimal ducts

Empty tears on the surface of the conjunctiva of

the upper eyelid.

o Lacrimal punctum

Two small openings where tears enter after

passing medially over the anterior surface of the

eyeball

o Lacrimal canals

Two ducts that leads to the lacrimal sac

o Nasolacrimal duct

Duct that carries lacrimal fluid into the nasal cavity

o Lacrimation

Lacrimal glands over secrete tears if there is

irritant present

Protective mechanism

Tears dilute and wash away irritating substance

Tears contains lysozyme(protective bactericidal

enzyme), salts and some mucus.

Lubricates ,protects and moistens the eyeball.

*Colds, obstruction of nasolacrimal ductsand bloacks

drainage of tears.

*Crying, response to parasympathetic stimulation. Lacrimal

gland produces excessive lacrimal fluid that spill over the

edges of the eyelids and fill nasal cavity with fluid (runny

nose).

EXTERIOR OF THE EYEBALL

Eyeball

o 2.5 cm in diameter, anterior 1/6 is exposed

o Composed of 3 Layers:

1. Fibrous Tunic

Superficial coat of eyeball.

Avascular.

Anterior cornea

Transparent coat, covers colored iris.

Focus light onto the retina.

Outer surface: nonkeratinized startified

squamous epithlium.

Middle surface: collagen fibers and

fibroblasts.

Inner surface: simple squamous epithelium.

Posterior sclera

“Scler-”= hard.

White of the eye.

Layer of dense CT (collagen fibers and

fibroblasts)

Covers entire eyeball except cornea.

Give shapes and rigidity to eyeball.

Protects inner part.

Scleral Venous Sinus

Canal of Schlemm

- Where aqueous humor drains.

2. Vascular Tunic

“Uvea”

Middle layer of the eyeball

Has 3 parts:

Choroid

Posterior portion of vascular tunic.

Lines internal surface of sclera.

Provides nutrients to the surface of

sclera.

Anterior: ciliary body.

Extends to ora serrata, jagged anterior

margin of retina.

Ciliary Body

A. Cilliary processes

Protrusions on the internal surface of the

retina.

Contains blood capillaries—secretes

aqueous humor.

Where zonular fibers extends.

B. Ciliary muscle

Circular band of smooth muscle that

alters shape of lens.

For adaptation to near and far vision.

Iris

Colored portion of eyeball.

Suspended between cornea and lens.

Consists of smooth muscles:

- Radial (dilator pupillae)

Sympathetic, dim light, iris

contract, increase in pupil size

- Circular (sphincter pupillae)

Parasympympathetic, bright

light, iris contract decrease pupil

size

Pupil

Hole at the center of Iris.

Autonomic reflexes regulate pupil

diameter in response to light levels.

3. Retina

Inner and 3rd coat of eyeball.

Lines posterior ¾ of eyeball.

Beginning of visual pathway.

Optic disc- where optic nerve, central retinal

artery and vein exits.

Blind spot (no rods or cones).

Cosnsists of:

- Pigmented layer

Sheet of melanin-containing

epithelial cells.

Melanin- absorbs

stray light.

Prevent reflection

and scattering of light w/in

eyeball

Location between choroid and

neural part of retina.

Other cell types: horizontal and amacrine cells.

Neural Layer

Outgrowth of brain that processes visual data.

3 layers: separated by outer and inner

synaptic layer

Bipolar cell layer

Ganglion cell layer

Photoreceptor layer

Rods: low light threshold, no

color vision

Cones: higher threshold,

produce color vision

Macula Lutea

At the exact center of the posterior portion of

retina.

Central fovea

Small depression at the center of

macula lutea.

Contains only cones.

Has the highest visual acuity or

resolution.

Lens

o Behind pupil and iris.

o Crystallins - make up the lens.

o Transparent and lacks blood vessels.

o Focus images of retina for clear vision.

INTERIOR OF THE EYEBALL

Divided in 2 cavities by the lens

o Anterior cavity

Filled with aqueous humor – watery fluid that

nourishes lens and cornea.

Produce intraocular pressure (16mmhg)-

maintains shape of eyeball and prevents it

from collapsing.

Drains at the canal of schlemm and are being

replaced.

Posterior chamber

Behind iris, in front of zonular fibers and lens.

Anterior chamber

Between cornea and iris.

o Vitreous membrane

Lies between lens and retina.

Has vitreous body.

Jelly-like substance , contributes to intraocular

pressure.

Vitreal floaters

Collection of debris, can cast shadow to

retina.

Harmless, common to old individuals.

Hyaloid canal

Narrow channel runs through vitreous

body from the optic disk to the posterior

of the lens.

Holds retina flush against choroid

For even surface of reception of ear

images.

Does not undergo constant replacement.

PHOTORECEPTORS AND COLOR BLINDESS

Photoreceptors

o Outer segments

Transduction of light energy into a receptor

potential occurs here.

They are easily replaced.

Rods:

Cylindrical/Rod-shaped.

1-3 new discs are added to the base of outer

segment every hour.

Cones:

Tapered/Cone-shaped.

Old discs go at the tip and phagocytized by

pigment epithelial cells.

o Inner segments

Cell nucleus, golgi complex and many

mitochondria.

o Proximal end

Expands into bulblike synaptic terminals.

Photopigments

o Integral colored proteins in the plasma membrane.

o Absorption of light leads to chemical changes.

o Two parts of photopigments:

Opsin

Glycoprotein.

Different amino acid sequence, different

colors are absorbed.

Retinal

Light-absorbing part.

Vitamin A derivative (from carotenoids).

o Rods:

Rhodopsin

Absorbs blue to green light.

Pleates pinch off from plasma membrane forming

discs.

o Cones:

3 different cone photopigments.

Absorbs blue, green, yellow orange

Blue/Short wavelength (S)

Green/Medium wavelength (M)

CONES RODS

Center (Macula) Periphery

Bright Dim

Iodopsin Rhodopsin

1 ganglion : 1 2 ganglion : 4

Form and color (Photopic) Intensity/Movement

(Scotopic)

Visual acuity and color

perception

Visual firlds, light and dark -

adaptatiom

Red/Long wavelength (L)

Plasma membrane folds back and forth in a

pleated fashion.

Photopigments – Visual Transduction

o Isomerization

Retinal is in bent shape (cis-retinal) fitted to

opsin.

When it absorbs light, retinal straightens (trans-

retinal).

Isomerization is the transformation from cis-to-

trans retinal.

Chemical stability is affected.

Leads to receptor potential.

o Bleaching

Occurs for about a minute.

Trans-retinal separates from opsin.

Final colorless product.

o After bleaching

Rods: Half of them regenerate in 5 minutes.

Cones: Half of them regenerate in 90 seconds.

o Enzyme retinal isomerase

An enzyme converting the trans retinal to cis

retinal again.

o Regeneration

Cis retinal back to opsin to form a functional

photopigment.

In rods,

Pigmented layer adjacent to photoreceptors

has high quantity of Vitamin A.

Regeneration of rods.

If retina detaches from pigmented layer,

regeneration of rhodopsin is low.

In cones,

Photopigments regenerate more quickly.

Less dependent on pigmented layer.

Light and Dark Adaptation

o From darks surroundings, light adaptation

Visual system adjusts into bright surroundings.

Visual system decreases sensitivity.

o Into a darkened room, dark adaptation

Visual system increases sensitivity over minutes.

o When light level increases

More photopigments are bleached.

In daylight, regeneration of rhodopsin cannot

keep up with the bleaching process.

Rods: Contribute little to daylight.

Cones: Regenerate rapidly.

Cis retinal always present

o When light level decreases

Increased sensitivity and then more slowly.

In complete darkness

A threshold, light flash is seen as having a

color.

Rhodopsin regenerates more slowly,

increasing the visual sensitivity.

Even a single photon can be detected.

At low light levels, only rods are functioning.

Release of Neurotransmitters by Photoreceptors

o Photoreceptor in the absence of light

Na inflow (“dark current”) into photoreceptor

outer segment.

Ligand-gated Na channels.

Guanosine monophosphate (cGMP).

o Inflow partially depolarizes the photoreceptor.

Membrane potential: -30 mV

Triggers release of NT at synaptic terminals

NT in rods and cones: Amino Acid Glutamate

(Glutamic Acid)

o Glutamate

Between rods and bipolar cells at synaptic

terminals.

Inhibitory NT

Inhibits postsynaptic potentials.

Hyperpolarizes bipolar cells.

o Photoreceptor in the presence of light

Cis retinal goes isomerization.

Enzymes are activated leading to breakdown of

cGMP.

Some cGMP ligand-gated Na channels

closes.

Na inflow decreases.

Membrane potential: -70 mV

Hyperpolarize receptor potential

Decrease in release of NT

o Dim Lights

Cause small and brief receptor potentials.

Partial shutdown of some NT release.

o Brighter lights

Elicit larger and longer receptor potentials.

Complete shutdown of NT release.

Color Blindness

o Inability to distinguish between certain colors.

o Absence or deficiency of one of three cone

photopigments.

o Red-green color blindness

Most common.

Photopigment sensitive to orange-red/ green

light is missing.

Person cannot distinguish between red and green.

o Vitamin A deficiency and consequent below normal

amount of rhodopsin

o Night blindness/ Nyctalopia.

Inability to see well at low light levels.

o Deuteranopia

Absence of green cones.

o Protanopia

Absence of red cones.

o Tritanopia

Absence of blue cones.

VISUAL PATHWAY AND VISUAL FIELDS

Neuronal cell types:

o Photoreceptors (rods and cones) – transmit signals to

the outer plexiform layer, where they synapse with

bipolar cells and horizontal cells.

o Horizontal cells – transmit signals horizontally in the

outer plexiform layer from the rods and cones to

bipolar cells.

o Amacrine cells – transmit signals in two directions,

either directly from bipolar cells to ganglion cells or

horizontally from axons of bipolar cells to dendrites of

the ganglion cells or to other amacrine cells.

o Ganglion cells – transmit output signals from the retina

through the optic nerve into the brain.

o Interproximal cell

Transmits signals in the retrograde direction from

the inner plexiform layer to the outer plexiform

layer.

The signals are inhibitory and control lateral

spread of visual signlas

Help control the degree of contrast in the viual

image.

Visual Pathway Process

RETINA

o Receptor potentials arise in rods and cones

o Spread through the inner segments to the synaptic

terminals.

o Neurotransmitter molecules (glutamate) are released.

o Neurotransmitters induce local graded local potentials

in bipolar cells and horizontal cells.

6 and 600 rods synapse with bipolar cells.

Increases the light sensitivity of rod vision but

slightly blurs the image perceived.

Stimulation of rods by light excites their

bipolar cells.

Cone more often synapses with just one bipolar

cell.

Less light sensitivity but has higher acuity due

to one-to-one synapses between cones and

their bipolar cells.

Stimulation of rods by light may either excite

or inhibit cone bipolar cells.

o Horizontal cells transmit inhibitory signals to bipolar

cells in the areas lateral to excited rods and cones.

Enhances contrasts in the visual scene between

areas of the retina that are strongly stimulated and

adjacent areas that are more weakly stimulated.

Assist in the differentiation of various colors.

o Amacrine cells are excited by bipolar cells, synapse

with ganglion cells and transmit information to them.

Signals a change in the level of illumination of the

retina.

o Ganglion cells become depolarized and initiate nerve

impulses.

OPTIC NERVE

o Axons within the optic nerve pass through the optic

chiasm.

Crossing point of the optic nerves.

o Medial half of the axons cross the opposite side and

the lateral half of the axons remained uncrossed.

o After passing to the optic chiasm, the axons, now part

of the optic tract, enter the brain and terminate in the

lateral geniculate nucleus in thalamus.

THALAMUS

o The axons synapse with neurons whose axons form

the optic radiations, which project to the primary

visual area in the occipital lobes of the cerebral cortex.

CORTEX

o Large number of optic fibers project to the lateral

geniculate nucleus of the thalamus, where information

from the different ganglion cell types is kept distinct.

o Receive input from the brainstem reticular formation

and input relayed back from the visual cortex.

Control the transmission of information from the

retinal to the visual cortex.

Involved in our ability to shift attention between

vision and the other sensory modalities

o Lateral geniculate nucleus sends action potentials to

the visual cortex.

Processed simultaneously in a number of

independent ways in different parts of the cerebral

cortex.

Reintegrated to produce the conscious sensation

of sight and the perceptions associated with it.

Constriction of pupil.

Suprachiasmatic nucleus: establishes pattern

of sleep and other activities in response to

intervals of light and darkness.

Brainstem and cerebellum: coordination of

head and eye movements.

o Cells are organized to handle information about line,

contrast, movement, and color.

Form a spatial and temporal pattern of electrical

activity.

Visual Field

o Visual area seen by an eye at a given instant.

o Nasal field of vision - area seen to the nasal side.

Light rays fall on the temporal half of the retina.

o Temporal field of vision - the area seen to the lateral

side.

Light rays fall on the nasal half of the retina.

o Extend farthest on the temporal sides

o Limited by:

Superiorly – Brows

Inferiorly – Cheeks

Medially – Nose

IMAGE FORMATION

1. Refraction

o As light rays enter the eye, they are refracted at the

anterior and posterior surfaces of the cornea.

75% of the total refraction of light

o Both surfaces of the lens of the eye further refract the

light rays so they come into exact focus on the retina.

25% of focusing power (changes the focus to view

near or distant objects)

o Image is focused on the retina: upside down and

undergo right to left reversal.

o Focusing power of the lens:

Object is 6 meters (20 feet) or more: light reflected

from the object are nearly parallel to one another.

The rays must be bent enough to be focused

on the retina.

Object is closer than 6 meters (20 feet): light rays

reflected from the object are divergent.

The rays must be refracted more to be

focused on the retina.

2. Accommodation

o When the eye is focusing on a close object, the lens

becomes more curved and refracts the light more.

The lens of the eye is convex on both its anterior

and posterior surfaces.

Increase curvature of lens (for near vision) =

Increase focusing power

o Near point vision: minimum distance from the eye

that an object can be clearly focused with maximum

accommodation.

3. Constriction

o Narrowing of the diameter of the pupil through which

light enters.

Contraction of the circular muscles of iris to

constrict the pupil.

o Occurs simultaneously with accommodation.

o Prevents light rays from entering the eye through the

periphery of the lens.

Contraction of ciliary muscle

Relaxation of zonular fibers

Relaxation of lens (becoming more spherical)

Near objects brought into focus

VISUAL ACUITY AND PUPILARY REACTION TO LIGHT

Visual Acuity

o Measure of the eyes’ ability to distinguish object details

and shape at a given distance.

o Normal Vision

Occurs when light is focused directly on the retina

rather than in front or behind it.

o Far Vision

Typically measured at twenty feet.

Rays of light from a distant object are

practically parallel.

Little accommodation is required.

o Snellen Chart

Numerator: the distance the patient is from the

chart

Denominator: the distance at which an normal eye

could see the optotype on the chart.

Visual Acuity

Eg. 20/50

A patient sees at twenty feet what the patient

with no refractive error or ocular pathology

would see at fifty feet.

20/20 visual acuity- “normal visual acuity”

i denominator value, the better the acuity; i

denominator value, the poorer the acuity.

20/40 vision in at least one eye is the vision

required to pass the driving test

20/200- “legally blind”

o Hyperopia

The eyeball is short relative to the focusing power

of the lens and cornea.

Timid or lazy lens.

Corrected by using eye glasses with convex lens.

o Myopia

The eyeball is too long relative to the refractive

power of the lens and cornea.

Enthusiastic lens.

Corrected by using eye glasses with concave lens.

o Presbyopia

Lens loses elasticity and thus its ability to

accommodate. Therefore, older people cannot

read print at the same close range as can

youngsters.

Usually begins in the mid-forties.

Age 40: 20 cm (8 in)

Age 60: 80 cm (31 in)

Pupillary Light Reflex

o A reflex that controls the diameter of the pupil, in

response to the intensity of light that falls on the retina

of the eye.

o When light is shown into the eyes, the pupils constrict.

o ↑ light intensity= ↑ intensity of signals transmitted by

the bipolar, horizontal, amacrine, and ganglion cells

(neural adaptation).

o Mechanism of Pupillary light reflex:

Optic nerve/ CN II- responsible for the afferent

limb of the reflex. It senses the incoming light.

Oculomotor nerve- responsible for efferent limb of

pupillary reflex. It drives the muscles to constrict.

Its pathway begins with retinal ganglion cells,

which convey information from photoreceptors to

the optic nerve.

OCULAR MOVEMENTS

Innervated by CN III, IV, VI

o Superior Rectus

Elevation, adduction and medial rotation of the

eyeball

o Inferior Rectus

elevation, adduction and lateral rotation of the

eyeball

o Lateral Rectus

Abduction of eyeball

o Medial Rectus

Adduction of eyeball

o Superior Oblique

Depression, abduction and medial rotation of

eyeball

o Inferior Oblique

ELevation, abduction and lateral rotation of

eyeball.

DISORDERS OF THE EYE

CATARACT

o Clouding of the eye's natural lens.

The lens is mostly made of water and protein. The

protein is arranged in a precise way that keeps the

lens clear and lets light pass through it. But as we

age, some of the protein may clump together and

start to cloud area of the lens.

o Most common cause of vision loss in people over age

40.

o Principal cause of blindness in the world.

Signs and Symptoms

o Vision is blurred a little.

Note on 1st bullet. like looking through a cloudy

piece of glass or viewing an impressionist painting

o May make light from the sun or a lamp seem too bright

or glaring

o The oncoming headlights cause more glare than

before

o Colors may not appear as bright as they once did

*A cataract starts out small and at first has little effect on your

vision

Causes

o Advancing of age

o Infection

o Trauma

o Ultraviolet radiation

o Diabetes

o Smoking

o Heavy alcohol consumption

Prevention

o Regular eye check-up

o Wearing of sunglasses

o diet high in antioxidants

Beta-carotene (vitamin A)

Selenium

Vitamins C and E

Treatment

o Severe condition

Surgical removal of lens and is replaced with an

artificial lens

*Plastic intraocular lens (IOL) – artificial lens

o For impaired vision

Visual aids i.e. glasses, bifocals, appropriate

lighting

MACULA DEGENERATION

o Common in older people.

o Central vision loss may occur.

Signs and Symptoms

o Yellowish spots (drusen)

form in the back of the eye or retina are an early

sign of "dry" macular degeneration.

It is believed these spots are deposits or debris

from deteriorating tissue.

o Early signs

shadowy areas in your central vision or unusually

fuzzy or distorted vision.

o Slow, painless loss of vision

rare case, however, vision loss can be sudden

Causes

o Hereditary disorders

o Infections

o Trauma

o Tumor

o Advancing age

o Smoking

o High blood pressure

o Obesity

o Lighter eye color

Like in the skin (melanin)

Prevention

o Diet with high levels of:

Antioxidants

Omega-3 fatty acids

Lutein (eggs, spinach, turnips)

o Amsler grid

straight lines, with a reference dot in the center

Treatment

o No satisfactory medical treatment

o Optical aids (i.e. glasses)

GLAUCOMA

o Silent thief of sight

Typically cause no pain and produce no symptoms

until noticeable vision loss occurs

o Excessive pressure build-up in the aqueous humor

Producing too much fluid, or it's not draining

properly

o Results from an interference with normal re-entry of

aqueous humor into the blood or from an

overproduction of aqueous humor

Pressure within the eye can close off the blood

vessels entering the eye and may destroy the

retina or optic nerve, resulting to blindness

*Normally, IOP should be below 21 mmHg

Signs and Symptoms

*The word "glaucoma" came from a Greek word which means,

"opacity of the crystalline lens." (Cataracts and glaucoma were

not distinguished until c.1705)

o Typically, none

o In a specific type of glaucoma

Blurry vision, halos around lights, intense eye

pain, nausea and vomiting

Prevention

o Exercise

Lowers OPP or ocular perfusion pressure

* OPP is a mathematical value that is calculated using a

person's intraocular pressure and his or her blood pressure.

o Gonioscopy

Make sure the aqueous humor (or "aqueous") can

drain freely from the eye

In gonioscopy, special lenses are used with a

biomicroscope to enable your eye doctor to see

the structure inside the eye (called the drainage

angle) that controls the outflow of aqueous and

thereby affects intraocular pressure.

o Visual field testing

to determine if you are experiencing vision loss

from glaucoma

o Imaging technology

create baseline images and measurements of the

eye's optic nerve and internal structures.

o Tonometer 

measure your intraocular pressure, or IOP

Treatment

o Depending on the severity

glaucoma surgery

Lasers

medications

o Glaucoma eye drops

Keeps IOP low