ESye and Vision Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.

Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.

ESye and Vision


Vision uses visible light which is part of the electromagnetic spectrum with wavelengths from about 400 to 700 nm.

Vision


Accessory structures of the eyes include the eyelids, eyelashes, eyebrows, lacrimal (tear-producing) apparatus and extrinsic eye muscles.

Vision


1 inch diameter

5/6 of eyeball inside orbit & protected

Eyelashes & eyebrows help protect from foreign objects, perspiration & sunlight

Palpebral muscles control eyelid movement and extrinsic eye muscles are responsible for moving the eyeball itself in all directions.

Eyeball


The conjunctiva is a thin, protective mucous membrane that lines the eyelids and covers the sclera.

The tarsal plate: a fold of connective tissue that gives form to the eyelids. Contains a row of sebaceous glands (tarsal glands/Meibomian glands) that keeps the eyelids from sticking to each other. (sty)

Vision


Vision


The lacrimal apparatus produces and drains tears. The pathway for tears is:

The lacrimal glands

The lacrimal ducts

The lacrimal puncta

The lacrimal canaliculi

The lacrimal sac

The nasolacrimal ducts that carry the tears into the nasal cavity.

Lacrimal Apparatus


• a watery physiologic saline, with a plasma-like consistency,

• contains the bactericidal enzyme lysozyme;

• it moistens the conjunctiva and cornea,

• provides nutrients and dissolved O2 to the cornea.

Lacrimal Apparatus


Lacrimal Apparatus


Six extrinsic eye muscles move the eyes in almost any direction. These muscles include the

4 recti : superior rectus, inferior rectus, lateral rectus, medial rectus,

2 obliques :superior oblique and inferior oblique.

Extrinsic Muscles of Eye


• Move eyeball in direction of name

• Superior rectus, inferior rectus, medial rectus supplied by Occulomotor or Cranial nerve III.

• Lateral rectus supplied by Cranial nerve VI / Abducens

Recti muscles of eye: movement and nerve supply


Extraocular Muscles


• Superior & inferior - rotate eyeball on its axis

• Superior rotates inferiorly and laterally

• Inferior rotates superiorly and laterally

• Superior oblique supplied by Cranial nerve IV / Trochlear

• Inferior oblique supplied by Cranial nerve III / Occulomotor

Oblique muscles


The eyeball contains three tunics (coats): the fibrous tunic, outermost (the cornea and sclera)

the vascular tunic/uvea, middle(the choroid, ciliary body and iris).

the nervous tunic, innermost( retina)

Three tunics of Eyeball


Vision


• Sclera

• Cornea

Fibrous tunic


• “White” of the eye, covers posterior ¾ of the eye

• Dense irregular connective tissue layer -- collagen & fibroblasts

• Provides shape & support

• At the junction of the sclera and cornea is an opening (scleral venous sinus)

• Posteriorly pierced by Optic Nerve (CNII)

Sclera


• Transparent, avascular

• Helps focus light(refraction); astigmatism

• 3 layers

- nonkeratinized stratified squamous

- collagen fibers & fibroblasts

- simple squamous epithelium

• Transplants: common & successful; no blood vessels so no antibodies to cause rejection

• Nourished by tears & aqueous humor

Cornea


1.choroid

2. ciliary body

ciliary muscle & ciliary process

3. iris

radial muscle & circular muscle

4. pupil

Middle vascular tunic or uvea


is a network of blood vessels that supply oxygen and nutrients to the tissues of the eye.

located deep to the sclera and is called the "choroid.“

contains a pigmented layer (melanin) that helps absorb excess light

anterior to the choroid is a circular structure called the "ciliary body."

ciliary body has ciliary muscles that act on suspensory ligaments which suspend the lens in the correct position.

Ciliary body is also made up of a ciliary process that makes a fluid called aqueous humor

Vascular tunic:


Vision


The suspensory ligaments are either taut or relaxed based on the action of the ciliary muscles.

The tension on the ligaments changes the shape of the lens, depending on the distance of the object being viewed.

This process is called "accommodation".

The iris is the colored portion of the eye and it is found in front of the ciliary body,

Vascular Tunic


it divides the front portion of the eye into two chambers – the anterior and posterior chambers.

the opening in the middle of the iris is called the "pupil," which appears as the dark center of the eye.

the iris either dilates or constricts the pupil to regulate the amount of light entering the eye. In bright light the pupil will be small, but in dim light the pupil will be very large to let in as much light as possible.

Iris of the Eye - controls the amount of light entering the eye

Constriction of pupils - contraction of the circular fibers – parasympathetic, bright light

Dilation - contraction of radial fibers – sympathetic, dim light

Iris


The iris (colored portion of the eyeball) controls the size of the pupil based on autonomic reflexes.

Vision


posterior ¾ of eye ball only

anterior margin – ora serrata retina

superficial layer of pigment epithelium:

nonvisual portion

absorbs stray light & helps keep image clear

optic disc – attachment of optic nerve / blind spot, no photoreceptors

deeper layer of neurons

- rods/cones are photoreceptor layer

- bipolar neuron layer

- ganglion cell layer

Inner sensory tunic/retina


The retina lines the posterior three-quarters of the inner layer of the eyeball. It may be viewed using an ophthalmoscope.

The optic (II) nerve is also visible.

The point at which the optic nerve exits the eye is the optic disc (blind spot).

The exact center of the retina is the macula lutea. In its center is the fovea centralis (area of highest visual acuity).

Vision


Vision


The retina contains sensors (photoreceptors) known as rods and cones.

Rods to see in dim light

Cones produce color vision

From these sensors, information flows through the outer synaptic layer to bipolar cells through the inner synaptic layer to ganglion cells. Axons of these exit as the optic (II) nerve.

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Vision


Pathway of light

Ganglion cells bipolar cells photoreceptor cells

Pathway visual impulses

Photoreceptor cells bipolar cells ganglion cells optic nerve

Visual and light pathways


• Light penetrates retina

• Rods & cones transduce light into action potentials

• Rods & cones excite bipolar cells

• Bipolars excite ganglion cells

• Axons of ganglion cells form optic nerve leaving the eyeball (blind spot)

Retina


Vision


The eye is divided into an Anterior Cavity and Posterior

Cavity.

The anterior cavity is divided into

- anterior chamber and a posterior chamber by the iris

(colored portion of the eyeball).

anterior chamber (between the iris and cornea)

posterior chamber lies behind the iris and in front of the

lens

the anterior cavity contains aqueous humor

Behind this is the posterior cavity (vitreous chamber) filled

with a transparent, gelatinous substance, the vitreous humor.

Vision


• filled with aqueous humor

Aqueous Humor

- Continuously produced by ciliary body

- Flows from posterior chamber into anterior through the pupil

- continually drained by scleral venous sinus or canal of

Schlemm / opening in white of eye at junction of cornea &

sclera

- drainage of aqueous humor from eye to bloodstream

Glaucoma

increased intraocular pressure that could produce blindness

problem with drainage of aqueous humor

Anterior Cavity


(posterior to lens)

• filled with vitreous body (jellylike)

• holds retina in place

• formed once during embryonic life

• floaters are debris in vitreous of older individuals

Posterior Cavity


Light passes through the cornea, the anterior chamber, the pupil, the posterior chamber, the lens, the vitreous humor, and is projected onto the retina.

Vision


Vision


1. Refraction of light

by cornea, aqueous humor, lens & vitreous humor

light rays must fall upon the retina

2. Accommodation of the lens

changing shape of lens so that light is focused

3. Constriction of the pupil

less light enters the eye/ pupil

Major Processes of Image Formation


Light refracts (bends) when it passes through a transparent substance with one density into a second transparent substance with a different density. This bending occurs at the junction of the two substances.

Refraction of light


Images focused on the retina are inverted and right-to-left reversed due to refraction. The brain corrects the image.

The lens must accommodate to properly focus the object.

The image is projected onto the central fovea, the site where vision is the sharpest.

Vision


Vision


The normal (emmetropic) eye will refract light correctly and focus a clear image on the retina.

Vision


In cases of myopia (nearsightedness) the eyeball is longer than it should be and the image converges (narrows down to a sharp focal point) in front of the retina. These people see close objects sharply, but perceive distant objects as blurry.

A concave lens is used to correct the vision.

Vision


Vision


In cases of hyperopia (farsightedness) also known as hypermetropia, the eyeball is shorter than it should be and the image converges behind the retina. These individuals can see distant objects clearly, but have difficulty with close objects.

A convex lens is used to correct this abnormality.

Vision


Vision


Astigmatism is a condition where either the cornea or the lens (or both) has an irregular curve. This causes blurred or distorted vision.

Vision


Rods and cones, the photoreceptors in the retina that convert light energy into neural impulses, were named for the appearance of their outer segments.

Vision


Rods and cones contain photopigments necessary for the absorption of light that will initiate the events that lead to production of a receptor potential.

Photopigments = opsin (protein) + retinal /vitamin A derivative

Rods contain only rhodopsin.

Cones contain three different photopigments, one for each of the three types of cones (red, green, blue).

Photopigments respond to light in a cyclical process.

Vision


Color blindness

inability to distinguish between certain colors

absence of certain cone photopigments

red-green color blind person can not tell red from green

Night blindness (nyctalopia)

difficulty seeing in low light

inability to make normal amount of rhodopsin

possibly due to deficiency of vitamin A

Color Blindness & Night Blindness


Vision


• Isomerization: light cause cis-retinal to

straighten & become trans-retinal shape

• Bleaching: enzymes separate the

trans-retinal from the opsin,

• colorless final products

• Regeneration: in darkness, an

enzyme converts trans-retinal back to

cis-retinal (resynthesis of a photopigment)

Physiology Of Vision


• Pigment epithelium near the photoreceptors contains large amounts of vitamin A and helps the regeneration process

• After complete bleaching, it takes 5 minutes to regenerate 1/2 of the rhodopsin but only 90 seconds to regenerate the cone photopigments

• Full regeneration of bleached rhodopsin takes 30 to 40 minutes

• Rods contribute little to daylight vision, since they are bleached as fast as they regenerate.

Regeneration of Photopigments


Light adaptation occurs when an individual moves

from dark surroundings to light ones. It occurs in

seconds.

Dark adaptation takes place when one moves from a

lighted area into a dark one. This takes minutes to

complete.

Part of this difference is related to the rates of

bleaching and regeneration of photopigments in rods

and cones.

Light causes rod photoreceptors to decrease their

release of the inhibitory neurotransmitter glutamate.

Vision


Vision


In darkness, rod photoreceptors release the inhibitory neurotransmitter glutamate. This inhibits bipolar cells from transmitting signals to ganglion cells which provide output from the retina to the brain.

Vision


In darkness: Na+ channels are held open and photoreceptor is always partially depolarized (-30mV); continuous release of inhibitory neurotransmitter onto bipolar cells

In light: enzymes cause the closing of Na+ channels producing a hyperpolarized receptor potential (-70mV); release of inhibitory neurotransmitter is stopped; bipolar cells become excited and a nerve impulse will travel towards the brain

Physiology of Vision


Retinal Processing of Visual Information

Convergence one cone cell synapses onto one bipolar cell produces best visual acuity

600 rod cells synapse on single bipolar cell increasing light sensitivity although slightly blurry image results

126 million photoreceptors converge on 1 million ganglion cells


The neural pathway for vision begins when the rods and cones convert light energy into neural signals that are directed to the optic (II) nerves. The pathway is:

The optic chiasm

The optic tract

The lateral geniculate nucleus of the thalamus

Optic radiations allow the information to arrive at the primary visual areas of the occipital lobes for perception.

Vision


Vision


The anterior location of our eyes leads to visual field overlap. This gives us binocular vision.

Vision


The two visual fields of each eye are nasal (medial) and temporal (lateral).

Visual information from the right half of each visual field travels to the left side of the brain.

Visual information from the left half of each visual field travels to the right side of the brain.

Vision


Vision


Visual information in optic nerve travels to

occipital lobe for vision

Midbrain for controlling pupil size & coordination of head and eye movements

Hypothalamus to establish sleep patterns based upon circadian rhythms of light and darkness

Processing of Image Data in the Brain

ESye and Vision Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.

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Transcript of ESye and Vision Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.