Neuro: Vision

• An eyeball is about 1 inch in diameter
• About 5/6 of the eyeball is inside the orbit and therefore protected

Eyelashes and Eyebrows

• Eyelashes and eyebrows help protect from foreign objects, perspiration, and sunlight
• Sebaceous glands at the base of the eyelashes, and if they become clogged and inflamed it is called a sty
• The palpebral fissure is the gap between the eyelids

Muscles of the Eye

• Six muscles insert on the exterior surface of the eyeball and are responsible for eye movement
• These muscles are innervated by cranial nerves III, IV, or VI
• There are 4 rectus muscles: superior, inferior, lateral, and medial
• Two oblique muscles: inferior and superior

Lacrimal Apparatus

• Approximately 1 ml of tears are produced per day, which spreads over the eye by blinking
• Tears contain a bactericidal enzyme called lysozyme

Layers of the Eye

• The eye has three layers
• These layer are called the Fibrous Tunic (outer layer), Vascular Tunic (middle layer), and Nervous Tunic (inner layer)

Fibrous Tunic - Cornea

• The cornea is transparent and colorless
• Light is focused by the cornea
• Five layers of epithelial tissue, connective tissue, and endothelial tissue comprise it
• Cornea transplants are common and successful
• The cornea has no blood vessels but instead is nourished by tears and aqueous humor

Fibrous Tunic - Sclera

• The sclera is the "white" of the eye
• The "cooperative eye" hypothesis relates to the sclera
• Its dense irregular connective tissue layer of collagen and elastic fiber provides shape and support
• The sclera and cornea meet to form the scleral venous sinus
• It is posteriorly pierced by other entities

Vascular Tunic - Choroid

• The choroid contains pigmented epithelial cells (melanocytes) and blood vessels
• Nutrients are provided to the retina by the colored tunic Black pigment in melanocytes functions to absorb scattered light

Vascular Tunic - Ciliary Body

• The ciliary body houses the ciliary muscle
• The ciliary muscle, a smooth muscle, can alter the shape of the lens
• It also has ciliary processes

Vascular Tunic -Iris

• The iris is the colored portion of the eye
• Shape is similar to a flat donut, suspended between the cornea and lens
• It contains muscle fibers, pigment, CT, and epithelial tissue
• The iris has a hole in the center called the pupil
• It regulates the amount of light entering the eye
• It also has autonomic reflexes
• Circular muscle fibers contract in bright light to constrict the pupil

Pupil Control

• Constrictor pupillae (circular) are innervated by parasympathetic fibers
• Dilator pupillae (radial) are innervated by sympathetic fibers.
• Response varies depending on levels of light

Vascular Tunic - Lens

• The lens is avascular

• Crystallin proteins are arranged like layers in onion

• Its clear capsule is perfectly transparent

• Suspensory ligaments hold it in place

• Light is focused by the lens and is projected on the retina

• As you age the lenses lose elasticity

• Suspensory ligaments attach the lens to the ciliary process

• Ciliary muscle controls tension on the ligaments

Nervous Tunic - Retina

• The retina is the posterior 3/4 of eyeball
• Contains the optic disc, where the optic nerve exits back
• It has Central retina blood vessels to supply nourishment to the retina
• Also exit through the optic disc
• It can be visualized during inspection to assist in diagnosis
• Hypertension & diabetes can be observed from its features
• The retina itself can become detached, due to trauma, diabetes or AIDS

Photoreceptors

• Rods and cones differ in shapes of their outer segments and differ in function
• Rods allow us to discriminate between different shades of dark and light
• Allow us to see shapes and movement and is specialized for dim light
• Cones specialize in high visual acuity and are specialized for bright light
• Cones are concentrated in the central fovea, a small depression in the center of macula lutea

Macula Lutea and Central Fovea

• The macula lutea is the posterior portion of the retina, corresponding to the visual axis of the eye
• Due to the high concentration of cones, the central fovea is a location of sharpest vision
• Rods are absent from the fovea and macula but increase in density towards periphery

Retina Layers

• The pigmented epithelium is the nonvisual portion and absorbs stray light which helps keep image clear
• Three layers of neurons exist, photoreceptor, bipolar neuron, and ganglion neuron layer
• Horizontal and amacrine cells modify and manage signals in the eye

Photoreceptor Shapes

• Rods are rod shaped and work in shades of grey in dim light
• Rods consist of 120 million cells which detect general shapes and movement and are distributed across the periphery
• Cones are cone shaped and are responsible for sharp color vision
• Six million cones exist which create very detailed outputs at fovea of macula lutea. and have Very high density at exact visual axis of eye allowing for the sharpest resolution, or acuity

Visual Data Pathway

• Light enters the retina where rods and cones transduce it into action potentials
• Rods and cones excite bipolar cells
• Bipolar cells excite ganglion cells, which then create information that is sent to the optic nerve
• Axons of ganglion cells form optic nerve which leave at the optic disc
• That nerve travels to the thalamus (LGN)
• And a third-order neuron ends at the primary visual cortex

Intraocular Cavities

• Light has to be able to travel through the anterior and posterior chambers within the eye
• Anterior cavity (anterior to lens) is filled with aqueous humour, and this fluid produced by ciliary body, continually drained. The eye is replaced with this fluid every 90 minutes
• The interior (or anterior) cavity has two chambers inside, the anterior area, and the posterior chamber: between lens
  • The posterior cavity (posterior to lens) is filled with vitreous humour, (jellylike) and only formed once during embryonic life.

Intraocular Pressure and Glaucoma

• Intraocular pressure is important for vision, is created by aqueous humour
• It allows the eye to keep the retina smoothly applied to the choroid so the retina forms clear images
• Glaucoma causes intra-ocular pressure to spike and put pressure on the choroid
• The drainage of aqueous humor is blocked, this may cause degeneration of the retina and eventual blindness

Importance of Aqueous Humour

• Aqueous humor is produced by the ciliary body
• It travels from the posterior chamber and then towards the anterior chamber and is drained at the pupil
• Scleral venous sinus exists, and has a canal of Schlemm. Also there is Lymphatic-like drainage at the junction of cornea & sclera
• Aqueous humor drains from eye to bloodstream

Image Formation - Light Refraction

• Image creation depends on light being able to travel in a straight line to your retina.
• This is done by the cornea & lens and the rays have to hit the retina for you to process visual stimulation
• The changing in strength in accomodation that is required for the image to travel in a straight line comes from "Accommodation"
• It is the action of being able to change lens shape so that its able to be put in to focus"
• Finally , constriction of the pupil limits the amount of light that comes in to the eye, so the eye can see more clearly.

Refraction of Light

• Refraction is the bending of the light
• This often occurs when light moves through substances with different refractive indexes at a direction that is not perpendicular to the ray

Refractive Index

• Refractive index refers degree to which a subject can be bent or not
• FYI refractive index is the ratio of the velocity of light in air to velocity of light in substance
• The speed of light thru air is 300km/s, and Speed of light thru glass is 200km/s
• Index of glass example is , 115 which is also 300,000/200,000

Light & The Eye

• The eye needs four refractive surfaces
• This includes the conrnea, aqueous humour, lens, and light
• A diagram in the text shows an approximate index number for above structures for comparison

Cornea & Lens

• Altho cornea is the one that bends light the most, it cannot change shape
• Instead the lense is what allows focus by shifting forward and backward to keep objects in focus/
• In this shifting, it has refractive power from 2-34 diopters

Diopter

• A diopter is a measure of the refractive power of a lens
• This number is defined as the reciprocal of the focal length of the lens (in meters)
• The distance to the focal "point" is referred to as the focal length.

Focal Point

• A focal point occurs through the use of parallel light being converged thru a lense

Retina & Image Formation

• The image focused on the retina is inverted reversed from left to right.
• Your brain automatically solves this to allow normal interpretation of the image.
• ~75% of refracion is provided is provided by the conrea, and the rest by the lense
• Light has to be parallel enough to be visible on the retina, and to achieve this is called accomodation

Image Formation and Mechanisms

To create an image:

• Your lense has to have more refractive power (called "accomodation")
• Your eyes have to properly be positioned
• And you have change the pupil size accordingly

Accommodation Reflex

• Accommodation is an increase in convexity of a lens, initiated by ciliary muscle

• Accommodation is more difficult when your muscles work "harder" to move the lens

• Convex has 2 types, where there has to be convergince and then

• When you are viewing a "flat" image, it is going to require a more rigid straight path"

• Or is it closer, there may be different curves so you need to be able to flex to view those images

Cones Again

It goes back to the first point Ability to focus on objects near us goes back to above point at the beginning.

The 1) lense refractive power has to improve, so this is done by increasing cones In other words this is the step "accomodation

Muscular Contraction and Eye Sight

Flat cilliary muscles have more tenion in the ligiments, relaxed is flat stretched musles

Contrated cilliary muscles have " slack ligaments "

Eye Position

• Have a classmate look at your eyes as you slowly increase a pen towards your eyes

• There will likely be " shifting " in your eyes to move pen in more focus"

• It should be known what muscles and nerves control eye postions

Having two eyes in front allows you to make sense of reality easier

Pupil Size and Image Formation

• Conttrolling size has an image in eye, is done by nerve stimulation
• There has to smaller stimulation at lower light and larger stimulation at higher light

Constriction

Constrict the pupil.

1. focus

2. is the relationship of focus with the pupil and nerve connections

Smaller depth to more light

Emmtropia

• An eye condition for "normal" vision is emmotropia

• This is where distance are able to focused on retina when ciliary muscles are lax

• This is "the defintion or way that an eye" can focus parellel beams without accomodating the lens -

Hypopia

• Is is an eye disorder where light of nearsighted objects are not converged by time, the light beams focus, when the muscles are

Hyperopia

• With hypertopia people can't focus at short lengths. With hypertopia one needs to shift lens all the way in so light waves from short objects have be stretched beyond

These individuals are long distance dominant at vision, since the focal point in eye Is as far away as possible, meaning your are likely to squint, or stretch eye to be able see nearby objects.

Note what lense would fix this

More on Eye Sight

• One can opt for laser correction.This involves resurfacing of surface, called " the stroma" inside and around lense.
• The cornea is what what gets reshaped, which bends light and this helps the person see to
• See the last statement in how this assists

Pressbyopia

• Also called the decline of accommodation by aging
• As you age, the point of focus changes on the lens so its harder to focus on objects as they get far away
• This typically occurs because of the way your eye is functioning
• In particular, with the cilliary, it can’t stretch and bend like you need it to.
• Typically, it's only helpful to those with a specific distance and it stops working beyond.
• Lens cannot fully flatten, so you can't see objects that are close to you.
• For bifocals, there's glasses that upper segments to create this visual effect

There are also near int glasses This refers to distance you can be, for example, with text or mobile devices

• Typical Points Of Failure*
• Four inches in a young adult
• Eight inches in a 40 year old, this increases. To becoming 31 inches in 60/80 year olds.
• At this stage, reading classes may be needed. Due to this, glasses that are not convex are likely what will be suggested."

Astigmatism

• Is also often an irregular curvature of the corner of the lense
• This means light that travels in can't converge at many planes it diverges and is offset, so many corrective actions can occur

NearPoint Stress and What to do

• Is a condition where cilliary muscles become unable to completely relax with the lens This is common where individuals cannot focus on nearby objects without the visual point of divergence"

• In this instance, looking up from tasks would be neccessary. This includes the near to the visual point so that the eye does not completely fatigue"

OPEN ANGLED and CLOSED ANGLE

• OPEN ANGLED
• Inadequate drainage occurs despite the opening
• Due to the meshwork of the trabecum and
• IN this you must reduce stress and tension
• There is also Closed angle where cornea and iris converge in what is considered near-stress
• And thus, for this case, it requires laser therapy (which can assist with OPEN as well)

Emmotripic Eye

Emmetropic have "normal" eyes that can refract light from 20 feet away Myyopic, or near sightedness, is where the eye is long from front to back so the individual wears glasses to correct this Hyermetropic means that th eye is not enough at 55 and uses contvex "coke" to fix the vision Lastly, there is astiggamtism which means the corneal surface is bad out of focus

Visual Physiology

There are three main stages Stage 1 Light conversion due to photopigments - vision Stage 2 - Rhodopsin /light transducstion vision Stage 3 Light regeneration or how much light is around in the first place and

From retina to CNS Receptors transducer Light converts into chemicl Photopiugnment

Lastly the Oospin

LIGHT is NEEDED

There is the light pigment relationship- that relates back to ops Four opsins are used to create this effect

• Photoreceptor Function* A diagram illustrates function, rods, and cones in a matrix table. Night is in high light for night but low for cones The color cones the oppositt With convergenc its high in rods, loww with convergenc in cones

Rods, Cones, and Stimulation

Diagram is shared, including light with rods- low convergence"

• Star Appears To Go Away* Rods, cones, etc. explain why this happens.

Rod vs Cones Rod has to be sensitiive, but if not then you cant see right.

• See the "angular seperation from fovea"* At "0". Angular, that is only where cones are. Beyond that, the light becomes less sensitive to This means that cones only stimulate the small are you look at, but its less sensitiive this means you can see more, but lose some peripheral vision (the light that stimulates the peripheral vision)

Cones

Cones create the effect of "color vision. And 3 specific colors" S is blue, m , is green, and l, is orange/red

HOW SO MANY COLORS

This occurs due to varying degrees. YOu take three cones to get so many colours like we'd see usually." Blue cone has about "100 cone", green has " 197", where all create the effect (in this example" The effect is" 83% red, 83% green, - % blue for the example"

• Sky Explanation* Ligh passes through the atmosphere, there are wavelengths in nitro /oxi There are what "we see" and that is why sky blue light has to short wavelengths which are easier/scatters most Lastly the violet explanation is also short", in what way does this make sense or in what way does this stimulate??

Bleaching

These both "regulate or manage sensitiivtiy". To bleaching or too much , or too liittke regeneration"

The eye cannot maintain a sensitiivtiy relationship

Isomerization

The action of being able to straighten light Light breaks the enzyme Breaks from enzyme and in darkness they work in back For what enzyme can assist, that we dont’ go blind, so we require what vitamin" a and the epithelium

• *WHAT FORM IS THE BLINDNESS, or what causes these failures to occur.

To distunguish color, we see that men are more likely (due to red color, or some Blindness is due to "absence of photopigmens

Pigment

These can require pigment to assist those in need for better light A full cycle goes through regeneration

Vitamin is needed to heal - cycle time takes (30 to 40 minutes Also its the light that causes the bleaching so robs

IN Light, IN DARKNESS!

In darkness 1 - channel is always " depolarized' 2. Continuous neurotransminter is stopped

In light The opposite occurs

1. Enzymes cause the closing of channels due the the hypo 2 - hyperpolarized receptors means transmission stoppedd