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For more information, please see full course syllabus of Anatomy & Physiology
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Lecture Comments (8)

2 answers

Last reply by: Justin Huang
Fri Jul 14, 2017 3:04 PM

Post by Justin Huang on July 10, 2017

I can't pause or view different parts of the lessons.

1 answer

Last reply by: Bryan Cardella
Mon Mar 23, 2015 4:11 PM

Post by Kayla Steiner on March 22, 2015

How does the nerve signal propagate as far as action potentials, as in depolarization/hyperpolarization? Is it similar to CNS and PNS?

1 answer

Last reply by: Bryan Cardella
Sun Mar 23, 2014 11:59 PM

Post by Jose Guerrero on March 23, 2014

Unable to view the lesson through. videos stops at 7min

0 answers

Post by ido montia on February 28, 2014

gr8 lecture thanks :)


  • The eye has numerous accessory structures that help it work and be protected: eyebrows, eyelids, eyelashes, skeletal eye muscles, conjunctiva, lacrimal glands, and orbital fat
  • The outer (fibrous) tunic of the eye includes the sclera and cornea (with the aqueous humor just posterior to the cornea)
  • The middle (vascular) tunic of the eye includes the choroid, iris, pupil, lens, ciliary bodies and suspensory ligaments
  • The inner (neural) tunic of the eye includes the retina (with photoreceptors), the vitreous humor, and the optic nerve
  • The blind spot is the part of the retina on top of the optic nerve exit that has no photoreceptors
  • Accommodation (the act of focusing) occurs because of actions of the lens, it gets more concave for focusing far away and more convex for focusing up close
  • Problems with accommodation include various degrees of myopia and hyperopia
  • Photoreceptors (rods and cones) contain bipolar cells, an inner segment, outer segment, and pigmented epithelium
  • Stereoscopic vision allows our visual fields to overlap, giving us depth perception
  • The optic chiasma, optic tracts, and occipital lobes are all involved in the optic pathways from the eyes to the brain
  • Vision conditions/problems include cataracts, glaucoma, astigmatism, and retinitis pigmentosa
  • Did you know…
    • Q: I’ve heard that some people can “see” sounds. How does that happen?
    • A: It’s called synesthesia, a disorder in which parts of the brain that interpret stimuli from the 5 senses are intimately connected within neuronal networks rather than segregated to different areas. Studies suggest that it is natural in infants for these parts of the brain to be wired together (perhaps hearing their mother’s voice may bring visions of glowing sunshine!) but as we age, the parts of the brain get separated as certain neurons in the brain die off. People with synesthesia may go to a dance club and see colors associated with various noises…maybe a black square appears every time they hear a deep bass drum beat. The precise sensations in these individuals vary from person to person. One person may hear the word “Friday” and see purple and another might see orange with the same word.


Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Accessory Structures of the Eye 0:04
    • Eyebrows
    • Eyelids
    • Eyelashes
    • Skeletal Muscles
    • Conjunctiva
    • Lacrimal Glands
    • Orbital Fat
  • Outer (Fibrous) Tunic 7:24
    • Sclera
    • Cornea
  • Middle (Vascular) Tunic 10:27
    • Choroid
    • Iris
    • Pupil
    • Lens
    • Ciliary Bodies
    • Suspensory Ligaments
    • Vitreous Humor
  • Inner (Neural)Tunic 19:31
    • Retina
    • Photoreceptors
    • Macula
    • Optic Disc
  • Blind Spot Demonstration 23:34
  • Lens Function 25:28
    • Concave
    • Convex
    • Clear Image
  • Accommodation Problems 28:31
    • Emmetropia
    • Myopia
    • Hyperopia
  • Photoreceptor Structure 34:15
    • Rods
    • Cones
    • Bipolar Cells
    • Inner Segment
    • Outer Segment
    • Pigment Epithelium
  • Visual Pathways to the Occipital Lobe 41:58
    • Stereoscopic Vision
    • Optic Nerves
    • Optic Chiasm
    • Optic Tract
    • Occipital Lobe
  • Vision Disorders / Conditions 48:03
    • Myopia / Hyperopia
    • Cataracts
    • Glaucoma
    • Astigmatism
    • Color Blindness
    • Night Blindness
    • Scotomas
    • Retinitis Pigmentosa
    • Detached Retina

Transcription: Vision

Hi and welcome to

This is the lesson on vision.0002

When we talk about the eyes themselves, we look at the accessory structures around the eye that have to do with the eye and protecting them in order to function properly.0003

First is the eyebrows, this is one of the areas that we have retained a lot of thick terminal hairs from our mammalian ancestors.0015

They are concentrated on the top of the head.0023

If you wonder why do we have a lot of hairs here?0027

Part of the reason is just expression, being able to see someone’s eyebrow movements0031

with respect to their facial expressions is important for the socialization that humans have.0038

Beyond that in terms of protecting the eye, when you sweat a lot above the brow,0044

the sweat will tend to get caught in these hairs and prevent a lot of it from coming into your eyes.0051

If you sweat a lot you are not going to be a lot to caught all of it but that is a nice function of eyebrows.0057

The eyelids they are made up of skin, muscles, and a little tiny bit of fat.0063

You can think of those as windshield wipers.0072

Every time you blink you get a little bit of fluid over the surface of the eye helping to clean the eye, lubricate, and protect the eye.0076

You have that automatic reaction where you blink when some thing comes to your face really quickly.0084

That is important in terms of protecting the surface of the eye.0090

The eyelashes also protect the eye.0093

They prevent a lot of dirt, micro organisms, little bugs, that we did not notice from coming into the eye itself.0096

Eyelash length is mainly genetics unless you are using some product to accentuate them.0103

An interesting thing about lashes are the tarsal glands.0111

Tarsal glands are modified kind of sebaceous glands.0114

Sebaceous glands are associated with hairs in general but the modification of the tarsal gland makes it so0118

that there is even more of the lipid fatty secretion that comes out of the eyelashes and not only protects them but it also make them not clamp together.0124

It helps keep them separated.0137

Those tarsal glands is associated with each little eyelashes you have.0139

Sometimes they can get infected.0143

Sometimes a little bit of bacteria and dirt gets into them and they can get inflamed and as a result you can get a sty.0145

Have you heard someone say they have a sty in there eye, it is because of the tarsal glands getting inflamed.0153

Skeletal muscles.0159

You have 6 extrinsic muscles attached to the outside of the eye.0162

On the next slide, you are going to see those better.0165

You cannot see it when looking at someone’s face but behind the eyelids, attached to the white part of the eye you have all of these skeletal muscles.0169

They allow you to move your eyes all over the place.0177

The conjunctiva is a layer that is around the surface of the eye and also curves up on the under side of the eyelids.0181

It is made up of a lot of epithelium that is meant to protect the eye.0190

It is another form of protection.0194

When it is healthy, when it is not infected, you do not notice it.0196

But when you get conjunctivitis, the infection of the conjunctiva, also called the pink eye, it is very noticeable.0201

Because of that infection you will get dilation of the blood vessels and swelling of that area.0208

It could be very irritating and very annoying.0213

That happens because of irritation of that layer of that tissue.0216

It could be bacteria or virus that is making it happen.0222

Generally it is something g that is going to last several days.0225

The lacrimal glands, if you remember from the skeletal lessons there is a lacrimal bone and it comes from a Latin word lagrima.0230

The lacrimal glands they are the tear producers.0242

The lacrimal gland itself is about a size and shape of the almond.0246

They are located in this region of the eye.0250

You are producing about a milliliter of tears per day.0255

You can go beyond that if there is a lot of crying going on.0260

Even when you go all day without shedding a tear, you still produce tear that is not exiting the eye.0262

Every time you blink you get a little bit of fluid going over the eyeball.0270

The nasal lacrimal duct is what leads those tears out of this area and through the lacrimal bone and part of the maxillary bone into the nasal conchae.0276

You have this thing that is called a lacrimal lake with each eye.0290

It is a space for excess tears to hang out in.0294

If that lacrimal lake gets over flowed with tears they are going to come out of the eye.0300

The active crying you are producing so much of those tears that lake over flows.0306

Your nasal lacrimal duct is not able to drain all of it out at that moment so they are going to come out in the eye down to the cheek, etc.0313

The duct since it takes tears down to the nasal conchae area that is why0321

when you are crying a lot you are going to get the sniffles at the same because you have a lot of tears going into that region.0328

Beside all of this you have orbital fat.0340

The orbit of the eye is formed by several bones that make that classic looking eye socket.0343

Cushioning the eye all around the exterior attached to the white part known as the sclera you have a lot of fat.0350

It is meant to cushion.0357

It makes sense you want that cushioning around the eye because you have hard bones that is right next to the eye.0359

There a little fat, nice cushioning.0367

We are going to talk about the eye in terms of the 3 tunics.0371

We are going to start with the outer tunic, this outer layer into the middle, to the inner.0375

The outer fibrous tunic is incredibly fibrous.0381

It is made up of a lot of connective fibers mainly collagen and elastin.0386

The white part of the eye is called the sclera.0390

It varies in terms of the thickness and hardness depending on what part you are at.0393

If you go to the back part of the eye and this is the superior view of the right eye, in the back where to the optic nerve it is a little bit harder.0399

It gets a little less hard when you get to the front or anterior part of the eye.0411

In general, the sclera is tough.0416

There is a lot of science fiction movies or tales where someone steeping on an eyeball and squishing it.0419

It is not that soft.0429

With enough force in the eye you are going to collapse it but it is very tough.0431

If you ever dissected a cow or sheep eye you will know that.0436

The sclera is very tough.0440

You have attachment for all the muscles.0442

You can see al of those extrinsic muscles down here.0445

Some of the others you cannot see because they are on the inferior side.0448

Beside this part of the outer tunic which is majority is the sclera, you have the cornea.0451

The most anterior frontal portion, that part that you put contact lens on, that is the cornea.0458

It is made up of some squamous epithelium about 5-7 layers thick and it is transparent.0465

It is supposed to be transparent.0472

You do not want to disrupt the rays of light that is coming through there.0473

That is the first layer, that is the window of the eye, which light passes through,0478

you do not want there to be anything getting in the way of light because it changes the perception of what is outside of your head.0483

It is hard to detach.0494

It is very tough.0496

Just posterior to it is a little cavity called the aqueous humor.0497

A lot of textbook call that the anterior cavity, because it is the most frontal space just behind the cornea.0502

There is a fluid in there and that should be clear.0510

If you look very close at someone’s eye, directly on the side of it, you can see that little aqueous humor just behind the cornea.0513

You produce fluid there and it is supposed to drain through the canal of schlem.0522

It drains excess fluid out of the aqueous humor.0531

If that gets back up and there is too much pressure in can lead to glaucoma.0539

The interesting thing about the cornea and aqueous humor is the cornea is one of those few places in the body where there are no blood vessels directly adjacent to it.0544

How do the cells of cornea get nourished?0559

It is through the aqueous humor.0562

You have little bits of gases and nutrients coming into that fluid cavity, since it is directly adjacent to it that is what nourishing the cornea.0564

The cornea can get scratch or damaged, with enough impact it can be completely destroyed.0572

One of the most common transplants done to human beings is corneal transplant.0579

You can get corneas from cadavers after death.0584

The majority of organs you need to take out of the donor while the cells are still getting oxygen and nourishment.0590

One of those tissues that you can take several hours after death is the corneas.0602

Corneal transplants typically you get them from cadavers.0607

When we go deeper, you have a lot of going on in the middle tunic also called vascular tunic.0611

Vascular means having blood transport and getting blood into an area.0619

There is a lot of circulation coming in of the eye and it is thanks to the middle tunic.0624

The choroid I am going to color this in red.0628

It is red in a living eye.0631

If you dissect a cow or sheep eye the choroid is black because once you do have all the oxygen and living blood cells going into there, it is no longer red.0634

It is very thin but this layer you can see it is right in between the sclera and the retina which is a part of that inner tunic.0648

You can see how it connects to this part called the ciliary bodies.0665

I will cover those momentarily.0671

The choroid is mainly for blood flow and it has a lot of pigment and melanin.0672

The purpose of that is when light enters the eye, you do not want to stick around.0680

You want light to enter the eye, be noticed by the retina on the back and not be bouncing around.0685

You do not want to be seeing something after a second or two after seeing it0692

You want to get new light coming in and going away.0697

One of the ways you can absorb a light and get rid of it once it has been first absorb by the retina is by having a very dark anterior that is going to absorb.0700

Having that pigment added to this blood layer is able to absorb light as it comes to the eye.0712

Like I have said, it is providing nourishment to the parts of the eye that need it.0720

Here is the iris.0725

The iris is made of smooth muscles.0727

Actually two layers of smooth muscles.0729

They are called smooth muscles not skeletal muscles because whether or not attached to your skeleton and you do not have conscious control over them.0732

They change based on how the light is outside of your face.0738

If there is lot of light, you do not need much of the light come of the eye.0742

You are going to have the muscles get more relaxed and not be constricted and that will make the iris bigger and the pupil smaller.0748

The pupil is just a hole.0759

I am not going to draw because the pupil is not something that contains matter.0762

It is not something that contains substance.0769

It is just a hole inside of the iris.0771

If you are wondering why the iris looks like this, it is not round, this is a cross section.0774

It is a transverse horizontal cross section through the left eye.0779

The reason I know that is because of the angle of the optic nerve which we are going to hear more about later.0783

The iris is made up of smooth muscles.0788

When there are not enough light, you want to get the iris expanded to make the pupil bigger.0790

You want to make the hole bigger so the little bit of light that is available, you want as much of it coming in.0799

The pupil size is connected to the autonomic branch of the nervous system.0804

The sympathetic and parasympathetic side which you can review in some of those nervous system lessons.0810

The iris let us make it blue.0818

I have blue eyes.0822

Smooth muscle and unconscious control.0827

The pupil, that black dot is just a hole and the reason why it is black because the inside of the eye is dark.0831

The lens I will color it yellow.0839

The lens should not be yellow but it should be clear.0841

The lens is very tough.0845

The lens is meant to last a lifetime.0847

It is tough made up of crystalline fibers and if you cut it apart it looks like an onion like it has different layers of crystalline protein.0851

It is meant to be clear because like I said when light is coming through the cornea which is clear0863

to the aqueous humor which is clear and to the lens which is clear, that is good.0869

You do not want the light to be influenced or anything added to it as it is coming in.0874

You want a clear lens.0878

If your lens gets a build of calcified compounds and other molecules, it can make it cloudy.0880

We are going to look at a picture of it later.0890

Cataracts is a build up of calcified products inside the lens and it makes your vision cloudy because you have this stuff in the way of the light coming into the eye.0893

Something you can do to get rid off that is cataract surgery.0902

They take the lens out and put a new lens back in.0906

The lens is what changes in size to focus near or far.0911

Your ability to look at something very far away or very close up, that has something to do with the lens.0916

The ciliary bodies which have ciliary muscles in them have to do with changing the lens shape.0922

Attached to these ciliary bodies you have these suspensory ligaments.0931

Let us do that in green.0940

The suspensory ligaments and those are all along the exterior part of the lens as well.0943

If those suspensory ligaments pull on the lens it makes it flutter and make you see farther away.0951

When the suspensory ligaments let go on the lens it makes it more bulging and that allows you to read something close to your face.0959

Those are the basic of the middle or vascular tunics.0968

Next up is the neural tunic, the most inner part of it.0972

It is neural because it has a lot to do with sensing the light that is coming in and getting those signals to the brain.0980

The most important part of this is the retina.0988

The retina is what allows you to see.0992

All those other parts are important but those parts can be working perfect but if the retina is not working,0999

if the retina falls off the back of our eye it does not matter how well your occipital lobe is working,1011

the lens, cornea, etc, the retina is the drape curtaining the back of the eye that has the photoreceptors.1017

Those cells that respond to those different wave lengths coming in.1024

Photoreceptors are the next point.1029

These are specialized cells all along the retina.1032

You have about 130 million photoreceptors per eye way more than rods and cones.1037

We will discuss the difference between rods and cones a little bit.1042

Those 2 kinds of photoreceptors are scattered all along the back of the eye.1045

Depending on what part of the retina we are looking at the amount of rods and cones may vary.1049

The macula lutea is an area right back here.1054

In the very center of this region is a point call the phobia centralis.1066

The phobia is where you want light rays coming into the eye to be perfectly projected.1074

You want light rays to be converging to that point.1084

If light rays are not coming of that point you are not going to see clearly.1087

The reason why I am wearing glasses is because when I do not have glasses on the light rays from looking towards you, they do not come to that point perfectly.1090

They come to a point converged in front of it.1101

When I put this lens in front of my eye it changes the defraction.1104

It changes the angle where the light rays come in and make it that they come right there.1108

I like being able to see clearly.1112

The phobia centralis is that part of the eye that you want light rays to be focused on.1115

We will talk more about vision, nearsightedness versus farsightedness in a bit.1120

The macula or the phobia is right there.1124

The optic disc is a part of the retina that is right on top of the optic nerve.1129

Notice that the optic nerve is slightly off to the side.1138

If we are looking down on the left eye, the right eye would actually have an optic nerve coming more medial as well.1141

We will look at this in a little bit how the optic nerves come together and this criss cross going on.1153

With each of the eye there is an area where there are no photo receptors.1160

It is a blind spot.1165

You do not notice the blind spot of the eyes when both eyes are open because1167

if I consider light rays bouncing of the object over there coming into my eye where my optic disc would be on my right eye.1171

The reason why I do not notice the blind spot is because my left eye is able to perceive what is there.1182

My left eye allow to cover up the blind spot of the right eye.1195

When you close one eye or the other eye, you are able to notice that blind spot and we are going to a test in a little bit1200

so that you can see that certain things may disappear when they are hitting that spot of the retina.1208

There are no photoreceptors on that part.1215

Photoreceptors are all along on this green part of the retina.1218

The vitreous humor in this case it is called vitreous gel.1223

It is a gel like sac that is taking up all the space that is just deep to the retina and just deep to the lens.1226

It is supposed to be clear as well because it is in the pathways of light rays.1235

You want it to be clear but it is a gel that is meant to give the eye some integrity.1239

To give it the right amount of pressure so it will not collapse and it is a nice medium for light rays to go through on the way to the retina.1244

If there is too much pressure in there it can result to glaucoma.1254

If you ever wonder what are those little floaters that you see?1257

You will be looking at something and there is a little white dot that is drifting along and you blink and it is still there.1262

What is that thing?1269

Floaters are little bit of cells that come of from parts of inner tunic and end up floating along the vitreous humor.1270

Eventually they go away and you will have white blood cells that eat up stuff that do not belong there.1282

It is funny, those floaters.1290

The optic nerve you have one in each eye.1294

All those different neurons associated with the retina and photoreceptors come together, they converge in the optic nerve1297

and come back at the back of the eye, move medially and end up going to the thalamus and to the occipital lobes at the back of the brain.1305

Here is that blind spot demonstration.1314

Here is how you do it.1318

You can switch, you do not have to close your left eye.1320

You can close your right eye.1322

If you close your left eye and stare at the star with your right eye, make sure that the star is in the center of your vision.1323

You do not want to have your right eye staring at the black dot because that black dot is what is going to be disappearing.1332

Make sure that your right eye is in front of that star and start with your head close to the screen.1339

With that left eye closed, gradually move your head back keeping you r right eye looking at the star.1345

Gradually and slowly moving your head back and at a certain point that black dot will vanish.1352

The reason why some people it does not work for them right away is because they are moving their head too fast they do not notice its disappearance.1358

Once you move past the point, that black dot, the light rays from it are not hitting that blind spot anymore.1368

Try this a few times if it does not work but eventually it will work for you.1378

If after trying this 10 times and it does not work there are a lot of optical illusion books out there1383

and there are other things you can look up online but it is fascinating.1390

If the light rays bouncing of this black disk hits your optic disk that is the blind spot.1393

Having both eyes open this demonstration will not work like I have explained before.1400

The lens function, the lens change shapes so that you can focus near versus far and that is called accommodation.1405

Some people’s accommodations are just not enough to cut it.1414

My lens is as I got older my lens is not cutting it.1418

My lens is not doing enough to focus light rays at the back of the eye and my doctor will say that my eyes are too long.1423

I will something in a little bit about that.1435

The reason why they say that my eye is too long is because no matter how much my lens get flat to look far away, it cannot get those light rays back to the retina.1439

If my eye is a little bit shorter then I will have a clear vision.1450

It is the opposite with being far sighted.1454

Those people’s eyes are in a sense too short for what the lens is doing.1456

As the lens get more concave meaning the opposite of bulging.1462

A biconcave lens will look like this.1466

That is what concave is.1474

Your lens cannot flip itself like that.1478

What this means is if this is convex getting more bulging, the more flat it gets closer to this inverse lens you can call that getting concave.1480

Getting concave means more flat and that what happens to focus on images far away.1490

The lens getting more convex as in this particular image right here is for images close to your face.1499

For an image to appear clearly on a viewer the image must be precisely projected on the phobia which is right there and there.1506

Accommodation problems this is why people need glasses and contact lenses.1515

Emmetropia means proper vision.1522

If you have 20/20 vision you do not have accommodation problem.1526

If you ever wondered about the 20/20 thing and why some people are 20/15 and 20/10.1531

Do not think 20/20 vision is perfect, that is the normal vision.1536

That is what you wanted to have as expected.1540

Here is how it works.1543

This is the way to explain it.1545

20/20 vision means that somebody with normal vision from 20ft away can read all the words properly1546

and the person next to them also from 20ft away can read the same image perfect.1556

20/15 means a person with normal vision has to be 15ft away to see that image but the person with 20/15 vision they can still be 20ft away and see it great.1563

20/10 is even more exaggerated that person with normal vision they have to be 10ft away from those words to read it.1582

That other person with 20/10 vision they can be 20ft away.1591

You could think that 20/15 or 20/10 is better than perfect.1595

The term perfect vision when we look at other species, there are predatory birds that have vision better than any human on Earth.1600

The term perfect vision is relative.1612

The opposite is true with a relative of mine.1615

I have a relative with a 20/100 vision.1619

A person with normal vision could read a sign from 200ft away but she must be 20ft away from it to read it.1624

After getting lasix surgery, that was corrected.1633

The lasix surgery involved lasering off parts of the cornea making it more flat to assist the lens with the lens incapability to get even more flatter.1637

The lasix made is so that now she has close to 20/20 vision but the problem with that is now she needs reading glasses.1649

When you make the cornea a little bit flatter to assist with seeing in a distance, that ends up being a problem when you are trying to read something close to your face.1659

At least you have that farsightedness1667

Back to this, myopia.1669

This demonstration here shows how to fix myopia and that is being nearsighted.1673

Like I have said before, that means that my lens cannot get flat enough to look at something far away.1679

As a result, the images are converging right here in front of the phobia.1696

When you put a lens like this in front of your face, a concave lens makes it so that you are assisting the lens with making those light rays hit the phobia.1704

You could imagine how a lens inside of the eye glasses could be shaped like this.1722

I have often heard the question what about the contact lenses?1729

When you put a lens on the surface of your cornea, it is not shaped like that.1732

You cannot even close your eyelid over that, it would hurt.1737

That is true.1740

The contact lens is shaped like this on the side but the way they get the lens to behave like this1741

is by playing with the densities within the material inside the contact lens.1748

If you change the densities with respect to the middle part versus the outer part, that is going to give it this property.1752

The opposite is true with hyperopia, it means you are farsighted.1761

You see stuff in a distance quite well but you have problems with up close.1766

That is not common as myopia.1771

Hyperopia is the exact opposite.1773

Imagine having the inability to make your lens bulging enough to focus in the light rays that is coming from here.1776

That makes it so that when you are hyperopic, the light rays want to go pass the retina which is not possible.1784

They are not going to be able to go pass the retina.1793

When you put a lens in front of somebody who is hyperopic, the lens happens to be convex then you are making it so that it hits right here.1795

The intensity of how convex or how concave it is has to do with how bad your vision is.1806

Somebody who is more myopic than me, like the relative I told you about, maybe that stuff is converging even closer to the lens.1813

When we look at the retina, all these photoreceptors that sense the light and react to it and send the signal to the brain,1822

we have 2 main varieties the rods and cones.1834

The rods and cones have similar structures but very different roles.1837

Here is how I remember it.1842

Rods have to do with light and dark.1843

Cones is colored.1846

Imagine color for the C, cones with C.1848

Rods are just the ability to see light versus dark in general.1852

The rods are not very good at distinguishing a wide range of colors.1857

When the light is dim it is hard to distinguish color than it is light out.1864

Looking at an object in a dark, you can say that things are there but in terms of red versus orange it is very hard to distinguish.1873

Cones is meant distinguish ROYGBV, the color of the rainbow.1884

There are 3 main kinds of cones.1894

It is similar to old projection TV, if you look at those TV that have those 3 different colors, it have red, blue, green light.1899

There are red cones, blue cones, and green cones.1911

The majority are red.1915

All 3 are there and depending on what combinations of those are stimulated that gives you a sense of all the different colors of the rainbow.1920

When we look at the retina, there are no rods on the phobia section.1931

It is just a bunch of cones concentrated there.1938

That is why when we look at something straight on, when we focus on something, it easy to see the detail, the rich color there.1941

Cones are enabling you to see that clear, bright image.1952

On the periphery where you have more rods, you can see things are there but whenever we want to observe shape, texture, and color, we are looking straight on.1962

When we look at the different rods and cones, this is a rod.1973

The main difference in structurally between a rod and cone is this.1979

If we look at this part here, you can see it looks like a rod, like a pole.1983

The outer segment of the cone is shaped like this.1991

You still would have these little lines and disk which I will talk about momentarily.1996

Imagine having another one of this right next door but instead of having the other segment long, it is triangular.2001

Structurally other than that they are very similar.2008

Bipolar cell is a kind of neuron that looks like this.2011

Here are dendrites, axon cell body, axon, and axon terminal.2015

Receiving in and sending out.2025

Bipolar cells are right next to the rods and cones.2027

If you think about the retina as having different layers of cells, the light first hits some other cells before entering the rods and cones portions.2033

The photons stimulate bipolar cells which then send the signal to the rods and cones.2044

Depending on how the photons hit the rods and cones that is how you see different colors in different images.2050

The inner segment is a little bit superficial it is like we are getting farther back in the retina and closer to the middle and outer tunic.2058

You got a lot of mitochondrion here.2071

The area that is doing most of the work in terms of giving you sight is the outer segment.2073

The outer segments are all these little disks just stacked on top of one another.2081

There disks are made up of different parts.2089

We tend to concentrate on the rodopcin.2092

The rodopcin is made up of 2 parts, opcin is a protein and there is pigment called retinal.2096

Retinal is made from vitamin A.2104

That is why they say eat your carrots.2108

Carrots compared to other fruits and vegetables have a lot of vitamin A.2111

It is not necessarily true that your vision is bad, eat your carrots so that you do not need glasses.2116

No, eating carrots allow you to maintain your eye health.2124

If you get a deficiency in vitamin A it can lead to problems over time.2128

Without vitamin A you are not going to make the pigment retinal.2133

Opcin is a protein depending on the structure of opcin that determines how it is responding to ROYGBV.2138

The red versus violet versus green, etc.2146

you are going to see different kinds of opcin within rodopcin in the cones and rods.2149

The pigment epithelium is just outside of that.2156

You are getting a little bit closer to those optic tracts that are going to be taking the signal to the optic nerve.2162

The pigment epithelium it have pigment, it has melanin in those regions.2169

There is an ability in those cells to eat up the older disks.2174

You can make new disks within the outer segments throughout your whole life.2181

You can make this new disk but if you damage the cell as a whole, if you destroy the cell it is going bye bye.2192

If you stare at the sun, that is way too much light rays.2201

That is way too much energy coming in to the eye and it is going to fry these cells.2208

Staring at the sun can definitely make you blind.2213

I have read some research suggesting that in the future we will be able to regenerate retina2217

but as it stands now, once you lose your retina, you are not getting one back.2224

In the future, we will be able to make retinas from stem cells.2230

When we look at the visual pathways to the occipital lobe, it is important to realize that we have stereoscopic vision.2237

It is like having vision in stereo.2243

If this is the visual field of my right eye and this is the visual field of my left eye, they overlap.2245

There are a few reasons why it is important.2252

If I close my left eye and I look around with my right eye, I can see that my nose is blocking a lot of stuff over there.2254

The orbit of my eye is doing a lot to block what is over on the left side here.2263

When both of my eyes are open, I do not notice my nose as much.2268

I can see it is still there but it is not as obvious to me.2273

The parts that were being blocked by my nose and part of the orbit of my eye when my right eye is open2277

and my left eye is also open I can see those parts clearly what is over here.2281

Along with that, having stereoscopic vision is good for depth perception.2287

Because the eyes on humans and primates are in the front, we can see around the sides of objects and we can tell how far something is in front of our face.2295

Unlike other animals that have eyes on the side, having eyes in the front as primates is important.2307

Because if you consider our ancestors that are all arboreal, they are in the trees, from million of years ago, swinging through the trees,2314

having depth perception is important so that you do not run into a tree and fall and get eaten by something.2320

It is important in terms of evolution and as being successful.2330

Even though now we are not in trees anymore, as with most apes in trees, having a depth perception has a lot of importance.2334

Just think about driving a car, without depth perception there will be a lot more accidents.2344

Stereoscopic vision, having vision in stereo, you can see that the visual fields do overlap.2350

Here is a little depiction of those visual fields overlapping.2358

When light rays come in, here is the cornea, light rays I will depict in yellow.2365

The light that is coming from the right side of your face into the right eye will end up hitting the left side of the retina.2372

It is going to end up hitting the medial side of the retina and vice versa.2383

The funny thing about that is, let us say we went back into the head where these neurons end up criss crossing.2389

We will get to those terms in a second.2401

If some reason there was trauma or infection that harmed these inner parts,2403

let us say that this got completely severed where these blue tracks overlap think about how this is laid out.2409

The blue parts art sensing light from the outer part, even though if this part of this chiasm, this little space in here,2417

if that got damaged in the middle it will cut out everything around the outside of your visual fields.2428

It will blacken and you cannot see anything here.2434

You will have a much thinner or less wide field of vision.2438

You could see that there is a criss crossing going on.2442

The red parts of the retina do not think that these correspond to cones.2446

They are depicting this part as red, you can see that the red part stick to the outside, the more lateral parts.2453

The optic nerves contain these little tracks.2460

You can see that this little black tube looking thing that is the nerve.2466

As I have mentioned before, the optic nerves come out from the posterior part of the sclera and they come in medially.2471

They come in towards the center of the head and there is a criss cross going on before it ends up at the occipital lobes which allows you to see.2481

Those were the optic nerves in green.2490

The optic chiasm, that is where you have the criss crossing of some of those optic tracks.2492

The optic tracks as you can see here, the red and blue lines, those are all of the tracks of neurons within the optic nerves that end up going back to the occipital lobe.2504

The occipital lobe is all of these here.2520

You can see that is the final destination for these tracks.2527

If you saw the nervous system, the one on the brain, there is the visual cortex that allows you just to be able to see and2533

there is the association part that allows you to make sense of what you are seeing.2544

This is the visual pathways that get from the eye to the occipital lobe.2549

When we look at vision disorders that affect the eye I already mentioned myopia and hyperopia.2554

Myopia is nearsightedness and hyperopia is farsightedness.2562

I am myopic I have this concave lens that is allowing me to see in the distance and it is vice versa with hyperopia.2568

Cataract is what this picture is depicting.2577

If you shine a light in this person’s eye you can see that through the pupil it is quite cloudy right here.2579

Just posterior to the pupil is the lens and it should be clear.2585

Build up of calcified compounds will make it cloudy.2591

Cataract surgeries are very common especially in older people.2595

You can have a cataract problem as a younger individual that would be not that common but an injury can cause cataracts to occur.2599

I have seen cataract in more older people.2611

As I have said before, they will remove the lens and put in a new one.2615

It is a very simple surgery these days.2621

Glaucoma is in increase in the intra ocular pressure inside of the eye where the vitreous humor is.2624

If you have too much pressure in there it is going to be pressing on a lot of the delicate parts of the eye that allows you to see.2636

It is not painful but it can make you blind.2643

One of the causes is if the aqueous humor in that anterior cavity gets too much of that fluid production2645

and not draining through the canal of Schlem that build up there can affect the pressure within the entire eye.2656

Glaucoma can result to blindness.2663

You have probably heard the one of the treatments for glaucoma is using cannabis or marijuana.2667

My good friend from college, he is an eye doctor now has told me before that the amount of2677

marijuana a person have to smoke to help with glaucoma is ridiculous.2683

People who has been given marijuana as a prescription for glaucoma, there is a lot of medication that2687

do much better job of treating that pressure within the eye and reducing the pressure than something like cannabis.2695

Speaking of which, glaucoma within the eye can also occur because of high blood pressure.2703

Somebody who has high blood pressure in general, they have a high blood because of genetic factors, poor diet, that can affect the blood pressure of the eye as well.2712

That is another source of glaucoma.2725

Astigmatism is something that affects your vision.2728

What supposed to happened is the cornea is supposed to be a nice perfect circle and the lens should be equal in terms of shape on the outside.2730

It should be much more of a baseball than a football.2746

If you imagine this not being a nice circle, if the cornea or lens is slightly off in terms of its circularness2749

that can cause astigmatism because the way that light is coming into the eye it is not going to have the refraction of light as it should be.2761

Some parts of light individual field the angle at which they come at the back of the eye is going to be a little bit off because of the misshaped.2769

Astigmatism can be corrected by having the lenses that you wear adjust for that.2778

Color blindness it is more common in males and if you have taken biology you might remember that2785

because of the sex chromosomes, that is one of the ways the color blindness affects male more.2792

Males have 1 x chromosome rather than 2 x chromosome that the female does.2798

That an affect how the genotypes are inherited and being a carrier versus expressing it.2802

Color blindness especially red and green is much more common in males.2808

The inability to see color at all is extremely rare.2813

That is in 1/1,000 of people.2820

It is very rare for your cones not having the ability to distinguish between all the colors of the spectrum.2822

If you heard that dogs see in black and white that is not true.2829

Dogs do not see quite as much colors as us but think of it quite more as sepia setting on a camera that gets more browns.2834

I am not saying that dogs see exactly like that but if dogs cannot see ROYGBV they still have the different kinds of cones that allow to see colors.2849

Maybe it is not quite to the point that humans can see it.2861

Speaking of which, like I have said before humans have 3 different cone varieties that allow to see ROYGBV.2863

There are some primates that have 4 cone varieties.2870

Some of those monkeys can see more color than human eye can which is amazing to think about.2874

We might be looking at 2 forms of purple and everyone agrees that those are the same purple but that monkey can actually distinguish between them.2880

Night blindness one of the causes of it is vitamin A.2889

Vitamin A is used to make retinal that is inside of the redopcin which is inside of the rods and cones.2897

It is recommended that you get vitamin A in your diet but somebody who does not get enough vitamin A in your diet,2904

you do have vitamin A reserves in your body but once those reserves run out you are going to notice that especially at night,2910

when there is less light coming into your eye it is going to be hard to see certain things especially when you are driving.2917

Night blindness one of the ways that you can actually cure it or fix it is get more vitamin A so that you can get those rodopcin molecules to what they should be.2924

Scotoma is a blind spot that is not normal.2936

If other than the one that is an optic disks, someone else is getting blind spot in their visual fields even if their both eyes are open.2941

That can be due to injury or due to lack of nourishment or genetic factors affecting a scotoma coming into play.2949

Retinitis pigmentosa has to do with a dying off those pigment disks inside of the redopcin2958

and that is something that could eventually make rods and cones not function at all.2971

This is something that could lead to blindness overtime.2980

A detached retina, this is something you do not want to happen.2984

A detached retina is usually from an injury.2987

Some kind of injury of the head makes the retina comes off at the back of the eye and that means you are not going to be able to see.2990

If that curtain against the back of the eye comes off, that is where your rods and cones are.2998

One of the ways that they can fix it if they do it soon enough after an accident is with a laser going though the cornea3005

and shutter on the sides of the retina at the back of the eye.3013

You can get it back on but the problem is when you laser on the side of the retina you are damaging those parts of the visual field.3018

That person is going to lose some of the light that is coming in terms of perceiving it but if I were me I rather have some vision than no vision at all.3027

Detached retinas in other cases, the retina comes off and it is not fixed in time.3038

It is a permanent thing that causes permanent blindness.3044

Like I have mentioned before, one day we can remake retina from stem cells.3048

If you ever dissected a cow eye or sheep eye, you will know this.3054

I suggest that you try.3058

The amazing thing is if you cut in the eye and look at the back where the retina is, it looks like wet tissue paper.3060

It looks very thin and fragile.3067

Hundred of years ago when they are first cutting into the eyes and looking at them and trying to figure out what is wet,3070

they thought that the retina was useless and just tossed it.3075

Because they look at the structure and thought that it was not important.3080

It was so tiny and fragile but the retina is the most important tissues in the entire eye.3084

Without it you cannot see.3091

Thank you for watching