In this episode, we discuss the basic eye anatomy of the globe. Topics include:
- tear film (3 layers)
- cornea (5 layers)
- anterior chamber
- lens (three layers)
- ciliary body
- retina (with layers)
- optic nerve
EPISODE 2: Eye Anatomy
Download the show: EB002-Anatomy.mp3
I could have gone into the external eye anatomy (lids, orbit, etc.) but thought that would be too much for one show. You can subscribe to this podcast using iTunes … just search for “Eye Basics” in the iTunes store, or go directly to the podcast page here.
Show Outline (transcript coming soon … comments below)
This is Eye Basics 101, Episode 2: Eye Anatomy
In a world without good eye education, a beacon of hope is born. That beacon is Eye Talk Radio.
Welcome to Eye Basics 101. This is an audio program dedicated to teaching you about the eye and ocular disease. No matter if you are a student, a practicing doctor … or just a curious person wanting to learn more about the eye… we can all learn through thoughtful discussion.
I’m Dr. Timothy Root … general ophthalmologist and cataract surgeon.
– You can find this show at EyeTalkRadio.com
– Questions or Comments about today’s discussion … visit the show notes at:
EyeBasics101.com … and look for episode 2
Eye Anatomy … specifically the anatomy of the eyebal itself. We’re not going to be talking about the orbital, bone, or eyelid anatomy, as I think that much anatomy would be overwhelming to hear all at once.
ANATOMY CAN BE CHALLENGING
– I struggled with this in medical school
– Language … just like Spanish, German, French … you need fundamental building blocks before you can get big picture
– without learning the relevance to real-life disease makes it hard to memorize underlying anatomy first
WHEN I LEARED THIS STUFF
I was in medical school in the late 90s …
– Learned most of my knowledge through a Netter Atlas and a borrowed skull model (I couldn’t afford my own) and gross anatomy
– internet wasn’t in existance yet …
… yahoo was the dominant search engine
… youtube wasn’t around yet
… everyone was viewing tiny streaming videos using the aweful realplayer codec
… weren’t any good anatomy sites around
ANATOMY PROFESSORS WERE TRYING
– anatomy professor … digital book on floppy disk. Not terrible but dense and unintuitive
– another one was experimenting with quicktime VR objects
– mid 90’s format for creating panoramas and spinning objects
– pelvis bone … one of the things that peeked my interest in new media education
– gross anatomy disgusting and unhelpful for many systems (like the eye)
– eyeballs were deflated, sunken in, gross
Good NEWS: THINGS ARE BETTER NOW!
– Technology has improved quite a bit
… Youtube videos
… 3D models … download it wirelessly to your ipad
– will cadavers really be necessary?
– brings us to this episode! I’ve already put several anatomy videos up on youtube, but today we’re going to try the “spoken word” to learn this material.
MORE GOOD NEWS: GETS EVEN BETTER!
– Eye anatomy (compared to weird structures like the kidney’s) is pretty straightforward
– Stable Field … occasional advances and new body parts “discovered” … but generally, the knowledge from 40 years ago is still applicable today.
– Hopefully, today’s discussion still applicable 40 years from now
– The same can’t be said of the treatment of glaucoma, cataracts, and refractive surgery
WAYS TO APPROACH ANATOMY
1. Systemic Approach (muscular, nervous, circulatory)
2. Location (prefer … closer to how we examine the eye)
3. Disease approach … how things relate to ocular diseases we treat
Mix things up a little bit. This will be a rambling discourse, will review pertinant anatomy in future episodes as they relate to the eye diseases we are looking at.
– No self-respecting eye doctoar calls the eye the eye “ball” … it’s a “globe”
– With the exception of the brain, most complex organ in the body …
– high level sensory component … with over 100 million rods/cones cells
– clear crystaline structures
– combination of voluntary and involuntary muscles
– the most powerful sensory organ, with a third of the brain devoted to processing the information the eye obtains
– All in the space of One-Inch … and we’ve got TWO of them. hooray!
– So complex and marvelous …
– Charles Darwin himself, in his book Origin of Species, confessed that the eye is such a marvelous structure that it is difficult to imagine such a thing evolving through evolution.
– the eye has been a central argument in the creationism versus evolutionary debate.
Purpose is not to argue for or against evolution … but to discuss the anatomy of this little organ. Because it is pretty neat
BASIC LEVEL – CAMERA
– The anatomy can be compared to the workings of a digital camera
– Like all cameras … lenses in the front, film in the back
– Cornea (lens)
– Lens (another lens)
– Retina (film)
– Optic Nerve (transmission wire)
OUTER GLOBE WALLS
– tough tissue made of collagen (type 2)
– gives eye its outer integrity
– insertion point for muscles that move the eye
– its about 1mm in thickness (or less)
– Continuous with external sheath of the optic nerve
– Flows foward to form the Cornea
CORNEA 5 LAYERS
– Also made primarily of collagen, but configured in sheets like a woven mat
– Precise matrix allows 99 percent of light pass through
– layer of cells on the surface of the eye. (fast healing, majority of nerve endings located)
– corneal abrasion … occurs here.
– hurts (nerve endings end here)
– heals quickly
2. Bowmans Membrane
– compact, strong layer of densely packed collagen layer that differs in orientation from the underlying stroma.
– strong collagen layer that is supposed to help maintain shape of the eye
– not present in many animals, like dogs
– purpose is not well understood, regulates the epithelium sticking to the stroma?
– if a scratch is deep enough to go through bowmans, scar
– when people have RK surgery (lasik without the flap) this layer is ablated away and people seem to do just fine.
– collagen layer
– thinly dispersed keratocytes … relatively dormant fibroblast cells that are mixed into the cornea, sandwhiched between the collagen sheets, and interconnected as a network.
– produce collagen and proglycans, keeping the cornea healthy and clear
– they also kick in to help heal during trauma or inflammation
4. Descemet’s Membrane
… thin basement membrane, made of a different type 4 collagen … and the endothelium layer sticks to it.
… sounds a little like Bowman’s membrane … its not, but if you’re trying to remember which is where … “Descemets is DEEP while Bowmans is high in the Bowry/Belltower”
a single layer of cells, hexagon shaped, single layer honeycomb
extremely important, as the endothelium keeps the cornea dry …
CORNEA IS DRY
– Relatively dehydrated – Transluccent Collagen Sheets
– Endothelial “Bilge Pumps”
– All born with a certain number … say 2400 – 3200 cells per square millimeter
– number drops slowly with age
– the endothelial pump cells don’t replicate … as a cell dies off, the neighboring cells spread and enlarge to cover the area … ultrimately having less pump power with
– if the number drops down to
– Fortunately, we have so many, that unless we live to 200, not a problem ..
1. After internal ocular surgery, percentage stop working
… a major cause of corneal decompensation was after early cataract surgery.
2. Fuchs Dystrophy … if the cell count drops down to 500, cornea swells
– cells can be counted with specular microscope … measure with pachymeter and guttae
-PK versus DSEK as a repair mechanism.
– no discussion of the cornea anatomy is complete without mentioning the tear film
– covers the eye, especially the cornea
1. Lubricate cornea, smoeeth surface for lids to cover and protec
2. Nourish avascular cornea
3. Refract light
– tear interface = majority of the refracting power of the eye.
– irregularities of the tear film can create significant visual blur, dry eye patients complain of a lot blur when reading and watching television.
PRODUCTION: produced by accessory glands in the conjunctiva and under the eyelids
– only reflex tears are produced by the lacrimal gland itself
– evaporate or drain via the nasolacrimal system into the nose
a. Mucin Layer – mucous layer that sits on the cornea surface itself
produced by goblet cells in the conjunctiva
hydrophobic (water loving layer) that helps the tears stick and spread over the eye surface
b. Aqueous Layer
– water, proteine, and saline, antibodies … the components that keep the cornea lubricated.
– cornea avascular, tears help nourish the epithelial cells
c. Surface lipid layer – oils layer produced by the meibomian glands that run along our eyelid margin. keep the tears form evaporating and are important for creating surface tension that keeps the tears from spreading over the lids and running down our cheeks.
– Fluid filled space behind cornea, in front of the iris
– Aqueous is here … important for nourishing the cornea and lens (avascular)
– Fluid pressure here responsible for maintaining the shape of the eye
– Glaucoma if high
– Only place in body you can see inflammation … white blood cells, macrophages, blood cells floating. Little aquarium.
– filter/drain that aqueous drains through before draining into the venous system (canal of Schlemm)
– Colored part of eye
– Muscular diaphragm or drum head with a hole in the middle (pupil)
– Pupil Size changes to control ambient light and some focusing effects via pinhole phenomenon
– controlled by autonomic nervous system
… sympathetic system dilating the eye (bear in woods)
… parasympathetic system constricting (helps focus while resting and engesting food)
– Iris has two muscles
– Spincter-like muscle along the pupil margin to constrict
– Dilator muscles along the edges that dilate
– Use eye drops, we typically use two
… a sympathetic (sympathomimetic) to stimulates the retractors
… anti-parasympathetic to relax the spincter muscles
– Color of iris is determined by how much pigment in the tissue
– Much of that pigment is on the BACK surface of the iris
– Punch hard, we can see a ring of pigment on the lens underneath like a dirty footprint
– Fine Focusing
– Many people think it is main light focusser … its not … cornea fixed lens, 2/3
– Change shape to allow adjustment
– Pancake … far away, Marble … reading close up
– Not every animal works this way … fish and sharks have solid lenses that move like telescopy
– Cataracts … insoluble crystaline proteins form in the cells causing opacities
– Three layers … like a peanut M&M
– Capsule (hard candy Shell)
– Cortex (chocolate)
– Nucleus (peanut)
– Relavence … cataracts form in the inner layers. During CE we remove inner two layers and leave the outer capsule husk. The new prosthetic implants we insert go INTO this capsule which holds it securely.
CILIARY BODY & ZONULES
– Ring of Muscle behind the iris
– Two functions
1. Supports the lens with zonules … springs
Ciliary body contracts like a sphincter. Zonules loosen, lens becomes rounder
2. Produces aqueous fluid … which nourishes the avascular lens and floats forward through the pupil to fill anterior chamber and nourish the cornea as well
– relavent because they can
genetically weak, such as in marfans
can break during surgery
… all leading to a lens dislocation
VITREOUS (PVD discussion)
– Gel Fluid behind the lens that fills the majority of the eye
– Solid in Youth
– Watery (saline) as Older
– Debris precipitates, castes shadows on the retina … cause of floaters
– PVD … flashes and floaters
– Not two as you might think
– Posterior chamber … behind iris, in front of lens
aqueous in the front two chambers, vitreous in the back
– Sulcus lens placement in challenging cataract surgeries
RETINA – MACROSCOPIC LEVEL
– Light Sensing Structure at the back of the eye
– Look at retina during a dilated eye exam … usually at the slit-lamp using a 90 diopter lens, or with a head-mounted indirect ophthalmoscope and a large 20-diopter lens.
– If you are a poor medical student stuck using the hand-held ophthalmoscope … you have my pitty as difficult.
– Visible Landmarks
– Optic Disk … insertion of the optic nerve
– Macula … the more pigmented area responsible for central vision
– Fovea – the exact middle of the macular … for our extreme central vision
– Foveola (pronounciation fo-VE-ola)
… anatomic pit in the middle of the fovea
… anatomic term, not a clinical one (we mainly talk about macula and fovea)
– Ora Serata (stops, has serrated or scalloped pattern)
… retina firmly adherrant to the underlying choroid at this point, near to the beginning of the ciliary body muscle.
… when have retinal detachments, the retina tends to stay attached at this spot
RETINA – MICRO LEVEL
– Many Layers (sucks)
– Many layers
– inner plexiform, outer nucleur,
– bipolar cells, amacrine, horizontal cells, Mueller processes
… these layers really stressed me out as a student
– Not that important. The only retina layers worry about are:
1. Photoreceptors (Rods … b/w night vision, Cones … color fine acuity, higher concentration in our central vision)
– Photoreceptors are at the bottom
– Light passes THROUGH
2. Signal Passes Up to ganglion nerves/fibers
– Surface Layers get from Central retinal artery … Branchs into superior and inferior divisions
– Drains away from Central retinal vein.
– a blockage (CRAO) is devastating to these superficial layers and ganglion nerves
– Deep Layers (important photoreceptors) get from underlying Choroid
– Bed of blood vessels
– nutrition percolates up to support photoreceptors
– waste products percolate down
– retinal detachment is big deal!
– blood grid, metastatic cancer like melanoma and lymphoma can land here and be visible on exam.
1. Retinal Pigment Epithelium
– The RPE
– Single cell layer thick
Dark – absorbs photons, keeps light from bouncing around in eye
Metabolism and maintenance for the rods/cons immediately adjacent
… facilitating the flow of nutrition/oxygen moving up from the choroid
… helping to dump waste products down into the choroid
– Schaffer Sign with RD
2. Bruchs Membrane
– Elastic barrier
… important in the pathogenesis of macular degeneration
a. fatty lipofuscin buildup decreases nutrition/waste movement of the photoreceptors causes atrophy
b. breaks allow blood vessels from the choroid to grow through and up into the retina where they bleed
– The Nerve that sends the signals to the brain
– 1.2 million ganglion nerves
– ganglion nerves course to the optic disk
– lateral geniculate nucleus in the brain
– Pipe analogy
– The Disk … round structure you can see
– The Cup … the indentation in the middle
– Lamina Cribosa? Mesh-like hole in the sclera through which the nerves pass through.
(mechanical theory of glaucoma)
SUMMARY … THE SIGNAL PATH PHOTON OF LIGHT
– Tear film (three layers oil, aqueous, mucous)
– 5-Layers of the Dry Cornea (epithelium, bowmans, stroma, descemet’s, endothelial pump cells)
– Aquous filled anterior chamber
– Through the iris pupil into the small posterior chamber also filled with aqueous
– Through the Lens with its three-layer peanut configuration
– Travels through the Vitreous gel,
– Strike the retina … preferably at the macula
– Travels through surface ganglion nerves
– Bunch of retinal layers
– Rod or Cone photoreceptors
– Converted into an electrical signal
– Signal is shot back UP the retina layers
– Surface ganglion nerves.
– Signal Travels along the surface of the retina toward the optic disk
– plunges over the edge of the disk, through the holes in the lamina cribosa in the sclera, into the optic nerve proper
– Sent back to the brain lateral geniculate nucleus, and eventually to the occipital lobe in the back of the brain. This brain visual pathway I’ll leave for a future neurology lecture
I heard there is a sixth layer of the cornea called Dua’s layer.
– Thin layer of collagen between the stroma and Descemet’s membrane.
– Paper in Ophthlamology 2013.
– Professor at University of Nottingham.
– May have been discovered in 1991 by Dr. Binder
– Does this qualify as a layer, or have any significance I don’t know … the few cornea doctor’s I’ve asked about it seem underwhelmed by the significance and shrug their shoulders.
– for now there are 5 layers to the cornea … plus a possible extra.
– when I first read about Dua’s layer … I kind of felt a lot of internal resistance. the way about it as I do removing pluto as a planet. Are you kidding me? You see there is a natural tendency for scientists to want to name things and go down in history. We see this in surgery as well … where cataracts doctors are constantly making minor changes to instrumentation and surgical techniques.
– Root forceps, root cataract chop, and wrote the root eye dictionary … the most interesting and entertaining eye dictionary ever written in the history of medicine, ever.
– maybe Duas layer will have some bearing … possibly with corneal transplants or understanding keratoconus.
What is the uvea?
– middle, pigmented layer of the eye
– includes iris, ciliary body, and the choroid
– all pigmented, and tend to become inflamamed … whole subspecialty, called uveitis
– uveitis doctors are rare and difficult to find outside of teaching hospitals or large cities as their generally isn’t enough atypical uveitis to support this kind of doctor in normal private practice.
That wraps things up folks.
As I promised at the beginning … we’ve only concentrated on the internal eye anatomy, and haven’t covered topics like the orbit, eyelids, or eye muscles. I think we’ll save those for future talks as they become relevant. Hope you found this episode useful.
Shows notes at eyetalkradio.com or eyebasics101.com … look for episode 2
Until next time … thanks for listening.