Microanatomy and physiology of the eye

Question 1

Layers and chambers of eye

Answer 1

• 3 layers (outer to inner): corneoscleral, uvea (iris, ciliary body, choroid), retina
• 2 chambers separated by lens: anterior, vitreous body

Question 2

Anterior chamber

Answer 2

• Sits btwn the cornea and lens, filled w/ clear aqueous humor (secreted by ciliary epithelium)
• Refractive index of cornea similar to the aqueous humor, thus most refraction (80%) occurs at the air-cornea junction
• Aqueous humor provides nutritional support to the avascular lens and cornea (also contains Igs)

Question 3

Vitreous body

Answer 3

• Occupies large chamber behind the lens
• Vitreous is transparent gel w/ collagen II fibers, which are anchored to the basal lamina of the retina, ciliary body and lens
• No blood vessels or nerves in vitreous

Question 4

Corneosclera

Answer 4

• Outermost layer of eye is mostly sclera (5/6), with the anterior portion (1/6) being the cornea
• Sclera is dense irregular CT (collagen + elastic fibers), and protects the eye from trauma, maintains shape, and is the site for attachment of EOMs
• Sclera is continuous w/ cornea, and the inner sclera is perforated at the optic disc to allow the passage of axons form retinal ganglion cells out of the eye
• The cornea (5 layers) is the anterior most covering, is avascular and receives nutrition from aqueous humor and tears

Question 5

Layers of the cornea

Answer 5

• Corneal epithelium: stratified non-keratinized epithelium
• Bowman’s membrane: basement membrane of corneal epithelium
• Corneal stroma: regularly arranged bundles of type I and type V collagen + ground substance
• Descemet’s membrane: basement membrane of corneal endothelium
• Corneal endothelium: simple squamous epithelium, controls hydration of the stroma (required to maintain transparency). Limited regenerative potential (damage= corneal transplant)

Question 6

Limbus

Answer 6

• The transitional zone btwn the cornea and sclera
• Common site of corneal epithelial neoplasms
• In this region the aqueous humor leaves the anterior chamber via trabecular meshwork, which then merge to form the canal of schlemm, which is dumped into the blood stream
• Blockage in the pathway or overproduction of aqueous humor leads to increased intraocular pressure (glaucoma)

Question 7

Conjunctiva

Answer 7

• Thin membrane that extends from the lateral surface of the cornea to overlie the sclera for the anterior part of the eyeball, and the internal surface of the eyelids
• Produces mucus and tears, which aid in bacterial surveillance
• Exposure to microorganisms, irritants and allergens can result in conjunctivitis

Question 8

Lens

Answer 8

• Lens capsule: hypertrophied basal lamina generated by the cells of the sub capsular epithelium
• Subcapsular epithelium: single layer of mitotically-active cuboidal cells (located only on anterior surface of lens). Ass cells divide they move down (toward equator of lens) where they mature into lens fibers
• Lense fibers: long and thin cells that are nucleated immediately below the subcapsular epithelium. As new layers form the deeper ones (older) lose their nuclei and organelles and cytoplasm is filled w/ crystallin proteins (responsible for transparency and refractive properties of the lens)

Question 9

Cataracts

Answer 9

• Crystallin proteins denature and degrade over time and give rise to cataracts (clouding of lens)
• Certain factors (diabetes, HTN, UV) accelerate cataracts
• Lens receives nutrients from aqueous humor

Question 10

Accomodation

Answer 10

• Lens is suspended to the ciliary body by suspensory ligament (zonule of zinn)
• Tension on these ligaments is constant (ciliary muscles relaxed) and far vision is enabled (lens is flat)
• When ciliary muscles contract, the tension on the lens is lost and the lens becomes more spherical (accommodation), allowing us to see near objects
• W/ older age there is reduced elasticity of lens and contractility of ciliary muscles, leading to decreased ability to focus on near objects (presbyopia)

Question 11

Choroid

Answer 11

Heavily vascularized and pigmented connective tissue that lies btwn the sclera and retina (melanocytes close to sclera)

• Most of choroid is blood vessels (size decreases as retina is approached)
• Veins are in the outer layer and arteries are in the inner layer
• These arteries supply the outer layers of the retina
• Boundary btwn choroid and retina (bruch’s membrane) is composed of choroid capillary endothelium, the basal lamina of the pigmented epithelium cells, and the elastic and collagen fibers btwn the two

Question 12

Ciliary body

Answer 12

• Triangular shaped anterior extension of the choroid, it abuts the sclera on the outer surface and abuts the vitreous on the inner surface
• The medial surface projects ciliary processes towards the lens, the suspensory ligaments are connected from the lens to the ciliary processes
• 2 main components of ciliary body: stroma and epithelium
• Sympathetic activation causes relaxation of ciliary muscles (far vision) and contraction of trabecular meshwork muscles
• Parasymp activation causes contraction of ciliary muscles (near vision) and relaxation of trabecular meshwork

Question 13

Stroma and epithelium of ciliary body

Answer 13

• Bulk of stroma is 3 groups of smooth muscle (ciliary muscles), 2 of which control the tension on suspensory fibers (accommodation) and the other facilitates drainage of aqueous humor (regulates trabecular meshwork)
• A vascular bed (supporting the aqueous humor-producing cells and ciliary muscles) lies under the epithelium
• The epithelium is bi-laminar, consisting of a superficial layer that is continuous w/ the neural retina and non-pigmented, and another layer which is deeper and pigmented (adjacent to stroma and continuous w/ retinal pigmented epithelium)

Question 14

Iris and pupil 1

Answer 14

• Iris (4 layers) arises from the anterior border of the ciliary body, projects over the lens, and its internal aperture defines the pupil
• Anterior surface: incomplete layer of fibroblasts
• Stroma: connective tissue containing blood vessels, nerves, and melanocytes (melanocyte number and arrangement dictates color of iris)

Question 15

Iris and pupil 2

Answer 15

• Discontinuous layer of smooth muscle @ medial margin (circular arrangement) called the sphincter pupillage muscle
• This is under parasympathetic control and when it contracts it causes the iris to expand inward, thus contracting the pupil
• In the outer (lateral) margin of the iris there is the dilator pupillage muscle (under symp control) which contracts to retract the iris outward and thus dilate the pupil
• Epithelium: double layer of pigmented cells that blocks light ensuring only the light through the pupil will pass thru the lens

Question 16

Retina

Answer 16

• Has 2 regions: photosensitive and non-photosensitive
• The non-photosensitive region covers the internal suffices of the iris, the ciliary body, and its processes
• Non-photosensitive region consists of inner non-pigmented epithelium and outer pigmented epithelium layers

Question 17

Neural retina (photosensitive region)

Answer 17

• Contains 5 classes of neurons: photoreceptors (rods and cones), bipolar cells, horizontal cells, amacrine cells, and ganglion cells
• Most of the neurons have typical morphology and function, but the photoreceptors are unique in both regards
• From outer to inner: sclera-> choroid-> retinal pigment epithelium (RPE)-> neural retina

Question 18

Photoreceptor morphology

Answer 18

• Outer segment characterized by stacks of horizontal membrane discs (separate from plasma membrane in rods, continuous w/ membrane for cones)
• In rods the discs and membrane are replaced regularly and they contain rhodopsin
• In cones they are not replaced regularly and contain one of 3 types of opsins (red, green, or blue)
• There is a connecting stalk containing cilium btwn the inner and outer layer
• The inner segment contains organelles, esp mito/ER/golgi
• Even further in is the nucleus and synaptic body (beneath the outer limiting membrane)

Question 19

Function of photoreceptors

Answer 19

• Form connections w/ bipolar and/or horizontal cells
• Rod density greater in peripheral retina (for seeing low-light)
• Cone cells are most dense in the fovea (center of retina), they see color and best function in high light
• The muller cell (a type of glial cell) forms the scaffolding for the neural retina and fill the extracellular space btwn neural processes

Question 20

Layers fo neural retina 1

Answer 20

• From out to in (signal transduction goes in but light comes from in traveling out, so they are going in opposite directions)
• Photoreceptors: outer and inner segments (synapse on outer plexiform layer)
• External limiting membrane: junction of photoreceptors and muller cells
• Outer nuclear layer: cell bodies/nuclei of photoreceptors
• Outer plexiform layer: processes of photoreceptors synapsing on bipolar and horizontal cells’ dendrites
• Inner nuclear layer: cell bodies of bipolar, horizontal, amacrine, and muller cells (extend their axons to the inner plexiform layer)

Question 21

Layers fo neural retina 2

Answer 21

• Inner plexiform layer: processes of bipolar, and amacrine cells synapsing on dendrites of ganglion cells
• Ganglion cell layer
• Nerve fiber layer: axons of ganglion cells coursing toward optic disc
• Internal limiting membrane: expanded terminal portions of muller cells w/ basal lamina. Separates retina from vitreous body
• Retinal pigment epithelium is just outside of photoreceptors (these two supplied by choroid)
• The remainder of the neural retina is supplied by central retinal arteries

Question 22

Retinal pigment epithelium (RPE)

Answer 22

• Cuboidal and columnar cells attached to bruch’s membrane and contain abundant melanin granules
• These reduce backscattering of light
• The RPE phagocytoses packets of outer segment membrane that are shed by rods
• Esterifies vitamin A derivatives
• Reactivates rhodopsin
• Retinal detachment occurs btwn RPE and photoreceptors due to lack of metabolic support to these two layers
• Tight junctions btwn RPE cells forms basis of blood-retina barrier

Question 23

Regional variations in the retina

Answer 23

• The optic disc is the exit site for the optic nerve (and blood vessels for inner retina), which is comprised of ganglion cell axons
• At the optic disc there are no photoreceptors and thus forms the blind spot
• Lateral to optic disc is the macula lutea, which is a protective filter over the fovea
• Damage to the macula results in loss of central vision
• Fovea is located in the center of the macular and appears as a small depression (inner layers not present). Visual acuity is highs here and it contains only cones
• The ganglion cells for these cones circle around the fovea since there is no inner layers (only cones)
• Both the macula and fovea receive nutrients from the choroid

Question 24

Phototransduction 1

Answer 24

• Both rods and cones use GPCRs to monitor the polarization of their membranes
• Rods/cones in the dark are constitutively firing, that is when they are not stimulated they have open cation (Na, Ca) channels which depolarize the cell
• In this ground state of depolarization, the rod/cone constitutively releases glutamate onto the bipolar cells
• The release of NT is due to voltage-gated Ca channels at the synaptic terminal, which influx Ca and initiate NT vesicle fusion
• Since Ca is constantly being evacuated, a decrease in the Ca influx reduces NT release (when there is light)

Question 25

Phototransduction 2

Answer 25

• When the rod/cone cell receives light, the visual pigments (containing rhodopsin) are activated and cause a closing of the cation channels
• This will hyperpolarize the cell (cations are still being pumped out from inner segment but are no longer being brought in) and result in a decrease in glutamate release on the bipolar cells
• This is a graded response, meaning that the intensity of light is replicated by the decrease in glutamate release. Meaning weak light only decreases the glutamate release slightly and thus the CNS can interpret if there is strong or weak light sources

Question 26

Photoactivation

Answer 26

• All pigments contain 11-cis retinal (vit A derivative) covalently linked to an opsin protein
• Rod cells only express rhodopsin, and cones express one of 3 opsins (red, blue, green)
• All opsins use 11-cis retinal and are GPCRs
• Activation of the receptors activates the G protein transducin, which binds to the inhibitory subunit of phosphodiesterase (PDE), activating PDE and allowing it to hydrolyze cGMP
• Hydrolysis of cGMP leads to closure of cGMP-gates channels @ plasma membrane, causing a hyperpolarization of the cell
• Deactivation of the photo cascade requires rhodopsin phosphorylation and arresting binding

Question 27

Regeneration of 11-cis retinal

Answer 27

• When a photon strikes 11-cis retinal it causes it to isomerize to 11-trans retinal, which is at a lower energy than the cis form
• The cis form must be regenerated, and this is a job for the retinal pigment epithelium which contains nzs for this process
• Mutations in these nzs causes blindness and other visual disorders

Question 28

Bipolar, horizontal, amacrine, and ganglion cells 1

Answer 28

• These cells relay and process signals from photoreceptors to the ganglion cells
• Bipolar cells are the first to receive signals from the photoreceptors
• There are 11 types of bipolar cells receiving signals from cones, and only 1 type that receives signals from rods
• Further, the cone bipolar cells are broken into 2 categories: On and Off
• On bipolar cells will depolarize in response to light (via signals from cones) by opening cation channels

Question 29

Bipolar, horizontal, amacrine, and ganglion cells 2

Answer 29

• Off bipolar cells will hyperpolarize in response to light (via signals from cones) by closing cation channels
• Rod bipolar cells will depolarize in response to light (like On cells)
• Horizontal cells are integrators that provide inhibitory feedback onto the rod and cone
• Amacrine cells are integrators (feedback the bipolar cells) and relayers from bipolar cells to ganglion cells
• There are many types of ganglion cells, which represent parallel processing channels that encode the visual scene (motion, contrast, color, luminance)

Question 30

Convergence

Answer 30

• Refers to how many photoreceptors converge their information onto a single ganglion cell
• Lower convergence means more ganglion cells per unit of photoreceptors and thus more ability to extract information from the receptors
• Rods have large convergence, meaning there is very low spatial detail from rods
• Cones have very low convergence, which results in great spatial detail from cones
• The fovea has the lowest convergence and thus the highest resolution
• Cone bipolar cells make direct synapses w/ ganglion cells w/o the need for amacrine cells (amacrines only in rod pathway)
• Therefore cones have a more direct and narrow pathway

Question 31

Receptive field (RF) types

Answer 31

• A receptive field of a ganglion cell is a region of space that the photoreceptors (communicating to that ganglion cell) receive stimuli from
• Therefore the presence of a stimulus in the RF will alter the firing of that ganglion cell
• Most ganglion cells have circular RFs with an activating center and a deactivating periphery (center-surround organization)

Question 32

On/Off center receptive fields 1

Answer 32

• RFs detect specific properties of stimuli such as contrast or motion
• On center ganglion cells increase their firing rate (above the basal level) when stimulated by light in the center of their receptive field
• These ganglion cells will decrease their firing rate (below the basal level) when stimulated by light in the periphery of their receptive field

Question 33

On/Off center receptive fields 2

Answer 33

• On center and off center cells both respond to a uniform light stimuli by slightly increasing firing rate
• Off center ganglion cells decrease firing rate from stimuli in the center of their RF
• They increase firing rate from stimuli in the periphery of their RF

Question 34

Suprachiasmatic nucleus (SCN)

Answer 34

• A subset of retinal ganglion cells express a light sensitive protein, and these cell project axons to the SCN in the hypothalamus
• These specialized ganglion cells are responsible for synchronizing mammalian circadian rhythms to environmental light as well mediating the light reflex

Question 35

Color vision

Answer 35

• We use rods to se by starlight (scotopic vision), both rods and cones by moonlight (mesopic vision) and only cones for anything brighter than moonlight (photopic vision)
• Less convergence of cone pathways means greater resolution/detail
• Cones utilize 3 wavelengths of light to generate the entire spectrum: long-wavelength (red), middle-wavelength (green), and short-wavelength (blue)
• Each type of wavelength is absorbed by a cone cell that expresses a particular type of opsin

Question 36

Color-blindness

Answer 36

• The genes for red and green cone pigments are located next to each other on the X chromosome
• Thus for males it is relatively common to wind up w/ a missing or defective red and/or green gene (due to unequal crossing over in meiosis)
• Blue gene is on chrom 7 and thus is rarely lost