The eye and visual system

Question 1

What does stereopsis mean?

Answer 1

Seeing in 3D

Question 2

The 3 layers of the eye

Answer 2

Outer layer - sclera and cornea

Middle layer - uvea

Inner layer: retina

Question 3

Sclera and cornea transparency

Answer 3

Sclera is white, made up of collagen fibres that are cross fibred so opaque.

Cornea is extension of cornea but is transparent to allow light in. Cornea fibres of collagen are parallel.

Question 4

Limbus

Answer 4

Area where sclera and cornea meet. Stem cells here.

Question 5

Function of sclera

Answer 5

Protection and maintains spherical shape

Question 6

Choroid

Answer 6

Blood supply and nutrition to retina.
Made of melanocytes.
Is coloured to prevent scatter of light entering eye.
Can have melanoma of the eye.

Question 7

Retina

Answer 7

Transducer, takes image you see and turns it into electrical signal.

Ora serrata - after this area cannot receive light as no more photoreceptors.

Question 8

Outer layer

Answer 8

Sclera - tough fibrous outer coat, collagen

Cornea - collagen

Light transmission -must be transparent
Barrier to trauma and infection, must be tough

Responsible for 2/3 refractive power of eye

Question 9

Lens

Answer 9

Never stops growing,

Becomes less pliable,

With age, less ability to see things up close (age related long sighted ness is called Presbyopia)

Question 10

The Outer Later

Answer 10

Epithelium
Bowman’s layer
Stroma
Descemet’s layer
Endothelium

(epithelium can grow back, but endothelium does not grow back so leads to vision loss)

Question 11

What does the endothelium do?

Answer 11

1 cell thick

Keeps cornea dehydrated, pumps water out of stroma, keeps it transparent.

Question 12

Middle Layer (uveal tract)

Answer 12

Uveal tract has 3 parts - choroid, ciliary and iris.

Choroid is blood supply to outer third of retina

Anything anterior of lens is anterior segment. Has chambers (anterior and posterior divided by iris).

Anterior segment filled with aqueous humor (made in ciliary body, moves through pupil to anterior chamber) maintains pressure of eye.

Drains at angle between iris and cornea.
If build up of pressure then can effect nerves.

Question 13

Inner layer

Answer 13

Retina, specialised organ of phototransduction, may layer.

Outer retina has photoreceptor cells, chemical reaction, synapses with other cells to send signal to occipital lobe.

Question 14

Rods and cones

Answer 14

Cones - colour perception, high resolution
Rods - shape and shades of grey, lower resolution

Macula lutea responsible for central vision, clarity. Fovea at centre of macula - highest visual acuity point.
Diabetes can cause damage

Question 15

Pathway

Answer 15

Photoreceptors –> bipolar cells –> ganglion cells (all these axons merge and form optic nerve).

Question 16

Supporting cells

Answer 16

Amacrine cells involved with first synapse

Horizontal cells involved with second synapse

Question 17

Phototransduction

Answer 17

Light photons cause chemical change in rods and cones.

Within photoreceptors, vitamin A derivative is transformed and then recycled through enzyme.

Question 18

Emmetropia

Answer 18

20/20 vision

Question 19

Myopia

Answer 19

Short sighted (so can see near but not far, refractive power too high, overpowered so can’t focus on far objects on retina)
Get a minus lens to focus image on retina

Question 20

Hypermetropia

Answer 20

Far sighted (so cannot see near, refractive power too low, underpowered to focus on near objects )
Get a plus lens

Could be due to shallow corneal curvature
Lens inflexibility
Axial length of eyeball too short

Question 21

Adnexae

Answer 21

Lids, conjunctiva, tear film

Lids - protect globe
Anterior skin
Eye lashes
Meibomian glands
Orbicularis oculi
Tarsal plate
Tarsal conjuctiva
Levator palpebrae superioris and sympathetic muscle

Question 22

Conjunctiva

Answer 22

Palpebral (on bottom eyelid part)
Bulbar (on eye)

Contains goblet cells and lymphoid cells for protection

Question 23

Tear film

Answer 23

3 layers: anterior lipid
middle aqueous
posterior mucous

Protective and nutrition for cornea

Question 24

Blood supply

Answer 24

Inner 2/3 retina = central retinal artery

Branches to super, inferior, temporal and nasal branches.

Question 25

Blood drainage

Answer 25

Vortex veins drain choroid, usually one for each quadrant.

Central retinal vein (superior goes to cavernous sinus, inferior goes to pterygoid venous plexus)

Infection could travel to brain.

Question 26

What do melanin granules in outer pigmented layer of retina do?

Answer 26

Absorb some light rays and prevent scattering of light and preventing reflection of light.

Question 27

Role of outer pigmented layer

Answer 27

Prevent reflection of light (melanin granules)

Barrier to control what nutrients reach photoreceptors from choroid blood vessel

Phagocytose any debris from photoreceptors (eg old discs)

Question 28

Rods

Answer 28

For scotopic vision

For dim lighting
Light in different shades of grey
Decreased acuity

Contain rhodopsin

Question 29

What composes rhodopsin?

Answer 29

Retinal (vitamin A derivative) and opsin (protein)

Question 30

Cones

Answer 30

For photopic vision

For colour vision
Good visual acuity
Good edge detection

Contain photopsin

Question 31

Photopsin

Answer 31

3 different types

Detect different wavelengths of visible spectrum

1 is blue
1 is red
1 is green

Consists of iodine and opsin

Question 32

Is the transduction process the same for rods and cones?

Answer 32

Yes

Question 33

Horizontal cells

Answer 33

Hold photoreceptors together.
Release GABA

Question 34

Bipolar cells

Answer 34

1 dendritic extension and 1 axon extension

Question 35

Amacrine cells

Answer 35

Modulate between bipolar and ganglion cells

Question 36

Ganglion cells

Answer 36

Axons from many cells come together to form optic nerve (CN II)