Lecture 23: The Eye

Cornea

clear outer covering

iris

controls the size of the pupil

pupil

hole that lets the appropriate amount of light in

lens

that which focuses the light on the retina

ciliary muscle

pulls on the lens to focus at different distances

sclera

outermost layer (white)

retina

the true sensory organ of the visual system

fovea

part of the retina with the greatest acuity

optic disk

where blood vessels and nerves leave the eye

What happens to the lens when you focus far away?

accommodated or unaccommodated?

lens is thin and flat

* unaccommodated

What happens to the lens when you focus near?

accommodated or unaccommodated?

lens is thick and round

* accommodated

What is myopia?

near-sightedness: occurs because lens cannot accommodate enough to focus far objects correctly (always focuses in front of retina)

What is hyperopia?

far-sightedness: occurs because lens cannot accommodate enough to focus near objects correctly (always focuses in back of retina)

What is emmetropia?

normal eyesight

Is it more prevalent to get hyperopia or myopia as you get older?

hyperopia

Where is the blind spot and what does that mean?

The blind spot is at the optic disc and its the part of the retina where all the outputs come together but no photoreceptors are present

Why do we not notice our own blind spot?

Because it is in a different spot in each eye so it gets filled in with normal vision

Where does an image land when you are focusing on it?

What is packed inside that part for fine grained processing?

* the fovea
* photoreceptors

How does LASIKS correct near/far-sightedness?

By changing the curve of the cornea

What are the only two cells in the eye that respond to photons?

rods and cones

What is the process of traveling light starting with photoreceptors and ending with output cells

photoreceptors → bipolar cells → retinal ganglion cells → output cells

What is the job of horizontal cells?

coordinate the activity within one layer of photoreceptors

What happens in regards to the cells polarization when light is detected by a photoreceptor?

When are the most neurotransmitters released?

When are the least neurotransmitters released?

Why?

* the cell hyperpolarizes
* lots of neurotransmitters are released in the dark
* less neurotransmitters are released in the light
* This is because light reduces cGMP activity which closes cGMP gated Ca2+ and Na+ channels

Which ion rushes in during depolarization?

Which ion rushes out?

* Na+
* K+

Phototransduction

the absorption of a photon in the retinal that kicks off a g-protein, turning the light energy into electrical potential

* light acts on rhodopsin which starts the G protein that interacts with phosphodiesterase that halts depolarization by hydrolyzing cGMP

What is photopigment made by?

retinal + an opsin

Rhodopsin

rod photopigment

What are cones responsive to and what are they responsible for?

Where are they?

They are responsive to bright light and are responsible for color vision/our ability to see fine detail

* mostly in the fovea (it only has cones, no rods)

What are rods most sensitive to and what are they mainly used for?

Where can they be found?

Are they less or more numerous than cones?

* more sensitive to dim light and are thus mainly used for night vision
* become more numerous the further you get from the fovea

Do rods or cones have higher light sensitivity?

Do rods or cones have better spatial sensitivity?

* rods have better light sensitivity
* cones have better spatial resolution

How does the inward current of a rod differ from the inward current of a cone after light stimulus?

The inward current of a rod becomes more positive and stays positive whereas the inward current of a cone depolarizes super quickly and the proceeds to hyper-polarize quickly after

Around what wavelength do rods absorb the most light at?

Around 500 nm

label

1. cone
2. rod
3. cone
4. rod
5. cone
6. horizontal cell
7. distal
8. proximal
9. lateral information flow
10. ganglion cell
11. to optic nerve
12. bipolar cell
13. amacrine cell

1. zonule fibers
2. iris
3. pupil
4. cornea
5. aqueous humor in anterior chamber
6. posterior chamber
7. ciliary muscle
8. choroid
9. sclera
10. retina
11. fovea
12. optic disk
13. optic nerve and retinal vessels
14. (big blue) = lens
15. (big brown) vitreous humor