lecture 15

Question 1

Name structures that light passes through on the way to retina

Answer 1

1. Cornea: translucent connective tissue light first passes through
2. Aqueous humour: fluid behind cornea
3. Lens: rounded structure or can be flattened to change directionality of light (malleable)
4. vitreous humour: jelly like behind lens
5. Retina: layer that contains photoreceptors (where light will fall behind eye)

Question 2

when light is sensed in the retina

Answer 2

photoreceptors transduce light energy into an electrical signal (AP) to the brain via the optic nerve

Question 3

what are the two regions that do the most bending

Answer 3

1. Cornea (2/3, right away)
2. Lens (1/3 malleable)

Question 4

what is the key point to see an image clearly

Answer 4

focal point needs to fall on the retina for clear vision

Question 5

mechanism of lens accommodation

Answer 5

1. when ciliary is relaxed ligaments are pulling causing lens to flatten (helps focus retina on far objects)
2. when ciliary muscle contracts ligaments are let go and lens assumes it’s normal rounded shape (helps focus retina on close objects)
   - loose accommodation with age

Question 6

the fovea and the macula

Answer 6

region of greatest focus and center of visual field
fovea within the maculla

Question 7

opsin

Answer 7

• light activated GCPRs
• start photo transduction cascade
• are proteins

Question 8

outer and inner segments of photoreceptors

Answer 8

outer segment: light sensing part of photoreceptors
inner segment: cell business part where you hav nucleus, mitochondria, ER, etc.

Question 9

preventing scatter means

Answer 9

preventing light from reflecting back

Question 10

Rods

Answer 10

• responsible for low light or night vision
• opsin GCPR expressed in rods is rhodopsin which is a photo pigment that is a signal transducer

Question 11

cones

Answer 11

• are responsible for sharp vision and color vision
• have 3 different types of cones each have a different photo pigment (related to rhodopsin) and responds to a specific color (red, blue, green)

Question 12

what can we do with just 3 cone types

Answer 12

• see 200 different hues (color)
• see 20 levels of saturation
• see 500 levels of brightness

Question 13

retinal is a derivative of

Answer 13

vitamin A

Question 14

what is required for opsin to become photosensitive

Answer 14

11-cis retinal

Question 15

what activates opsin

Answer 15

the conformational change from 11-cis retinal to all-trans retinal

Question 16

rhodopsin=

Answer 16

opsin + retinal

Question 17

when opsin and retinal are tightly bound

Answer 17

rhodopsin is inactive

Question 18

in outer segment

Answer 18

• there is intracellular flattened disks where we find the opsin

Question 19

signal transduction in rods (darkness)

Answer 19

1. 11-cis retinal is bound to opsin which mean rhodopsin is inactive and transducin is off
2. cyclic GMP levels are high because transducin is inactive so phosphodiesterase is inactive
3. high levels of cGMP lead to opening of CNG Na+ channels which cause an influx of Na+ and rod cell is depolarized
4. leading voltage gated Ca2+ channels to open
5. Ca2+ enters cell and causes neurotransmitter glutamate release onto bipolar cells so they are inhibited

Question 20

photo transduction in rods (in light)

Answer 20

1. a photon of light is absorbed by 11-cis retinal and it photoisomerizes to all trans retinal (kink—> unkink)
2. this causes a conformational change that activates rhodopsin and opens up a binding site for transducin to be activated
3. transducin activates phosphodiesterase that breaks down cGMP
4. cGMP levels decrease which causes CNG channels to close
5. which causes cell to hyperpolarize
6. voltage gates Ca2+ channels close which results in less Ca2+ to enter
7. this causes a decrease in neurotransmitter glutamate release so we remove inhibition on bipolar cells
8. this causes bipolar cells to excite which causes a receptor potential which then cause release of neurotransmitter glutamate
9. retinal ganglion cells will excite and cause and action potential

Question 21

ganglion cells axons form _________ and project to _______

Answer 21

1. optic nerve
2. CNS

Question 22

Pathways for vision

Answer 22

• optic nerve
• then we cross at optic chiasm
• optic tract
• thalamus
• visual cortex (occipital lobe)

Question 23

Loss of accommodation is called

Answer 23

Presbyopia