Phototransduction

Question 1

major trade off between compound and refractive eyes

Answer 1

compound- wide range of wave lengths

refractive- high resolution

Question 2

what is the major refractive element in the eye

Answer 2

cornea

Question 3

choroid layer

Answer 3

pigmented epithelium w/ blood vessels

includes iris

Question 4

aqueous humor circulation

Answer 4

produced by ciliary epithelium (replaced 10-20x per day)- circulates in the anterior chamber and is cleared through the canal of schlemm

build up can lead to pressure and damage to the eye- called glaucoma

Question 5

describe the passage of light from outside the eye to retinal processing

Answer 5

cornea- anterior chamber- iris/pupil - lens - vitreous humor - retina

Question 6

ciliary muscles

Answer 6

control the shape of the lens. when the muscle contracts, lens gets rounder, enables close vision

controlled by parasympathetics- cn3 (contraction) and sympathetics (superior cervical ganglion- dilation)

Question 7

accomodation loss

Answer 7

w/ age, lens looses its elasticity, and cannot reform rounded shape

pronounced by age 45

Question 8

myopia

Answer 8

eyeball too long or cornea too curved, focal plane is in front of retina and you cant see far away

nearsighted– concave lenses

Question 9

hyperopia

Answer 9

focal plane falls behind retina, not enough fractive power d/t short eyeball or flat lense

farsighted- convex lenses

Question 10

babies are born w/ what condition?

Answer 10

hyperopia

Question 11

cataracts

Answer 11

lens gets opaque and it interferes with vision- can be d/t crystallins

Question 12

what happens to light not captured by retina

Answer 12

absorbed by pigmented epitherlium on the back of the eye

Question 13

why does the fovea have the greatest acuity?

Answer 13

1 it has cones
2 tightly packed photoreceptors
3 other neuronal cells pulled to the side, resulting in less distortion

Question 14

tapetum lucidum

Answer 14

contains guanine crystals- reflects light back thru retina and gives photoreceptors a second chance

increases sensitivity- night vision

decreases acuity

Question 15

retinal detachment

Answer 15

a separation forms b/t pigmented epithelium and photoreceptor layers- brings photoreceptors out of focal plane and away from nutrients

occurs b/c of development- optic stalk invaginates and forms an optic cup- w/ outer layer becoming neural pigmented epithelium and inner layer becoming neural retina

Question 16

macular degeneration

Answer 16

wet- involves tissue degradation and abnormal blood vessel growth

dry- deposition of yellowish protein and lipid aggregates called drusen

Question 17

describe the 3 main parts of a photoreceptor

Answer 17

synaptic terminal- vesicles designed for transmitter release

inner segment- includes nucleus and most of cellular synthetic machinery

outer segment- connected by cilium, segment is filled w/ disks containing photopigments.

Question 18

are there more rods or cones?

Answer 18

100 mil rods

5 mil cones

Question 19

photoreceptors do not divide. what do they do instead?

Answer 19

produce new disks- 3/hour

Question 20

how are rods specialized

Answer 20

sensitivity- have more photoreceptors and are longer, so better at capturing light. rods amplify light signals more than cones. large degree of convergence on a given bipolar cell, increasing sensitivity but at the expense of resolution

night vision- most stimuli too weak to excite cones, but rods threshold of activation is about starlight

Question 21

how are cones specialized

Answer 21

temporal resolution- cones respond faster

spatial resolution- less convergence on bipolar cells and conical shape gives the fovea/cones better resolution

color vision- 3 types of cones w/ photoreceptors sensitive to different wavelengths. rods only have one photopigment

Question 22

what is the response to light from either rods or cones

Answer 22

hyperpolarization

Question 23

dark current and response to light

Answer 23

in the outer segment of rods/cones. cGMP gated channels allow a depolarizing current of Na and Ca into the cell.

this depolarization allows voltage gated Ca to remain open, which causes glutamate to be continuously released

in response to light, the cGMP disappears and those channels close, hyperpolarizing the cell, turning off Ca voltage gated channels, and halting the release of glutamate

Question 24

biochemistry of phototransduction

Answer 24

rhodopsin is in the disk membranes, and activation by light causes it to isomerize from cis to trans.

it then interacts w/ transducin. transducin then takes up GTP, and GTP-transducin activates phosphodiesterase

phosphodiesterase hydrolyzes cGMP, turning off the dark current

Question 25

rhodopsin kinase

Answer 25

usually inhibited by cGMP and Ca, decreases in those solutes cause it to phosphorylate rhodopsin. this causes rhodopsin to bind arrestin, which dissociates retinal from opsin. the retinal is not very soluble and so is transported to the pigmented epithelium. the all-trans retinal becomes all trans retinol, which is the precursor to 11-cis retinal vitamin a is not synthesized so it is important that it is recycled

Question 26

photoreceptor adaptation

Answer 26

cGMP synthesis is tied to Ca. A decrease in Ca signals bright light and cGMP lacks. however, guanate cyclase responds to the lack of Ca by producing more cGMP, which eventually opens up dark current channels, inhibiting the eye and "adapting" it to a brighter baseline

Question 27

what determines the optimal wavelength for a visual pigment

Answer 27

the opsin moiety determines the wavelength

Question 28

what wavelengths do cones absorb best?

Answer 28

420, 531, and 558 nm

Question 29

why are blue and red/green color blind syndromes inherited differently?

Answer 29

red/green are on the x chromosome and blue is on the 7th chromosome

Question 30

m v p cells

Answer 30

ganglion cells magnocellular- large receptive fields- movement detection parvocellular- small receptive fields- color vision

Question 31

characteristics of ganglion receptive fields

Answer 31

roughly circular non homogenous- center and surround center and surround cause opposite results

Question 32

w type ganglion

Answer 32

not center/surround contain melanospin- sensitive to blue important for circadian rhythms- see best at dawn and dusk

Question 33

difference between on and off center bipolar cells

Answer 33

on center on center cells have bipolar cells with metabotropic, g protein coupled glutamate receptors. when stimulated but glutamate, lead to k channel activity and hyperpolarization off center cells bipolar cells with nmda/ampa ionotropic receptors, causing glutamate to open channels and allow cations to depolarize

Question 34

what information can be gained from the on/off center organization?

Answer 34

where the edges are

Question 35

strategies for treating vision loss

Answer 35

virus mediated expression of photoreceptor genes photoreceptor precursors gene therapy lens endogenous stem cells artificial retinas

Question 36

retinitis pigmentosa

Answer 36

photoreceptor degeneration

Question 37

suprachaismatic nucleus

Answer 37

nucleus in hypothalamus receiving w type ganglion cell innervation playing a role in circadian rhythms

Question 38

inputs to superior colliculus

Answer 38

coordinates visual, somatic and auditory info in a topographic organization responsible for adjusting movements of the head, neck and eyes

Question 39

projections to pretectal area

Answer 39

w cells project to ew nucleus, thru CN3 to the ciliary ganglion (pupillary sphincter) to generate the pupillary reflexes

Question 40

lateral geniculate

Answer 40

receives most retinal projections- topographic representation of the retina it has 6 layers, 3 from each eye. outer 4 are P layers, inner 2 are M layers