vision iii (transdution)

Question 1

outer and inner segments for rods and cones

Answer 1

outer segment

• contains photopigments
• transduction of light energy into receptor potential occurs here

inner segment
-contains the nucleus, golgi complex, and mitochondria

Question 2

rods

Answer 2

• outer segment is rod-shaped
• dim light –> sensitive to light
• black and white vision
• contain single photopigment –> percieved input in grey tones
• peripheral vison –> located in peripheral zone of retina
• pathways converge, causing fuzzy, indistinct images
• 20 rods: 1 cone

Question 3

Cones

Answer 3

• outer segment is cone-shaped
• need bright light for activation (have low sensitivity)
• color vision: have 3 photopigment (red, greed, blue)
• many shades of color- overlap of wavelength domains
• dense in fovea centralis (sharpest vision and best color perception)

-nonconverging pathways for detailed, hi res image

Question 4

photoreceptor cells

Answer 4

• vulnerable to damage
• degenerate if retina detaches
• destroyed by intense light
• outer segment renewed every 24 hr
• tips fragment off and are phagocytized

Question 5

photopigments

Answer 5

• 2 parts: opsin and retinal
• opsin = 3 types in cones; 1 in rod
• retinal = light absorbing part
• rhodopsin = photopigment in rods

absorption of light by a photopigment leads to structural changes

Question 6

Rhodopsin

Answer 6

• deep purple pigment of rods
• 11-cis-retial + opsin
• 3 steps of rhodopsin formation and breakdown (happens in discs and triggers elec impulse)

1. pigment synthesis
2. pigment bleaching
3. pigment regeneration

Question 7

pigment synthesis

Answer 7

rhodopsin forms and accumulates in dark

Question 8

pigment bleaching

Answer 8

when rhodopsin absorbs light, retinal changes to all-trans isomer

-retinal and opsin separate (rhodopsin breakdown)

Question 9

pigment regeneration

Answer 9

• all-trans retinal converted to 11-cis isomer by retinal isomerase
• rhodopsin regenerated in outer segments
• rod regeneration takes much longer than cones

Question 10

Light transduction reactions

Answer 10

• light activated rhodopsin activates G-prot transducin (NOT A TYPO)
• Transucin activates photodiesterase (PDE) which breaks down cGMP to GMP
• in the dark, cGMP holds channels of outer segment open –> Na and Ca depolarize cell
• In light cGMP breaks down, channels close, cell hyperpolarizes
• hyperpolarization is sigmal

Question 11

phototransduction in cones

Answer 11

• similar process as in rods
• cones are far less sensitive to light, so it takes more light to activate cones
• also regenerate faster

Question 12

Function of Glutamate

Answer 12

• glutamate is released by photoreceptors all the time
• glutamate inhibits bipolar neurons
• more glutamate will hyperpolarize bipolar membrane (IPSP)
• glutamate is not released by photoreceptors when the photoreceptors are hyperpolarized
• less glutamate will depolarize bipolar cell membrane (EPSP), leading to AP in ganglion cells

Question 13

Info-processing in retina

Answer 13

• photoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs)
• when light hyperpolarizes photoreceptor cell, glutamate isn’t released, so bipolar cells depolarize, releaseing NTs onto ganglion cells, which can now generate APs which are tranmitted in optic nerve to brain

Question 14

different wiring for rods and cones

Answer 14

rods

• many rods: 1 bipolar cell
• many bipolar cells: 1 ganglion neuron –> convergence
• spatial summation (unclear about source of activation)–> vision not as clear

Cones

• 1 cone: 1 bipolar cell: 1 ganglion neuron
• higher visual acuity of a small area of visual field

Question 15

Light and Dark Adaptation

Answer 15

• light adaptation: dark to light = faster
• dark adaptation: light to dark = slower
• cones regenerate rapidly whereas rhodopsin regenerates more slowly

Question 16

horizontal and amacrine cells

Answer 16

• lateral pathways that modify messages being transmitted from rods
• horizontal cells enhance contrast (connected to rods/cones)
• amacrine cells signal change in illumination (connected to ganglion cells)

Question 17

color blindness and night blindness

Answer 17

Color blindness

• inherited inability to distinguish b/t certain colors
• x-linked recessive
• result from the absense of one of the 3 types of cones
• most common type: red/green

Night blindnness

• “Nyctalopia”
• vit A deficiency

Question 18

main visual pathway

Answer 18

• axons of ganglion cells form optic nerve
• optic nerve to optic chiasma (NO SYNAPSE)
• fibers from each eye can cross here or stay on same side
• optic tracts to thalamus where ganglion cells synapse on lateral geniculate nucleus
• thalamus to visual cortex (occipital lobe)

Question 19

other places visual stimuli can go

Answer 19

• midbrain: superior colliculus = reflexes of head, eye, neck
• hypothalamus = circadian rhythms
• association areas (all lobes but insula) –> more detailed processing, like object identification