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PSL 300 > Lecture 22 > Flashcards

Lecture 22 Flashcards

1
Q

Photoreceptors

A

light-sensitive neurons that convert light energy into electrical energy in cells and are located in the retina

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2
Q

Phototransduction

A

The conversion between light energy to electrical energy

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3
Q

What are the 2 main types of photoreceptors in our retinas?

A

cones and rods

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4
Q

Each retina contains about ___________ cones and __________ rods.

A

6 million
120 million

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5
Q

How do rods and cones respond to stimuli?

A

with graded membrane potentials (they DO NOT fire action potentials)

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6
Q

True/False: rods and cones are neurons

A

True

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7
Q

What segments are rods and cones made up of?

A

outer segment and inner segments

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8
Q

Outer Segments

A
  • contain the membrane (which folds into disk-like layers which contain the visual pigments that respond to light)
  • These segments face the epithelial cells close to the back of the eye
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9
Q

Inner Segments

A
  • are the nucleus and organelles (ER, mitochondria etc) for protein synthesis; and in a basal layer
  • contain the synaptic terminal that releases glutamate.
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10
Q

What is the dominant neurotransmitter in the retinal network?

A

Glutamate
(Has both excitatory and inhibitory effects based on what receptors it binds to)
Ie: excites on-Center bipolar cells when it binds the the photoreceptors in the Center rather than the surrounding

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11
Q

What 2 cells are affected by glutamate binding?

A

OFF-bipolar cells
ON-bipolar cells

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12
Q

What happens to bipolar cells when there is no light?

A
  • photoreceptors continuously release glutamate which
  • HYPERpolarizes ON-bipolar cells (making them less likely to fire) BUT
  • DEpolarizes OFF-bipolar ones (making them more likely to fire).
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13
Q

What happens to bipolar cells when there is light?

A
  • photoreceptors stop releasing glutamate; this causes
  • DEpolarization of ON-bipolar cells (they start firing) and
  • HYPERpolarization of OFF bipolars (they stop firing).
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14
Q

What is Rhodopsin?

A

a transmembrane visual pigment protein

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15
Q

How many pigments does each type of photoreceptor contain?

A

one

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16
Q

What is the name of the pigment in the rod photoreceptor

A

rhodopsin

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17
Q

What pigments are in the 3 cones?

A

blue
green
red
(protein- iodopsin)

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18
Q

What are more sensitive to light; rods or cones?

A

cones

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19
Q

What are cones responsible for?

A

they are responsible for vision in bright light and for distinguishing colors

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20
Q

What type of photoreceptors don’t operate in dim conditions?

A

cones

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21
Q

What type of photoreceptors only operate in low light?

A

Rods in daylight they are “bleached out”- their rhodopsin is broken down so they can’t sense light.

22
Q

Why can’t rods operate in normal light

A

their rhodopsin is broken down so they can’t sense light and in daylight, they are “bleached out”

23
Q

How do rods adapt to dim light?

A

hey rebuild their stores of rhodopsin over ~30 minutes

24
Q

Where are photoreceptors most densely packed?

A

The Macula a central disk and especially in its central pit called the fovea

25
Q

What de we use the fovea for?

A

deatiled vision

26
Q

What is the blind spot?

A
  • a region with no photoreceptors
  • also known as the optic disc or optic nerve head
  • it is the point of exit for ganglion cell axons leaving the eye
27
Q

The fovea contains almost exclusively __________. Whereas more-peripheral retina contain mainly _________.

A

cones

rods

28
Q

What are the 3 layers of neurons in the retina?

A

(front of the eye/light) Ganglion cells > Bipolar cells > receptors (back of the eye)

29
Q

Up to ____ photoreceptors may converge on a single bipolar cell

A

45

30
Q

Bipolars in turn converge on ganglion cells, so in each eye, the signal from 126 million receptors is condensed into ______________.

A

1 million ganglion cells

31
Q

Convergence is greatest in the _________________ and least in the ______

A

peripheral retina

fovea

32
Q

What are the 2 types of bipolar-cell receptive fields?

A

On center
Off center

33
Q

On-center cells

A

are excited by light in the center of their field, and inhibited by light in the surround.

These cells respond most when a light spot fills their center and the surround is dark

34
Q

Off-center cells

A

are inhibited by light in the center, and excited by light in the surround.

They respond best when a dark spot fills their center and the surround is light.

35
Q

What happens to bipolar receptive fields when the lighting is uniform

A

neither type of bipolar cell responds, because the effects of the center and surround cancel, leaving the cell at its resting level of activity (only weakly active)

36
Q

the Chevreul illusion

A

a brightness illusion that consists of adjacent homogeneous grey bands of varying luminance that appear inhomogeneous.

37
Q

the Hermann grid

A

an optical illusion in which the crossings of white grid lines appear darker than the grid lines outside the crossing

38
Q

Large, magnocellular ganglion cells, (M cells)

A

provide information that is used by the brain to infer the movement of objects. These cells are phasic. ~10% of retinal ganglion cells are M.

39
Q

Small, parvocellular ganglion cells, (P cells)

A

provide information that is used to infer form and fine detail, such as texture. ~70% of retinal ganglion cells are P.

40
Q

What are the least common retinal ganglion cells?

A

melanospin cells

41
Q

A ganglion cell near the _____ gets input via bipolars from only a few photoreceptors, mostly _____

A

fovea
cones

42
Q

Farther from the _____, each ganglion cell gets input from many receptors (up to 75,000), mostly ____

A

fovea
rods

43
Q

And near the fovea, ganglion cells are _____ sensitive to light but have better ______ because each one gets input from just a few densely packed cones

A

less
spatial resolution

44
Q

The cranial nerve II

A
  • Also known as the optic nerve
  • responsible for transmitting visual information.
  • contains only afferent (sensory) fibers, and like all cranial nerves is paired
45
Q

Chiasm

A

an intersection or crossing of two tracts in the form of the letter X

46
Q

Fibers from the _____ half of each retina cross; those from the _______ retinas do not.

A

nasal
temporal

47
Q

What are the optic tracts?

A

The nerve bundles emerging from the chiasm

48
Q

How do signals make their way to the cortex?

A
  • through the chiasm to the optic tracts
  • The to the LGN in the thalamus
  • Then project via the optic radiations to the primary visual cortex
49
Q

In the eye, the right side of the scene (the right visual hemifield) projects onto the _________ of each retina

A

left side

50
Q

retinotopic organization

A

the mapping of visual input from the retina to neurons, particularly those neurons within the visual stream.

51
Q

neurons close to each other in ______ get information from close-together parts of ________

A

the brain
the retina

PSL 300 (24 decks)

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