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Fundamental Neurobiology > eye/visual pathway > Flashcards

eye/visual pathway Flashcards

1
Q

3 functions of pigment epithelium

A
  1. takes up old pigment disks (12 days)
  2. replace photopigment molecules
  3. provide nourishment for photoreceptors
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2
Q

structure of rods and cons

A

have an inner segment and outer segment
- outer segment is the part that actually senses the light
- the other end contains the synaptic terminal

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3
Q

visual pigments

A

light-sensing molecules in photoreceptor cells
- rhodopsin - visual pigment in rods
rods: pigment molecules are embedded in the bilipid membrane of the outer membrane which makes up the disks
cones: pigment molecules are embedded amongst the teeth of the comb (outer segment)

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4
Q

how rods become hyperpolarized

A

photoreceptors have both cation-conductive channels (which conduct primarily Na+) and K+ channels. In the dark, many of the cation channels are open, allowing Na+ to flow into the cell - dark current - serves to keep photoreceptors in a depolarized state
via a biochemical cascade, light triggers closing of the cation-conductive channels. the K+ channels which have remained open throughout, now dominate, dragging the membrane potential toward the K+ equilibrium potential
hence light hyperpolarized photoreceptors

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5
Q

structure of visual pigments

A

rods: rhodopsin - consists of an opsin molecule and a retinal molecule. the retinal is the actual light sensor
cones: opsin molecule affects the light wavelength sensitivity of the neuron

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6
Q

molecular mechanism of light transduction in rods

A

rhodopsin works just like a G-protein coupled receptor but instead of being triggered by ligand binding, it responds to light
the transducin molecule activates PDE which breaks down cGMP, reducing its conc
reduced cGMP leads to Na+ channel closing and cell hyperpolarization

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7
Q

transducin

A

the g-protein used as a signaling molecule in rods
light allows replacement of a bound GDP with GTP, activating transducin
once activated the alpha subunit starts the signal cascade

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8
Q

steps of light transduction in rods

A
  1. light stimulation of rhodopsin leads to activation of a G-protein, transducin
  2. activated G-protein activates cGMP PDE
  3. PDE hydrolyzes cGMP, reducing its conc
  4. this leads to closure of Na+ channels
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9
Q

steps in phototransduction

A

light -> photoisomerization -> reduced cGMP -> closing of Na+ channels -> hyperpolarization

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10
Q

steps in recycling of retinol isomers

A

trans-retinal is transported into pigment epithelium, which converts it to the cis isomer and sends it back into the outer segment

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11
Q

luminance sensitivity of rods

A

rods are used to detect changes in overall luminance (black and white vision)
highly sensitive to light but saturate in bright light and are no longer useful

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12
Q

luminance sensitivity of cones

A

cones enable color vision
require more light to respond but enable accurate color vision in bright light
non-functional in very dim light

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13
Q

distribution of rods and cones on the retina

A

fovea is covered with cones with virtually no rods
rods are more prevalent in the periphery
cones in the fovea are smaller and more densely packed, this leads to increased acuity

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14
Q

color constancy

A

two objects returning different spectra to the eye can appear to be the same color

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15
Q

receptive field

A

the area of the retina from which the activity of a neuron can be influenced by light

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16
Q

on-center receptive field

A

a spot of light at the center of the receptive field leads to a strong response

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17
Q

off-center receptive field

A

cells are inhibited by a light spot in the center of their receptive field and excited by light in the surround

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18
Q

photoreceptors have only

A

graded potentials

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19
Q

ganglion cells are excited by

A

glutamate and fire action potentials

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20
Q

two flavors of bipolar cells

A
  1. d bipolar or “on-center” - expresses a metabotropic glutamate receptor which is inhibitory. these cells depolarize when light is received by their photoreceptor
  2. H bipolar or “off-center” - these cells have a normal excitatory synapse with the photoreceptor. light decreases glutamate release and causes them to hyperpolarize
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21
Q

neural responses leading up to the on-center ganglion cell response

A
  1. light causes reduction in glutamate release from photoreceptor
  2. D bipolar cell is released from inhibition and depolarizes
  3. bipolar cell releases glutamate which activates ganglion cell
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22
Q

neural responses leading up to the off-center ganglion cell response

A
  1. light causes reduction in glutamate release from photoreceptor
  2. H bipolar cell is normally depolarized by glutamate release from photoreceptor. now, it has reduced input and hyperpolarizes
  3. bipolar cell reduces glutamate which inhibits ganglion cell
23
Q

horizontal cells

A

release GABA at synapses with rods and cones and are inhibitory
they extend dendrites over large areas of the retina and are thought to mediate surround inhibition
also receive an excitatory input from rods and cones
have graded potentials

24
Q

when light shines in the surround of a particular on-center ganglion cell

A
  1. the surround cone hyperpolarizes, reducing its glutamate release
  2. the horizontal cell becomes hyperpolarized and reduces its GABAergic suppression of the center cone
  3. response of the center cone to light is reduced (it reverts to the resting, depolarized state). it releases more glutamate
  4. a D bipolar cell is suppressed (hyperpolarized) by increased glutamate. this cell reduces its glutamate released onto the ganglion cell
  5. the response of the ganglion cell is suppressed
25
Q

hyperpolarization of the surround causes

A

depolarization of the center

26
Q

a center surround receptive field causes

A

suppression on one side of a dark-light boundary and excitation on the other side
this amplifies the effect of the luminance edge and enhances edge detection
- the brain cares about differences

27
Q

retinogeniculostriate

A

mediates seeing
retina -> LGN -> V1

28
Q

striate cortex

A

primary visual cortex (V1)

29
Q

macula

A

highly pigmented part of the retia that includes the fovea
has the highest density of receptors and best spatial acuity

30
Q

3 types of neurons in the LGN

A

Magnocellular
parvocellular
koniocellular

31
Q

magnocellular

A
  • large cell bodies; large receptive fields
  • fast conduction velocities
  • short processing time, but with little detail
  • input from all cone types (S, M, L) so they are not color selective
  • fast motion detection/processing
32
Q

parvocellular

A
  • small cell bodies; smaller receptive fields
  • center and surround have input from different cone types, so they are color selective
  • slow conducting velocities
  • long processing time, but carries lots of info
  • picture/scene analysis
33
Q

koniocellular

A
  • very small cell bodies
  • located in between m and p layers
  • function is unclear, linked with integrating visual info with other sensory info and color perception
34
Q

foveal

A

great color and detail

35
Q

peripheral

A

good luminance contrast and temporal frequency sensitivity

36
Q

pinwheels

A

neighboring neurons have similar orientation selectivity

37
Q

ocular dominance columns

A

bands of neurons that receive input from one eye
found in LGN and V1
they alternate, and are a way for the brain to integrate binocular input

38
Q

ocular dominance columns

A

injecting an anterograde tracer into the retina of one eye produces strips in the primary visual cortex

39
Q

hypercolumn

A

the slab of material which satisfies all the criteria for ocular dominance in V1
V1 is organized so that for every retinotopic location, there is a complete set of cells to represent ocular dominance, all orientations, and both blob and interblob regions

40
Q

stereopsis

A

depth perceptions
disparities between the images falling on the two retinas is the basis of stereopsis

41
Q

extrastriate

A

regions around V1

42
Q

two general pathways for visual info

A

dorsal (where and how)
ventral (what)

43
Q

ventral pathway

A

what pathway
concerned with object identity and has neurons sensitive to color and form

44
Q

dorsal pathway

A

where pathway
concerned with movement and where things are in space
respond selectively to motion

44
Q

area MT

A

specialized for motion processing
motion maps form pinwheels

45
Q

akinetopsia

A

focal damage to area MT
an inability to perceive motion
patient perceives the world as a series of still images, as if viewed via a strobe light

46
Q

achromatopsia

A

damage to certain extrastriate areas can induce a selective deficit in color vision

47
Q

color opponent cells

A

color sensitive ganglion cells and LGN cells often have centers and surrounds that respond to opposing colors
these are the first step in coding color in the visual system

48
Q

descending inputs

A

connections are glutamatergic but contact both primary LGN relay cells and inhibitory interneurons
cortex can shape its inputs which in turn changes cortical activity and so forth in a feedback loop

49
Q

illusory contours

A

visual illusion
visual cues yield the perception of an object boundary where no physical boundary cues actually exist

50
Q

neurons that respond to illusory contours

A

some neurons in V2 and V4 but not V1
top-down processing

51
Q

face-responsive neurons

A

neurons in the temporal cortex that respond selectively to intact faces
these neurons are near the top of the visual cortex processing hierarchy

52
Q

fusiform gyrus

A

region of the human cortex which responds selectively to faces

53
Q

prosopagnosia

A

a selective deficit in perceiving faces
produced from lesions in the fusiform gyrus

Fundamental Neurobiology (16 decks)

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