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4. BIOL257 > Special senses IV: vision > Flashcards

Special senses IV: vision Flashcards

(64 cards)

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1
Q

name main features of the eye: (11)

A
  • lens
  • cornea
  • vitreous humour
  • aqueous humour
  • pupil
  • retina
  • ciliary mm
  • iris
  • sclera
  • fovea
  • choroid
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2
Q

optics: cornea and lens

A
  • cornea is main refractive element (40 dioptres fixed)
  • bends (refracts) light due to refractive index btw air/ cornea
  • convex surface: converge light rays
  • lens adjusts point of convergence by changing shape (20 dioptres variable)
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3
Q

define dioptres:

A

1/ focal length (m)

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4
Q

dioptres eg:

A
  • 1D- focuses parallel light rays at 1m

- 40D focuses at 2.5cm

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5
Q

accomodation: distant vision

A
  • ciliary mm relaxes
  • suspensory ligs tighten
  • lens stretched (less curved)
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6
Q

accomodation: near vision

A
  • ciliary mm contract
  • suspensory ligs relax
  • lens becomes rounder (more curved)
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7
Q

retina: anatomical structure- features

A
  • part of CNS (outpocketing of diencephalon)

- connected to rest of brain via CN II = axons of retinal ganglion cells

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8
Q

retina: anatomical structure- inverted retina

A
  • light must pass through other retinal layers before reaching the photoreceptors
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9
Q

retina: anatomical structure- list retinal layers

A
  • axons of optic nerve
  • ganglion cells
  • amacrine cells
  • bipolar cells
  • horizontal cells
  • photoreceptors
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10
Q

retina: anatomical structure- duplex

A
  • contains photoreceptors for low light (rods), bright light (cones)
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11
Q

retina: anatomical structure- primary afferent nn

A
  • bipolar cells
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12
Q

retina: anatomical structure- secondary afferent neurons

A
  • ganglion cells
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13
Q

photoreceptors: rods general features

A
  • dim light (scotopic) vision
  • higher sensitivity
  • slower to respond
  • quickly saturate in bright light
  • 95%
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14
Q

photoreceptors: cones general features

A
  • bright light (photopic vision)
  • lower sensitivity
  • faster to respond
  • do not readily saturate
  • responsible for colour vision
  • 5%
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15
Q

fovea: general features

A
  • small depression in retina
  • highest density of cone receptors for high acuity vision
  • represents 1% of total retinal area, but 50% of area of visual cortex
  • avascular: so blood vessels don’t obscure image projected onto photoreceptors
  • other neural layers pushed out of the way to maximise incoming light= image quality
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16
Q

where are rods abundant:

A
  • periphery of retina

- absent from fovea

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17
Q

where are blue cones absent:

A
  • from fovea
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18
Q

photoreceptors: outer segment

A
  • stack of membranous discs

- visual pigment molecules embedded in disc membranes

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19
Q

photoreceptors: visual pigment molecule

A
  • protein called opsin
  • linked to chromophore derived from vit A (11-cis retinal)
  • visual pigments: metabotropic cell surface receptors aka GPCRs
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20
Q

visual pigment: photoisomerisation

A
  • chromophore absorbs visible light (photons)
  • photon absorption causes isomerisation of chromophore -> causes opsin to change shape
  • conformational change activates the opsin, activating G protein (transducin)
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21
Q

phototransduction cascade: features

A
  • phototransduction: conversion of light energy into biochemical signal
  • ligand= light for visual pigment (= metabotropic receptor protein)
  • phototransduction cascade: biochemical pathway -> amplifies the visual signal (eg. allows rod to respond to and signal absorption of single photon of light)
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22
Q

phototransduction cascade: mechanism

A
  • each photoisomerised opsin molecule activates many molecules of G protein transducin
  • alpha subunit of transducin -> activates many molecules of phosphodiesterase (PDE)
  • each PDE molecule -> converts many molecules of 2˚ messenger cGMP to 5’ GMP
  • cystosolic levels of cGMP are critical for controlling membrane permeability via cGMP-gated cation (Na+) channels
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23
Q

photoreceptors: in the dark mechanism

A
  • cGMP levels in cytosol high
  • Na channels open
  • Na enter cell, depolarisation spreading from outer segment to terminal
  • Vm= -10 to -40mV
  • Ca open responding to depolarisation
  • Ca enters cells, triggering exocytosis of transmitter
  • transmitter causes graded potentials in bipolar cell
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24
Q

photoreceptors: light induced hyperpolarisation

A
  • light absorbed by photopigment
  • retinal and opsin dissociate
  • transducin activated
  • phosphodiesterase activated
  • cGMP levels in cytosol decrease
  • Na channels close
  • w less Na entering cell = hyperpolarises
  • Vm hyperpolarises in light
  • Ca channels close
  • transmitter release decreased
  • graded potential in bipolar cell gets smaller
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25
principle of univariance:
- visual pigment spectral sensitivity determines probability of photon absorption - receptor output depends upon total quantum catch regardless of photon wavelength - individual photoreceptors can't signal colour -> requires comparison of different spectra types of photoreceptor
26
spectral tuning:
- each opsin protein (GPCR) consists of 350ish aa - arranged into 7 transmembrane domains (integral polytopic protein) - specific aa residues (spectral tuning sites) surrounding chromophore binding pocket 'tune' spectral sensitivity of visual pigment by altering shape of chromophore molecule
27
opsin aa sequence determines:
- chromophore shape/ orientation and thus visual pigment spectral sensitivity - output of cones expressing different visual pigments can be compared by other neurons to give colour vision
28
bipolar cells:
- bipolar types 1˚ afferent neurons - respond to changes in rate of glutamate release of photoreceptors - produce graded potentials (not APs) and when depolarised release glutamate onto ganglion cell dendrites - relay signals from photoreceptors to ganglion cells - receive inhibitory input from horizontal cells - first stage of spatial info processings (spatial summation) - first stage of temporal info processing (tonic/ phasic types)
29
ganglion cells:
- output neurons of retina - multipolar 2˚ afferent neurons - axons travel via CN II to rest of brain - encode visual info to frequency modulated spike trains (action potentials) - receive excitatory signals (glutamate) from bipolar cells - receives inhibitory signals from amacrine cells (GABA, glycine) - diverse morphology (size dendritic extent)
30
ganglion cells: diversity functions
- brightness contrast - colour contrast (R/G, B/Y) - motion detectors - uniformity detectors - edge detectors - etc.
31
spatial summation: convergence
- bipolar cells can show one to one synaptic relationships to photoreceptors -> preserves image detail (acuity) at expense of absolute sensitivity - or bipolar cells can pool signals from many photoreeptors (summation) -> enhances absolute sensitivity at cost of acuity - same is true for bipolar-to-ganglion cell connectivity
32
spatial resolution: visual acuity
- ability to detect fine detail (distinguish objects as separate) - human foveal acuity - 20/20 vision (30 cycles/ ˚ of vision)
33
spatial resolution: visual acuity depends on
- photoreceptor density, receptive field size (cross sectional area) - degree of summation
34
spatial resolution: visual acuity no/ low summation
- central retina (fovea) = high acuity - 1:1:1 - photoreceptor: bipolar cell: ganglion cell
35
spatial resolution: visual acuity high spatial summation
- peripheral retina= low acuity | - many photoreceptors: few bipolar cells: 1 ganglion cell
36
temporal resolution: define
- ability to detect changes in stimulus brightness
37
temporal resolution: think of
- think of detection of flashing light (= flickering stimulus)
38
CFFF:
- critical flicker fusion frequency (Hz) - fastest flicker rate that is still perceived as flashing - fastest shutter speed of the eye
39
temporal resolution related to:
- perception of moving objects | - if image moves across retina too fast, will be blurred
40
temporal resolution dim and bright light; Hz
- dim using rods: 5Hz | - bright light using cones: 60Hz
41
temporal resolution: Ferry-Porter Law
- CFFF changes w retinal illumination (stimulus brightness)
42
temporal resolution limited by:
- properties of photoreceptors and bipolar cells
43
name types of bipolar cells:
- on centre | - off centre
44
on centre bipolar cells:
- metabotropic glutamate receptors (mGluR6) - glutamate causes mGluR6 to hyperpolarise bipolar cell - light causes DECREASE in glutamate release by photoreceptor = light depolarises cell
45
off centre bipolar cell:
- ionotropic glutamate receptors (AMPA and kainate) - glutamate causes IGRs to depolarise cell - light causes DECREASE in glutamate release by photoreceptor - light causes hyperpolarisation of cell - (cell activated by dark)
46
bipolar cell- centre surround receptive field
- lateral inhibition from horizontal cells generates opponent centre surround receptive field
47
bipolar cell: lateral inhibition- mechanism GABA
- inhibitory NT released by horizontal cell when depolarising - GABA hyperpolarises photoreceptors = reduce glutamate release onto bipolar cells
48
bipolar cell: general functions - lateral inhibition
- enhances detection of edges and fine detail in image - cells signal relative (vs absolute) intensity - colour opponency
49
bipolar cell: lateral inhibition- detection of edges
- cells respond most strongly to small spots of light illuminating RF centre - respond most weakly/ not at all to uniform illumination covering centre and surround of receptive field
50
bipolar cell: lateral inhibition- relative signalling
- response to light falling in RF centre relative to light in RF surround
51
bipolar cell: lateral inhibition- colour opponency (colour vision)
- RF centre receives input from 1 spectral cone type (eg. red) - lateral inhibition by horizontal cells contact different spectral cone type (eg. green cones) in RF surround
52
bipolar cell: lateral inhibition- general feature
- creates opponent/ antagonistic response btw centre and surround
53
parallel processing: define
- visual info split into separate ON and OFF pathways= parallel processing
54
parallel processing: synapsing
- ON centre bipolar cells synapse w ON centre ganglion cells | - OFF centre bipolar cells synapse w OFF centre ganglion cells
55
ganglion cells- opponency: achromatic
- centre surround receptive field structure of bipolar cells is transmitted to ganglion cells - achromatic opponent ganglion cells detect brightness contrasts- edges, shape, motion - uses combined green + red cone signals
56
ganglion cells- opponency: chromatic
- chromatic opponent ganglion cells detect colour contrasts = colour vision - centre and surround receive segregated input from different spectral cone types
57
ganglion cells- opponency: chromatic eg combo
- red ON centre, green OFF sur - red OFF centre, green ON sur - green ON centre, red OFF sur - green OFF centre, red ON sur - blue ON centre, yellow (R + G) OFF sur
58
primary visual pathway: retino-thalamo-cortical pathway
- each hemisphere has input from both eyes - given hemisphere gets info from contralateral visual field, (R hemisphere gets visual info from L visual field- both eyes) - ganglion cell axons cross at optic chiasm, project (via optic tract) to lateral geniculate nucleus (LGN) in thalamus - LGN neurons project into Primary visual cortex (Area V1)
59
primary visual pathway: primary visual cortex (V1)
- afferent from LGN travel to V1 in occipital lobe via optic radiation - V1 (striate cortex) is first visual area in cerebral cortex that processes visual signals - like most neocortex, V1 has 6 distinct functional layers - processes visual info and relays to other visual and nonvisual brain areas
60
primary visual cortex (V1): retinotopic organisation
- retinotopically organised spatial map of visual world - each point in visual space processed in parallel by separate chromatic/ achromatic circuits - amount of cortex utilised is related to retinal eccentricity/ cone density (visual hommunculus)
61
primary visual cortex (V1): columnar organisation
- reflects underlying functional organisation - occular dominance columns: process input from each eye separately (position, depth) - orientation columns: orientationselective neurons - blobs: local areas within each column that contain colour sensitive neurons
62
primary visual pathway:
- V1 -> visual association cortex (V2,3,4,5) - V2, V4: integration of visual modalities (eg. colour and motion) - visual cortex projects to other association areas: dorsal 'where' stream - ventral 'what' stream
63
primary visual pathway: dorsal where stream
- object location - depth perception - coordination of eye, head and body movements
64
primary visual pathway: ventral what stream
- object identification - reading - visual learning - memory - emotions

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