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4. BIOL257 > Special senses IV: vision > Flashcards

Special senses IV: vision Flashcards

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
Q

principle of univariance:

A
  • 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
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26
Q

spectral tuning:

A
  • 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
Q

opsin aa sequence determines:

A
  • 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
Q

bipolar cells:

A
  • 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
Q

ganglion cells:

A
  • 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
Q

ganglion cells: diversity functions

A
  • brightness contrast
  • colour contrast (R/G, B/Y)
  • motion detectors
  • uniformity detectors
  • edge detectors
  • etc.
31
Q

spatial summation: convergence

A
  • 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
Q

spatial resolution: visual acuity

A
  • ability to detect fine detail (distinguish objects as separate)
  • human foveal acuity - 20/20 vision (30 cycles/ ˚ of vision)
33
Q

spatial resolution: visual acuity depends on

A
  • photoreceptor density, receptive field size (cross sectional area)
  • degree of summation
34
Q

spatial resolution: visual acuity no/ low summation

A
  • central retina (fovea) = high acuity
  • 1:1:1
  • photoreceptor: bipolar cell: ganglion cell
35
Q

spatial resolution: visual acuity high spatial summation

A
  • peripheral retina= low acuity

- many photoreceptors: few bipolar cells: 1 ganglion cell

36
Q

temporal resolution: define

A
  • ability to detect changes in stimulus brightness
37
Q

temporal resolution: think of

A
  • think of detection of flashing light (= flickering stimulus)
38
Q

CFFF:

A
  • critical flicker fusion frequency (Hz)
  • fastest flicker rate that is still perceived as flashing
  • fastest shutter speed of the eye
39
Q

temporal resolution related to:

A
  • perception of moving objects

- if image moves across retina too fast, will be blurred

40
Q

temporal resolution dim and bright light; Hz

A
  • dim using rods: 5Hz

- bright light using cones: 60Hz

41
Q

temporal resolution: Ferry-Porter Law

A
  • CFFF changes w retinal illumination (stimulus brightness)
42
Q

temporal resolution limited by:

A
  • properties of photoreceptors and bipolar cells
43
Q

name types of bipolar cells:

A
  • on centre

- off centre

44
Q

on centre bipolar cells:

A
  • metabotropic glutamate receptors (mGluR6)
  • glutamate causes mGluR6 to hyperpolarise bipolar cell
  • light causes DECREASE in glutamate release by photoreceptor
    = light depolarises cell
45
Q

off centre bipolar cell:

A
  • 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
Q

bipolar cell- centre surround receptive field

A
  • lateral inhibition from horizontal cells generates opponent centre surround receptive field
47
Q

bipolar cell: lateral inhibition- mechanism GABA

A
  • inhibitory NT released by horizontal cell when depolarising
  • GABA hyperpolarises photoreceptors = reduce glutamate release onto bipolar cells
48
Q

bipolar cell: general functions - lateral inhibition

A
  • enhances detection of edges and fine detail in image
  • cells signal relative (vs absolute) intensity
  • colour opponency
49
Q

bipolar cell: lateral inhibition- detection of edges

A
  • 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
Q

bipolar cell: lateral inhibition- relative signalling

A
  • response to light falling in RF centre relative to light in RF surround
51
Q

bipolar cell: lateral inhibition- colour opponency (colour vision)

A
  • 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
Q

bipolar cell: lateral inhibition- general feature

A
  • creates opponent/ antagonistic response btw centre and surround
53
Q

parallel processing: define

A
  • visual info split into separate ON and OFF pathways= parallel processing
54
Q

parallel processing: synapsing

A
  • ON centre bipolar cells synapse w ON centre ganglion cells

- OFF centre bipolar cells synapse w OFF centre ganglion cells

55
Q

ganglion cells- opponency: achromatic

A
  • 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
Q

ganglion cells- opponency: chromatic

A
  • chromatic opponent ganglion cells detect colour contrasts = colour vision
  • centre and surround receive segregated input from different spectral cone types
57
Q

ganglion cells- opponency: chromatic eg combo

A
  • 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
Q

primary visual pathway: retino-thalamo-cortical pathway

A
  • 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
Q

primary visual pathway: primary visual cortex (V1)

A
  • 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
Q

primary visual cortex (V1): retinotopic organisation

A
  • 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
Q

primary visual cortex (V1): columnar organisation

A
  • 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
Q

primary visual pathway:

A
  • 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
Q

primary visual pathway: dorsal where stream

A
  • object location
  • depth perception
  • coordination of eye, head and body movements
64
Q

primary visual pathway: ventral what stream

A
  • object identification
  • reading
  • visual learning
  • memory
  • emotions

4. BIOL257 (18 decks)

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