Special senses IV: vision Flashcards
name main features of the eye: (11)
- lens
- cornea
- vitreous humour
- aqueous humour
- pupil
- retina
- ciliary mm
- iris
- sclera
- fovea
- choroid
optics: cornea and lens
- 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)
define dioptres:
1/ focal length (m)
dioptres eg:
- 1D- focuses parallel light rays at 1m
- 40D focuses at 2.5cm
accomodation: distant vision
- ciliary mm relaxes
- suspensory ligs tighten
- lens stretched (less curved)
accomodation: near vision
- ciliary mm contract
- suspensory ligs relax
- lens becomes rounder (more curved)
retina: anatomical structure- features
- part of CNS (outpocketing of diencephalon)
- connected to rest of brain via CN II = axons of retinal ganglion cells
retina: anatomical structure- inverted retina
- light must pass through other retinal layers before reaching the photoreceptors
retina: anatomical structure- list retinal layers
- axons of optic nerve
- ganglion cells
- amacrine cells
- bipolar cells
- horizontal cells
- photoreceptors
retina: anatomical structure- duplex
- contains photoreceptors for low light (rods), bright light (cones)
retina: anatomical structure- primary afferent nn
- bipolar cells
retina: anatomical structure- secondary afferent neurons
- ganglion cells
photoreceptors: rods general features
- dim light (scotopic) vision
- higher sensitivity
- slower to respond
- quickly saturate in bright light
- 95%
photoreceptors: cones general features
- bright light (photopic vision)
- lower sensitivity
- faster to respond
- do not readily saturate
- responsible for colour vision
- 5%
fovea: general features
- 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
where are rods abundant:
- periphery of retina
- absent from fovea
where are blue cones absent:
- from fovea
photoreceptors: outer segment
- stack of membranous discs
- visual pigment molecules embedded in disc membranes
photoreceptors: visual pigment molecule
- protein called opsin
- linked to chromophore derived from vit A (11-cis retinal)
- visual pigments: metabotropic cell surface receptors aka GPCRs
visual pigment: photoisomerisation
- 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)
phototransduction cascade: features
- 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)
phototransduction cascade: mechanism
- 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
photoreceptors: in the dark mechanism
- 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
photoreceptors: light induced hyperpolarisation
- 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
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
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
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
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)
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)
ganglion cells: diversity functions
- brightness contrast
- colour contrast (R/G, B/Y)
- motion detectors
- uniformity detectors
- edge detectors
- etc.
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
spatial resolution: visual acuity
- ability to detect fine detail (distinguish objects as separate)
- human foveal acuity - 20/20 vision (30 cycles/ ˚ of vision)
spatial resolution: visual acuity depends on
- photoreceptor density, receptive field size (cross sectional area)
- degree of summation
spatial resolution: visual acuity no/ low summation
- central retina (fovea) = high acuity
- 1:1:1
- photoreceptor: bipolar cell: ganglion cell
spatial resolution: visual acuity high spatial summation
- peripheral retina= low acuity
- many photoreceptors: few bipolar cells: 1 ganglion cell
temporal resolution: define
- ability to detect changes in stimulus brightness
temporal resolution: think of
- think of detection of flashing light (= flickering stimulus)
CFFF:
- critical flicker fusion frequency (Hz)
- fastest flicker rate that is still perceived as flashing
- fastest shutter speed of the eye
temporal resolution related to:
- perception of moving objects
- if image moves across retina too fast, will be blurred
temporal resolution dim and bright light; Hz
- dim using rods: 5Hz
- bright light using cones: 60Hz
temporal resolution: Ferry-Porter Law
- CFFF changes w retinal illumination (stimulus brightness)
temporal resolution limited by:
- properties of photoreceptors and bipolar cells
name types of bipolar cells:
- on centre
- off centre
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
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)
bipolar cell- centre surround receptive field
- lateral inhibition from horizontal cells generates opponent centre surround receptive field
bipolar cell: lateral inhibition- mechanism GABA
- inhibitory NT released by horizontal cell when depolarising
- GABA hyperpolarises photoreceptors = reduce glutamate release onto bipolar cells
bipolar cell: general functions - lateral inhibition
- enhances detection of edges and fine detail in image
- cells signal relative (vs absolute) intensity
- colour opponency
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
bipolar cell: lateral inhibition- relative signalling
- response to light falling in RF centre relative to light in RF surround
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
bipolar cell: lateral inhibition- general feature
- creates opponent/ antagonistic response btw centre and surround
parallel processing: define
- visual info split into separate ON and OFF pathways= parallel processing
parallel processing: synapsing
- ON centre bipolar cells synapse w ON centre ganglion cells
- OFF centre bipolar cells synapse w OFF centre ganglion cells
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
ganglion cells- opponency: chromatic
- chromatic opponent ganglion cells detect colour contrasts = colour vision
- centre and surround receive segregated input from different spectral cone types
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
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)
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
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)
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
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
primary visual pathway: dorsal where stream
- object location
- depth perception
- coordination of eye, head and body movements
primary visual pathway: ventral what stream
- object identification
- reading
- visual learning
- memory
- emotions
