Brain and Behaviour 3 Flashcards
What is macular pigment?
Blue absorbing pigment
Where is macular pigment found?
The fovea and surrounding region
Why is macular pigment important for the fovea?
Reduces chromatic aberrations
Where does rod intensity peak?
20 degrees either side of the fovea
What is the parafoveal region?
Area of most sensitive vision under mesopic and scotopic conditions
Where is the blind spot?
On the optic disk where the optic nerve exits the retina
How closely are rods spaced in the parafoveal region?
As closely as cones in the fovea
Cones in the fovea and rods in the parafovea are spaced equally, but what differences are their in sensitivity?
Rod signal acuity is much reduced because they are summed or pooled
What is papilloedema?
Increased ICP leads to optic disk swelling
What are the two fundamental segments of photoreceptors?
Outer and inner segments, joined by cilium
What is the function of the outer segment of a photoreceptor?
Transduction
What is the function of the inner segment of a photoreceptor?
Normal cellular functions
What is the cilium?
Connect outer and inner segments of photoreceptors
What does ROS stand for?
Rod outer segment
What does the ROS consist of?
Stacked membranous discs containing visual pigment and enzymes of the transduction cascade
What does COS stand for?
Cone outer segment
What does the COS consist of?
Continuous folds of invaginating lamellae
What is rhodopsin?
Visual pigment
Describe the structure of rhodopsin.
Membrane protein, 7TM
What does rhodopsin bind?
Small chromophore molecule, 11-cis retinal
What are opsins?
Light sensitive GPCR
When is 11-cis retinal usually absorbed?
When in ultraviolet light
When is 11-cis retinal actually absorbed, and why?
Around 500nm due to opsin bonding (moiety)
What happens to the chromophore once a proton is absorbed?
Isomerisation from 11-cis to all-trans retinal
What is photoisomerisation?
Isomerisation in response to proton absorption
What does photoisomerisation of 11-cis retinal lead to?
The catalytically active form - metarhodopsin II (RH*)
What is Rh*?
Metarhodopsin II - catalytically active form of rhodopsin
What is metarhodopsin II?
Catalytically active form of rhodopsin
What happens after Rh* has played its role?
All-trans retinal dissociates from the protein slowly (100-1000 sec)
What does Rh stand for?
Rhodopsin
What form is Rh said to be in after all-trans retinal has dissocated?
Bleached form
Describe the bleached form of Rh.
All-trans retinal has dissociated and regeneration must occur before the Rh can be used again
What performs the regeneration of Rh?
Retinal pigment epithelium
What does RPE stand for?
Retinal pigment epithelium
What does RPE do?
Regenerates bleached Rh and dissociated all-trans retinal
What happens to all-trans retinal after dissociation?
Reduced to all-trans retinol and transported to RPE where it is converted back into 11-cis retinal
What happens to 11-cis retinal after being regenerated?
Transport back to the photoreceptor joining the bleached opsin to form Rh
How long can it take for complete regeneration of Rh after complete bleaching?
30 minutes
What retinal a derivative of?
Vitamin A
What can a deficiency of Vitamin A cause?
Night blindness
What is retinitis pigmentosa?
Progressive hereditary retinal degeneration
What does RP stand for?
Retinitis pigmentosa
What is the incidence of RP?
1 in 3000
Characterise RP.
Gradual onset of night blindness in adolescence leading to total loss of all periphery and in extreme cases, blindness
What is the cause of RP?
No single cause, but 5-10% caused by Rh mutations
Describe the general phototransduction pathway.
Rh* activates transducin (GPCR); this activates PDE; this hydrolyses cGMP into 5’GMP; reduction in [cGMP] results in channel closure, thus hyperpolarisation; GC resynthesises cGMP to terminate response
What does PDE stand for?
Phosphodiesterase
What does GC stand for?
Guanylate cyclase
How and why does cGMP affect the rod plasma membrane?
Membrane contains high density of cGMP gated cation channels; reduced cGMP closes them leading to negative hyperpolarisation
How is the phototransduction pathway amplified?
Each rhodopsin activates 100s of GPCR, each PDE hydrolyses 100s of cGMP
Describe the dark current
Open channels due to high [cGMP] causes Na and Ca ions to flow into the ROS causing depolarisation to -30mV
What completes the dark current circuit in the inner segment?
K channels and Na/K ATPase
At what point is the ROS saturated?
All cGMP channels are closed at -75mV
Define light adaptation
If flashes of indentical light intensity are superimposed ona background, photoreceptor responses get smaller
What mediates light adaptation?
Ca ions
How do Ca ions prevent saturation, and cause adaptation?
To prevent saturation, cGMP must be resynthesised by GC; GC is inhibited by Ca ions thus a decrease in [Ca] leads to the de-inhibition of GC
What is vision under photopic conditions mediated by?
Cones, exclusively
How are cone responses different to rod?
50x less sensitive, and are much faster
What type of vision do cones mediate?
Photopic colour vision
Define colour vision.
Ability to distinguish different objects on the basis of their spectral reflectance independently of their intensity
What are the three wavelengths cones are optimised to detect?
420, 534, 564nm
How is information on the wavelength of light extracted?
Comparison of the output of at least two cones tuned to different wavelengths
Why must two cones of different tuning be compared to detect wavelength?
If a green cone absorbs 10x less red photons, it will still output the same for 100 red photons as 10 green
What is a system with three differently tuned cones called?
Trichromatic
What does trichromatic mean?
System is based on three differently tuned cones
How is colour deriven from a trichromatic system?
Ratio of excitation in the three cones
What is the theory of how colour is deriven from a trichromatic system?
Young-Helmholz trichromacy theory
What is the Young-Helmholz trichromacy theory?
Theory of how colour is deriven from a trichromatic system
What is perceived when equal excitation of all three cone tunings occurs?
White light
What is the lack of red cones called?
Protanopia
What is protanopia?
Lack of red cones
What is the lack of green cones called?
Deuteranopia
What is deuteranopia?
Lack of green cones
What is the incidence of protanopia?
2% males
What is the incidence of deuteranopia?
2% males
What is the lack of blue cones called?
Tritanopia
What is tritanopia?
Lack of blue cones
What is the incidence of tritanopia?
<1%
What is a shifted red/green cone range called?
Anomalous trichromacy
What is anomalous trichromacy?
Shifted red/green cone ranges
What is the incidence of anomalous trichromacy?
2-6%
What is the complete lack of cones called?
Rod monochromacy
What is rod monochromacy?
Complete lack of cones
What is the incidence of rod monochromacy?
1 in 30000
What cones do non-primate mammals express?
Only blue and yellow
Why is red/green colour blindness most common?
Both lie on the same arm of X-chromosome, thus UHR can easily occur to create anomalous hybrids
What does UHR stand for?
Unequal homologous recombination
What are the four main classes of interneurons?
bipolar, horizontal, amacrine and ganglion
What other, non-neuronal cell type is present in the retina?
Glial Muller cells
How many layers are in the retina?
5 - 3 nuclear and 2 plexiform
What is meant by a nuclear cell layer?
Contains cell bodies
What is meant by a plexiform cell layer?
Contains axons and neuronal processes
What are the retinal layers, deep to superficial?
ONL, OPL, INL, IPL, GCL
What does ONL stand for?
Outer nuclear layer
What does OPL stand for?
Outer plexiform layer
What does INL stand for?
Innel nuclear layer
What does IPL stand for?
Inner plexiform layer
What does GCL stand for?
Ganglion cell layer
What is in the ONL?
Photoreceptor cell bodies
What is in the OPL?
Synapses between photoreceptors, bipolars and horizontal cells
What is in the INL?
Bipolar, horizonatl and amacrine cell bodies
What is in the IPL?
Synapses between bipolar, amacrine and ganglion cells
What is in the GCL?
Ganglion cell bodies
What is the direct pathway of photoreception?
Photoreceptor - bipolar cell - ganglion cell; all using excitatory NT, glutamate
What is lateral inhibition in the retina mediated by, and by what NT?
Horizontal cells - GABAergic
What do horizontal cells do, and by what NT?
Lateral inhibition by GABA
What do amacrine cells do, and by what NT?
Diverse interactions, many different NTs
What are diverse interactions in the retina mediated by?
Amacrine cells
What do photoreceptors synapse with?
Bipolar and horizontal
What do ganglion cells receive inputs from?
Bipolar and amacrine cells
What is the overall output from the retina carried by?
Ganglion cells, which become the optic nerve
What is retinal processing characterised by?
Convergence and divergence
What does retinal divergence result in?
Parallel processing
What does retinal convergence result in?
Spatial summation
What type of potentials do most retinal neurons used?
Graded potentials
What is the only type of retinal neuron to use action potentials?
Ganglion cells
What type of potential do ganglion cells use?
Action potentials
What type of potentials do amacrine cells use?
Fire spikes in response to strong stimuli
What is the only type of retinal neuron to use fire spikes?
Amacrine cells
Why are graded potentials better?
More efficient means of transmitting information over short distances
What is characteristic of photoreceptor synapses?
Presynaptic ribbon; postsynaptic targets always include processes from both bipolar and horizontal cells
What do cone acons end with?
Cone pedicle - large synaptic swelling
How many synapses can there be at each cone pedicle?
Up to 30
What does the number of synapses at each cone pedicle convey?
Divergence to numerous bipolar cells
What is the rod synaptic terminal called?
Spherules
What is the difference between spherules and cone pedicles?
Spherules show no divergence - one spherule, one synapse
Define a cell’s receptive field.
The area on the retina from which its activity can be influence by light
What establishes the centre-surround receptive field?
Synaptic interaction at the conde pedicle
What are the two general classes of centre-surround receptive field?
On-centre and off-centre
What do off-centre bipolar cells do to a central stimulus?
Hyperpolarise
What do off-centre bipolar cells do to a surround stimulus?
Depolarise
What happens to an off-centre bipolar cell when in the light?
Photoreceptors hyperpolarise, glutamate release diminishes, bipolar cell also hyperpolarises
What happens to an off-centre bipolar cell when in the dark?
Photoreceptors release glutamate, bipolar cell depolarises by opening of glutamate gated cation channels
What do on-centre bipolar cells do to a central stimulus?
Depolarise
What do on-centre bipolar cells do to a surround stimulus?
Hyperpolarise
How do on-centre bipolar cells act in the opposite way to off-centre?
Use a metabotropic receptor; like rhodopsin it activates intracellular transduction cascade resulting in hydrolysis of cGMP and closure of ion channels
What generates the antagonistic surround for on & off centre bipolar cells?
Lateral inhibition mediated by horizontal cells
What two types of bipolar cells are there?
Midget and diffuse
How are bipolar cells classified?
Receptive field size and dendritic morphology
Characterise midget bipolar cells.
Most bipolar are midget, receive one input from a single cone to the centre of their receptive field
Characterise diffuse midget bipolar cells.
Sum inputs from several cones - more sensitive but sacrifice spatial and chromatic detail
How many bipolar cells does each foveal cone contact?
10 to 15
What does the divergence of the fovea establish?
Parallel streams in the visual system
What are parallel streams in the visual system?
Different aspects of the image are coded by different cells with overlapping visual fields
Characterise the bipolar-ganglion cell synapse.
Excitatory
What is the result of the excitatory nature of the bipolar-ganglion cell synapse?
Ganglion cells have a similar receptive field structure as bipolar cells
What is the difference between the receptive field structure of bipolar cells and ganglion cells?
Ganglion cells are modified by lateral interaction from amacrine cells
What is the most important subdivision of ganglion cells?
Magnocellular or parvocellular classes
Characterise M ganglion cells.
PARASOL - large field, transient signal, fast conduction, high gain
How are M ganglion cells different to P?
Respond more rapidly, generating transient responses to changes in intensity, more sensitive, larger diameter axons
Why are ganglion cells classed as M or P?
Project to different layers in the LGN and cortex
What does LGN stand for?
Lateral geniculate nucleus
Characterise P ganglion cells.
MIDGET - the majority, small field, sustained response, slow conduction velocity, low gain
What do M ganglion cells convey?
Motion detection
What do P ganglion cells convey?
Form and colour
How many distinct anatomical classes of amacrine cells are there?
20-30
What is the role of amacrine cells?
Little is known - contribution to lateral inhibition, modulatory functions for adjusting the eye to different light sensitivities
What is the distribution of amacrine cells?
M ganglion cells receive significantly more
What is the only amacrine cell whose function we understand?
Rod, or AII amacrine cell - mediates signals from rods under scotopic conditions
Outline the parvocellular stream.
Single cone > midget BP cell > midget ganglion > fine grain, colour
Outline the magnocellular stream.
Many cones > diffuse BP cell > parasol ganglion > coarse grain, motion
What is rod pooling?
Convergence of rod signals, resulting in extra sensitivity but lower spatial resolution
What is the Purkinje shift?
Shift of light sensitivity in scotopic vision from 560nm to 500nm
What type are ALL rod BP cells?
ON-centre
What do rod BP cells connect to?
AII amacrine cells exclusively
What outputs do AII amacrine cells make?
Inhibitory (glycine) to off-ganglion cells, excitatory to on-ganglion cells
Outline the massive convergence of the rod pathway.
1500 rods > 100 rod-BP > 5 AII amacrine > 4 cone-BP > 1 ganglion cell
What is the problem with the optic nerve?
Bottle neck in the visual system
How is the bottle neck in the visual system overcome?
Convergence of 100 million photoreceptors into 1.5 million ganglion cells, followed by a massive divergence into 200 million neurons in the PVC alone
What does PVC stand for?
Primary visual cortex
What is the primary projection from the retina?
To the LGN
Where do the two optic nerves converge before the LGN?
Optic chiasm
What the two optic nerves do at the optic chiasm, and why?
Decussate so that axons representing the same half of the visual field ar combined
Where does the optic radiation pass after the optic chiasm and LGN?
Fans out in a broad path through the internal capsule, ending in the PVC
What is the PVC also known as?
Striate cortex or V1 or area 17
Where is the PVC?
Buried in the medial aspect of the hemispheres - the calcarine sulcus
What is cortical magnification?
Gross overrepresentation of the fovea in the PVC
What does the LGN consist of?
4 parvo cellular layers and 2 magnocellular
Describe the layers of the LGN, ventral to dorsal
1 - Magno, contra; 2 - Magno, ipsi; 3 - Parvo, ipsi; 4 - Parvo, contra; 5 - Parvo, ipsi; 6 - Parvo, contra
Describe the outputs and inputs of the LGN
60% of input is cortical feedback, many local neuronal connections, direct excitatory projections to the cortex
How thick is the grey matter of the visual cortex?
2mm
Where do LGN fibres end, primarily, in the PVC?
Spiny stellate neurons in layer 4C-alpha and 4C-beta
Where do outputs for higher visual areas exit the PVC?
Layers 2 & 3 - pyramidal neurons
Where do PVC projections back to the thalamus exit?
Layer 6
Where do PVC projections to deep brain structures leave?
Layer 5
What are RFs?
Receptive fields
How are the PVC RFs different to centre-surround organisation?
They are in bars or edges, with a particular orientation
What are the two classes of PVC organisation?
Simple and complex
Characterise simple PVC organisation.
Simple cells respond only to an edge of a particular orientation in a very well-defined position
Characterise complex PVC organisation.
75% of cells - also respond like simple cells, but position is not so critical - they respond to directionally specific moving edges over a larger field
What is ‘end-stopping’?
Some PVC cells become inhibited when an edge exceeds a critical length
What theories have been designed to explain PVC organisation?
Hubel & Wiesel speculated simple RFs generated by combining centre-surround LGN inputs, and complex RFs generated by combination of simple RFs
Briefly outline the architectural organisation of the PVC.
Ocular dominance columns - alternating slabs 0.5mm thick, one eye input then the other; Ortientation columns - cells with same orientation, adjacent columns slightly different; Blobs - highly selective areas for colour, rich in cytochrome oxidase
What are ocular dominance columns?
Alternating slabs of 0.5mm thick cortex - receive dominant input first from one eye, then the other
What are orientation columns?
Perpendicular to cortical surface with same preferred orientation; adjacent slabs have shifted preference
What are PVC blobs?
Concentrations of cytochrome oxidase unresponsive to orientation by highly chromatically selective
What are amblyopias?
Permanent defects in cortical function due to retinal problems in early infant life
What are the two higher order visual pathways?
Dorsal - where; ventral - what
What type of input projects into the dorsal visual pathway?
Magno
Describe the dorsal where pathway.
V1 > V2 > V5 > Posterior parietal cortex
