Vision Flashcards
What is the range of wavelengths of visible light
What does this correspond to
~390 (violet) to 700 (far red) nm
The spectrum of sunlight
What are the 4 basic variables analysed by the visual system
Intensity (1) and Wavelength(2) and their variation in space (3) and time (4)
What is light from a light source quantified as
As illuminance (lux)
How is light reflected from objects quantified
Luminance (cd/m^2)
From visual threshold to saturation, by how much can light intensity in the environment vary
What about for reflectance of natural objects
By a factor of ~10^10
Reflectance it natural objects varies nobly ~20 fold and is independent of illuminance
What is the most commonly used measure of light stimulus strength
Relative intensity or contrast
ΔI / I
Where I= mean background
And ΔI= increment in intensity
Briefly give an overview of how an image is formed in the eye
An inverted image is focussed by the cornea and lens on to the retina, with an aperture controlled by the pupil
How are size and distance in the outside world expressed in the retina
In angular terms
As size and distance are expressed in angular terms on the retina, what is 1 degree equivalent to
1 degree = 60 arc minutes
This is about the width of a thumbnail at arm’s length or 300μm on the retina
What is 1 radian in degrees
~57 degrees
What is diffraction
The spreading of waves around objects
Such as light spreading out as it passes through an aperture
What is the point spread function
Express this as an equation
Because of diffraction, even with perfect lenses, the image of a point source is a blurred circle
The angular diameter of point spread function is:
d~ λ/D
Where D= diameter of lens (or aperture if limited by diaphragm or pupil);
λ= wavelength
d~λ/D
What does this mean for the diffraction limit in relation to the lens size
The smaller the lens/ aperture, the larger the diffraction limit
True or false
Lens aberrations degrade the image
True
Other than diffraction, the image can, in practise, be further degraded by optical imprecisions including spherical and chromatic aberration and glare
What is spherical aberration
For a spherical surface, rays towards the edge are more strongly refracted
What is chromatic aberration
Different colours are focussed at different depths
The human eye is well focussed for green but poorly for blue light
Describe how glare can affect image formation
Small particles in the optical media scatter light in all directions, reducing contrast of the image
When does the point spread function approach the diffraction limit
At small pupil diameters when spherical and chromatic aberration is modest
What happens to the point spread function when the pupil dilates
Off axis rays contribute to image formation and aberrations become more significant, broadening the point spread function despite the reduced effects of diffraction
Do eyes have refractive defects
Some eyes may have refractive defects such as ametropic
What does emmetropic mean
Can focus sharply on an object at infinity
If an eye cannot do this it is ametropic
What is myopia and what is its incidence
Short sight
20% of population
What is hypermetropia
How common is it
Long sight
30% of population
What does myopia predispose
Retinal detachment, degeneration and glaucoma
What leads to bifocal lenses being required
Combination of myopia and presbyopia
True or false
Myopia is inherited
False
It has both genetic and environmental factors
What sets an absolute limit on our spatial resolution
Optical quality
WHat does the detail in an optical image have to be matched by
The grain (receptor spacing) of the retina
Theoretically what should the receptor spacing be in order not to sacrifice the detail in the image
Where is this achieved
~half the width of the point spread function
In the fovea
How are adjacent cones arranged in the fovea
Separated by 0.5 arc minutes
Arranged in a precise mosaic to maximise packing density
What are the 4 optical surfaces in the eye
How are they arranged
Front and back surfaces of the cornea and lens
In series
Describe the structure of the cornea
Consists of a 650μm thick layer of transparent collagen fibrils (stroma) encloses between an epithelium and an endothelium
What are stroma in the cornea
Transparent collagen fibrils enclosed between epithelium and endothelium
How is the eye lens structured
Built from long ribbon like cells, packed with transparent protein (crystallin)
Is the lens exactly the same as an adult and child
No
Cells are added to the periphery of the lens throughout life
The lens absorbs increasingly strongly in the blue with age
Where are the oldest cells of the lens
Oldest cells in the core
Where is the greatest refractive index of the eye lens
Why
At the centre
This is to correct for spherical aberration
True or false
The lens absorbs weakly in the UV
False
Absorbs strongly in the blue
What are cataracts
When is this common
Clouding of the lens
In old age
How are the lens and cornea supplied with blood
They are avascular so are supplied with metabolites by the aqueous humour
What is the aqueous humour of the lens and cornea secreted by
How is it drained
What is the primary cause of a glaucoma
The epithelium of the ciliary body
Through the trabecular meshwork and Canal of Schlemm.
A reduction in the rate of flow of the aqueous humour
How is the Power (P) of a lens expressed
Dioptres
1
——————-
Focal length in meters
To a first approximation, how is the power of a lens system in series calculated
By the sum of the powers of the individual components
What is accommodation
The lens changing its focal length to focus on objects at different distances
How is accommodation accomplished
By a combination of radial elastic ligaments (suspenseful ligaments or zonule) and a circular ciliary muscle
What happens when the ciliary muscle is relaxed
The suspension ligaments stretch the lens, reducing optical power
What is focused on the retina when a normal eye is at rest
When contracted?
An object at infinity ♾
Nearer objects
How does lens elasticity change with age
What does this lead to
Decreases with age
Presbyopia- a reduction in accommodating power
True or false
The ciliary muscle is under PS control
True
Via the oculomotor nerve
What is the near reflex
Accommodation combined with simultaneous constriction of the pupil to improve depth of focus and also convergence of the 2 eyes to fixate on the new target
How many muscles comprise the iris
2 antagonistic smooth muscles under ANS control
Sphincter (parasympathetic)
Dilator (sympathetic)
What is pupil diameter largely determined by
What is the control circuit
The sphincter muscle
Involves projection from the retina to the pretectum in the midbrain, which projects bilaterally to preganglionic PS neurons in the Edinger- Westphal nucleus that project via the oculomotor nerve to the ciliary ganglion, innervating the pupillary sphincter muscle
True or false
Illumination of one eye evokes pupil constriction in both eyes
What kind of response is elicited
True - there is a bilaterally projection from the pretectum in the midbrain
It is a direct and consensual response
Where is the pretectum
In the midbrain
What does the direct, consensual response of both eyes constricting for illumination of one eye provide the basis for
A standard neurological test for intact brainstem function in unconscious patients
What is Argyll-Robertson pupil characteristic of
What happens
Neurosyphilis
Pupil does not react to light but does react to accommodation
Which layer are the photoreceptors in the retina
The layer furthest from the incident light
The remaining layers contain visual interneurons
What can glare in the visual interneuron layers do
What acts to reduce the effect of this
Degrade the image
Retinal glial cells (Müller cells) which act as optical waveguides to aid transmission of light
Where are the visual interneurons in the primate fovea
What is the size of the primate fovea
Displaced to the side
1.5mm (5 degrees in diameter)
True or false
Come density increases in the fovea
True but at the expense of rods
What is the foveola?
Is it avascular
How many rods are here
The central 1 degree (260 μm) of the fovea with the highest acuity
Yes - to avoid/ minimise light scattering
The foveola is completely rod free
When is the minimum come spacing reached in the fovea
At the centre of the foveola
How does the fovea and surrounding region reduce effects of chromatic aberration
Contain blue absorbing macular pigment
When does primate rod density peak in the retina
What is the area called
20 degrees either side of fovea
Parafoveal region - this is the area of most sensitive vision under mesopic and scotopic
How are the cones and rods spaced in the parafoveal region
Rods are spaced as closely here as cones in the retina
But rod signals are summed or pooled together, reducing spatial acuity
Where is the eye’s blind spot
Subtending 5 degrees at the optic disk where the optical nerve exist the retina
Why can papilloedema occur
What can happen
Because CSF in the optic nerve is in continuity with that of the brain
The optic disk can appear swollen with raised intracranial pressure
What do both rods and cones consist of
An outer segment specialised for transduction joined to an inner segment with more normal cellular machinery
They are connected by a cilium
What do rod outer segments consists of
Stacked membranous discs containing the visual pigment and enzymes of the transduction cascade
What do cone outer segments consist of
Continuous folds of invaginating lamellae
What is the visual pigment
Rhodopsin
A protein with 7 transmembrane helices embedded in the disk membrane (a prototypical GPCR)
What does rhodopsin bind
A chromophore molecule, 11-cis retinal
True or false
Free retinal only absorbs ultraviolet
True
This changes when retinal is bound:
Interactions between covalently bound retinal and opsin shift the wavelength of peak absorption to ~500nm
Do cone opsins bind an identical chromophore to rods?
Chromophore is identical but different charge interactions tune the absorption to different wavelengths
What happens following absorption of a single photon
What does this lead to
The chromophore isomerise from 11-Cis to all-trans retinal
This photoisomerisation leads to a catalytically active form of rhodopsin known as metarhodopsin II
What is R*
Catalytically active metarhodopsin
What happens once R* has completed its role
all-trans retinal dissociates slowly from opsin
Rhodopsin is now in its bleached form and must be regenerated before it can be used again
How long does it take for all trans retinal to dissociate
100-1000s
Can regeneration of bleached rhodopsin occur within the photoreceptor itself
No
It must be performed in the retinal pigment epithelium (RPE)
What is the RPE
Retinal pigment epithelium
The cells of which encompass the apical processes of the outer segments
Describe the process of photo pigment regeneration
Retinal is reduced to all-trans-retinol, which is transported out of the photoreceptor to the RPE
It is then converted back to 11-cis-retinal
It is then transported back to the photoreceptors where it rejoins the bleached opsin to form rhodopsin
After bright light how long can it take before rhodopsin is fully regenerated
30 minutes or more
What is retinal derived from
Vit A
What can vitamin A deficiency lead to
Night blindness
What is vitamin A
11-cis-retinol
Where can rapid rhodopsin regeneration occur
There is an alternative pathway via Müller cells which allows rapid regeneration in cones
How does phototransduction take place
Overview and specifically (give cascade)
Via a GCPR cascade, culminating in the destruction of cGMP (a negative internal transmitter)
R* activates transducin (a G protein)
Transducin and rhodopsin diffuse freely in the disc membrane
Each transducin molecule activates 1 PDE
PDE hydrolyses cGMP into inactive 5’GMP
Reduced cGMP results in closure of cyclic nucleotide gated cation channels, leading to a hyperpolarising response
How many transducin molecules can R* activate
by random collisions R* can activate >1000 transducin molecules
What is a PDE
Phosphodiesterase
Which molecule is in high concentrations in rod cytosol in the dark
Why is this
cGMP
cGMP continuously opens cyclic nucleotide gated cation channels, preventing a hyperpolarising response
How is a response to light terminated in rods
Guanylyl cyclase resynthesises cGMP
Is rhodopsin the only intracellular signal transduction cascade involving G proteins?
Eg?
No
Other cascades use GPCRs homologous to rhodopsin but which are activated by a ligand rather than light
β adrenergic and muscarinic receptors and odorant receptors in the olfactory system
How many G proteins can 1 rhodopsin molecule activate per second?
Each molecule of PDE hydrolyses how many cGMPs?
How many molecules are hydrolyses per photon?
150/s
600 cGMP molecules/ second
10^5 per photon
What percentage of the cyclic nucleotide gated cation channels that are open in the dark are open in light
This blocks the entry of how many cations?
As low as 3%
10^6 cations
What is retinitis pigmentosa (3)
A progressive hereditary retinal degeneration
Gradual onset of night blindness in adolescence, leading to loss of all peripheral vision by adulthood or even total blindness
PDE and the light sensitive channel can also be affected in other cases of hereditary retinal disease
What causes retinitis pigmentosa
No single cause of hereditary RP but 5-10% of cases are caused by mutations in the gene for rhodopsin
What is the dark current
In darkness, when [cGMP] is high, many channels are open and there is a continuous current of Na+ and Ca2+
What is the voltage of a rod or cone when the dark current is circulating
Depolarised to -30mV
What is the outward arm of the circulating dark current
What happens to these channels during illumination
Efflux of K+ in the inner segment
K+ continues to exit through these channels - this results in hyperpolarisation
Is an ion pump needed in rods and cones for the dark current?
A Na/K pump maintains ionic gradients
What happens to photoreceptor channels with flashes of increasing intensity
More and more channels opens causing a graded hyperpolarisation
What is the saturated hyperpolarisation of photoreceptors
When is this reached
How is saturation avoided
-75mV
When all cGMP gated channels are closed
Photoreceptors light adapt by resynthesising cGMP via guanylyl cyclase
What equipment would you use to measure the dark current
Suction electrode
How can you detect single photons
With very dim lights
Define what it means to light adapt
Why would you do this
Reduce sensitivity as the steady light intensity increases
In order to avoid saturation and to allow operation over a wide range of background intensities
What ion mediates photoreceptor light adaptations
What does this ion inhibit which is relevant to light adaptation
Ca2+
Guanylyl cyclase
Describe the regulatory feedback loop of calcium entry by light sensitive channels
In darkness, calcium enters via cyclic nucleotide gated channels, inhibiting GC
When cGMP is reduced by light, the channels close, less calcium enters the cell, but continues to be extruded by a Na/Ca/K exchanger
The resulting drop in calcium relieves the inhibition of GC so more cGMP is synthesised to counteract the effect of excitation
True or false
Rods saturate at relatively high intensities
False
rods saturate at relatively low intensity and vision under photopic conditions is mediated entirely by cones, which can continue adapting under the brightest conditions
How do the basic processes of transduction and adaptation appear to be between rods and cones
The same, with important quantitive differences
What are the quantitive differences between transduction and adaptation in the rods and cones (2)
Cones are 50 times less sensitive than rods and cannot detect single photons
Conal responses are much faster than those of rods
Do rods or cones mediate colour vision
Cones
Colour -Cones (think C)
Define colour vision
The ability to distinguish different objects on the basis of the spectral reflectance independently of their intensity
Each cone absorbs maximally at a particular wavelength. What are the possible wavelengths?
420, 534 or 564nm
Can a single cone be useful for colour vision
Explain
What is this principle
No, alone it provides no colour information
A green: might absorb 10 times fewer read photons than green photons, but 100 incident read photons would nevertheless be indistinguishable from 10 green photons
The principle of univariance
Given the principle of univariance, how is colour vision therefore achieved
By comparing the output of different cones
Describe the trichromatic system
How many colours can it see
Based on 3 cone classes (red, blue, green) with different photopigments
2 million colours
How can we define colour
What theory is this the basis of
By the ratio of excitation in the three current colour classes
Young Helmholz trichrome Theory
Equal excitation of all 3 cones results in what colour
White
What are the commonest cases of hereditary colour blindness
Where either red or green pigment is missing resulting in dichromatic vision
The blue may be missing but this is rare
Describe colour vision in non-primate mammals
Dichromatic, expressing only blue and yellow opsins
How did the red opsin evolve
Why do we think this
By duplication of the green opsin 35 million years ago in Old World Monkeys
Red and green opsin genes share extensive homology (98% identical) and are adjacent on the X chromosome
Because of the proximity of the red and green genes, what can happen
It is not unusual for the DNA strand to align incorrectly resulting in unequal homologous recombination
When this happens between genes (unequal intergenic recombination) loss/ duplication of genes can occur
Or hybrid genes form, which may be functionless, but may form novel rhodopsins with shifted absorption spectra
How can deuteranopia be explained
The absence of the green gene (green-dichromacy)
How can protanopia be explained
Loss of red gene
Red-dichromacy
When do anomalous trichromats emerge
When hybrids are formed with shifted spectra sensitivities
Hence a hybrid green-red gene may give rise to deuteranomaly whereas a red- green hybrid may cause protanomaly
What is tritanopia
Loss of blue pigment
It is v rare
Blue cones are excluded from the fovea, resulting in foveal tritanopia
Blue cones comprise what % of cones
10%
What does the retina contain (6)
photoreceptors four major classes of interneurons: bipolar cells (BC), horizontal cells (HC), amacrine cells (AC) and ganglion cells (GC). There are also glial elements known as Müller cells (MC)
How are the cells in the retina organised
distributed
in five well-defined layers, which can be subdivided into three nuclear layers (containing cell bodies) and two plexiform layers (containing axons and cell processes).
What are the 5 layers of the retina
Outer Nuclear layer Outer plexiform layer inner nuclear layer inner plexiform layer ganglion cell layer
What cells are in each of the layers of the retina
Outer nuclear layer (ONL) - photoreceptor cell bodies.
Outer plexiform layer (OPL) - synapses between photoreceptors, bipolars and horizontal cells.
Inner nuclear layer (INL)- bipolar,
horizontal and amacrine cell bodies
Inner plexiform layer
(IPL) - synapses
between bipolar, amacrine and
ganglion cells.
Ganglion cell layer (GCL)- cell bodies of
ganglion cells.
What is the direction of information flow in the retinal layers?
What are the neurotransmitters involved?
from photoreceptors to bipolar cells (IB/FB)
to ganglion cells (G).
All of these cells use glutamate as neurotransmitter
Which cells mediate lateral inhibition in the retina
Which NT is used
horizontal cells
GABA
What are amacrine cells? What are they used for and where are they found?
mediate a diverse collection of interactions in the inner retina, and use many different transmitters.
Which cells photoreceptors synapse with?
ONLY with bipolar cells and horizontal cells
What cells do ganglion cells in the retina receive input from
bipolar and amacrine cells
Which cells mediate the output of the retina
carried by the axons of ganglion cells which together form the optic nerve
Do retinal neurons use action potentials?
Most retinal neurons use graded potentials.
Only ganglion cells use action potentials, whilst amacrine cells usually only fire spikes in response to strong stimuli
Why are graded potentials used in the retina and not action potentials
they are a more efficient means of transmitting information over short distances
Does the retina exhibit divergence or convergence
both divergence (for parallel processing) and by convergence (for spatial summation).
Overall, exhibits convergence,
possessing 6 million cones, 120 million rods, but only 1.5 million ganglion cells.
Where is the highest resolution on the retina
Does convergence or divergence take place here
fovea
there are ~3x more ganglion
cells than cones (net divergence)
Describe how the cells of the peripheral retina reflect its lower resolution
there is only one ganglion cell for every ~16 cones (net convergence and loss of spatial information).
What is characteristic of the photoreceptor synapses in the retina
presynaptic ribbon
What is the presynaptic ribbon characteristic of photoreceptor synapses
a modified presynaptic
density characteristic of synapses that
transmit graded signals.
Why is the term synaptic triad used when discussing photoreceptor synapses
The postsynaptic targets always include processes from both bipolar and horizontal cells
Describe the synapse of cones in the eye (2)
Cone terminals end in a large synaptic
swelling - the cone pedicle.
Cones form both invaginating
and flat synaptic contacts
How many synapses can be associated with 1 cone pedicle
What does this reflect
up to 30
divergence of the cone signal to numerous bipolar cells
Describe the synapse of rod synaptic terminals
called spherules
smaller than cones - only 1 synapse per rod (ie no divergence)
How many rods can lead into 1 bipolar cell
many
indicates convergence and a loss of spatial resolution
Define the receptive field of a visual cell
the area on the retina (or its projection in the visual field) from which its activity can be influenced by light.
What is the purpose of a receptive field
serves to define the position of a stimulus within the
retina, while its size corresponds to the degree
of convergence.
What does a visual cell’s receptive field usually contain
both excitatory and
inhibitory regions. Different spatial regions
of the receptive field may also differ in their
sensitivity, for example to wavelength.
What kind of receptive field is typical of a retinal neuron
centre surround, with a circular receptive field centre and a concentric surrounding annulus
Illumination of the receptive field centre and the surround have antagonistic effects on the cell’s overall response.
