Vision Flashcards
What is the range of visible wavelengths?
The visible wavelengths in humans range from 400-700nm
What is the human range of light intensity
The human range of light intensity is 10-1010 photons
What is the greatest site of refraction?
Cornea
What is the focal distance?
The distance from where the refractive surface lies (cornea) and the point where parallel light rays converge is called the focal distance.
What are dioptres?
Reciprocal of focal distance (considering refractive index)
What is the refractive power of the cornea?
42 dioptres
Myopia
Shortsight: If the focal point lies before the retina - the individual is shortsighted
This can be caused by myopia - a too long eyeball
Hyperopia
Longsight: If the focal point is (theoretically) behind the retina the individual is longsighted
This can be caused by hypermetropia - a too short eyeball
Myopia corrective lens
This is corrected with a concave lens - extending the focal length
Hyperopia corrective lens
This can be corrected by a convex lens - shortening the focal length
Astigmatism
A visual astigmatism creates different focus in different planes of the cornea. Often due to irregular cornea shape.
How can you test for an astigmatism
Can be tested for using a hemicircle of radial lines
How many dioptres is the lens responsible for in accomodation?
A few
Mechanism of accommodation
Increase in power of lens caused by contraction of annular ciliary muscle (parasympathetically innervated), which reduces tension in radial zonular fibres, allowing lens to relax to a more convex state (especially on anterior surface).
Presbyopia
Failure of accommodation with age. Usually considered to be caused by lens material becoming stiffer and less elastic with age.
Effects of presbyopia
Longsight, fixed with a convex lens
Cataracts
Condition in which the lens becomes cloudy
Straightforward to treat by removing the lens surgically and replacing it with an artificial lens BUT cataracts still cause millions to be blind.
Glaucoma
Aqueous humor normally flows and circulates around the eye through trabecular network (at the drainage angle)
If this becomes blocked (canal of schlemm), the drainage of fluid is affected
Normal intraocular pressure (IOP), maintained by production of aqueous humour, is about 16.5mmHg, glaucoma, associated with elevated IOP (>21mm Hg).
Amblyopia
Lazy eye, reduced focus
Perimetry
Used to detect defects in peripheral vision
Where is the blind spot
The blind spot in each eye is 10-15deg horizontally on the temporal side of each eye
Cone wavelength and colour
Small, medium and long wavelengths associated with blue, green and red respectively.
How is colour determined?
Relative excitation of different cone types is the basis for colour vision - independent of intensity.
Deuteranomaly
Abnormal middle wavelength (green-absorbing) pigment (5%), (most common).
Protanomaly
Abnormal long wavelength pigment, red, protanopia (absence of that pigment)
Trichomat
Normal colour vision
Anomalous trichromat
One of the cones altered slightly, affects all colours slightly, examples (deuteranomaly and protanomaly)
Why is colour blindness mainly in males?
Genes responsible for the most common forms of colour blindness are on the X chromosome.
How is visual acuity tested?
Snellen chart
Normal visual acuity?
6/6 or 20/20
Letters with gaps about 1 min arc (1/60 degree) can just be read. But under ideal conditions, gaps of 0.5 min can be resolved.
Layers of cells in the retina
Inner limiting membrane Nerve fibre layer Ganglion cell layer Inner plexiform layer Inner nuclear layer Outer plexiform layer Outer nuclear layer External limiting membrane Inner segment / outer segment layer Retinal pigment epithelium
Inner limiting membrane
BM elaborated with Muller cells
Nerve fibre layer
Axon of ganglion cell bodies
Ganglion cell layer
Nuclei of ganglion cells, axons of which become optic nerve fibres, some amacrine cells
Inner plexiform layer
Synapse between bipolar cell axons and dendrites of ganglion and amacrine cells
Inner nuclear layer
Contains the nuclei and surrounding cell bodies (perikarya) of the amacrine cells, bipolar cells, and horizontal cells (send information laterally between cells).
Outer plexiform layer
Projections of rods and cones ending in the rod spherule and cone pedicle, respectively.
These make synapses with dendrites of bipolar cells and horizontal cells.
In the macular region, this is known as the Fiber layer of Henle.
Outer nuclear layer
Cell bodies of rods and cones
External limiting membrane
Layer that separates the inner segment portions of the photoreceptors from cell nuclei
Inner segment/ outer segment layer
Inner segments and outer segments of rods and cones. The outer segments contain a highly specialized light-sensing apparatus.
Retinal pigment epithelium
Single layer of cuboidal epithelial cells.
This layer is closest to the choroid, and provides nourishment and supportive functions to the neural retina, The black pigment melanin in the pigment layer prevents light reflection throughout the globe of the eyeball; this is extremely important for clear vision
Photoreceptor cell common structure
Outer segment located at distal surface of the neural retina
Inner segment, located more proximally
Cell body
Synaptic terminal
Rod vs cone structure
Rods have a long cylindrical outer segment within which discs are stacked
Cones have shorter more tapered segment, continuous with the outer membrane.
Rod vs cone function
Rods signal the absorption of a single photon (responsible for vision under dim illumination) but as light increases, the rod response becomes saturated and they no longer respond to changes in intensity.
Cones are much less sensitive to light but are solely responsible for vision in daylight. They have faster responses - dense packing of cones in the fovea determines our visual acuity.
Primates have three types of cone cells differentiated by the wavelength they can respond to (thus colour)
Photoreceptor resting potential state in the dark
Na+ ions flow through non-selective cation channels into the photoreceptor.
This means the resting potential sits around -40mv
What is the visual pigment in rod cells and its properties?
Rhodopsin (membrane embedded opsin part and light absorbing retinal part)
Rhodopsin upon light exposure
Absorption of proton leads to conformational change - it becomes unstable and splits into opsin and retinal, which changes from cis-retinal to trans-retinal.
What happens to trans-retinal
Trans-retinal is transported from rods to pigment epithelial cells where its reduced to trans-retinol (vitamin A)
Isomerisation of photopigment activates transducin (a G-protein)
Transducin activates a phosphodiesterase (PDE)
PDE reduces the level of cyclic GMP
What does reduced cGMP cause?
Reduced cGMP causes sodium channels to close, and therefore the photoreceptor hyperpolarizes.
This change in the cell’s membrane potential causes voltage-gated calcium channels to close.
This leads to a decrease in the influx of calcium ions into the cell and thus the intracellular calcium ion concentration falls.
What does decreased intracellular calcium lead to?
Less glutamate is released via calcium-induced exocytosis to the bipolar cell.
What does reduction in glutamate release cause?
Reduction in the release of glutamate means one population of bipolar cells will be depolarized and a separate population of bipolar cells will be hyperpolarized, depending on the nature of receptors (ionotropic or metabotropic) in the postsynaptic terminal
What happens when glutamate binds to an ionotropic receptor?
Bipolar cell will depolarize (and therefore will hyperpolarize with light as less glutamate is released).
What happens when glutamate binds to a metabotropic receptor?
Hyperpolarization, so this bipolar cell will depolarize to light as less glutamate is released.
Dark adaptation depends on
Regeneration of the visual pigment from opsin and 11-cis retinal.
Time required for dark adaptation and pigment regeneration is largely determined by the local concentration of 11-cis retinal and the rate at which it is delivered to the opsin in the bleached rods.
Different dark adaptation rates in rods and cones
Rods are more sensitive to light and so take longer to fully adapt to the change in light (maximum sensitivity ~ two hours, 5-10 mins for some vision).
Cones take approximately 9–10 minutes to adapt to the dark.
How are OFF ganglion cells generated?
When glutamate binds to an ionotropic receptor, the bipolar cell will depolarize (and therefore will hyperpolarize with light as less glutamate is released). THUS ROD HPYERPOLARIZATION LEADS TO BIPOLAR HYPERPOLARIZATION.
This hyperpolarization does not activate ganglion cells - thus these are OFF ganglion cells
How are ON ganglion cells generated?
Binding of glutamate to a metabotropic receptor results in a hyperpolarization, so this bipolar cell will depolarize to light as less glutamate is released. THUS ROD HYPERPOLARIZATION LEADS TO BIPOLAR DEPOLARZATION
This depolarization activates ganglion cells - thus these are ON ganglion cells.
Horizontal cells
Make antagonistic lateral connections between receptors and bipolar cells
Horizontal cells are depolarized by the release of glutamate from photoreceptors, which happens in the absence of light. Depolarization of a horizontal cell causes it to hyperpolarize nearby photoreceptors.
Though in the presence of light, the lower glutamate leads to horizontal hyperpolarization and leads to depolarizing of nearby photoreceptors.
Hence, bipolar cells have opposing input from surrounding receptors, and hence respond to local contrast rather than to light per se.
Amacrine cells
Lateral antagonism between bipolar cells and ganglion cells - inhibition interneurons.
Pupillary light reflex
Photosensitive retinal ganglion cells, convey information via the optic nerve.
Some axons connect to the pretectal nucleus of the upper midbrain instead of the cells of the lateral geniculate nucleus (which project to the primary visual cortex).
Axons synapse on (connect to) neurons in the Edinger-Westphal nucleus. Parasympathetic preganglionic neuronal axons in the oculomotor nerve synapse on ciliary ganglion neurons.
Short post-ganglionic ciliary nerves leave the ciliary ganglion to innervate the Iris sphincter muscle of the iris.
Where does optic nerve project to?
Chiasm (partial decussation of visual fields), optic tract, LGN thalamus, optic radiation, V1
Hemianopia
Loss of vision in half of visual field (optic tract lesion)
The artery supplying the primary visual cortex
Calcarine branch of the posterior cerebral
Dark current
Depolarisation caused by inner Na+ current
Colour-opponent cells project
mostly to the
Parvocellular layers
Cells which are insensitive to wavelength project to the
Magnocellular layers
Unlike retinal or thalamic neurons, cortical neurons are often sensitive to the
Orientation/motion of lines and can be sensitive to Binocular disparity.
What are P cells
Parvocellular cells, are neurons located within the parvocellular layers of the lateral geniculate nucleus (LGN) of the thalamus - respond to colour
What are M cells
Magnocellular cells, magnocellular layer of the lateral geniculate nucleus of the thalamus, not colour, contrast detection instead
Properties developed in V1
Orientation, binocular integration and disparity, colour contrast (though note colour starts via Parvocellular in thalamus but not CONTRAST)
Parvocellular pathway projects to
Cytochrome oxidase blobs
Magnocellular pathway projects to
Interblob region
What colour contrasts do retinal ganglion cells respond to?
Red on, green off
Green on, red off
Yellow on (red and green), blue off
Blue on, yellow off
Which LGN layers receive which eyes information?
1,4 and 6 receive the contralateral eye vision, 2,3 and 5 receive ipsilateral
What is the opponent process theory, where does it occur?
Theory of colour vision generation (occurs in the retinal ganglion cells)
Describe opponent process theory when centre colour is activated, use an example.
Red light shines
L cone hyperpolarises
ON ganglion with L cone in centre of receptive field
ON ganglion cell depolarized by red light (activated)
M cone in surround depolarises - depolarises horizontal cell - hyperpolarises OFF ganglion
Describe how colour opponent theory works anatomically.
ON ganglion with L cone in centre of receptive field has mixture of L and M cones in the surround area
M cone synapses onto OFF ganglion cells via horizontal cell
(central ON area in which stimulation tends to excite neural responses and a surrounding OFF area in which stimulation tends to suppress neural responses by lateral inhibition)
Describe opponent process theory when surround colour shone, use an example
Green light shines
L cone depolarises
ON ganglion hyperpolarises
M cone hyperpolarises horizontal cell hyperpolarises OFF ganglion depolarises
How do visual fields decussate?
Fibres from the nasal hemiretina (vision from temporal area), cross the midline to proceed to the contralateral hemisphere.
As temporal hemiretina of one eye sees the same half of the visual field as the nasal hemiretina of the other, partial decussation ensures all the information from one hemifield is processed in the cortex of contralateral hemisphere.
How is the visual cortex split into R/L eye/vision?
L cortex deals with right visual field
R cortex deals with left visual field
Homonymous hemiopia
Half visual field missing (same half) lesion in contralateral tract
Bitemporal hemianopia
Lesion at chiasm (temporal visual field missing)
How is colour processing more complex at the cortex?
‘Double-opponent’ cell is excited by green and inhibited by red in its receptive field centre, and excited by red and inhibited by green in its receptive field surround.
Do single opponent retinal ganglion or parvocellular LGN cells respond selectively to colour contrast?
No, only the cortex
Colour contrasts in the cortex?
RG YB BW
Thick stripes
Contain neurons selective for direction of movement and binocular disparity as well as cells responsive to illusionary contours and disparity cues.
Pale stripes
Contain orientation selective neurons
Thin stripes
Hold cells specified for colour
What does V2 contain?
Thick, thin and pale stripes
Ventral pathway
Identifying what the object is
Dorsal pathway
Movement and identifying location
What sort of ion channels are modulated during visual transduction?
Non specific Na+ channels, Ca2+ channels
What neurotransmitter is released by photoreceptors?
Glutamate
Which cells generate the centre-surround properties of visual receptive fields?
Ganglion cells
Which midbrain structure do you associated with visual localisation?
Superior colliculus
Which is the major thalamic relay for visual information to the cerebral cortex?
Optic radiation
Which cells have their axons within the optic nerve?
Ganglion cells
Which part of the human retina has axons that cross to the contralateral side of the
brain in the optic chiasm?
Nasal
Lesions of right temporal lobe (meyer’s Loop) of the optic radiation on one side produces
A loss of the upper, outer quadrant of vision on the same side in both eyes, known as homonymous superior quadrantanopia or superior quadrantic hemianopia.
The effect of a lesion of the left optic tract
Right homonymous hemianopia
