Case 3 - Vision Flashcards
What are the 3 layers of the eye?
Fibrous tunic - outer layer, sclera and cornea (avascular layer)
Vascular tunic - includes iris, choroid and ciliary body
Neuroretina tunic - neural layer, contains the retina, where images are formed
Where is visual acuity the highest?
Fovea
Compare rods and cones
Rods: low-threshold for light, sensitive to low-intensity light (as has more opsin) therefore function well in darkness. Responsible for low-acuity, non-colour vision
Cones: high-threshold for light, operate best in daylight. Higher visual acuity and colour vision
Where are rods and cones found?
Rods - round periphery of retina
Cones- round the fovea (centre of macula)
What is the proportion of rods and cones to neurones?
1 cone to 1 neurone
Multiple rods to 1 neurone
What controls the size of the pupil?
The iris - circular muscles contract to reduce the pupil in size (bright light), and radial muscles contract to increase pupil size (dim light)
What is a consensual response?
When you shine light in one eye, the pupil of the other eye will constrict (= a consensual response)
What happens when there is an efferent defect of the pupillary light reflex?
The ipsilateral eye doesn’t react to light but the other eye does
What happens when there is an afferent defect of the pupillary light reflex?
What happens in one eye also happens in the other
What is the pupillary response to light if a patient has a relative afferent pupillary defect (Marcus Gunn pupils)?
Both pupils will remain dilated (no constriction) when light is shone into the ipsilateral eye to the lesion (i.e. L optic nerve lesion, no response of either eyes if shine light onto L eye)
However, if shine light onto the other eye (i.e. R), both eyes will constrict (normal response)
What can cause a relative afferent pupillary defect?
Tumours, glaucoma, optic neuritis, nerve compression
What is the test used to diagnose Marcus Gunn pupils?
Swinging light test - shows normal constriction with light over unaffected eye but swinging to the affected side leads to dilation of eyes as there is no response
What is the pupillary response to light if someone has third nerve palsy (efferent defect)?
The affected side will have a dilated pupil that does not respond to light shone in either eye.
As afferent is normal, the contralateral pupil will contract when light is shone into the affected eye and in response to its own light
How does the shape of lens change with near vs far vision?
Far vision (distant object) - lens is flattened as ciliary muscles relax and increase tightness in suspensory ligaments
Near vision (close object)- lens is rounded as ciliary muscles contract and decrease tension in suspensory ligaments
What condition is caused when the lens is too convex? How is it corrected?
Myopia: near-sightedness, the lens is round so struggles to focus light for far objects, i.e. light convergence point falls short of the retina
Corrected with a concave lens
What condition is caused when the lens is too concave? How is it corrected?
Hyperopia: far-sightedness, light convergence point falls behind the retina so near objects are blurry
Corrected with a convex lens
How are each of the 6 extra-ocular muscles tested?
SR: look up and out
MR: look in
LR: look out
IR: look down and out
IO: look up and in
SO: look down and in
How are the extra-ocular muscles innervated?
All innervated by oculomotor nerve apart from LR = CN VI (abducens) and SO = CN IV (trochlear)
What is strabismus?
Squinting due to misalignment of visual axis - a lack of coordination between the eyes. It may be linked to amblyopia (poor vision in one eye), but stabismus does not necessarily mean the patient has have poor vision
What can cause stabismus?
Muscle weakness, genetics, brain lesions, uncorrected hypermetropia (i.e. in attempt to accomodate the eye turns in)
What are the symptoms of stabismus?
Double vision, headaches, eye strain
How is stabismus treated?
Glasses/ contact lenses, eye patches, surgery
What is version of the eyes?
When they move in the same direction as each other, i.e. supraversion = both eyes looking up
What is vergence of the eyes?
When eyes move in opposite directions, i.e. lines of sight don’t remain parallel
What is the difference between estropia and exotropia?
Estropia = convergent squint, where one eye turns in
Exotropia = divergent squint, where one eye turns out
What is the difference between hypertropia and hypotropia?
Hypertropia - one eye turns up
Hypotropia - one eye turns down
What does the snellen test for and how is it done?
Tests visual acuity
Chart of letters with a number above each line, which refers to the distance from which a person with normal vision should be able to read, i.e. largest letter numbered 60 = should be able to see at 60m
Chart is always viewed at 6m (20 ft)
How is the snellen fraction calculated?
Snellen fraction = viewing distance (m) / number on test line
What is a normal snellen fraction? What is a better than average fraction?
Normal = 6/6
Better than normal = 6/5
Describe the layers of the retina in order that light hits
1: Ganglion cell layer
2: Inner plexiform layer
3: Inner nuclear layer
4: Outer plexiform layer
5: Photoreceptor layer (outer nuclear)
What do the plexiform layers contain?
Connections between cells/ neurons
i.e. the inner plexiform layer = cell bodies of bipolar cells and amacrine cells, which synapse with ganglion cells
What is rhodopsin?
Protein that absorbs light, composed of opsin and retinal
How does retinal change with absorption of light?
Changes from 11-cis retinal isoform to all-trans retinal, i.e. removes the kink in the carbon chain (between 11th and 12th carbons)
What is opsin?
A G-protein coupled receptor, which amplifies isomerisation of retinal into a signal.
Name the agonist and inverse agonist for opsin and what the effects of them binding are
Inverse agonist = 11-cis retinal, binding suppresses signalling by the receptor (keeps opsin inactive, i.e. when its dark)
Agonist = all-trans retinal, binding initiates signalling (I.e. when its light)
State the steps of the phototransduction cascade
1: photon of light absorbed by rhodopsin; 11-cis retinal converted into all-trans retinal
2: activated opsin binds to G-protein, leading to dissociation of transducin and frees the alpha subunit
3: transducin/ alpha subunit binds to GTP which activates cGMP phosphodiesterase
4: Hydrolysis of cGMP, thus cGMP concentration decreases and ion-gated channels close
5: Na+ channels close leading to hyperpolarisation (IPSP)
6: Reduced glutamate release
7: glutamate stimulates bipolar cells (EPSP) to generate a receptor potential = release of glutamate
How are on/off patterns established?
Glutamate can bind to 2 types of receptors - ionotropic and metabotropic.
Binding to ionotropic receptors leads to hyperpolarisation and thus inhibition of bipolar and horizontal cells = OFF pattern
Binding to metabotropic receptors leads to depolarisation and thus excitation of bipolar and horizontal cells = ON pattern
How do photoreceptors respond to light?
With graded hyperpolarisation:
Photoreceptors in dark = depolarised, release lots of glutamate
Photoreceptors in light = hyperpolarised, release less glutamate
Give an example of metabotropic and ionotropic glutamate receptors
Metabotropic = mGluR6
Ionotropic = AMPA, Kainate
What is the range of wavelength for visible light?
400-758nm
What wavelength are rods sensitive to?
peak sensitivity = around 500nm
What is the role of horizontal cells?
Enable lateral interactions between photoreceptors and bipolar cells that maintain the visual systems sensitivity
What is the role of amacrine cells?
Post-synaptic to bipolar cells and presynaptic to dendrites of ganglion cells
How do horizontal cells modulate photoreceptors?
Photoreceptors release glutamate onto the horizontal cells, so they then release GABA which inhibits the photoreceptors
What are the next steps in image formation following the reduction in glutamate from phototransduction?
1: Little glutamate release stimulates bipolar cells (EPSP)
2: Generates a receptor potential, which leads to release of glutamate
3: ganglion cells stimulated to produce an action potential, which increases AP sent down the optic nerve
4: signal transferred to primary visual cortex in the occipital lobe for perception
How are lighter/ darker areas of an image distinguished?
‘ON’ population:
Lighter areas = high firing of ganglion cells
Darker areas = low firing of ganglion cells
‘OFF’ population:
Darkest areas = high firing
What is the purpose of horizontal cells?
From light to dark, they help us to adapt to changes in light levels = maintains sensitisation of photoreceptors
How do amacrine cells modulate action potentials of ganglion cells?
Release chemicals such as dopamine, GAAB, ACh. These are thought to inhibit ganglion cells = modulate AP
How do cones establish colour vision?
They vary in sensitivity to different colours, i.e. a ‘red cone’ = more sensitive to red light (than green light) would be hyperpolarised in response to red light.
It is the comparison between different coloured cones that distinguishes colour
Are horizontal cells excitatory or inhibitory?
Inhibitory, but the degree of inhibition changes due to intensity of light (i.e. bright light = more inhibition)
What happens when you move from a dark to light area?
1: pupil constricts to focus light rays onto the centre of the macula (fovea), where there are high numbers of cones
2: rhodopsin is continuously broken down into all trans retinal
3: rods turn off and retinal sensitivity to dim light decreases
4: cones turn on so visual acuity and colour vision increases
How long can it take for your eyes to fully adjust when moving from a dark to light environment?
5-10 minutes
What happens when you move from a light to dark area?
1: pupils dilate so light rays can spread out and hit the outer peripheral parts of the retina, where rods are located
2: rhodopsin accumulates and retinal sensitivity increases (to dim light)
3: rods turn on and cones turned off - this is because low-wavelengths of light (low light intensity) cannot activate cones due to their high threshold
4: visual acuity and colour vision decreases
How long can it take for your eyes to fully adjust when moving from a light to dark environment?
Why does it differ to moving from dark to light areas?
20-30 minutes = takes longer as rhodopsin has to accumulate before retinal sensitivity can increase
What is colour blindness caused by?
X-linked recessive disorder - lack certain types of photopsin pigments, usually red and green = thus cannot see red or green wavelengths of light
What is sensation?
Low-level psychological process; a detection of stimuli via sensory organs (transmits information to the brain)
What are the 5 modalities of sensation?
Taste, olfaction, somatosensation, vision, audition
What does somatosensation include?
Covers our body’s senses:
- Detects touch
- Thermoception (temperature)
- Vestibular sense (balance)
- Proprioception (internal feedback from muscles)
- Nociception
What is perception?
Interpretation of what is represented by a sensory input, i.e. recognising objects, sounds and people. This occurs unconsciously but can be influenced by higher level cognitive processes (i.e. expectations)
What are the principles of Gestalt’s psychology?
- Ground principle: perceive objects as in foreground or background
- Adjacency/ proximity principle: elements of a visual scene that are close together tend to be grouped together
- Similarity principle: similar elements perceived as together
- Good continuation: elements that follow a line tend to belong together
- Law of closure: supplies missing information if figure is complete
- Principle of common fate: if on same movement trajectory then perceived together
What can a pituitary tumour lead to?
Bi-temporal hemianopia (information from the temporal visual fields is lost) because it presses on the optic chiasm
What are the 2 types of tumour and how may they differ in upon clinical presentation?
Hormone-secreting: patients normally seek advice prior to visual defects due to excessive secretion of hormones = systemic symptoms
Non-hormone secreting: vision loss occurs first
What is felt stigma?
May manifest as shame, guilt or depression, and behaviours such as self-stigmatism, withdrawal from society and unwillingness to speak up. It is based on how you act to a perceived threat without evidence of it
What is enacted stigma?
May be direct, i.e. person treated less favourably due to a characteristic, or indirect, where people from groups are more likely to suffer effects of rules, systems or procedures.
Consequences = loss of employment, denial of opportunities
What is courtesy stigma?
By association, i.e. a parent/ child of someone with a disability not feeling included. Family may cope with their own feelings of shame or guilt
What is the social model of disability?
Disability is caused by how a society treats disabled people, i..e a persons activities are impaired by the environment and not by their condition
What are the steps in the visual pathway to the cortex?
1- Light enters eye: nasal visual fields to temporal hemiretina, and temporal visual fields to nasal hemiretina
2- Info transmitted in the optic nerves
3- At the optic chiasm, nasal hemiretinal fibres decussate, whereas temporal hemiretinal fibres stay ipsilateral
4- Forms the L and R optic tracts (both nasal and temporal fibres)
5- Reaches the lateral geniculate nucleus in the thalamus:
- Ipsilateral fibres synpase at 2, 3, 5
- Contralateral fibres synpase at 1, 4, 6
6- Goes to optic radiations:
- Superior retinal fibres (Barums loop) go through the parietal lobe
- inferior retinal fibres (Meyers loop) go through the temporal lobe
7- Enters the striate cortex (primary visual cortex)
What would a lesion of the R optic nerve cause?
Right monocular blindness (full blindness in R eye)
What would a lesion of the optic chiasm cause?
Bi-temporal hemianopia (loss of temporal visual fields)
What would a lesion of the R optic tract cause?
L homonymous hemianopia: loss of vision through left visual fields (i.e. temporal field of L eye and nasal field of R eye)
What would a lesion of R optic radiation -superior retinal fibres cause?
Left inferior homonymous quadrantopia, i.e. loss of vision from left inferior visual fields
What would a lesion of R optic radiation - inferior retinal fibres cause?
Left superior homonymous quadrantopia, i.e. loss of vision from left superior visual fields
What would a lesion in the R occipital lobe cause?
Left homonymous hemianopia with macula sparing (as macula has other sources of blood, i.e. MCA)
Compare the use of a spherical vs cylindrical lens
Spherical = used to correct near or far sightedness, i.e. myopia or hypermetropia
Cylindrical = used to correct astigmatism, i.e. uneven cornea
How do spherical and cylindrical lenses refract light?
Spherical lenses= focus light onto a specific point
Cylindrical lenses= focus light into a linear/ vertical line
