Vision Flashcards
Where is the supraorbital notch? What passes through the notch?
half way of the supraorbital margin
supraorbital artery, vein and nerve
What forms the roof of the orbit?
frontal bone, lesser wing of sphenoid
what forms the floor of the orbit
maxilla, zygomatic and tiny pit of palatine
What forms the lateral wall of the orbit?
zygomatic, greater wing of sphenoid
what forms the medial wall of the orbit
maxilla, lacrimal bone, ethmoid, body of the sphenoid
Which bones are very susceptible to fracture
ethmoid and lacrimal
How much of the eyeball is formed by the sclera?
5/6
What is the function of the sclera
maintain shape of globe
offer resistance to internal and external forces
provide attachments for extra-ocular muscles
why is sclera so hard to dissect?
there is a lot of collagen laid down in whirls to offer extra strength
What is the function of the cornea? Where is it in the eyeball?
does 60% of the refraction for the eye
it’s found on the anterior 1/6th of the eye
What makes the cornea ideal for vision?
it’s avascular and transparent
how many layers are in the cornea? What are the names of the layers
five epithelium Bowman's Stroma descemet's membrane endothelium
the epithelium is a ______ membrane
the stroma is full of _______ and is continuous with _______
the endothelium is very specialised because it does not _____ or _______. It controls _________, and any damage will cause the cornea to _______
mucus collagen sclera divide regenerate water balance swell
What is the makeup of the cornea? How does it make the cornea transparent?
packets of collagen laid down in organised layers
adjacent lamellae lie at angles to each other but fibrils within a lamella run in parallel
this creates “destructive interference”, where there is no interruption for light to pass
The collage fibrils of cornea are _______ spaced and ______ in diameter. There are about ________ lamellae in the stroma.
evenly
uniform
200-300
why is the sclera is opaque
all collagen fibres are at different size and length
the layers are not organised
When there is damage to the cornea, to what extent will there be scarring?
if the stroma is damaged, the ordered layers might be disrupted, so you may get corneal scar
damage to endothelium is irreversible
Where is the anterior chamber angle
it’s at the junction of iris and cornea
what is the function of anterior chamber angle?
it’s the draining site for the aqueous humour of the eye
what happens if the fluid is not drained?
pressure builds up inside the orbit
How is the aqueous humour produced and where does it travel to?
It is produced in the ciliary body, passes through the pupil, and drains via the anterior chamber angle, through trabecular meshwork, into the canal of Schlemm then to the exterior
what is the uvea made of?
ciliary body, iris, and choroid
What are the functions of ciliary body
formation of aqueous humour
tethers lens via ciliary process
accommodation via ciliary muscle
what are ciliary processes
epithelial-like cells that form aqueous humour
the most medial area has ligaments that attach to the lens
What are the ligaments of ciliary processes called?
zonules
what is the most important fibre of ciliary muscle? What is it innervated by?
circular muscle, innervated by PNS from CNIII
When we try to look close up, the ciliary muscle ______, and the ______ in the zonules is lost, making the lens ______
When we try to look in the distance, the ciliary muscle _____, providing tension on the ________, which ______ the lens
contracts
tension
bulgy
relaxes
zonules
flattens
What is presbyopia
the loss of accommodation with age because the lens is inflexible and cannot change shape as well
Iris is the ________ of the eye which forms the _______. There are two muscles - SNS innervates __________, which ______ the pupil, and PNS innervates ___________, which _______ the pupil
aperture pupil dilator pupillae dilates sphincter pupillae constricts
What is the function of choroid?
has blood supply and provides nutrition for mostly the outer retina
How many layers are there in the choroid? Which layer is the most important layer
3 layers
choriocapillaris
what is the edge of the retina called?
Orra serrata
what is the relationship between the fovea and macula
macula is a yellow colour spot in the retina, whereas fovea is an area within the macula which has highest density of cones
T/F the retina is made of layers of neurons
True, with 6 principle types of neurons
List 4 properties of the fovea
high visual acuity
avascular
high density of cones
no rods
T/F The fovea contains layers of neurons
False, everything is shuffled aside except for photoreceptors
What is the lamina cribosa?
A patchy section of sclera that crosses the path of the optic nerve
How much sclera continues as the lamina cribosa? How much continues parallel with the optic nerve
1/3 forming the lamina cribosa
2/3 parallel to optic nerve
how is lamina cribosa related to eye pathology?
Because the sclera is patchy, it is more sensitive to change in intra-ocular pressure, hence can contribute to optic nerve damage where there is increased pressure
Which arterial branch gives tributaries for the blood supply of the eye
opthalmic artery, first branch of the internal carotid
What are the main arteries supplying the eye?
central retinal artery
long and short posterior ciliary artery
anterior ciliary artery
Which artery pierces the optic nerve? Which arteries pierces the globe?
central retinal artery
central retinal, long and short posterior ciliary artery
where the the anterior ciliary artery pierce the globe?
travels with a muscle and pierces at the conjunctiva
Describe the dual blood supply of the retina
the inner retina is supplied by the central retinal artery
the outer retina is supplied by the posterior ciliary artery
what structures can be found on the eyelids?
skin glands and eyelashes conjuntiva muscles lacrimal apparatus
what are the muscles of the eyelid and what are their function?
orbicularis oculi - sphincter muscle to depress upper lid
levator palpebrae superosis - elevates upper lid
What are the two muscles of the eyelid supplied by? Do they ever work with each other?
LPS supplied by CNIII
Orbicularis Oculi supplied by CNVII
No, when one contracts, the other always relaxes
What is the macula densa important for?
central vision
What are the two fundamental factors affecting visual acuity?
neural and optical factors
What is visual acuity
the ability to see fine detail
How is visual acuity tested?
by the Snellen or LogMAR chart
What does a Snellen result of 6/12 mean?
what a normal person sees at 12m distance, a 6/12 person needs to be at 6m to see it
What are the optical factors affecting visual acuity?
pupil size
clarity of optical media
refractive error
What is cataract?
clouding of the lens
Define each term Myopia Hypermetropia Astigmatism Presbyopia
Myopia = short-sighted Hypermetropia = long-sighted astigmatism = deviation from spherical curvature presbyopia = long-sightedness due to ageing and lack of elasticity of the lens
Define
1) photopic
2) scotopic
1) related to daytime
2) related to night time
What is vision at the fovea determined by?
the number of cones
where is the
1) cones
2) rods
most concentrated?
1) at the fovea
2) at ~8 degrees off the centre of retina
What are the six principle retinal neurons?
rods, cones, bipolar, horizontal, amacrine, ganglion cells
What cells are found in the outer and inner plexiform layer of the retina
outer: photoreceptors and horizontal cells
inner: bipolar cells and amacrine cells
Why do cones provide better visual acuity when rods outnumber cones 20:1
connectivity - cones have one to one relationship with ganglion cells, whereas many rods integrate signals onto a ganglion cell
Which cells are responsible for lateral interactions
horizontal and amacrine cells
How many types of bipolar cells can be found in the retina?
10 - 1 type of rod bipolar, and 9 types of cone bipolar
There are two broad types of bipolar cells - the off BCs ________ when excited, and the on BCs _______ when excited
hyperpolarise
depolarise
Horizontal cell is important for _________ . It receives inputs and outputs to the _________ and signals via the neurotransmitter ________. Hence, HC is mostly _______
lateral inhibition
photoreceptors
GABA
inhibitory
There are many types of amacrine cells, but structurally, they are ________ and is important for ________ . They use neurotransmitters like _____ and _____ , and is important for detecting ________
axonless lateral inhibition GABA glycine movements
There are many types of Ganglion cells, but _____ and ______ are the most important types. _______ is the main neurotransmitter. GCs are the first cell of visual pathway to __________
magnocellular
parvocellular
glutamate
fire action potentials
T/F the retina is purely a light detector
False, it also detects contrast
How do ganglion cells respond to light?
increase or decrease the rate of firing action potentials
Do neuro signals differ when light is shown onto different parts of the receptive field? If so, how does it differ?
Yes
light on the centre of receptive field will increase AP rate
light on the surrounding of receptive field will decrease AP rate
How does light get transduced to neural impulse?
the opsin, attaching to retinal, is activated by light. This changes the conformation of cis-retinal to trans-retinal, and dissociates the opsin so it activates transducin, which activates phosphodiesterase
phosphodiesterase decreases cGMP level, and closes Na channel and reduces Na influx, so the cell hyperpolarises
T/F Only ON bipolar cells activate with light signal
False, OFF and ON both get switched on at the same synapse
When light is on, the photoreceptor will be ________, leading to a _______ of glutamate release. Glutamate is _______ to the ON cell and _______ to the OFF cell. Therefore, the _______ of glutamate will lift the _______ on the ON cell, and it will ________. The opposite will occur to OFF cell, and it will _________ .
hyperpolarised reduction inhibitory excitatory reduction inhibition depolarise hyperpolarise
When light shines on the surrounding photoreceptor, the photoreceptor _______ and there is a _______ in glutamate. Glutamate is ________ to horizontal cell, so a ______ of glutamate will _______ the horizontal cell and ______ the amount of GABA released, therefore further _______ the ON bipolar cell
hyperpolarises reduction excitatory reduction hyperpolarise reduce depolarise
what is glaucoma
group of ocular disorder that ultimately leads to increased intra-ocular pressure
What do M cells look like and what do they detect?
they have large cell body and large dendritic tree
it detects motion
what do P cells like look like? What proportion of GCs are P cells? What do they detect?
P cells are small cells with small dendrites. 80% of the GCs are P cells. They detect colour and visual acuity
Where do most of the GCs go to make synaptic connection?
the lateral geniculate nucleus in the thalamus
Where is the optic chiasm?
at the base of the brain anterior to the pituitary, with internal carotid arteries on either side
where do information from the temporal visual field go to in the retina?
the photoreceptors on the nasal side
where GCs cross at the optic chiasm?
the nasal GCs, carrying information for the temporal visual fields
What is the most common cause of optic chiasm lesion? What happen to the visual field?
pituitary macroadenoma
bitemporal hemianopia
What does the LGN act as for the visual neurons?
a relay station
What is the segregation of visual information at the LGN based on 1) L/R eyes 2) types of cells
1) Left side goes to layers 1,4,6, right side to 2,3,5
2) P cells to 3,4,5,6 M cells to 1,2
What is the name of the axon tract after the LGN? Where does it go to?
optic radiation to the occipital lobe primary visual cortex
What area number is V1?
area 17
Where is the occipital lobe? Where is V1 in particular?
occipital lobe is around the calcarine fissure
V1 is in the medial occipital lobe, the most posterior area of the brain
What does each contralateral visual cortex represent?
one half of the visual field
What is the most posterior part of the primary visual cortex for?
Macula cortex for central vision
The more anterior it gets, the more peripheral the vision
Is the visual signal inverted in the visual cortex?
Yes, the top left quadrant of the visual field is encoded in the bottom right part of the visual cortex (V1)
What is occipital sparring
When there is avascular problem in the visual cortex, resulting in homonymous peripheral hemianopia and the central vision is sparred
This is because the macula cortex is supplied by a separate artery
What will happen if lesion occurs in
1) before the chiasm
2) at the chiasm
3) post chiasm
1) unilateral blindness
2) bitemporal hemianopia
3) homonymous hemianopia
Can GC cells still fire AP when the subject is blind? What do these cells target?
A proportion of the cells can (~4000 cells) as long as the retina is intact
There isn’t enough of them for vision, but they project to areas other than LGN
What is the name of the GC that projects to areas other than the LGN?
Melanopsin ganglion cells, which contains photosensitive melanopsin which depolarises the GC
They are the visual pigments in invertebrates
Why does migraine pain get worst with light exposure
Because of the intrinsically photosensitive melanopsin GCs that increases the sensitive of pain fibres
Where do the GCs related to migraine project to?
posterior nucleus of the thalamus
What is the pain sensation in migraine carried by?
from the dura, carried by CNV, which project to brainstem, and to posterior nucleus of thalamus (where GCs act)
Is the pupil reflex lost in a blind person?
No, not if the retina is intact and the ipGCs are still present
What is the pathway of the pupil reflex?
ipGCs leaves half way to the LGN, runs infero-medially to optical pretectal nucelus (OPN)
OPN sends two secondary interneurons to Edinger-westfal nuclei on both sides, and a third neuron carries info back anteriorly to the ciliary ganglion
Finally, an efferent neuron leaves the CG to control the Iris
What is the function of the GCs that project to the suprachiasmatic nucleus of the hypothalamus?
driving circadian rhythm
what is gaze?
stabilising an image of an object on the retina
What are the two components controlling gaze?
oculomotor and head-movement systems
What is saccadic eye movement
shifts the fovea rapidly to a new visual target
what is smooth pursuit?
keeps the image of a moving target on the fovea in a slower fashion
What is intorsion
movement that involves twisting the eyeball to nose
What is extorsion
movement that involves twisting the eyeball away from nose
What is the course of superior oblique muscle
projects medially from the tendinous ring, runs through the trochlea and turns laterally, goes under the superior rectus muscle and insert close to the lateral rectus
What is the primary action of oblique muscles?
torsion, which is something that we can’t see. We see either elevation of depression
how can ocular muscle actions be divided into primary, secondary, and tertiary?
because of the insertion angle. For example, the superior rectus inserts at a 23 degrees angle, which allows elevation and also a twisting action
How do we isolate the movement of superior and inferior rectus muscles?
test the movement in eye abduction
How do we isolate the movement of superior and inferior oblique muscles?
test the movement in eye adduction
What is the action of superior oblique?
depress the eyeball (note that the insertion is posterior to the trochlea, so the pulley system depresses the eyeball)
Describe the course of the inferior oblique
runs laterally from the trochlea. There is no pulley system for IO, and it inserts medially near the medial rectus
How is the course of CNIV different to the two other ocular lower motor neurons?
CNIV crosses the midline before exiting the brainstem, hence innervates the contralateral lateral rectus muscle
What is the function of medial longitudinal fasciculus
it is a white matter tract that connects the various cranial nerve nuclei
If you can’t coordinate eye movement in the vertical direction, where would the lesion be?
in mesencephalic paramedian reticular formation of the midbrain
What would happen if you have a lesion in the pontine paramedian reticular formation (PPRF)
lose coordination between medial and lateral rectus, thus cannot move in the horizontal direction
What is the pattern of action potential for burst neurons
they fire at high frequency just before movements in saccade
Burst neurons drive movements
what is the pattern of action potential for omnipause neurons
fires continuously during saccade. They fire all the time unless burst neurons are firing
What is the role of omnipause neurons?
Omnipause neurons are GABA inhibitory. They tonically inhibit the action of lower motor neurons in order to fix the eye on a target
When moving the eye to the left in saccade, the cortical neuron first reaches the burst neurons in _______, which further projects to ______ on the ipsilateral side. From there, one lower motor neuron will excite the _______ on the ________ eye, while another neuron will project to ________ oculomotor nucleus via ________, which excites the __________ medial rectus
PPRF abducent nucleus lateral rectus ipsilateral (left) contralateral medial longitudinal fasticulus contralateral
T/F the left cortex controls the right brainstem in terms of eye movement
True, the decussation occurs just under the inferior colliculus.
Which system provides information about the position of the head
vestibular system
What is the role of
1) semicircular canal
2) otolith
1) send information regarding head position
2) send information regarding linear acceleration
T/F with acceleration, the neurons of the vestibular centre will decrease firing rate
False, it will increase the firing rate
When the head is turn to the right, the right vestibular system will ______ firing rate, while the left vestibular system will ______ firing rate
increase
decrease
Which muscles do we want to contract in order to maintain central eye field if we move our head to the right? Which vestibular nucleus will be excited? Which abducens nucleus inhibited?
left lateral rectus and right medial rectus
right vestibular nucleus (in the direction of head movement)
left abducens nucleus (in the direction of excited lateral rectus)
Where do LGN neurons project to?
the primary visual cortex
Which zone is the major input zone of V1
layer 4
which zone do M cells terminate at?
layer 4Ca
which zone do P cells terminate at?
layer 4Cb
Where do cells in layer 4Ca (motion) project to?
to layer 4B, which has special cells tuned for motion
What special property do cells of layer 4B have?
some neurons show preference for directional movement. They respond best to bars of light moving in a specific directly
What’s the proportion of the cortex responsible for vision?
40%
Where does the dorsal stream project to and what does it process?
Where does it receive information from?
projects to area MT (middle temporal lobe), specialised for processing object motion
V2, V3, and 4B of V1
In area MT, neurons have ________ receptive field and respond to _________. Almost all cells in area MT are ________, which respond to ________ or ________. A lesion in area MT will cause the visual images to be _______, and can only see ______ motion
large movement directional selective drifting spots waves of light intermittent slow
Which three primary colours create the images we see?
red, green, blue
What determines the colour we see?
how much of each kind of photoreceptors are activated
Red is always compared with ______
_______ is always compared with yellow
green
blue
How do we maximally inhibit a red photoreceptor signal?
shine green light in the periphery
T/F Shining blue light in central vision will maximally inhibit yellow from peripheral vision
True, blue and yellow are contrasting colours, and brain detects colour based on contrast
What happens when red light is shone onto both central and peripheral visual fields?
red is detected and peripheral signals are deleted
Area V4 is part of the ________ stream which receives inputs from _____ and V3 via _______. Neurons of V4 have ______ receptive fields that are both _______ and _______ selective. It is important for recognising object’s _____ and ____
ventral V2 V1 (4Cb) large orientation colour shape colour
Where is area V4?
in the medial aspect of inferior temporal lobe
Where do neurons from V4 project to? What is this area important for?
area IT
important for visual memory and perception of faces
neurons here are tuned to see faces
What are the three groups of colour blindness?
Monochromacy - patient only has one type of cone
Dichromacy - patient has two functional cones
Anomalous trichromacy - most common, all cones working but one is problematic
What is protanope? What is protanomal?
protanope - no red cone
protanomal - abnormal red cone
What is deutanope?
no green cone
what is tritanope?
no blue cone
