Vision Flashcards
what forms the supraorbital and infraorbital margins
supraorbital margin - frontal bone
infraorbital margin - zygomatic bone (laterally) and maxilla (medially)
what forms the roof of the orbit
frontal bone and the lesser wing of sphenoid
what forms the floor of the orbit
the maxilla, zygomatic bone and palantine bone
what forms the lateral wall of the orbit
the zygomatic bone and the greater wing of sphenoid
what forms the medial wall of the orbit
the maxilla, the lacrimal bone, ethmoid bone and body of sphenoid
which parts of the orbit are the most vulnerable to fracture
lacrimal bone and the ethmoid bone
what are the functions of the sclera
- maintains the shape of the globe
- offers resistance to internal and external formces
- provides attachments for the EOMs
what gives the sclera the resistance and structure it has
the way that the collagen is laid down within - laid down in whirls = tough
where is the cornea
the most anterior part of the sclera
what is the function of the cornea
principal refracting component of the eyes
what is the special function of the endothelial layer of the corne
it controls the water balance in the eye
why is the cornea transparent while the sclera is not
because the corneal collagen is laid down in a very organised way, uniform in diameter and evenly spaced - where each layer is laid down at slightly different angles
what determines the opacity of the sclera
- the composition of the stroma
- the hydration of the stroma
- the size and distribution of the collagen
where is the anterior chamber angle
the junction between the iris and the cornea
what is the function of the anterior chamber angle
it is the outflow for the aqueus humour to drain out of the eye
what are the key components of the anterior chamber angle that are involved in the drainage of the aqueous humour
the trabecular meshwork and the canal of schlemm
what are the functions of the ciliary body and which parts of the body do each function
- forms the aqueous humour (ciliary epithelium
- tethers lens (ciliary processes)
- accomodation (ciliary muscle)
what is the importance of aqueous humour
- maintains the health of the lens and cornea
- creates intraoccular pressure
which to structures are used for eye accomodation (focussing)
ciliary muscle
zonules
what are zonules
the ligaments that attach between the ciliary processes and the lens
how does the eye focus
the ciliary muscle changes the tension on the zonules that attach to the lens –> influences the shape of the lens
(contraction of muscle - less tension on the ligaments and vice versa)
innervation of the ciliary muscle
innervated by the PNS - non-voluntary muscle
what is presbyopia
refers to the loss of accomodation that occurs with age
what causes presbyopia
the reduction in flexibility in the lens, capsule and zonules (has become quite inflexible)
what are the two muscles that control the iris? What are their function and what are there innervation?
spincter pupillae - constricts the pupil - innervated by the PNS in CNIII
dilator pupillae - dilates pupil - innervated by the SNS
explain the structure and function of the choroid
3 layers of blood vessels underneath the retina - supplies nutrients to the retina
what is the optic disc
it is where the optic nerve enters the back of the eye and where the BVs exit
function of the fovea and macula
allows you to see centralls
what is the orra serrata
the edge of the retina
explain the structure of the fovea
avascular region of the retina where there is a high density of cones and no rods
the optic nerve is formed by….
the axons of ganglion cells as they exit the retina
what is the lamina cribosa
band of dense connective tissue made of collagen fibres that forms a sieve like structure that is part of the slcera that provide some structure as the axons pass out of the eye
all of the blood supply of the eye comes from which vessel
the opthalmic artery (first branch of the carotid artery)
what is the first branch of the opthalmic artery
the central retinal artery - pierces the optic nerve and spans out across the retina
what are the other main arteries of the eye other than the central retinal artery and where are they
- long posterior ciliary artery - travel around to the front of the eyeball
- short posterior ciliary artery - dont travel very far - supplies the optic nerve head and the photoreceptors
- anterior ciliary artery - dont pierce the globe, just come forward with one of the muscular arteries and feed the anterior structures when they get close to the conjunctivae
explain the dual blood supply of the retina
central retinal artery supplies the inner retina
posterior ciliary artery supplies the outer retina (photoreceptors)
what are the 2 muscles of the eyelids and explain their function and innervated
- orbicularis oculi - innervated by facial nerve - depresses the upper lid
- levator palpebrae superiosis - elevates the upper lid - innervated by CNIII
what forms the outer, middle and inner coats of the eye, and what are their general functions?
outer - cornea and sclera –> strength
middle - uvea –> nutrition
inner - retina –> vision
what is visual acuity
the ability to resolve fine detail
what are the optical factors affecting visual acuity
- pupil size (smaller the aperture, the better the VA)
- clarity of optical media
- refractive errors (sually determined by the legnth of the eyeball
what is the visual acuity across the retina? And why is there a change?
fovea - 6/6 - due to the high level of cones
at 5 minutes of arc away from the fovea –> demostrable VA loss- due to the very low level of cones and the high number of cones
what are the 6 neurons of the retina
rods cons horizontal cells bipolar cells amacrine cells ganglion cells
which, rods or cones, allow colour signals
cones
what is the ratio of rods and cones at the retina
rods:cones = 20:1
why is it that even though the rods outnumber the cones that our night vision is poor and our vision through cones is so good
because of the connectivity - how the retina is wired up
what is the “pathway” through the retina?
go through photoreceptors –> bipolar cells –> ganglion cells
(has modulation by the horizontal cells and the amacrine cells)
what are the two main types of bipolar cells
OFF-bipolar cells –> hyperpolarizing
ON-bipolar cells –> depolarizing
how do horizontal cells modulate the optic pathway
they respond to light by using inhibitory neurotransmitter GABA to cause hyperpolarization
how do amacrine cells modulate the optic pathway
release inhibitory NT for lateral inhibition
What is the only cell in the retina that fires action potentials?
the ganglion cells
what is the function of the ganglion cells
the axon asses down the optic nerve to higher brain centres
how do ganglion cells respond to light
by increasing or decreasing their action potential firing rate
what is the receptive filed of a ganglion cell or bipolar cell
the area of retina that when stimulated with light changes the cells membrane potential
what is the photopigment in rods and cones
rods - rhodpsin
cones - cone-opsins
(both attached to vitamin A molecule)
what happens to photoreceptors with light
they are hyperpolarized –> releasing less glutamate
in the dark… explain how the RMP is maintained in the photoreceptors
open cGMP gated sodium channels causing continuous influx of Na ions into the cell –> depolarisation
in the light…. explain the RMP in photoreceptors
light activates rhodopsin –> cascade of events that activates PDE –> cGMP breaks down to GMP - no longer keeps the sodium gate open –> flow of Na ions ceases –> cell is hyperpolarised
what determines the central response and the surround response of the retina
central response - the “through pathway” - Ph-BC-GCs
surround response - determined by inputs from horizontal cells
other than the classification of ON and OFF ganglion cells, what is the other classification of ganglion cells and what are their projections and functions
M ganglion cells –> project to the M layers of the LGN and give information regarding motion
P ganglion cells –> project to the P layers of the LGN and give information regarding colour and visual acuity
where does the optic nerve travel to
the lateral geniculate nucleus of the thalamus
where is the optic chiasm
lies over the sella turcica and pituitary at the base of the brain
which optic fibres cross at the optic chiasm
the nasal fibres taking information from the peripheral visual fields
what other important structures lie around the optic chiasm
pituitary gland
cavernous sinus
internal carotids
explain the layering system of the lateral geniculate nucleus
layers 1 and 2 = magnocellular layers - where M ganglionic cells terminate
layers 3-6 = parvocellular layers - where P ganglionic cells terminate
layers encode different eyes - do not mix information here
what are the optic radiations
the white matter tracts that project from the lateral geniculate nucleus of the thalamus to the visual cortex
where anatomically is the primary visual cortex (V1)
on the medial surface of the occipital lobe around the calcarine fissure
what is the organisation of the primary visual cortex
central vision is encoded on the most occipital/posterior part of V1
As you go more anterior it is encoding more peripheral vision
what visual field defect will you have with a lesion before the chiasm
defect of one eye only
what visual field defect will you have with a lesion at the chiasm
defect in both eyes on the temporal sides (bitemporal)
what visual field defect will you have with a lesion in the optic radiation
affects both eyes on the same side
how is it that if someone is blind, some cells in the retina are still activated
some ganglion cells are intrinsically photosensitive (dont rely on photoreceptor activation) and depolarise in response to light )due to containing melanopsin)
what are melanopsin ganglion cells responsible for
- circadian rhythm
- sleep regulation
- pupil responses
- general information about light levels in the environment
- light allodynia
what is the pathway for pupil constriction
light detected by melanopsin ganglion cells –> through the optic chiasm –> leaves the pathway before the LGN, goes to the pretectal nucleus in the midbrain –> sends out a process to each side of the brainstem to the Edinger-Westfall nuclei –> process goes to the ciliary ganglion and then to the intraoccular eye muscle that controls constriction of the pupils
where will the lesion be if you have an afferent or an efferent pupillary defect
afferent - in the optic nerve or the eyeball
efferent - somewhere between the EW nucleus and the eye
which nucleus of the hypothalamus receives information from the melanopsin ganglion cells
the suprachiasmatic nucleus
what is the importance of melanopsin ganglion cells projecting to the suprachiasmatic nucleus
important for driving circadian rhythms
what is the pain pathway for migraine
dura –> trigeminal nerve –> brainstem –> posterior nucleus of the thalamus
two components of the control of our eye gaze
occulomotor system -moves the eyes in the orbit (whilst head is still)
head-movement system -moves the eye sockets as a whole (whilst head moves)
what are the 5 types of eye movements
saccadic eye movements smooth pursuits vergence vestibular ocular optokinetic
what is saccadic eye movements
shift the fovea very quickly from one place to another
what is smooth pursuits eye movements
keeps the image of a moving target on the fovea
what is vestibular ocular eye movements
holds the image still on the retina during brief head movements
what is optokinetic eye movements
holds the image still on the retina during sustained head rotation or translation
what are the 6 eye extraocular eye muscles
superior rectus inferior rectus lateral rectus medial rectus superior oblique inferior oblique
what are the primary actions of the recti muscles of the eye
superior rectus - elevation
inferior rectus - depression
medial rectus - adduction
lateral rectus - abduction
where is the insertion of superior oblique
close to the lateral rectus
what is the primary action of superior oblique
torsion (rotation inwards and downwards)
where is the insertion of inferior oblique
close to the lateral rectus
what is the primary action of inferior oblique
torsion (rotation outwards and upwards)
why is that all extraoccular muscles have secondary and tertiary actions
because they insert at an angle
which muscles are tested during horizontal eye movement
medial and lateral rectus muscle
what muscles are testing during vertical eye movements while abducted
Superior rectus (upwards) and inferior rectus (downwards)
what movement tests the superior and inferior oblique muscles
vertical eye movements while the eyes are adducted
superior (downward), inferior (upward)
which cranial nerves are involved in eye movements
III - SR, IR, MR, IO
VI - LR
IV - SO
what is special about the trochlear nerve
it is the only nerve that crosses the midline and it exits the brainstem from the dorsal surface –> innervates the SO on the contralateral side
what two structures are involved in the coordination of one eye with the other
medial longitudinal fasiculus (MLF) - white matter tract that connects the various cranial nerve nuclei
reticular formation
what two parts of the reticular formation are involved in the coordination of one eye with the other, and what are their main functions
pontine paramedian reticular formation - horizontal gaze centre (coordination of MR and LR - CNIII and CNVI)
mesencephalic paramedian reticular formation - vertical gaze centre (coordination of SO and SR - IV an CNIII)
what are the two types of neurons in the pontine paramedian reticular formation and when are they activated
burst neruons - fire at high frequency just before movement
omnipause neurons - fires continuously except for when burst neurons are firing
where do the burst neurons project to?
provide:
- excitatory connections with ipsilateral abducens nucleus
- inhibitory connections to the suppress the activity of the contralateral abducens
where do the omnipause neurons project to?
the contralateral abducens nucleus
what is the order of steps of abduction of the R eye beginning from cortical stimulation
- cortical stimulation of the burst neurons in the pontine paramedian reticular formation
- burst neurons stimulate the R abducens nucleus while inhibiting the L abducens nucleus –> R lateral rectus contraction and L lateral rectus relaxation
- PPRF stimulation of contralateral occulomotor nucleus via the medial longitudinal fasiculus while inhibiting ipsilateral occulomotor nucleus via the MLF –> L medial rectus contraction and R medial rectus relaxation
where does the cortical control of eye movements come from
- contralateral frontal eye fields
- posterior parietal cortex
- superior colliculus
- basal ganglia
what is the function of the vestibular system of the ears
- provides info about the position of the head in space
- coordinates the position of the head and eyes
what are the two parts of the vestibular system involved in coordination of eye and head movements
semicircular canals: head position
otolith: linear acceleration
how is the vestibular system activated
movement of the head moves the fluid within towards one direction –> increasing the firing rate in the vestibular system in one ear and decreasing the firing rate in the other
where does the information go to from the vestibular system
carried by the vestibulocohlear nerve to the vestibular nuclei in the medulla
for coordination of head and eye movements.. the vestibular nuclei needs to coordinate with which cranial nerves
III and VI
explain the communication of the vestibular nuclei with cranial nerve nuclei III and VI
vestibular nuclei stimulates contralateral abducens nucleus (which itself stimulates contralateral (again) occulomotor nucleus)
what is the “pathway” for colour vision
P ganglion cells –> optic nerve –> optic chiasm –> optic tract –> LGN –> 4Cb layer of visual cortex –> ventral stream –> area IT
what is the “pathway” for motion vision
M ganglion cells –> optic nerve –> optic chiams –> optic tract –> LGN –> 4Ca layer of visual cortex –> 4Blayer of visual cortex –> dorsal stream –> area MT
which cells are directionally selective
the cells in layer 4B of the visual cortex
cells in area MT
which information travels down the dorsal and ventral streams from the primary visual cortex
dorsal = motion ventral = colour
where does the dorsal stream project to
area MT (middle temporal lobe)
function of area MT
specialised for processing object motion
how is “colour” determined
by the relative activity of the 3 different “coloured” cones in the retina
where does the ventral stream project to
area IT (inferior temporal lobe)
function of area IT
important for visual memory and perception
important for perception of faces
what is anomalous trichromacy
person has all 3 cones, but one expresses an abnormal pigment and doesnt work the same as normal cones
what are the types of colour vision deficiencies
protan - red cone problem
deutan - green cone problem
tritan - blue cone problem
what test is used to check for colour blindness
pseudoisochromatic plates
what is the part of the eye called that is the intersection between the sclera and the iris
the limbus
what gland keeps the anterior surface of the eye moist
the lacrimal gland (innervated via the superior salivatory nucleus)
when looking at a fundoscopy… how can you tell if it is a R or L eye
the optic disc is on the medial side of the retina
the fovea is on the lateral side of the retina
where is the conjunctiva
anteriorly covers the sclera
what is the anterior CHAMBER
the cavity between the cornea and the iris
what can a blockage of the canal of Schlemm cause
glaucoma (increase in pressure)
what is the relative size of the lens of the eye during close and distant accomodation
close - thicker lens
distant - thin lens
what are 8 layers of the retina
choroid photoreceptors outer nuclear layer outer plexiform layer inner nuclear layer inner plexiform layer ganglion cell layer nerve fibre layer
what is in the outer nuclear layer
contains photoreceptor nuclei
what is in the outer plexiform layer
contains synapses between the photoreceptors, bipolar cells and Horizontal cells
(synapses here are glutamate)
what is in the inner nuclear layer
contains nuclei of horizontal cells, bipolar cells and amacrine cells
what is in the inner plexiform layer
contains synapses between biplar cells, amacrine cells and ganglion cells
what is in the ganglion cell layer
contains nuclei of ganglion cells and displaced amacrine cells
what is in the nerve fibre layer
contains axons of ganglion cells
what is the NT used by photoreceptors
glutamate
what happens with retinitis pigmentosa
usually a mutation in rhodopsin gene –> causes gradual loss of rods, night blindness and development of tunnel vision
what type of receptors do ON and OFF bipolar cells have
ON - metabotropic glutamate Rs
OFF - ionotropic glutamate Rs
what type of receptor do ganglion cells have
ionotropic glutamate Rs
how does sympathetic innervation get to the dilator pupllae muscle for dilation of the pupil
via the long ciliary nerves (come with the nasociliary branch of V1)
What causes Horner’s syndrome
interruption of the SNS supply to the eye
what is the clinical “triad” of symptoms of a patient with Horner’s syndrome
- miosis (constricted pupil)
- partial ptosis
- loss of hemifacial sweating
what part of the visual pathway is split into upper and lower visual fields
optic radiations
- temporal white matter convey upper visual field
- parietal white matter convey lower visual field
what is papilloedema
swelling of the optic disc
what causes internuclear opthalmoplegia
lesion in the medial longitudinal fasiculus (connecting CNIII and CNVI nuclei)
explain the pathway onto off and on bipolar cells in the dark
dark:
OFF: glutamate released onto OFF bipolar cells –> releases NT onto off ganglion cells –> brain (dark)
ON: glutamate released onto ON bipolar cells–> inhibits release of NT
explain the pathway from photoreceptor to OFF and ON bipolar cell in the light
light:
OFF - less glutamate released from photoreceptor –> OFF bipolar cell no longer stimulated
ON - less glutamate therefore ON bipolar cells no longer told not to fire –> releases NT onto ON ganglion cell –> light
