Vision Flashcards
What are the extraoccular muscles?
muscles in the eye socket that are outside from the eye: rectus and oblique
What shape is the orbital cavity?
pyramidal
Where is the macula compared to the optic nerve?
lateral
How many extraoccular muscles are there in each cavity?
6 (4 rectus and 2 obliques)
Where is the origin of the rectus muscles in the optic cavity?
annulus of Zinn
Where does the inferior oblique originate from?
anterior medial orbital floor
Where does the superior oblique originate from?
sphenoid bone
What is the primary position of the eyes?
look straight ahead
What is the secondary positions of the eyes?
looking up, down, left, right
What are the tertiary positions of the eyes?
eyes directly diagonally
What are the three axes the eyes can rotate around?
- vertical (infero-superior)
- sagittal (antero-posterior)
- transverse (medio-lateral)
What are the three movements of rotation and to which axe do they correspond?
- yawing (vertical)
- rolling (sagittal)
- pitching (transverse)
What happens if you pitch upwards through the transverse axis?
you elevate the eyeball (in opposition to depression of the eyeball)
What happens if you yaw towards the medial side of the orbit though the vertical axis?
adduction of the eyeball
in opposition to abduction
When we talk about rolling, which part of the eyeball are we using to compare direction?
directly the superior part of sclera is moving in
What is outwards rotation of the eyeball through the sagittal axis?
extorsion
opposite: intorsion
Which are the extraoccular muscle that only have primary movements?
lateral and medial rectus
What are the primary, secondary and tertiary movements of the superior rectus?
- elevates
- intorsion
- adducts
What are the primary, secondary and tertiary movements of the inferior rectus?
- depression
- extorsion
- adducts
What are the primary, secondary and tertiary movements of the superior oblique?
- intorsion
- depression
- abduction
What are the primary, secondary and tertiary movements of the inferior oblique?
- extorsion
- elevation
- abduction
What position must the eyeball be in to test the pitching movement of the inferior and superior rectus?
abducted to 23°
What position must the eyeball be in to test the pitching movement of the inferior and superior oblique?
adduction between 50-55°
What do the H test show?
- if can’t adduct: medial rectus problem
- if can’t abduct: lateral rectus problem
- if eyes abducted to 23° and can’t elevate/depress: superior/inferior rectus problem
- if eyes adducted at 51° and can’t elevate/depress: inferior/superior oblique problem
What is versions?
the eyeballs move in the same direction: the lines of sight of each eyeball remain parallel
What is vergences?
the eyeballs move in opposite directions: the lines of site of each eyeball do not remain parallel
What are examples of versions?
supraversion
infraversion
extroversion
levoversion
What are examples vergences?
convergence
divergence
What is the innervation of the extraocular muscles?
oculomotor, trochlear and abducens nerves
What are the modalities of the:
- oculomotor nerve
- trochlear nerve
- abducens nerve
- all general somatic motor
- CN3: general visceral motor (parasympathetic)
- sensory: proprioception: sensory fibres pass through the mesencephalic nucleus of the trigeminal nerve
What are their path from the brainstem to the superior orbital fissure?
How is it clinically relevant?
go through the cavernous sinus (clinically relevant in case of bleeding in cavernous sinus)
What is the main oculomotor nuclei?
Where is it located?
somatomotor nuclei in midbrain of brainstem: located at level of superior colliculus
Where does the main oculomotor nuclei receive information from and what does it do?
- superior colliculus (therefore visual cortex)
- medial longitudinal fasciculus (internuclear neurones connecting nuclei of 4, 6 and 8)
- -> important for co-ordination of eye movements
What are the different branches of the oculomotor nerve and what do the innervate?
- -> superior branch:
- superior rectus (contralateral)
- levator palpeerde superioris (both
- -> inferior branch:
- medial rectus (ipsilateral)
- inferior rectus (ipsilateral)
- inferior oblique (ipsilateral)
- ciliary muscle for accommodation (from E-W nucleus)
- smooth muscles of the iris for pupil constriction (E-W nucleus)
Where is the E-W nucleus located compared to main nucleus?
posterior to main nucleus
Where do the E-W parasympathetic fibres synapse?
ciliary ganglion
Where do the sympathetic fibres innervating dilator pupillae?
thoracic nerves via sympathetic chain
What are the three aspects of the accommodation reflex?
- pupils constrict/dilate
- lines of sight converge/diverge
- lens bulges antero-posteriorly (contraction of ciliary muscles) or flattens (relaxation of ciliary muscles)
What is the accommodation reflex?
reflex action of the eye, in response to going from near to far distant objects
Where is the trochlear nerve nuclei located?
at the border of pons and the midbrain
located in grey matter surrounding cerebral aqueduct
Where does the trochlear nucleus receive fibres from?
- superior colliculus (therefore visual cortex)
- medial longitudinal fasciculus (internuclear neurones connecting nuclei of 3, 6 and 8)
- corticonuclear fibres from both cerebral hemispheres
- -> important for co-ordination of eye movements
What muscle(s) does the trochlear nerve innervate?
superior oblique (contralateral)
Where does the abducens nerve originate?
beneath floor of fourth ventricle
Where does the abducens nucleus receive fibres from?
- superior colliculus (therefore visual cortex)
- medial longitudinal fasciculus (internuclear neurones connecting nuclei of 3, 4 and 8)
- corticonuclear fibres from both cerebral hemispheres
- -> interneurons to contralateral main oculomotor nucleus via MLF important because medial rectus is antagonist of lateral rectus)
Which muscle(s) are innervated by the abducens nerve?
lateral rectus (ipsilateral?)
What is diplopia?
double vision
What does diplopia indicate?
- fatigue
- cranial nerve dysfunction
- raised intracranial pressure
- cerebellar dysfunction
- blow out fractures of the orbit
What is the course of the optic nerve?
retina –> optic canal –> (runs along middle cranial fossa) –> optic chiasm –> optic tracts –> lateral geniculate nucleus –> optic radiation –> Brodmann area 17
From what retinal quadrant does the upper optic radiation carry fibres from?
superior retinal quadrant (inferior visual field quadrants)
From what retinal quadrant does the lower optic radiation carry fibres from?
inferior retinal quadrants (superior visual field quadrants)
What is the lower optic radiation called?
Meyer’s loop
Where does the lower optic radiation travel through?
temporal lobe
What are the -1st order neurone -2nd ON -3rd ON of the optic nerve?
- bipolar cell
- ganglion cells
- LGN
What is the divide along the x axis of the retina?
horizontal raphe
Where are the ganglion cells in relations to the horizontal raphe on the retina?
superior and inferior
What is the clinical significance of a horizontal visual problem compared to a vertical visual problem?
horizontal: problem on retinal field
vertical: problem in the brain/optic nerve
Which 4 subcortical regions of the brain does the optic nerve project to and what do they do there?
primary visual pathway: -LGN (image formation) secondary visual pathway: -hypothalamus (circadian rhythms) -pretectum (pupil) -superior colliculus (eye movements)
What is the pupillary light reflex?
the reflex that controls the diameter of your pupils
What is the afferent pathway in the pupillary light response?
optic nerve until pretectal nucleus
What is the efferent pathway in the pupillary light response?
starts at pretectal nucleus and goes to E-W nucleus
How is the E-W nucleus innervated (bilaterally, contra laterally or ipsilaterally) and what is the significance?
bilaterally: both pupils constrict
What is the swinging flashlight test?
swing a light from one eye to the other: they should stay constricted if there is no problem (it is to see if there is a problem in the optic nerve)
What is stereoscopic vision?
single perception of a slightly different image from each eye
What is binocular visual field?
eyes aligned so image in nasal field of one eye matches image in temporal field in the other eye
What is the extend of the vital field (one eye)?
compared to line of site: 60° above 75° under 100° laterally 60° medially
What happens to the circular and radial muscles who there is papillary constriction and what is the innervation?
- circular muscles contract
- radial muscles relax
- -> parasympathetic innervation of the circular muscles
What happens to the circular and radial muscles who there is papillary dilation and what is the innervation?
- circular muscles relax
- radial muscles constrict
- -> sympathetic innervation of the radial muscles
What is responsible for accommodation?
- cornea: 75% of refraction of light
- lens: contraction of ciliary muscles/thickening of the lens
- convergence of the eye
What are common causes of visual field defects related to the retina?
- vascular: retinal artery occlusion, retinal vein occlusion
- retnal detachment
- macular lesion: central defect
What are common causes of visual field defects related to optic nerve (before chiasm)?
- glaucoma
- optic neuritis
- optic atrophy
- optic nerve compression (thyrotoxicosis, tumour)
What are common causes of visual field defects relating to lesion at chiasm?
stroke
tumour
What are common causes of visual field defects relating to occipital cortex
stroke
trauma
Where do the superior optic radiations go through?
parietal lobe
What kind of visual fields do defects in the visual cortex produce?
perfectly congruous (identical in both eyes)
What kind of visual fields do defects in the optic tracts produce?
incongruous defects
What is binocular depth perception?
stereopsis
What is the funds of the eye?
interior surface of the eye opposite the lens
What are the layers of the retina?
deep to superficial:
- photoreceptor layer
- outer plexiform layer
- inner nuclear layer
- inner plexiform layer
- ganglion cell layer
What are the three functions of the retina?
- translate light into a biological signal
- extract visual information
- transmit that signal to the brain
What does the inner nuclear layer do?
extract visual information (such as contrast)
What does the retinal ganglion cells (optic nerve) do?
transmit that signal to the brain
What do the photoreceptors do?
translate light into a biological signal
What are the inner and outer plexiform layers?
synapses between neurones
What is the vasculature of the retina?
- retinal: supplies inner retina, disrupted in glaucoma
- choroidal: supplies photoreceptors, disrupted in retinal detachment
What are the different parts of a photoreceptor cell?
- outer segment (invaginate in plasma membrane)
- inner segment
- synaptic ending
Where are the photopigment found in photoreceptor cells?
in plasma membrane of outer segment
What is the photo pigment called in rods and what type of protein is it?
rhodopsin
7 transmembrane G protein linked receptor (opsin)
What absorbs light in a cone cell? Where can it be found?
- retinaldehyde
- inside the 7 transmembrane domain G protein coupled receptor (opsin)
from what is retinal/retinaldehyde made?
vitamin A
- What is the name of the conformational shape of retinaldehyde in the dark?
- Where is the change?
- 11-cis retinal
- between the 11th and 12th carbon
- What is the name of the conformational shape of retinaldehyde in the light?
- Where is the change?
- How does it change?
- all-trans retinal
- straitening of the molecule between the 11th and 12th carbon
- absorption of a photon of light induces change
what is the reaction of retinaldehyde going from 11-cis retinal to all-trans retinal?
photochemical reaction (photoisomerisation)
What is an opsin protein?
a 7 transmembrane domain G protein coupled receptor
What are the functions of opsin protein?
- translate and amplify isomerisation of retinal into a ‘biological’ signal
- shifts spectral efficiency function to find the best wavelength for retinaldehyde absorption
What is the visible wavelength?
400-800 nm
What is the maximal sensitivity of retinaldehyde on its own?
400 nm
What happens when the retinaldehyde binds to the opsin protein?
shifts it into a visible wavelength
What is the peak sensitivity for rhodopsin?
500nm
What is the peak sensitivity of cone opsin?
- blue (short-wave cone): 433 nm
- green (middle-wave cone): 533 nm
- red (long-wave cone): 564 nm
What keeps opsin in its inactive state?
11-cis retinal (acting as a ligand): acts as an ‘inverse agonist’
What activates opsin?
photoisomerisation to all-tarn and retinal dissociation (from the pigment) and triggers signalling
What is the phototransduction cascade and what are the different steps?
(for rods but cones are basically the same)
opsin protein signal amplification:
- photon absorption
- G-protein dissociation
- phosphodiesterase activation
- closure of cGMP-gated channels
What are the phototransduction steps in more detail?
look at notes (G coupled protein receptors case 3)
Why is the phototransduction cascade so important?
- cells response depends on ion concentration
- cascade= one single photon absorption activates loads of tranducins which causes hydrolysation of loads of cGMP which closes loads of cation channels
What happens in the dark in regards to the channels and current of the photoreceptor cell?
- cGMP binds to the cations channels in outer segment to keep them open: Na ions enter depolarising cell
- dark current: Na+ enters cell in outer segment and K+ leaves cell in inner segment + Na/K pump (Na out, K in)
- (dark potential: -40mV)
- glutamate constantly released causing IPSP in bipolar cell: hyper polarisation of bipolar cell
- no EPSE occurs in ganglion cell
- no AP occurs along optic nerve
What happens in the light in regards to the channels and current of the photoreceptor cell?
- cGMP no longer present so the Na+ channels are closed
- K+ still leaving cell in the inner segment: membrane potential hyper polarised (graded): stops releasing glutamate (cells are silent/inactive) and the bipolar cells are no longer activated (lack of IPSP) resulting in depolarisation
- neurotransmitters release from bipolar cells causing EPS: AP released along optic nerve
What is graded hyperpolarisation(/potential)?
depends on the light exposure: the more light absorbed, the more hyper polarised it will be–> translated into a graded release of glutamate from synaptic terminal (less excited, the less glutamate release)
–> not the release of an AP but graded amount of neurotransmitters
Why are rods more sensitive to light?
have bigger outer segments: capture more photons so have larger signal amplification
How do cones have higher acuity?
adjust sensitivity to be active under any light level
What is the receptive field organisation?
centre: surround
(centre: photoreceptor directly below the ganglion cell
surround: photoreceptors around the centre)
How does light affect the electrical signalling in centre:surround?
- the ganglion above the centre will be excited if the centre is receiving photons and the surround is not. But it will not be stimulated if he centre is not receiving photons but the surround is.
- -> spot excites, annulus (outside the centre) does not: centre:surround antagonism
- if centre and surround are both lit up: there will be no effect on excitement: the light in the centre that would have excited it, is inhibited by the light that falls on the surround
Why is the centre:surround organisation that extracts spacial pattern of light exposure important?
- ignore ‘boring’ parts of visual scene
- map ‘edges’ with high resolution
What is a ‘on centre’ cell?
- increases firing rate in response to illumination at centre and decrease firing in response to illumination of periphery
- ‘on’ bipolar cells that are sign inverting: METABOTROPIC GLUTAMATE RECEPTORS
What is an ‘off centre’ cell?
- firing increase with surround illumination and stops with light on the centre
- ‘off’ bipolar cells that are sign conserving: INOTROPIC GLUTAMATE RECEPTORS
What is a sign inverting synapse?
inhibitory synapse
–> depolarisation in the presynaptic cell drives hyperpolarisation in the postsynaptic cell (and vice versa)
What is a sign conserving synapse?
excitatory synapse
–> depolarisation in presynaptic cell will drive depolarisation in the pot-synaptic cell (and vice versa)
What is the definition of red in terms of electrical activity?
hyper polarises your red cones more than the green or blue ones
What is the definition of yellow in terms of electrical activity?
hyper polarises your red and green cones the same but not your blue ones
How is the ganglion cell response regarding colour stimulation?
graded excitation (throughout the visible spectrum depending on ganglion cell responding to red:green or blue:yellow)
How is blue:yellow colour stimulated?
blue:
-stimulation of blue ‘on’ bipolar cell
-inhibition of red:green ‘off’ bipolar cells
–> all linked to 1 ganglion cell which is excited by blue
balance of excitatory input> inhibitory
yellow:
-stimulation of red:green ‘off’ bipolar cell
-inhibition of blue ‘on’ bipolar cell
–> all link to the same ganglion cell which is inhibited by yellow
balance of inhibitory > excitatory
What does the firing pattern of ganglion cells show specifically?
encodes visual information (rather than the amount of light falling on the photoreceptors):
- spacial contrast in grey scales (centre:surround) and/or colour
- movement in a particular direction
- local motion
- brightness
What is the function of the retinal ganglion cell?
- second order neurone
- 1% are photoreceptors (melanopsin based) concerned with circadian rhythms, pupil responses, adaptive responses to overall lighting condition
What are horizontal cells?
interneurones that connect photoreceptors: contrast detection via lateral inhibition
What are amacrine cells?
- interneurones between ganglionic and bipolar cells: signal modulation
- no axons but many neurites: have properties of both axons and dendrites –> use wide range of neurotransmitters to modify neurotransmission
Which are the cells that fire AP in the retina?
ganglionic cells
What is the rods:cones ratio?
20:1
What is photopic vision?
vision of eye under very well lit conditions: cones
What is scotopic vision
vision of the eye under low light conditions (see shades of grey)
What is the integration of cones to ganglion cells compared to rods? What does this signify?
-less integrated: less cones linked to one ganglion cell compared to rods:
in fovea the non-convergence is at its highest: 1:1 cone:ganglion cell
–> high visual acuity (cones) vs high sensitivity (rods)
Which cones are the in the least amount?
blue (S)
How are the cones distributed in the retina?
- no S (blue) cones in the fovea
- most in the fovea
- in patches: grainy colour vision
What is used to detect movement?
rods
What happens to the rods in daylight?
saturated and unresponsive
What is dark adaptation?
rods become unsaturated in twilight (vision is reduced for 10-15 minutes until rods are restored: regeneration of rhodopsin)
+ transducin returns to outer segment
What is pigment bleaching?
light allows retinaldehyde and opsin to separate
What is pigment regeneration?
enzyme within pigmented epithelium reconverts it to 11-cis isomer and rhodopsin regenerates when 11-cis retinal rejoins to opsin
What is dioptre?
unit of the power of refraction
1/focal distance (m)
What does the retina come from in embryonic development?
diencephalon
What is mesotopic vision?
when both cones and rods are used
What are people called who have lost their cone function?
legally blind
What are people called that have lost their rod function?
night blind
