Neuro 11: Structure + Function of the eye Flashcards
Identify the anatomical components of the eye
draw eye + muscles
-
The eye sits in the _______
Orbit
The lacrimal system produces 3 types of tears :
- basal tears
- reflex tears
- emotional tears
What are basal tears?
- tears that are produced at a constant level
- -> even in absence of irritation/stimulation
What are reflex tears?
- increased tear production in response to irritation
What is the tear reflex made up of?
made up of:
- afferent pathway
- CNS
- efferent pathway
- lacrimal gland
irritation of cornea = detected by sensory nerve fibres via _____________
irritation of cornea = detected by sensory nerve fibres via the ophthalmic branch of the trigeminal nerve (CN V)
efferent pathway = mediated by ______ nerve
–> which innervates the lacrimal gland
efferent pathway = mediated by a parasympathetic nerve
–> which innervates the lacrimal gland
tear films drain through the 2 ______
on the upper + lower medial lid margins
tear films drain through the 2 puncta
on the upper + lower medial lid margins
the puncta form the opening to the ____ and _____ ______
the puncta form the opening to the superior + inferior canliculi
the superior + inferior canaliculi converge –> as a single common canaliculus –> and this drains the tears into the ______
the superior + inferior canaliculi converge –> as a single common canaliculus –> and this drains the tears into the TEAR SAC
What is the function of the tear film?
tear film:
- maintains a smooth cornea- air surface
- important to maintain clear vision
- important for removing surface debris during blinking
- source of O2 + nutrient supply to anterior segment
What is a tear film?
tear film:
- thin layer of fluid that covers the cornea
- it is a bactericide
The Tear film consists of what 3 layers?
- superficial oily layer
- aqueous tear film
- mucinous layer
What does the Superficial Oily Layer do?
and what produces it?
- it reduces tear film evaporation
produced by: Meibomian Glands (along the lid margin)
What is the aqueous tear film?
what does it do?
Aqueous Tear film = main bulk of the tear film
–> contains bactericide
- it delivers oxygen + nutrients to the surrounding tissue
What does the mucinous layer do?
- it maintains surface wetting
- and ensures that the tear film sticks to the eye surface.
How do the mutinous layer work?
- the mucin molecules act by binding water molecules to the hydrophobic corneal epithelial cell surface
What is the conjunctiva?
conjunctiva = thin transparent tissue that covers the outer surface of the eye
–> very vascularized
What special feature does the conjunctiva have?
- it has goblet cells that produce mucin
What is the average anterior posterior diameter in adults?
24mm
What are the 3 layers that forms the coat of the eye?
- Sclera
- Choroid
- Retina
What are the characteristics of each layer that forms the coat of the eye?
- Sclera
- Choroid
- Retina
- Sclera
- hard + opaque - Choroid
- pigmented + vascular - Retina
- neurosensory tissue
What is the sclera ?
- white of the eye
- tough opaque tissue that serves as the eye’s protective outer coat
–> it has high water content
What is the Cornea?
cornea = transparent, dome shaped window
that covers the front of the eye
- it acts as a powerful refracting surface
What are main characteristics of the cornea
- it is the front most part of the anterior segment
- it is continuous with the scleral layer
- transparent
- provides 2/3 of eye’s focusing power
- has physical + infection barrier
- -> it has low water content
What are the 5 layers of the Cornea?
- Epithelium
- Bowman’s memb
- Stroma
- Descemet’s memb
- Endothelium
What is the significance of the stroma layer ?
- regularity of stroma contributes towards transparency
What is the significance of the Endothelium?
- it pumps fluid out of the corneal
- and prevents corneal oedema
note:
- endothelial cell density decreases with age
What happens if you hydrate the cornea?
?????
What is the Uvea?
Uvea = vascular coat of eyeball that lies between the sclera + retina
What are the 3 parts that the uvea is composed of?
- Iris
- Ciliary body
- Choroid
Where does the choroid lie?
- lies between the retina + sclera
What is the IRIS?
and what does it do?
- colored part of the eye
- embedded with small muscles that dilate + constrict the pupil size
- the iris controls light levels inside the eye
Describe the structure of the lens
Outer: has acellular capsule
inner: regular elongated cell fibres
What happens if the lens loses transparency with age?
cataract
What is the function of the lens?
- provides transparency
- has refractive power (responsible for 1/3 of refractive power of eye)
- provides accommodation
- -> through elasticity .
Lens = suspended by fibrous ring known as ___ _____
which consists if a passive connective tissue
Lens = suspended by fibrous ring known as lens zones
which consists if a passive connective tissue
What is the retina?
- retina = thin layer of tissue that lines the inner part of the eyes
What is the retina responsible for?
- responsible for capturing light rays that enter the eye
- the light impulses are then sent tp the brain for processing via the optic nerve
What is the function of the optic nerve?
- optic nerve transmits electrical impulses from the retina to the brain
–> it connects to the back of the eye (near the macula)
The visible part of the optic nerve = called the __________
optic disc.
What is the blind spot ?
- blind spot = where the optic nerve meets the retina
- -> there are no light sensitive cells
Where is the macula located at?
- located roughly at the centre of the retina, temporal to the optic nerve
-
What is the macula?
what is its function?
- small + highly sensitive part of the retina
- which is responsible for detailed central vision (e.g reading)
FOVEA = very centre of macula (forms a pit)
What is the difference between central vision + peripheral vision?
Central:
- provided detailed day vision / color vision
- for reading / facial recognition
- assed by visual activity assessment
Loss of Foveal vision –> leads to poor visual acuity
Peripheral:
- allows recognition of shape/ movement and night vision
- allows navigation vision
- assessed by visual field assessment
Loss of visual field –> leads to not being unable to navigate in environment
Fovea has the highest conc of ___________
- cone photoreceptors
describe the organisation of the retinal layer
outer: photoreceptors
middle: bipolar cells
inner: retinal ganglion cells
What are the 2 main classes of photoreceptors in the retina?
how do they differ in:
a) sensitivity to light
b) speed of response
c) night vision / daylight vision
- Rod photoreceptor
- 100x more sensitive to light than cones
- slower response
- responsible for night vision - Cone photoreceptor
- less sensitive to light
- faster response
- responsible for day light fine vision + color vision
Where can you find the highest conc of rod photoreceptors in the retina?
- 20 to 40 deg away from fovea
What is the Ishihara test?
- it is a color perception test
- tests for red-green deficiencies only
What is dark adaptation?
why is it a biphasic process?
Dark Adaptation:
- increase in light sensitivity in the dark
- it is a biphasic process
- -> 7 mins cone adaptation
- -> 30 mins rod adaptation
What is light adaptation?
how does it occur?
Light Adaptation: - adaptation from dark to light - occurs over 5 mins - involves bleaching of photo pigments - neuro adaptation -
What is Pupil adaptation?
- minor process
- that allows construction of pupil with light
What is the most common form of color vision deficiency in humans?
red-green confusion
How do you calculate Index of refraction?
n = speed of light in vacuum / speed of light in medium
What are the 2 basic types of lenses?
- converging lens
- -> which brings them to a single point - Diverging lens
- -> which takes light rates and spreads them outwards
What is Emmetropia?
- adequate correlation between axial length + refractive power
- parallel light rays fall on the retina (no accommodation)
What is Ametropia? (refractive error)
- There is a mismatch btw axial length + refractive power
–> so parallel light rays don’t fall on the retina
list 4 examples of ametropia
- myopia
- hyperopia
- astigmatism
- presbyopia
What is the mechanism of myopia?
- parallel rays converge at a focal point anterior to the retina
What are 2 main causes of myopia?
- excessive long globe (axial myopia) - common
2. excessive refractive power
What are symptoms of myopia?
- blurred distance vision
- squinting to improve uncorrected visual activity when gazing into distance
- headache
What is the mechanism of hyperopia?
- parallel rays converge at focal point posterior to the retina
What are 2 main causes of hyperopia?
1 . excessive short globe - common
2. insufficient refractive power
What are symptoms of hyperopia?
- visual acuity at near –> blurs relatively early
- has asthenopic symptoms (e.g eye pain, headache in frontal region, burning sensation in eyes)
- amblyopia
What is the mechanism of astigmatism?
- parallel rays come to focus in 2 focal lines rather than a single focal point
–> usually due to heredity
What is the main cause of astigmatism?
- refractive media = not spherical
- -> light rays are refracted differently forming –> 2 focal points on the retina
What are the main symptoms of astigmatism?
- asthenopic symptoms
- blurred vision
- distorted vision
- head tilting + turning
How would you treat astigmatism ?
REGULAR ASTIGMATISM:
–> cylinder lenses w/wo spherical lenses
IRREGULAR ASTIGMATISM
- -> rigid CL
- -> surgery
What are the near response triad?
- pupillary miosis
- -> to increase depth of field - convergence
- -> to align both eyes towards a near object - accommodation
- -> to increase refractive power of lens for near vision
What is presbyopia?
- naturally occurring loss of accommodation
- onset: from 40 years
- distant vision = intact
How would you treat presbyopia?
- use convex lenses for near vision
e. g reading/bifocal/trifocal/progressive power glasses
What are the different types of optical correction?
- spectacle lenses
- contact lenses
- intraocular lenses
- surgical correction
compare adv + dsadv for the different methods of optical correction:
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Describe the mechanism for accommodation
- contraction of circular ciliary muscle inside the ciliary body
- this relaxes the zones that are normally stretched between the ciliary body attachment + the lens capsule attachment
- in the absence of zonular tension –> lens returns to its natural shape
- this increases the refractive power of the lends
–> accommodation = mediated by the efferent 3rd cranial nerve
Where do retinal ganglion axons coming down the optic nerve synapse?
- Lateral geniculate nucleus
Where is the lateral geniculate nucleus found?
- Thalamus
What are the fibres leaving the lateral geniculate nucleus called?
- Optic radiation
Describe the convergence and receptive field sizes of rods.
- rods = have high convergence + large receptive fields
Describe the convergence and receptive field sizes of cones.
- cones = have low convergence + small receptive fields
- What is the benefit of having high convergence and a large receptive field?
- High light sensitivity
- What is the benefit of having low convergence and a small receptive field?
- Fine visual acuity
which parts of the visual field is it responsible for?
a) upper division of the optic radiation
- Responsible for inferior visual quadrant
which parts of the visual field is it responsible for?
b) lower division of the optic radiation
- Responsible for superior visual quadrants
- The left primary visual cortex = responsible for the right/left visual field from both eyes
- ## The right primary visual cortex = responsible for the left/right visual field from both eyes
- The left primary visual cortex = responsible for the right visual field from both eyes
- The right primary visual cortex = responsible for the left visual field from both eyes
Visual cortex above the calcarine fissure = responsible for the inferior / superior visual field
- Visual cortex below the calcarine fissure = responsible for the superior / inferior visual field
Visual cortex above the calcarine fissure = responsible for the inferior visual field
- Visual cortex below the calcarine fissure = responsible for the superior visual field
How is it possible for the macula to be spared by a stroke in the primary visual cortex leading to homonymous hemianopia?
area representing macula in the primary visual cortex = has dual blood supply (from both right and left posterior cerebral arteries) –> so less vulnerable to ischaemia
describe the pathway of consensual light reflex
- Retinal Ganglion Cell –> Pretectal Nucleus –> Edinger-Westphal Nucleus –> Ciliary Ganglion –> Short Ciliary Nerves –> Sphincter Pupillae
What test would you do to identify RAPD?
what is RAPD?
RAPD = Relative afferent pupillary defect
- Swinging torch test
What would you see in a patient with RAPD? in a swinging torch test?
- When the light is shone on good eye –> there is direct and consensual response
- When the light shone on bad eye –> there is paradoxical dilation of iris in bad eye
- This is because there is weaker constriction response in the bad eye than the consensual response elicited by the good eye
define - Dextroversion
right gaze
define – Levoversion
left gaze
define - Infraversion
depression of both eyes
define- Supraduction
elevation of one eye
What are the two types of eye movement
- Saccade = short fast burst
- Smooth pursuit = sustained slow movement
describe the innervation of extrinsic eye muscles
Lateral Rectus = Abducens (CN VI)
Superior Oblique = Trochlear (CN IV)
Medial Rectus, Superior Rectus, Inferior Oblique, Inferior Rectus and Levator Palpebrae Superioris = Oculomotor (CN III)
Where do all the rectus muscles originate
- Common tendinous ring at apex of orbit
What would you see in a patient with 3rd nerve palsy?
- Eye points down and out
- Due to unopposed lateral rectus + superior oblique
- Ptosis –> due to loss of innervation of lavatory palpebrae superioris
- Pupil dilation –> due to loss of parasympa innervation to eye via oculomotor nerve
What would you see in a patient with 6th nerve palsy?
- When asked the abduct the affected eye, they eye will stop around midline
- because the lateral rectus isn’t functioning and can’t abduct the eye
- This can lead to blurred vision
What is the function of Medial longitudinal fasciculus (MLF) ?
allows paired eye movements
what can happen to the eyes when MLF is damaged ?
- Internuclear opthalmoplegia
E.g. right abduction wont be accompanied by left adduction
- Could be accompanied by nystagmus on right gaze
