Neuro 12: Structure and function of the eye Flashcards
Which structures pass through:
- Sup. orbital fissure
- Optic foramen
- Supraorbital notch
- Infraorbital fissure
- Infraorbital groove
- Infraorbital foramen
- Trochlear, abducens, oculomotor (superior and inferior division) and ophthalmic (lacrimal, frontal and nasociliary branches of ophthalmic) cranial nerves, opthalmic vein (supoerior and inferior division)
- Optic nerve and ophthalmic artery
- supraorbital nerve (from frontal from opthalmic) and vessels (supraorbital artery and supraorbital vein.)
- Zygomatic branch of the maxillary nerve and the ascending branches from the pterygopalatine ganglion. Infraorbital vessels pass from here. Inferior division opthalmic vein
- Infraorbital vessels (infraorbital artery from the maxillary artery from external carotid) (through infraorbital groove, canal, and out via infraorbital foramen)
- Infraorbital vessels emerge and infraorbital nerve (branch of V2)
Corners of eye name
Normal AP diameter of eye
Lateral and medial canthas
Eye -Anterio-Posterior Diameter -24mm in adults
Name of pink bit on medial side of eye
Caruncle
What separates iris from sclera
Limbus
What is in the limbus
(corneal stem cells)
Function of tear film
Tear film maintains smooth cornea-air surface
Oxygen Supply to Cornea – Normal cornea has no blood vessels
Removal of Debris (Tear film and Blinking)
Bactericide
Maintaining clear vision
Where is the lacrimal gland located
In orbit, latero-superior to the globe, produces watery tears
Outline the reflex tear production
V1 opthalmic sensory, effernt is parasympathetic (CN7- acetycholine),`
Where to tears drain
Tear drains through the two puncta, opening on medial lid margin
Tear flows through the superior and the inferior canaliculi
Tear gathers in the Tear Sac
Tear exits the Tear Sac through the tear duct into the nose cavity (into inferior nasal meatus)
Why is pressure not releaved from puncta during sneezing
Valve in canaliculi (prevents retrograde reflux of fluid from the sac into the canaliculi)
3 layers of tear film
Lipid layer= meibum (superficial to reduce film evaporation, produced by Meibomian Glands along the lid margins)
Aqeuous (from tear gland)
Mucinous layer on corneal surface for surface wetting
How does the mucin layer work
The mucin molecules (produced by goblet cells) act by binding water molecules,
to the hydrophobic corneal epithelial cell surface.
(make it wettable)
Define conjunctiva
The conjunctiva is the thin, transparent tissue that covers the outer surface of the eye.
Where does conjunctiva extend
t begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids.
3 layers of the eye (from superficial to deep, at the back)
retina, choroid, sclera
What is the optic disk
surface manifestation of optic nerve
Characterise sclera, choroid and retina
Sclera – Hard and Opaque (protective outer coat)
Choroid – Pigmented and Vascular
Retina – Neurosensory Tissue
Which layer surrounds the optic nerve
retina
Compare water content of sclera and cornea
sclera- high cornea- low
What is cornea
the transparent, dome-shaped window covering the front of the eye.
What is the front most part of anterior semgnet
cornea
Which layer of eye is cornea continuous with
Sclera
What is the survature of cornea
Convex
Why is cornea refractive
Convex curvature
Higher refractive index than air
Other function of cornea other than refraction
Physical Barrier
Infection Barrier
5 layers of cornea
1 – Epithelium 2 – Bowman’s Membrane 3 – Stroma – regularity contributes towards transparency 4- Descements membrane 5. Endothelium
Epstein Bar Sung Down Eltham
What contributes to corneal transparency
Regularity of stroma
And the lack of water because endothelial cells pump it out
What is and is not present in stroma
is- corneal nerve endings providing sensation and nutrients for healthy tissue
Is not- blood vessels
What does endothelium of the cornea do
pumps fluid out of corneal and prevents corneal oedema,
1 layer
Why, with age, can you get corneal oedema and cloudiness
Endothelial layer pumps water from the more superifical stromal layer
Only 1 layer of endothelial cell, and they have no regeneration power
Endothelial cell density decreases with age
Endothelial cell dysfunction may result in corneal oedema and corneal cloudiness
What happens if you hydrate hte cornea
It goes opaque
What is the uvea
Vascular coat of eye ball
In which layers does the uvea lie
lies between the sclera and retina.
3 parts of the uvea
t/f all parts of the uvea are intimately connected, so a disease of one part affects all parts
Iris
ciliary body
choroid.
T, though not necessarily to the same degree.
What nourishes the outer and inner part of the retina
Outer part: choroid
Inner part: central radial artery
Define iris
coloured part of the eye
What controls light levels inside eye
iris (embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size. )
What is responsible for refractory power of the eye
2/3- cornea
1/3 lens
structure of lens
- Outer Acellular Capsule
2. Regular inner elongated cell fibres – transparency
Functiion of lens
- Transparency
-Regular structure - Refractive Power
-1/3 power
-Higher refractive index than aqueous fluid and vitreous - Accommodation
Elasticity
What is cataracts
Lens loose transparency with age
What is the lens zonules
Lens is suspended by a fibrous ring known as lens zonules, consists of passive connective tissue
Anchors lens to ciliary body
What allows focusing
- Action of ciliary muscle on lens
2. Iris aperture
What is the blind spot
Where the optic nerve meets the retina there are no light sensitive cells. It is a blind spot.
Where is the macula vs optic disk
Macula=roughlyin centre, temple to optic nerve(dark)
Optic disc=nasal side
The optic nerve contains axons, from which cell body
The retinal ganglion cells
What is the macula responsible for
detailed central vision, appreciate detail
Centre of macula is called
fovea (slightly thinner than retina) so where there is a dip in the retina, this is fovea
How is eye divided
Anterior and posterior segment… divided by lens
Boundaries of anterior chamber
Between Cornea and Lens
What produces aqeous humor
Ciliary body EPITHELIUM
Where does ciliary body sit compared to ciliary muscle
Medially
What sits immediatey anterior to ciliary bodyand lens
iris
Passage of aqeous himor
through lens zonules, arround lens into anterior chamber and then reabsorbed by trabecular meshwork
Where does trabelcular meshowrk lie with respect to th eiris
Anterior to iris
How is aqeous reabsorbed
passive= uveal-scleral outflow Active= TM canal of schlemm (80%)
Define glaucoma
Optic neuropathy with characteristic structural damage to the optic nerve, associated with:
- progressive retinal ganglion cell death,
- loss of nerve fibres and
- visual field loss (starting from periphery to centre) and blindness
Is glaucoma pressure in the eye?
No-it’s apoptosis of retinal ganglion cells
What is optic disk look like in retinal cell death in glaucoma
Enlargement of Optic Disc Cupping
What does pressure have to do with glaucoma
Sustained Raised Intraocular Pressure is risk factor
Types of glaucoma
Primary (=open angle) –> TM dysfunction
Closed (acute or chronic) –> increased pressure pishing iris/lens complex forward blocking TM
Risk factors of closed angle glaucoma
small eye (hypermetropia), narrow angle at trabecular meshwork
(treat w laser iridotomy to create a drainage hole on the iris)
Which glaucoma could be symptomatic
Primary asymptomatic until late stages,
closed angle- sudden painful red eye,
and drop in vision.
Differnetiate what an arteriole looks like vs venule on fundoscopy
venules more dilated
Highly sensitive part of the retina
Fovea of Macula
Why do things look brighter when in centre of your eye than out of periphery
only your fovea has the concentration of cones to perceive in detail.
How are cenral and peripheral vision assessed
Central: Visual Acuity Assessment
periph: Visual Field Assessment
What constitutes ‘central vision’ and where is this perceived on the retina
(Fovea has the highest concentration of cone photoreceptors)
Detail Day Vision, Colour Vision Reading, Facial Recognition
What constitutes Peripheral vision and where i this perceived on the retinal
Movement, Night Vision
Navigation Vision
Problems with navigating due to…
Peripheral vision loss
Structure of retina: outline outer layer (i.e. furthest from the centre of the eye)
Outer Layer – Photoreceptors (1st Order Neuron) – Detection of Light
Outline middle layer of retina and function
Middle Layer – Bipolar Cells (2nd Order Neurons) – Local Signal Processing to improve contrast sensitivity, regulate sensitivity
Outline the inner layer of retina (this means closest to the centre of the eye)
Retinal Ganglion Cells (3rd Order Neurons) – Transmission of Signal from the Eye to the Brain
t/f layer containing photoreceptors lays closest to the lens
F: it lies close to the choroid
What is the macula lutea
=macula. (yellow patch), pigmented region at the centre of the retina of about 6 mm in diameter
What forms the pit at the centre of the macula and why
Fovea- due to absence of the overlying ganglion cell layer
Fovea has the highest concentration of photoreceptors for fine vision
Differentiate structure of rod and cone
Rod Photoreceptor has Longer outer segment with photo-sensitive pigment
Differentiate functional property of rods and cones
Rod is more sensitive to light than cones (but cones faster)
DEEP IT!
Which is responsible for:
scototopic
phototopic vision
scoto- night- rod
photo-day-cone
More rods or cones
much more rods``
Outline photoreceptor distribution
Rods: wide dist. all over retina, high density just outside macula (20-40 degrees from fovea) . Completely absent in the macula
Cones- high distrivution onyl in the macula
T.F Rods have multiple peak light sensitivity and cones have just one
F- there are 3 cone photopigment subtypes:
S-blue
M-green
L-red
S for small waves (short wavelength is blue)
M for medium
and
L for large (long wavelength is red!)
only one rod (with peak sensitivity at 498nm, between cone S and M)
What is Deuteranomaly
caused by the shifting of the M-cone sensitivity peak towards that of the L-cone curve,
causing red-green confusion.
T/f colour vision deficit higher in males
true -8% males, 0.5% female
What is Anomalous Trichromatism (i.e. deuteranomaly)
Colour Vision Deficiencies due to shifting of photo-pigment peak sensitivity
What are dichromatism and monochromatism
In Dichromatism, only two cone photo-pigment sub-types are present.
In Monochromatism, there is complete absence of colour vision.
Differentiate blue and red cone monochromatism
blue- normal day light visual acuity
red- no functional day vision
What is ishihara test and what deficiency can they test for
Colour Perception Test
Ishihara Isochromatic Plates can test for red-green deficiencies only
Outline process of dark adaptation
BIPHASIC- Increase in light sensitivity in
dark
Biphasic Process
• Cone adaptation 7 minutes
• Rod adaptation 30 minutes – regeneration of rhodopsin
The threshold intensity reduces so less stimulus required to activate these photoreceptors???
Cones adapt first (7 minutes), then rods overtake cones at the rod-cone break
How long does it take for rods to reach maximum dark adaption, and what is regenerated
30 mins (it is extremely sensitive to light, and is photobleached when exposed to light, then regenerates when dark to allow some vision in dark)
How long does light adaptation take
5 mins
Mechanism of light adaptation
suppression of light sensitivity:
photo-pigment bleaching by bright light,
and neuro-adaptation inhibiting rod and cone function
+
The pupil also provides a minor degree of light and dark adaptation,
by acting as an adjustable aperture,
regulating the amount of light into the eye.
What happens to rod function in light adaptation
Suppressed, and cone function takes over within one minute.
Principle of refraction
- As light goes from one medium to another, the velocity CHANGES!
- As light goes from one medium to another, the path CHANGES!
What is the index of refraction
speed of light in vacuum/speed of light in medium
denominator will always be smaller, as light travels fastest in vacuum, so IR always bigger or equal to 1
T/f: angle of incidence=angle of refraction
f… Angle of incidence = Angle of Reflection
….
angle of incidence can be bigger or smaller than angle of refracrtion depending on direction of the light
What does a converging and diverging lens do
converging: takes light rays and bring them to a point
diverging|: takes light rays and spreads them outward.
What shape is a converging and divergin lens
converging- convexed
diverging- concave
What is emmetropia
Adequate correlation between axial length (=length of eyeball) and refractive power
Parallel light rays fall on the retina (no accommodation)
What is ametropia
Mismatch between axial length and refractive power
Parallel light rays don’t fall on the retina (no accommodation)
4 types of ametropia
Nearsightedness (Myopia)
Farsightedness (Hyperopia)
Astigmatism
Presbyopia
What is myopia
NEAR sightness.
Can see near. Can’t see far.
Parallel rays converge at a focal point anterior to the retina
2 causes of myopia
excessive long globe (axial myopia) : more common
excessive refractive power (refractive myopia)
Symptoms of myopia
Blurred distance vision
Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
Headache
What type of lens do you correct myopia with
you want the rays to converge furhter away (as they’re converging anterior to retina in this condition) so you give a diverging (i.e. concave) lens for correction
What is hyperopia
Long sightedness
Can see far, not near
Parallel rays converge at a focal point posterior to the retina
Causes of hyperopia
excessive short globe (axial hyperopia) : more common
insufficient refractive power (refractive hyperopia)
Symptoms of hyperopia
- visual acuity at near tends to blur relatively early
- asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
- Amblyopia – uncorrected hyperopia > 5D
What is visual acuity
clarity of vision
What type of lens do you correct hyperopia with
Rays convering too far back, so you want a converging lens (to make rays converge sooner), so a convex lens
Define astigmatism
Parallel rays come to focus in 2 focal lines rather than a single focal point
Cause of astigmatism
refractive media is not spherical–>refract differently along one meridian than along meridian perpendicular to it–>2 focal points ( punctiform object is represent as 2 sharply defined lines)
Symptoms of astigmatism
asthenopic symptoms ( headache , eyepain)
blurred vision
distortion of vision
head tilting and turning
Treatment of astigmatism
regular: cylinder lenses with or without spherical lenses(convex or concave), Sx
irregular: rigid CL , surgery
What is the near response triad
Pupillary miosis (sphincter pupillae constricts) to increase depth of field
Convergence (medial recti from both eyes) to align both eyes towards a near object- i,e. adduct medially
Accommodation (Circular Ciliary Muscle) to increase the refractive power of lens for near vision
What is prebyopia
Naturally occurring loss of accommodation (focus for near objects)
Onset from age 40 years
Distant vision intact
Corrected by reading glasses (convex lenses) to increase refractive power of the eye
lens loses elasticity
Treatment of presbyopia
convex lenses in near vision Reading glasses Bifocal glasses Trifocal glasses Progressive power glasses
4 types of optical correction
- Spectacle lens (monofocal or multifocal)
2, Contact lenses (higher quality of optical image and less influence on the size of retinal image than spectacle lenses
) - Intraocular lenses (cataract crystalline lens)
- Surgical correction Keratorefractive surgery or
Intraocular surgery : clear lens extraction (with or without IOL), phakic IOL
What is clear lens extraction + IOL
like removing opaque lens in cataracts, except lens is clear (just not bending light well), so means takoing out clear lens and putting in IOL (intraocular lens)
Problem with clear lens extraction woith IOL
Lose accommodation (patient will need reading glasses).
Outline process of accommodattion
Contraction of the Circular Ciliary Muscle inside the Ciliary Body
This relaxes the zonules that are normally stretched between the ciliary body attachment and the lens capsule attachment
Note that zonules are passive elastic bands with no active contractile muscle
In the absence of zonular tension, the lens returns to its natural convex shape due to its innate elasticity
This increases the refractive power of the lens
What is accommodation controlled by
CN3
