Visual System Flashcards
Eye anatomy
In what situations are tears produced by the lacrimal gland? Define these.
basal- tears produced at a constant level,
even in the absence of irritation or stimulation
reflex- increased tear production,
in response to ocular irritation.
emotional responses
what is the control of a tear reflex i.e. a bug lands on eye
Afferent – cornea, cranial nerve V1 – ophthalmic trigeminal (to CNS)
Efferent – parasympathetic (to lacrimal gland)
Neurotransmitter - acetylcholine
Where is the lacrimal gland located?
The lacrimal gland is located within the orbit,
latero-superior to the globe.
what is the route of a tear from lacrimal gland to nasal cavity?
Tears produced by lacrimal gland
Drain through the two puncta, openings on medial lid margin
Flow through superior and inferior canaliculi
Gather in tear sac
Exit tear sac through tear duct into nasal cavity
what is the role of the 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
what are the 3 layers of the tear film? Which layer forms the bulk of the tear film?
lipid outer layer, aqueous layer, mucinous layer on cornea surface
Aqueous layer forms bulk of tear film
Function of lipid layer, aqueous layer and mucionous layer of tear film?
What glands secrete the lipid layer?
The lipid layer on the top responsible for protecting the tear film from rapid evaporation. The lipid layer is secreted by the Meibomian Glands,
situated along the eyelid margins.
The Aqueous Tear Film Layer delivers oxygen and nutrient to the surrounding tissue. It contains factors against potentially harmful bacteria.
The bottom Mucinous Layer ensures that the tear film sticks to the eye surface. This renders the surface of the eye “wettable”. The mucin molecules act by binding water molecules, to the hydrophobic corneal epithelial cell surface.
what is the conjunctiva?
What does it cover?
thin, transparent tissue covering outer surface of the eye
It begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids
Is the conjunctiva vascularised?
Yes- It is nourished by tiny blood vessels that are nearly invisible to the naked eye
Antero-posterior diameter of the eye in adults?
24mm
what are the 3 tissue layers of the eyeball?
sclera- hard and opaque
choroid- pigmented and vascularised
retina- neurosensory tissue
what is the sclera?
What is its function?
What is the water content of the sclera?
the white of the eye
tough, opaque outer coat
serves as the eye’s protective outer coat.
high water content
what is the cornea?
What is its water content?
What is the function of the cornea?
transparent, dome shaped window covering front of eye
low water content
Powerful refracting surface, providing 2/3 of the eye’s focusing power.
what are the layers of the cornea?
1 – Epithelium
2 – Bowman’s membrane
3 – Stroma – its regularity contributes towards transparency
4- Descemet’s membrane
5- Endothelium – pumps fluid out of cornea and prevents corneal oedema
what is the uvea?
What are its three parts?
Vascular coat of eyeball and lies between the sclera and retina.
choroid, iris and ciliary body
what is the choroid?
Lies between the retina and sclera. It is composed of layers of blood vessels that nourish the back of the eye
What is the function of the iris? How is this achieved?
Controls light levels inside the eye
Embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size (Round opening in the centre is the pupil)
what is the structure of the lens?
outer acellular capsule
regular inner elongated cell fibres
what is the function of the lens?
Transparency
Regular structure
Refractive Power
1/3 of the eye focusing power - higher refractive index than aqueous fluid and vitreous
Accommodation
Elasticity
What is a cataract?
Loss of transparency of the lens with age
what is the retina?
What is its function?
What is the function of the optic nerve?
thin layer of tissue lining the inner eye
Responsible for capturing the light rays that enter the eye
These light impulses are then sent to the brain for processing, via the optic nerve
Where does the optic nerve connect to the eye and what is the visible portion of the optic nerve called?
connects to the back of the eye near the macula
visible portion is called the optic disc
what is the blind spot and why is it a blind spot?
where the optic nerve meets the retina
there are no light sensitive cells
what is the macula?
Where is it located?
A small and highly sensitive part of the retina responsible for detailed central vision
Located roughly in the centre of the retina, temporal to the optic nerve
what is the centre of the macula called?
fovea
what is the role of the fovea?
Allows to appreciate detail and perform tasks which require central vision
Give examples of tasks that require central vision
Reading
Facial recognition
what is the fovea?
What concentrations of photosensory cells are found in the retina
most sensitive part of retina - centre of macula
highest concentration of cones, low concentration of rods
What is central vision?
What part of the retina is responsible for this and why?
Detail day vision, colour vision
fovea has the highest concentration of cone photoreceptors
How is foveal vision assessed and what would a loss of foveal vision lead to?
Assessed by visual acuity assessment
Loss of foveal vision – Poor visual acuity
what is peripheral vision?
shape, movement, navigation and night vision
how is peripheral vision tested?
WHat would a loss of peripheral vision lead to?
Assessed by visual field assessment
Extensive loss of visual field – unable to navigate in environment, patient may need white stick even with perfect visual acuity
what is the three cell layers of the retina?
outer - photoreceptors (1st order neuron)
middle - bipolar cells (2nd order)
inner - retinal ganglion cells (3rd order)
what is the function of retinal photoreceptors?
detection of light
what is the function of bipolar cells of the retina?
Local signal processing to improve contrast sensitivity, regulate sensitivity
what is the function of retinal ganglion cells of the retina?
transmission of signal from eye to brain
what are the classes of photoreceptors?
rods
cones
Structure, sensitivity and response speed of response of rods?
What are rods responsible for and how many rods are found in the retina
Longer outer segment with photo-sensitive pigment
100 times more sensitive to light than cones
Slow response to light
Responsible for night vision (Scotopic Vision)
120 million rods
Structure, sensitivity and response speed of cones
What are cones responsible for and how many cones are found in the retina?
shorter outer segment
Less sensitive to light, but faster response
Responsible for day light fine vision and colour vision (Photopic Vision)
6 million cones
Photoreceptor distribution
Rod photo-receptors are widely distributed all over the retina, with the highest density just outside the macula. (20- 40 degrees away from the fovea)
The density of rod photo-ceptors gently tails off towards the periphery. Rod photo-receptors are completely absent within the macula
Cone photo-receptors are distributed only within the macula.
which cones detect blue wavelengths?
s cones
which cones detect green light?
M cones
which cones detect red light?
L cones
Why do we experience yellow as a combination of red and green light biologically?
Yellow light has a wavelength between the peak sensitivity wavelengths of M-Cones and L-Cones.
Yellow light stimulates both M-cones and L-cones equally.
Biologically, we experience yellow light as a combination of green and red light.
Are colour vision deficiencies more common in men or in women?
In men
What causes colour vision deficiencies and what is this known as?
Colour Vision deficits can be caused by a shift in the photo-pigment peak sensitivity.
This is called Anomalous Trichromatism.
What is dichromatism?
What is monochromatism?
Give two examples for monochromatism
In Dichromatism, only two cone photo-pigment sub-types are present.
In Monochromatism, there is complete absence of colour vision.
This can be caused by Blue Cone Monochromatism,
with the presence of only blue L-cones.
Or by Rod Monochromatism,
in which there is a total absence of all cone photo-receptors.
How do blue cone monochromatism and rod monochromatism affect daylight visual acuity?
Patients with Blue Cone Monochromatism have normal day light visual acuity,
whereas Patients with Rod Monochromatism have no functional day vision.
what is the most common colour blindness?
Deuteranomaly (a.k.a. Daltonism)- not being able to perceive the colour red
what is achromatopsia?
full colour blindness
what is the ishihara test used for?
to detect colourblindness
what is the index of refraction (n) ?
ratio of the speed of light in vaccum (air) and speed of light in new medium
what happens when light meets a boundary?
Compare angle of incidence to angle of reflection and angle of refraction.
Some of the light reflects off the boundary and some of the light refracts through the boundary.
Angle of incidence = Angle of Reflection
Angle of Incidence > or < the Angle of refraction depending on the direction of the light
what are the two basic types of lenses and how do they differ?
convex/ converging lens - takes light rays and brings them to a point e.g. camera lens which focuses an image on a photographic film
concave/ diverging lens - takes light rays and spreads them outwards
what is emmetropia?
adequate correlation between axial length and refractive power
parallel light rays fall on the retina (no accommodation)
what is ammetropia?
mismatch between axial length and refractive power
parallel rays dont fall on retina (no accommodation)
what are the types of ametropia?
myopia
hyperopia
astigmatism
presbyopia
what is myopia?
Where do parallel light rays converge in relation to the retina?
near-sightedness
Parallel rays converge at a focal point anterior to the retina
Etiology of myopia
not clear- genetic factor
what are the causes of myopia?
excessive long globe (axial myopia) more common
excessive refractive power (refractive myopia)
what are the symptoms of myopia?
Blurred distance vision
Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
Headache
how is myopia treated?
a. Correction with diverging lenses (negative lenses)
b. Correction with contact lens
c. Correction by removing the lens to reduce refractive power of the eye
what is hyperopia? Where do light rays converge in relation to the retina?
Etiology?
far sightedness
Parallel rays converge at a focal point posterior to the retina
Etiology : not clear, inherited
what are the causes of hyperopia?
excessive short globe (axial hyperopia)- more common
insufficient refractive power (refractive hyperopia)
what are the symptoms of hyperopia?
visual acuity at near tends to blur relatively early
nature of blur is varied from inability to read fine print to near vision is clear but suddenly and intermittently blur
blurred vision is more noticeable if person is tired , printing is weak or light inadequate
asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
Amblyopia – uncorrected hyperopia > 5D
how is hyperopia treated?
a. Correction with converging (positive lenses)
b. Correction with positive lens + cataract extraction
c. Correction with contact lens
d. Correction with intraocular lens
what is astigmatism?
Etiology?
parallel rays come to focus in two focal lines rather than a single focal point
Etiology: hereditary
what are the causes of astigmatism?
refractive media (cornea) 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)
what are the symptoms of astigmatism?
Asthenopic symptoms (headache , eyepain)
blurred vision
distortion of vision
head tilting and turning
how is astigmatism treated?
Regular astigmatism : cylinder lenses with or without spherical lenses (convex or concave), Sx
Irregular astigmatism : rigid cylinder lenses, surgery
what is the near response triad?
Adaptation for Near Vision
Pupillary Miosis (Sphincter Pupillae) to increase depth of field
Convergence (medial recti from both eyes) to align both eyes towards a near object
Accommodation (Circular Ciliary Muscle) to increase the refractive power of lens for near vision
what is presbyopia?
What is its onset
naturally occuring loss of accomodation/focus of near objects
distant vision intact, onset from 40+
how is presbyopia treated?
convex lenses in near vision- Reading glasses, bifocal glasses, trifocal glasses, progressive power glasses
Spectacle lenses- monofocal lenses (spherical lenses , cylindrical lenses), multifocal lenses
Contact lenses
Advantages of contact lenses
Indications for use of contact lenses
higher quality of optical image and less influence on the size of retinal image than spectacle lenses
indication : cosmetic , athletic activities , occupational , irregular corneal astigmatism , high anisometropia , corneal disease
what are the drawbacks of contact lenses?
careful daily cleaning and disinfection needed
expense
risk of complications
what are the complications of contact lenses?
infectious keratitis
giant papillary conjunctivitis
corneal vascularisation
severe chronic conjunctivitis
what are intraocular lenses used for?
Advantages?
replacement of cataract crystalline lens
best optical correction for aphakia (no lens)
avoids significant magnification/distortion as seen with spectacle lenses
Types of surgical correction
Keratorefractive surgery :RK, AK, PRK, LASIK, ICR, thermokeratoplasty
Intraocular surgery : clear lens extraction (with or without IOL), phakic IOL
what is the process of LASIK surgery?
Pre operative eye
Initial cutting of corneal flap
Cutting of corneal flap
Flipping of corneal flap
Photorefractive treatment (laser)
Corneal stroma reshaped post laser
Corneal flap back in position
Treatment completed
give an example of an intraocular lens and its use
Staar intra-collamer lens (ICL)
correction of myopia and astigmatism
What is clear lens extraction + IOL
Disadvantage
Same as cataract extraction.
Implantation of artificial lens.
Lose accommodation (patient will need reading glasses).
describe the process of clear lens extraction
natural lens is removed using a phaco tip
artificial intraocular lens inserted
Visual Pathway Landmarks
Eye
Optic Nerve – Ganglion Nerve Fibres
Optic Chiasm – Half of the nerve fibres cross here
Optic Tract – Ganglion nerve fibres exit as optic tract
Lateral Geniculate Nucleus – Ganglion nerve fibres synapse at Lateral Geniculate Nucleus
Optic Radiation – 4th order neuron
Primary Visual Cortex or Striate Cortes – within the Occipital Lobe
where, how many and which retinal ganglion cell fibres decussate?
53% decussate in the optic chiasm (nasal retina)
how do lesions anterior to the optic chiasm present?
affect visual field in one eye only
how do lesions posterior to the optic chiasm present?
affect visual field in both eyes
which fibres decussate at the optic chiasm?
nasal retina aka those responsible for temporal visual field
which fibres don’t cross at the optic chiasm?
temporal retina responsible for nasal visual field
how does a lesion at the optic chiasm present?
Damages crossed ganglion fibres from nasal retina in both eyes
Temporal Field Deficit in Both Eyes – Bitemporal Hemianopia
how does a right sided lesion posterior to the optic chiasm present?
left homonymous hemianopia in both eyes
how does a left sided lesion posterior to the optic chiasm present?
right homonymous hemianopia in both eyes
Disorders of visual fields due to lesions at following locations
what are the causes of bitemporal hemianopias?
pituitary gland tumour compressing optic chiasma
what are the causes of homonymous hemianopia?
stroke
what is the cause of homonymous hemianopia with macular sparing?
Damage to Primary Visual Cortex
Often due to stroke
Leads to Contralateral Homonymous Hemianopia with Macula Sparing
what blood vessel supplies the part of the visual cortex which is responsible for representing the macula?
What is the benefit of this
posterior cerebral arteries (dual blood supply from both sides arteries)
therefore macula is likely to be spared in strokes
What are the effects of pupillary constriction in response to light
decreases spherical aberrations and glare
increases depth of field
reduces bleaching of photo-pigments
What effect does pupillary dilation in the dark have?
increases light sensitivity in the dark by allowing more light into the eye
What type of nerve stimulation causes pupillary constriction and dilation respectively?
Pupillary constriction mediated by parasymapthetic nerve (within CN III)
pupillary dilatation mediated by sympathetic nerve
how does pupillary constriction occur (muscle contraction)?
parasympathetic stimulation causes circular muscles to contract, radial muscles relax
Describe the afferent part of the pupillary reflex
Afferent pathway (Red & Green):
Rod and Cone Photoreceptors synapsing on Bipolar Cells synapsing on Retinal Ganglion Cells
Pupil-specific ganglion cells exits at posterior third of optic tract before entering the Lateral Geniculate Nucleus
Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides in the brainstem
Describe the efferent pathway of the pupillary reflex
Efferent pathway (Blue):
Edinger-Westphal Nucleus -> Oculomotor Nerve Efferent ->
Synapses at Ciliary ganglion ->
Short Posterior Ciliary Nerve -> Pupillary Sphincter
what is the direct pupillary reflex?
constriction of pupil in light stimulated eye
What is the consensual pupillary reflex?
Constriction of Pupil of the other (not stimulated by light) eye
Neurological basis of direct vs consensual pupillary reflex
Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides
what response to light is seen a right afferent defect?
What can cause a right afferent defect?
No pupil constriction in both eyes when right eye is stimulated with light
Normal pupil constriction in both eyes when left eye is stimulated with light
optic nerve damage
Pupillary response to light in right efferent defect
no right pupil constriction when either left or right eye stimulated
left eye still constricts when right eye is stimulated
how do right efferent defects occur?
damage to oculomotor nerve
Difference in light responsiveness with unilateral afferent vs unilateral efferent pupillary defects
Unilateral afferent: Difference response pending on which eye is stimulated
Unilateral Efferent Defect: Same unequal response between left and right eye irrespective which eye is stimulated
what is a swinging torch test used for?
What happens when this is conducted in presence of defect?
to test relative afferent pupillary defects
Both Pupils constrict when light swings to left undamaged side
Both Pupils paradoxically dilate when light swings to the right damaged side
what are relative afferent pupillary defects?
partial pupillary response still present when damaged eye stimulated
so semi-damage to optic nerve
how would you test the back of someones eyes
fundoscopy
what are all of the types of eye movements
Duction – Eye Movement in One Eye
Version – Simultaneous movement of both eyes in the same direction
Vergence – Simultaneous movement of both eyes in the opposite direction
Convergence – Simultaneous adduction (inward) movement in both eyes when viewing a near object
what are the 2 speeds of eye movement
Saccade – short fast burst, up to 900°/sec
Smooth Pursuit – sustained slow movement up to 60°/s
Types of saccade movement
Reflexive saccade to external stimuli
Scanning saccade
Predictive saccade to track objects
Memory-guided saccade
What drives smooth pursuit
Driven by motion of a moving target across the retina.
what is the optokinetic nystagmus reflex
Nystagmus – Oscillatory eye movement
Optokinetic Nystagmus = Smooth Pursuit + Fast Phase Reset Saccade
Optokinetic Nystagmus Reflex is useful in testing visual acuity in pre-verbal children by observing the presence of nystagmus movement in response to moving grating patterns of various spatial frequencies
Presence of Optokinetic Nystagmus in response to moving grating signifies that the subject has sufficient visual acuity to perceive the grating pattern
What are the 6 extraocular muscles of the eye and what is their general function
Four straight muscles- Lateral rectus, medial rectus, superior rectus, inferior rectus
Two obliques- Superior oblique, inferior oblique
Functions- Attach eyeball to orbit, straight and rotatory movement
Attachment and function of superior and inferior rectus
Superior rectus: Attached to the eye at 12 o’clock, moves the eye up.
Inferior rectus: attached to the eye at 6 o’clock, moves the eye down.
Attachment and function of lateral (external) rectus
Attaches on the temporal side of the eye
Moves the eye toward the outside of the head (toward the temple)
Attachment and function of the medial rectus of the eye
Attached on the nasal side of the eye
Moves the eye toward the middle of the head (toward the nose)
Attachment and function of superior oblique of eye
Attached high on the temporal side of the eye.
Passes under the Superior Rectus.
Moves the eye in a diagonal pattern down and out
Travels through the trochlea
Attachment and function of inferior oblique
Attached low on the nasal side of the eye.
Passes over the Inferior Rectus.
Moves the eye in a diagonal pattern - up and out.
Innervation of Extraocular Muscles
- Third Cranial Nerve (oculomotor)
a) Superior Branch
Superior Rectus – elevates eye
levator palpebrae superioris - raises eyelid
b) Inferior Branch
Inferior Rectus – depresses eye
Medial Rectus – adducts eye
Inferior Oblique – elevates eye
Parasympathetic Nerve – constricts pupil - Fourth Cranial Nerve (trochlear)
Superior Oblique – depresses eye - Sixth Cranial Nerve (abducens)
Lateral Rectus – abducts eye
How are extraocular muscles tested
Isolate muscle to be tested by maximizing its action and minimizing the action of other muscles
Abduction – Lateral Rectus
Adduction – Medial Rectus
Elevated and Abducted – Superior Rectus
Depressed and Abducted – Inferior Rectus
Elevated and Adducted – Inferior Oblique
Depressed and Adducted – Superior Oblique
appearance of 3rd nerve palsy
what causes this
Affected eye down and out
Unopposed superior oblique innervated by fourth nerve (down)
Unopposed lateral rectus action innervated by sixth nerve (out)
Dilated pupil (loss of parasympathetic innervation)
Droopy eyelid (loss of elevator palpebrae superioris)
appearance of 6th nerve palsy
Affected eye unable to abduct and deviates inwards
Double vision worsen on gazing to the side of the affected eye
appearance of 4th nerve palsy
when looking towards unaffected eye, affected eye moves upwards (IO takes over from LR)
what are the lens changes in the near response triad mediated by
ciliary muscle attached to the lens via suspensory ligament contracts
reduces tension on the suspensory ligaments, so the lens relaxes and becomes thicker, causing greater refractive power
where does the right visual field travel to in the brain
left hemisphere primary visual cortex
includes nasal retina from right eye and temporal retina from left eye
What is mydriasis and miosis? What innervation and muscles are responsible for this?
Mydriasis- pupillary dilation, sympathetic innervation to dilator pupillae
Miosis- pupillary constriction, parasympathetic innervation to sphincter pupillae
How can lesions affecting occulomotor nerve be classified?
Lesions affecting the oculomotor nerve are usually classified into medical and surgical.
Medical lesions, typically affecting the vasculature to the nerve (and hence the central portion of the nerve), tend to not affect the pupil (‘pupil sparing’) as the parasympathetic fibres running to the eye are in the outer portion of the nerve.
Microvascular disease due to hypertension, diabetes mellitus.
Surgical lesions, usually in the form of a posterior communicating artery aneurysm, tend to affect the nerve. The aneurysm typically compresses the outer portion of the nerve fibres, which is where the parasympathetic nerves run.
In relation to the Circle of Willis: CN3 runs in between the posterior cerebral and the superior cerebellar artery.
How is third nerve palsy treated?
Treatment will depend on the cause of the cranial nerve palsy. In this case, the cause is likely to be a posterior communicating artery aneurysm. Hence a neurosurgical opinion is advised, and assessment of the aneurysm is needed (this may involve clipping).
For patients with microvascular disease, they will need better metabolic control of their condition.
