VISUAL SYSTEM Flashcards
What is the function of the lacrimal system?
Production of tears - basal, reflex and emotional
Drainage of tears
What are the afferent divisions of the lacrimal system?
Cornea CN V (ophthalmic trigeminal)
What is the efferent supply of the lacrimal system?
Parasympathetic
Which neurotransmitter is used in the lacrimal system?
acetylcholine
Describe the journey of a tear in the lacrimal system
- Produced by lacrimal gland
- Drains through the upper/lower punctum
- Flows through superior/inferior canaliculus
- Gather in tear sac
- Exit tear sac through tear duct into nasal cavity
What is the tear film and its functions?
Thin fluid layer covering eye
- Maintains smooth cornea-air surface and supplies
oxygen to cornea (no blood vessels supply cornea) - Removal of debris
- Bactericide
What are the 3 layers of the tear film and their functions?
Superficial lipid layer - reduces tear film evaporation
Aqueous layer
Mucinous layer - maintains surface wetting
What glands produce the superficial lipid layer?
Row of Meibomian glands along lid margins
What is the conjunctiva?
Thin transparent tissue which covers the outer surface of the eye. Begin at outer edge of cornea, covers visible part of eye and lines the inside of the eyelids
How does the conjunctiva get its blood supply?
Nourished by tiny blood vessels that are nearly invisible to the naked eye unless conjunctivitis
What are the 3 layers of the eyeball and label them on a diagram
Sclera - hard and opaque - “white of the eye”
Choroid - red and vascular
Retina - neurosensory tissue
What is the function of the sclera?
Tough, opaque tissue which serves as the eye’s protective outer coat
As you move to front of eye, the sclera seamlessly changes to the cornea
Name the different layers of the cornea from outer to inner
Epithelium Bowman's membrane (basal membrane) Stroma Descemet's membrane Endothelium
What is the water content of the sclera?
High
What is the water content of the cornea?
Low
What is the function of the endothelium of the cornea?
Pumps fluid out of cornea and prevents cornea oedema
What is the function of the cornea?
Powerful refracting surface - 2/3 of the eye’s focusing power
Gives clear window to look through
What is the uvea and describe its structure?
Vascular coat of eyeball and composed of 3 parts:
- Iris
- Ciliary body
- Choroid
Intimately connected so disease of one part also affects the other portions
Where does the uvea lie?
Between sclera and retina
What is the function of the choroid?
Composed of layers of blood vessels that nourish the back of the eye
What is the function of the iris?
Controls light levels inside eye
Embedded with tiny muscles that dilate and constrict the pupil size
Describe the structure of the lens
Outer acellular capsule
Regular inner elongated cell fibres - transparency
What is a cataract?
When a lens looses its transparency with age
What are the functions of the lens?
Transparent
Regular structure
Refractive power - 1/3 of eye focusing power
What is the retina and its function?
Thin layer of tissue which lines inner part of posterior eye
Captures light rays and then sends impulses to the brain via optic nerve
What is the name of the area where the optic nerve joins the retina and what does it create?
Optic disc
Creates a blind spot since no light sensitive cells there
What is the macula?
Small and highly sensitive part of the retina responsible for detailed central vision
Located roughly in centre of retina, temporal to optic nerve
What is the fovea?
Centre of the macula
Most sensitive part of retina
What allows the fovea to perceive in detail?
Highest concentration of cones
Low concentration of rods
What is central vision assessed by?
Visual acuity assessment
What is peripheral vision assessed by?
Visual field assessment
What is central vision important for?
Detail day vision, colour vision, reading, facial recognition
What is peripheral vision important for?
Shape, movement, night vision, navigation vision
What is the structure of the retina?
3 layers: outer, middle and inner
What cells are present in the outer layer of the retina and thus its function?
Photoreceptors (1st order neurons) allowing detection of light
What cells are present in the middle layer of the retina and thus its function?
Bipolar cells (2nd order neurons) providing local signal processing to improve contrast sensitivity and regulate sensitivity
What cells are present in the inner layer of the retina and thus its function?
Retinal ganglion cells (3rd order neuron) which transmit the signal from the eye to brain
Describe rod photoreceptors
100x more sensitive to light than cones
Have a longer outer segment with photo-sensitive pigment
Slow response to light
Responsible for night vision (scotopic vision)
Describe cone photoreceptors
Less sensitive to light
Fast response to light
Responsible for day light fine vision and colour vision (photopic vision)
What are the different types of cones and their functions?
S-cones: blue wavelength of light
M-cones: green wavelength of light
L-cones: red wavelength of light
What is the most frequent form of colour blindness?
Deuteranomaly AKA Daltonism
What is irregular about patients with deuteranomaly?
They don’t perceive the colour red
What is the name for full colour blindness?
achromatopsia
What is the equation for the index of refraction
Speed of light in vaccuum/speed of light in a medium
What does a convex lens do to light?
Converges light rays to a point
What does a concave lens do to light?
Diverges light rays, spreading them outwards
Focal point in front of lens
What does emmetropia mean?
Adequate correlation between eyeball’s axial length and refractive power meaning object at distance is focused exactly on retina resulting in perfect vision
Parallel light rays fall on retina (no accomodation)
What does ametropia mean?
Mismatch between eyeball’s axial length and refractive power.
Parallel light rays don’t fall on retina
What conditions fall under the umbrella of ametropia?
Myopia
Hyperopia
Astigmatism
Presbyopia
What occurs in myopia and its causes (2 types)?
Parallel rays converge at focal point anterior to retina
Caused by excessive long globe (axial myopia) - more common
Or 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 distance
Headache
What treatments are there for myopia?
Correction with diverging lens
Correction with contact lens
Correction by removing lens to reduce refractive power
What occurs in hyperopia and its causes??
Parallel rays converge at focal point posterior to retina
Caused by excessive short globe (axial hyperopia) - more common
Or insufficient refractive power (refractive myopia)
What are the symptoms of hyperopia?
Visual acuity at near tends to blur relatively early
Asthenopic symptoms (eyepain, headache, burning in eyes)
Amblyopia - uncorrected hyperopia > 5D
What is the nature of the blur in hyperopia?
- blurred vision is more noticeable if person is tired,
printing is weak or light inadequate - varies from inability to read fine print to near vision is
clear but suddenly and intermittently blur
What treatments are there for hyperopia?
Correction with converging lenses
Correction with converging lenses and cataract extraction
Correction with contact lenses
Correction with intraocular lenses
What occurs in astigmatism and its causes??
Parallel rays come to focus in 2 focal lines/points rather than a single focal point (hereditary)
Caused by a non-spherical refractive media
- refract differently along one meridian than along
meridian perpendicular to it
What are the symptoms of astigmatism?
Asthenic symptoms (headache, eyepain)
Blurred vision
Distortion of vision
Head tilting and turning
What is the treatment for regular astigmatism?
Cylinder lenses with or without spherical glasses
What are the treatments for 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) to align both eyes towards
near object
- Accomodation (circular ciliary muscle) to increase
refractive power of lens for near vision
What is presbyopia?
Naturally occurring loss of accommodation (focus for near objects)
Distant vision is still intact
Onset from 40+ years old
How is presbyopia corrected?
Convex lenses in near vision:
- Reading glasses
- Bifocal glasses
- Trifocal glasses
- Progressive power glasses
- Contact lenses
What are the positives of contact lenses?
Higher quality of optical image
Less influence on the size of retinal image than spectacle lenses
Cosmetic
Athletic activities
What are the negatives of contact lenses?
Careful daily cleaning and disinfection
Expense
Complications e.g. infectious keratitis
What are intraocular lenses and what condition are they most used for?
Used to replaced cataract crystalline lens
Best optical correction for aphakia (lack of lens) - avoids significant magnification and distortion caused by spectacle lenses
Detail the steps of laser surgical correction
Cutting of corneal flap Flipping of corneal flap Photorefractive treatment (laser) Corneal stroma reshaped post laser Corneal flap put back into position
What is the Staar intra-collamer lens (ICL) and when is it used?
Lens which is inserted over the natural lens of the eye and underneath the iris to correct myopia and astigmatism
What is the problem with the Staar intra-collamer lens (ICL)?
Eventually natural lens will become cloudy (cataract) and then you have to remove both lenses
How are intraocular lenses (IOL) implanted in patients?
First clear lens extraction (same as cataract extraction):
- Use phaco tip which uses ultrasound to break up the
retinal lens and sucks it up
Then implant artificial IOL
What do patients with an intraocular lens lose?
Accommodation (patient will need reading glasses)
At what visual pathway landmark do ganglion nerve fibres synapse?
Lateral geniculate nucleus
What is the optic radiation?
4th order neurone
What is the optic tract?
Ganglion nerve fibres exit as optic tract
List the visual pathway landmarks in order
Optic nerve Optic chiasm Optic tract Lateral geniculate nucleus Optic radiation Primary visual cortex
Draw a diagram showing the decussation of nerve fibres at the optic chiasm
Do it bitch
What percentage of ganglion fibres cross at the optic chiasm?
53%
How do lesions anterior and posterior to the optic chiasm affect the visual fields respectively?
Anterior lesion - visual field in one eye only affected
Posterior lesion - visual field in both eyes affected
What does a lesion at the optic chiasm cause?
Bitemporal hemianopia
What does a right sided lesion posterior to the optic chiasm cause?
Left homonymous hemianopia in both eyes
What does a left sided lesion posterior to the optic chiasm cause?
Right homonymous hemianopia in both eyes
What is bitemporal hemianopia typically caused by?
Enlargement of pituitary gland
What is homonymous hemianopia typically caused by?
Stroke (cerebrovascular accident)
What does a lesion at the primary visual cortex cause?
Contralateral homonymous hemianopia with macular sparing
Why does macula sparing occur?
Area in primary visual cortex that represents the macula gets dual blood supply from the posterior cerebral arteries from both sides
What is damage to the primary visual cortex often caused by?
Stroke
What does pupil constriction do?
Increases depth of field
Decreases spherical aberrations and glare
Reduces bleaching of photo-pigments
What nerve mediates pupillary constriction?
Parasympathetic nerve of CN III
Which muscles contract to cause pupil constriction?
Circular muscles
Which muscles contract to cause pupil dilation?
Radial muscles
What does pupil dilation do?
Increases light sensitivity in dark by allowing more light into eye
What nerve mediates pupillary dilation?
Sympathetic nerve of CN III
Detail the afferent pathway of the pupillary reflex
- Photoreceptors –> bipolar cells –> retinal ganglion cells
- Decussation at optic chiasm
- Pupil-specific ganglion cells exit at posterior third of
optic tract - Enter lateral geniculate nucleus
- Each pathway from each eye synapses on both Edinger-
Westphal nuclei on both sides in brainstem
Detail the efferent pathway of the pupillary reflex
- Edinger-Westphal nucleus –> oculomotor nerve efferent
- Synapses at ciliary ganglion
- Short posterior ciliary nerve –> pupillary sphincter
Draw out the pupillary reflex pathway with landmarks
Afferent:
Photoreceptors —> Bipolar cells —> Retinal ganglion cells —> Pupil specific ganglion cells exit at posterior optic tract —> Lateral geniculate nucleus —> Each eye synapses at Edinger-Westphal nuclei on both sides of brain stem
Efferent:
Edinger-Westphal nuclei —> oculomotor nerve —> synapse at ciliary ganglion —> short posterior ciliary nerve —> pupillary sphincter
What is the difference between direct and consensual pupillary light reflex?
Direct: constriction of pupil of the light stimulated eye
Consensual: constriction of pupil on the other eye
What is the neurological basis behind the consensual pupillary light reflex?
Afferent pathway on either side alone will stimulate efferent pathway on both sides
What happens when there is a right afferent defect to the optic nerve?
No pupil constriction in both eyes when right eye is stimulated with light
Normal consensual pupil constriction in both eyes when left eye is stimulated with light
What happens when there is a right efferent defect to CN III (oculomotor)?
No right pupil constriction whether right or left eye is stimulated with light
Left pupil constricts whether right or left eye is stimulated with light
How do you test for afferent pupillary defect?
Swinging torch test - alternating stimulation of right and left eye with lightt
How do you test for RAPD and what do you expect to see?
Swinging torch test
Partial pupillary response still present when damaged eye is stimulated
- Both pupils CONSTRICT when light swings to undamaged side
- Both pupils paradoxically DILATE when light swings to the damaged side
What does duction mean?
Eye movement in one eye
What does version mean?
Simultaneous movement of both eyes in the same direction
What does vergence mean?
Simultaneous movement of both eyes in the opposite direction
What does convergence mean?
Simultaneous adduction movement in both eyes when viewing a near object
What does saccade mean?
Short fast burst of eye movement up to 900 degrees/sec:
- Reflexive saccade to external stimuli
- Scanning saccade
- Predictive saccade to track objects
- Memory-guided saccade
What is smooth pursuit?
Sustained slow movement, up to 60 degrees/sec:
- Driven by motion of a moving target across retina
List all the extraocular muscles
Medial rectus
Superior rectus
Lateral rectus
Inferior rectus
Superior oblique
Inferior oblique
What does contraction of the superior oblique cause?
Moves eye in diagonal pattern down and out
Travels through trochlea
What does contraction of the inferior oblique cause?
Moves eye in diagonal pattern up and out
Travels through trochlea
Which extraocular muscles does CN III (oculomotor) innervate?
Superior branch:
- Superior rectus
- Levator palpebrae superioris
Inferior branch:
- Inferior rectus
- Medial rectus
- Inferior oblique
- Parasympathetic nerve constricts pupil (circular)
Which extraocular muscles does CN IV (trochlear) innervate?
Superior oblique
Which extraocular muscles does CN VI (abducens) innervate?
Lateral rectus
How do you test extraocular muscles?
Isolate muscle by maximising its action and minimising the action of other muscles
What is torsion?
Rotation of eye around anterior-posterior axis of eye
If you wish to test the superior rectus what position should it be in?
Elevated and abducted
If you wish to test the inferior rectus what position should it be in?
Depressed and abducted
If you wish to test the inferior oblique what position should it be in?
Elevated and adducted
If you wish to test the superior oblique what position should it be in?
Depressed and adducted
What are the symptoms of 3rd nerve palsy?
Affected eye down and out Droopy eyelid (loss of elevator palpebrae superioris)
What are the symptoms of 6th nerve palsy?
Affected eye unable to abduct and deviated inwards
Double vision worsen on gazing to the side of affected eye
What is the optokinetic nystagmus reflex?
Nystagmus - oscillatory eye movement
Reflex where smooth pursuit tracks an object whilst a fast phase reset saccade also occurs causing a grating motion
What/who is the optokinetic nystagmus reflex test used for?
Used to test visual acuity in pre-verbal children
Observe the presence of the nystagmus movement in response to moving grating patterns of various spatial frequencies
