Vision Flashcards
What bones make up the bony orbit?
maxilla, zygomatic, frontal, ethmoid, lacrimal, sphenoid and palatine bones
What bones make up the roof of the bony orbit?
orbital part of the frontal bone.
What bones make up the medial wall of the bony orbit?
maxilla, lacrimal, ethmoid and sphenoid bones.
What bones make up the floor of the bony orbit?
orbital part of the maxilla (small contributions from the zygomatic and palatine bones)
What bones make up the lateral wall of the bony orbit?
zygomatic bone anteriorly and greater wing of the sphenoid posteriorly
Label bones of the bony orbit (diagram).
What kind of tears can the lacrimal system produce?
Basal tears: constant fluid secretion by the lacrimal gland
Reflex tears: response to irritation (afferent V1, efferent PSNS)
Emotional tears
Explain the normal movement of basal tears.
move across the eyeball as the eyelids blink, accumulating medially in the lacrimal lake
Label diagram of eye.
Where are the lateral and medial Canthi located? Where is the caruncle located?
Canthi: the corners of the eye.
Caruncle present in medial canthus.
What are the Puncta?
medial openings that drain fluid into the lacrimal canaliculi to the tear sac
What is the role of the tear duct/sac?
fluid accumulates in the sac before draining to the nose via the tear duct
What the three layers of tear film?
Superficial oily layer: reduces evaporation (produce by glands in eyelid)
Aqueous tear film: produced by lacrimal gland
Mucinous layer: on corneal surface - maintains surface wetting (produced by conjunctival goblet cells)
What are the functions of tear film?
Maintain smooth cornea-air surface
Supplies oxygen to cornea (not normally vascularised)
Removes debris
Bactericidal
What is the conjunctiva?
thin, transparent tissue that covers the eye, beginning at the other edge of the cornea to cover visible eye and inside eyelids - nourished by tiny blood vessels
Diagram of cross section of the eye
What are the components of the outside coating of the eye (3 layers).
Sclera: hard and opaque
Choroid: pigmented and vascular
Retina: neurosensory tissue
What is the role of the sclera?
Protect the eye. Tough and opaque. High water content.
Explain the role of the cornea.
transparent, dome-shaped window in front of eye, providing 2/3 eye’s focusing power; continuous with scleral layer and physical/infection barrier. Low water content.
What are the 5 layers of the cornea?
Epithelium
Bowman’s membrane (basal membrane for the epithelium)
Stroma (nerve endings for sensation/nutrition)
Descemet’s membrane (basal membrane for the endothelium)
Endothelium (removes fluid and prevents corneal oedema)
Cross sectional diagram of eye.
What is the uvea? Where is it found? What is it composed of?
vascular coat of the eyeball, lying between sclera and retina - composed of iris, ciliary body and choroid
What is the choroid? Give its composition and function.
lies between retina and sclera - comprised of blood vessels that nourish eye
What is the iris? What is its function?
coloured part of the eye, controlling diameter of pupil using pupil constrictor/dilator muscles to adjust light levels
what are lens zonules?
fibrous rings that suspend the lens in the eye
What is the function of the lens?
1/3 refractive power of the eyes and accommodation for near vision
Explain the structure of the lens.
outer acellular capsule. Regular inner elongated cell fibres (regularity = transparency).
Contains collagen fibres
What is the retina and its function?
Thin layer of tissue lining inner eye. Captures light entering eye.
What is the role of the optic nerve? where is it located?
trasmit impulses fdrom retina –> brain. Connected at the back of the eye near the macula.
Where is the macula located? What is its role?
located centrally, anterior to the optic nerve. Small + highly sensitive region for detailed central vision.
Where is the fovea located?
pit at centre of the macula.
Diagram showing the anterior vs posterior segment of the eye.
What is the boundary of the posterior/anterior segments?
the lens
Where is the anterior chamber found? What is it filled with?
Berween lens and cornea.
Clear aqeous fluid which supplies nutrients.
Where is the posterior chamber found?
between lens zonules and the back of the iris.
What segment are the anterior and posterior chambers found in?
anterior chamber.
what segment is the vitreous chamber found in?
posterior segment.
Where is the vitreous chamber found?
between the lens and retina
What is the role of the ciliary body?
secrete AqH to eye to supply nutrients.
How does AqH flow?
anteriorly to anterior chamber. Drained by trabecular network.
How is AqH drained?
80-90% –> trabecular network (Canal of Schlemm).
Rest via uveal-scleral outflow (passive reabsorption).
What is AqH’s function?
supply cornea with oxygen and glucose.
What is glaucoma? What consequences can it lead to?
Sustained rasied intraocular pressure.
Causing retinal ganglion cell death and enlarged optic disc cupping - leads to visual field loss and blindness
What can cause open angle glaucoma?
trabecular network dysfunction.
What causes closed angle glaucoma?
increased pressure pushes iris/lens forward, blocking the TM and creating a vicious cycle; presents with painful red eye and acute drop in vision
What is optic nerve blind spot?
where optic nerve meets retina, no light sensitive cells
What is the role of the fovea?
Highest concentration of photoreceptors for fine vision. No overlying ganglion cell layer.
Why do we have a blind spot?
Vertebrates have an inverted retina. Light bounces off the back of the eye before activating photoreceptors, so photoreceptors are located behind retinal ganglion cells.
Blind spot occurs where the optic nerve passes the photoreceptors, leaving an area where no light sensitive cells are present.
What are the layers of the retina?
- photoreceptors
- bipolar cells.
- retinal ganglionic cells.
What is the role of photoreceptors?
1st order neurones that detect light (rods/cones)
What is the role of bipolar cells?
2nd order neurones that perform local signal processing to improve and regulate contrast sensitivity
What is the role of retinal ganglion cells?
transmit signal from eye to brain via optic nerve.
Compare rod and cone cells.
Rod - longer outer segment. 100 times more sensitive to light. Slower response. responsible for night vision - 120m rods in eye; highest concentration 20-40deg from fovea.
Cone - cone shaped outer segment. Less sensitive to light, but has a faster response; responsible for colour vision - 6m cones in eye; S cones = blue, M cones = green, L cones = red
Compare central and peripheral vision.
Central - detailed day and colour vision because the fovea has the highest concentration of cone cells - allows for reading and facial recognition
Peripheral - allows for shape, movement and night vision
Outline the 7 main colour vision deficiencies.
Deuteranomaly: green-weak; sensitivity of green (M) cones shifted towards red
Deuteranopia: green-blind; all green sensitive (M) cones are missing
Protanomaly: red-weak; red (L) cones present but have a mutation
Protanopia: red-blind; all red sensitive (L) cones are missing
Tritanomaly: blue-weak; blue (S) cones present but have a mutation
Tritanopia: blue-blind; all blue sensitive (S) cones are missing
Achromatoplasia: no colour vision whatsoever
How does the Ishihara test work?
isochromatic plates test for red-green deficiencies.
What is the near reflex triad?
action of the eye for adjustment to fixate on a near object.
What are the components of the near reflex triad?
Pupillary miosis: increases depth of field
Convergence: eyes aligned using medial recti to a near object
Accommodation: circular ciliary muscle increases refractive power of lens for near vision
How does accomodation of the lens work?
circular ciliary muscle inside ciliary body contracts –> relaxing the zonules that are normally stretched, allowing the lens to return to natural convex shape due to innate elasticity.
Lens’ refractive power increased.
What nerve mediates accomodation?
CN III
What is the difference between a converging (convex) lens and a civerging (concave) lens?
Convex - focuses rays to a point.
Concave - spread rays outwards.
What is emmetropia?
Refers to an eye with no visual defects.
Axial length and refractive power adequately correlated.
What is ametropia?
mismatch between axial length and refractive power –> imperfect vision.
Compare myopia and hyperopia.
Myopia - rays converge anterior to retina - excessive refractive power/long eyeball.
Hyperopia - rays converge posterior to retina - insufficient refractive power/short eyeball.
What is astigmatism?
Dysfunctional cornea distorts focal point of light away from the retina, so vision appears blurry.
What is presbyopia?
naturally occurring loss of accommodation, with intact distant vision - corrected by reading glasses with convex lenses to increase eye’s refractive power
What are the disadvantages of using contact lenses?
can lead to corneal vascularisation/conjunctivitis.
Outline the visual pathway that transmits signals from the eye to the visual cortex.
Optic nerve (ganglion nerve fibres)–>
optic chiasm (half of the fibres cross) –>
optic tract –>
lateral geniculate nucleus (fibres synapse at LGN)–>
optic radiation (4th order neurones connecting to primary visual/striate cortices.
What is a receptive field?
retinal space within which incoming light can alter firing pattern of a neurone; light hitting receptive field stimulates photoreceptors and inhibits surrounding
Explain convergence.
the number of lower order neuones synapsing to the equivalent higher order neurones.
Compare cone vs rod convergence. What is this indicative of?
cones - 1:1, rods - many:1. (rod > cone).
Low convergence - small receptive field, fine visual acuity, low light sensitivity.
High convergence - large receptive field, coarse visual acuity and high light sensitivity
Compare on and off centre ganglions.
On-centre: stimulated by light at centre of field and inhibited by light at edge of field.
Vice versa.
What is the role of on and off centre ganglions?
Allow for contrast sensitivity and enhanced edge detection.
What do 53% of ganglionc fibres cross at the optic chiasm?
ensure that one side of brain is responsible for contralateral visual field
Which retinal fibres cross at the optic chiasm?
Those originating from the nasal retina.
What visual field defects are casued by damage anterior, at and posterior to the optic chiasm?
Anterior - monocular blindness.
At - bitemproal hemianopia.
Posterior - contralateral homonymous hemianopia
What can cause bitemporal hemianopia?
enlargement for pituitary gland/pituitary tumours.
What is macular sparing homonymous hemianopia?
damage to primary visual cortex due to stroke, leading to contralateral homonymous hemianopia - macular area supplied by PCAs so spared
Where is the primary visual cortex found?
within occipital lobe. Characterised by distinct stripe due to fibres of optic radiation.
How is the primary visual cortex represented?
Superior visual field: below calcarine fissure
Inferior visual field: above calcarine fissure
Right hemifield: projects to left cortex
Left hemifield: projects to right cortex
How is the primary visual cortex organised?
organised as columns with unique sensitivity to visual stimulus, with right and left eye dominant columns interspersed with each other
What is the extrastriate cortex?
area around primary visual cortex within occipital lobe, converting basic information, orientation and position to complex information
What is the role of the dorsal communication pathway from the primary visual cortex to the posterior parietal cortex?
motion drection + visually guided action.
What is the role of the ventral communicaiton pathway betwee the primary visual cortex and the inferiotemporal cortex?
object/face recognition + detailed fine central and colour vision.
What is the role of the pupil?
regulate light input into eye.
Compare pupillary action in light and dark environments
Light - constriction to decrease spherical aberration and glare, while increasing depth of field; mediated by PSNS within CN III
Dark - pupil dilation to increase light sensitivity in dark; mediated by SNS
Explain the nervous pathway that controlls pupillary constriction/dilation.
Afferent - rods/cones synpase –> retinal ganglion cells. Pupil specific ganglion cells exit posterior third of tract –> LGN –> synapse to brainstem in pretectal nucleus.
Efferent - Edinger–Westphal nucleus –> occulomotor efferent –> ciliary ganglion –> posterior ciliary nerve –> pupillary sphincter.
Diagram to show nervous pathway of pupillary control.
Why does the constriction of one eye due to the direct light reflex cause contriction of the other eye?
Consensual reflex.
afferent pathway on either side stimulates efferent on both sides.
Compare the effects of L/R afferent and efferent defects on pupillary constriction.
Afferent - optic nerve damage. Affected eye stimulated –> no pupil constriction in either eye. Healthy eye stimulated –> constriction in both.
Efferent - no constriction on effected side regardless. Normal response from healthy eye regardless.
explain relative afferent pupil defect.
partial pupillary response when damaged eye stimulated, with both pupils constricting when on unaffected side, and paradoxical dilation on damaged side
Define duction, version, vergence, and convergence.
Duction: eye movement in one eye
Version: movement of both eyes in same direction
Vergence: movement of both eyes in opposite direction
Convergence: adduction movement of both eyes to view near object
Compare saccade and smooth pursuit eye movement.
Saccade: short, fast burst movement - allowing for reflexive responses to external stimuli, scanning and tracking objects
Smooth pursuit: sustained slow movement, driven by motion of a moving target across the retina
Name the muscles of the eye. Explain their attachments and direction of movement facilitated.
Superior / inferior rectus: attached to superior/inferior surface to move up and down
Lateral rectus: temporal attachment for horizontal movement
Medial rectus: nasal attachment for horizontal movement
Superior oblique: attached to eye high on the temporal side, passing under superior rectus, moving eye down and in
Inferior oblique: attached low on nasal side, passing over inferior rectus, moves eye up and out
Diagram showing muscles of right eye and which nerve innervates them.
What ocular muscles does the superior branch of SNIII innervate?
superior rectus and lid levator
What ocular muscles does the inferior branch of CNIII innervate?
inferior/medial rectus and inferior oblique
+ PSNS to constrict.
What ocular muscle is innervated by CN IV
superior oblique
What nerve innervates the lateral rectus muscle?
CN VI
Explain the pathology of CNIII PALSY
CNIV and CNVI innervation = unopposed. Superior oblique pulls down + lateral rectus pulls out.
–> affected eye down and out with droopy eyelid.
Explain the pathology of CNVI palsy.
affected eye unable to abduct and deviates inwards, double vision worsens on gazing to side of affected eye
What is nystagmus?
oscillatory eye movement
