Vision 1 + 2 Flashcards
What process underpins the eye projecting a sharp image onto the retina
Refraction
What is refraction
bending of light rays when it passes from one optical medium to another
A biconvex lens bends an incoming beam of light and converges or diverges the light rays
Converges them to one specific point
A biconcave lens bends an incoming beam of light and converges or diverges the light rays
Diverges them to several points
Anatomical components of the eye that are involved in refraction (4) + which 2 of these are the main components responsible for refracting light
Cornea - main
Lens - main
Aqueous humor
Vitreous humor
If an object is far away, only the … rays from the object hit the cornea, … rays wouldn’t hit the cornea therefore less … … is needed to invert the image onto the retina
parallel
divergent
bending power
If an object is up close, the … rays from the object hit the cornea as well as the … rays, however the … rays need extra … … to form an inverted image on the retina
divergent
parallel
bending power
Does the lens become thicker with constant viewing of close or distant objects
Close
Is the cornea or the lens a more powerful ‘bender’ of light
Cornea
Anything beyond 6m of vision is considered
infinity
Define accommodation in respect to the eye
When the eyes change focus from a distant to a close object
What 3 things occur to allow accommodation of the eye
Lens changes shape - becomes more spherical & thicker
Pupils constrict - to sharpen focus
Eyes converge
How does ciliaris contraction cause the lens to become thicker + more spherical (4)
Ciliaris contraction makes the ciliary body bulge out –> which decreases the space between the 2 ciliary bodies –> suspensory ligaments therefore become loose/lax so decreasing the pull on the lens –> lens no longer under stretch so becomes THICKER + MORE SPHERICAL
Are the lateral or medial recti thicker
Medial
What is myopia
Short sightedness (i.e. need glasses to see distance)
What is hyperopia
Long sightedness (i.e. need glasses to read a book)
What is presbyopia
Normal loss of near focusing ability that occurs with age
Pathophysiology of myopia (Short sighted)
- is eyeball too short or long AP
- why are distant objects blurry
- what is wrong with the shape of the cornea
too long
blurry because light rays are refracted (bent) TOO MUCH by the cornea + lens –> focusing an image IN FRONT OF THE RETINA instead of on the retina
cornea too curved so refractive power of cornea and lens combined is too much for the length of the eyeball
What is emmetropia
Perfect sight
Do myopes have excessive or not enough refractive power
Excessive
How does the excessive refractive power of myopes work to their advantage compared to emmetropes
With a close object, the light rays coming from it are divergent which allows the excessive refractive power to come to use –> forms an image on the retina without needing to increase lens curvature (WITHOUT USING ACCOMODATION) whereas emmetropes would have to use accommodative powers
Symptoms/signs of myopes (short sighted) (2)
Headache when looking at distant objects
Divergent squint - one eye abducted, the other normal (in infants)
Correction of myopia
- what needs to be done to bending power
- management options (2)
Bending power needs to be decreased so want to use a lens that diverges light rays
Biconcave lens (glasses or contacts)
or
Laser eye surgery
Pathophysiology of hyperopia (long sighted)
- is eyeball too short or long AP
- what is wrong with the lens
- why are close objects blurry (5)
Too short
lens too flat
Light rays of distant objects are not refracted enough by the cornea + lens –> so image is focused BEHIND THE RETINA –> hypermetropes then automatically start to use their ACCOMODATIVE POWER to make lens thicker to allow the image of the distant object to form on the retina (which normally doesn’t need to be used) –> effectively lose/use up one’s accommodative power
So, when hypermetropes then need to see close objects, don’t have any accommodative power to do this
Symptoms (1) /signs (1) of hyperopia
Eye strain after reading/looking at screen for too long
Convergent squint - one eye adducted, the other normal (in infants)
Correction of hyperopia
- what needs to be done to bending power
- management options (2)
Bending power needs to be INCREASED
Biconvex lens (to converge the light rays into one point, allowing accommodative power to rest) - glasses, contacts
or
Laser eye surgery
What is astigmatism + what sight is affected
NON-SPHERICAL CURVATURE OF CORNEA/ LENS
Both short and long sight affected
Pathophysiology of astigmatism
Corneal surface has different curvatures in different meridians so the refraction of light rays along one axis is not the same as that of the other axis
One meridian is flatter than the other so light rays will refract slightly more on one meridian than the other
Symptoms of astigmatism (2)
Headache
Eye strain
Correction of astigmatism (3)
Needs special type of lens that contain different refractive powers at different meridians
Cylindrical lens (correct the difference between the refractive powers of the 2 principal meridians [vertical + horizontal])
or
Toric lens - special contact lenses, ring donut shape
or
Laser eye surgery
What are toric lenses (3 components that allow it to work) and what are they used for
special contact lenses used for astigmatism, not the typical spherical shape but a torus shape (ring donut shape)
Contain a sphere power (to correct myopia/hyperopia), cylinder power and an axis designation (enables them to rotate into position so that the meridians of power are lined up with the meridians of your eyes)
Pathophysiology of presbyopia (lose short sightedness due to old age)
As you age, lens gets less mobile/elastic and more RIGID (as collagen fibres become more rigid)
So, when ciliaris contracts, lens is not as capable as before to change shape (to change spherical) (LOSE ACCOMODATIVE POWERS)
Correction of presbyopia
Biconvex lens - to increase refractive power
What is phototransduction
conversion of light energy to an electrochemical response by photoreceptors
How is light energy converted into a neural signal
light energy stimulates photoreceptors (rods and cones) –> cascade reactions which hyper polarises the rods and cones creating an electrical impulse –> transmitted to CN II from retina –> becomes an AP in the nerve
What part of rods and cones do light waves activate (2)
light waves activate the OUTER SEGMENT of rods and cones (photoreceptors) which is made up of stacks of lamellae
Integrated into the cell membrane of these lamellae are VISUAL PIGMENTS - rhodopsin and opsin
What is the visual pigment of
- rods
- cones
Rhodopsin
Opsin
Rhodopsin is aka + component parts (2)
visual purple
opsin (colourless protein) + 11-cis retinal (vitamin A compound)
How is vitamin A involved in phototransduction (3)
Hint: conformation change, bleaching
When eye is exposed to light, the 11-cis retinal component (vitamin A derived compound) of rhodopsin converts into all-trans retinal which means rhodopsin undergoes a conformational shape
The change in conformation initiates a phototransduction cascade within the rod, whereby light is converted into an electrical signal (AP) that is then transmitted along the optic nerve to the visual cortex.
The change in conformation also causes opsin to dissociate from retinal because all-trans retinal can’t fit inside the opsin protein –> resulting in bleaching
What is bleaching and how does it affect vision
Bleaching is when opsin dissociates from retinal because all-trans retinal can no longer fit inside the opsin protein
Bleaching limits the degree to which the rods are stimulated, decreasing their sensitivity to bright light and allowing cone cells to mediate vision in bright environments
The change in conformation of rhodopsin after being exposed light also causes opsin to dissociate from retinal because all-trans retinal can’t fit inside the opsin protein (bleaching), however the retina needs to be ‘unbleached’ or else we wouldn’t be able to see after 5 mins
So how do we ‘unbleach’ the retina
Need to constantly convert all-trans retinal BACK TO 11-cis retinal (regenerating visual pigment) which requires energy in the form of vitamin A (taken from vitamin A stores in liver)
What’s more abundant - rods or cones
rods
Signs of vitamin A deficiency (3)
Bitot’s spots (in conjunctiva)
Corneal ulceration
Corneal opacification
Colour blindness is due to a defect of what
Cones
Gold standard investigation of glaucoma
Automated perimetry
Describe the path the electrical signal takes from the retina to the visual cortex
Optic nerve –> optic chasm –> optic tract –> lateral geniculate body –> optic radiation –> visual cortex
What sits under the optic chiasm
Pituitary gland
What is the optic tract
axons/fibres of the optic nerve
Fibres of the optic tract synapse where
LGB
At the optic chiasm, what fibres cross over
Medial/nasal fibres cross over (which view the temporal field)
Optic tract contains fibres from … (2)
(lateral) temporal half of the ipsilateral eye
crossed-over nasal fibres from the contralateral eye
Right visual cortex sees the ….
left visual field
Left visual field is composed of what fibres (2)
LEFT NASAL RETINAL NERVE FIBRES and RIGHT TEMPORAL RETINAL NERVE FIBRES
Right visual field is composed of what fibres (2)
RIGHT NASAL RETINAL NERVE FIBRES and LEFT TEMPORAL RETINAL NERVE FIBRES
Define a tract
Bundle of myelinated axons creating a nerve fibre pathway
LGB is part of what
Thalamus
Nasal retinal nerve fibres view what field
Temporal
Temporal retinal fibres view what field
Nasal
If R optic nerve damaged, what happens
Blindness in R eye because no signal from right eye passed back to visual cortex
Disruption of optic chiasm causes what
Bitemporal hemianopia
If R optic tract damaged, what happens
Contralateral homonymous hemianopia - can’t see nasal field in R eye and temporal field in L eye
If right optic radiation damaged, what happens
Same as damage to R optic tract
Contralateral homonymous hemianopia - can’t see nasal field in R eye and temporal field in L eye
Binocular vision allows good perception of what, whereas uniocular vision doesn’t
Depth
What axis are extrinsic ocular muscles attached along
Orbital axis, not optical axis
-orbital axes are not parallel to each other, optic axes are
When the eye is ABDUCTED,
- what muscle elevates
- what muscle depresses
- what muscle intorts
- what muscle extorts
SR
IR
SO
IO
When the eye is ADDUCTED,
- what muscle elevates
- what muscle depresses
- what muscle intorts
- what muscle extorts
IO
SO
SR
IR
What is strabismus
Squint eyes
-one of the eyes may turn in, out, up or down while the other eye looks ahead
What is esotropia
Convergent squint (cross eyed)
What is exotropia
divergent squint (one or both eyes turn outwards)
What is amblyopia
Lazy eye
- childhood condition where an eye fails to achieve normal visual acuity (vision doesn’t develop properly), even with glasses
- eye and brain not working well together
What can be attempted to correct amblyopia in the early years
patching of the “good” eye to force the brain to pay attention to the visual input from the amblyopic eye and enable normal vision development to occur in that eye
Consequences of strabismus (Squint)
Amblyopia
Diplopia
Increased light to pupil stimulates sympathetic or parasympathetic –> pupils subsequently dilate or constrict
Parasympathetic
Constrict
Decreased light to pupil stimulates sympathetic or parasympathetic –> pupils subsequently dilate or constrict
Sympathetic
Dilate
Usually, visual impulses travelling along the optic nerve –> optic chiasm –> optic tract ultimately synapse at the LGB, however fibres destined to activate the pupillary light reflex DO NOT GO TO THE LGB - where do they synapse instead (essentially asking about AFFERENT pathway of the pupillary light reflex)
Leave the optic tract to go to synapse at the CN III nucleus in the midbrain, specifically the edinger-westphal (EWN) nucleus
The edinger-westphal nucleus (EWN) is part of the nucleus of what CN
CN III
Describe the EFFERENT pathway of the pupillary light reflex
Once synapsed in the edinger-westphal nucleus (EWN), parasympathetic fibres pass through CN III into the orbit and synapse at the ciliary ganglion –> ultimately constricting pupils
Define anisocoria
Pupils are different sizes
Name 2 common pupil abnormalities
Anisocoria
Abnormal light reflex
Common causes of abnormal pupillary reflex (3)
Retinal disease, e.g. retinal detachment
CN II disease, e.g. optic neuritis
CN III disease
In CN III palsy, there’s usually no damage to sympathetic or parasympathetic fibres so pupils should still …
Parasympathetic
constrict
pupillary reflex should still be present in CN III palsy, if absent it’s unlikely to be CN III palsy and could possibly be due to …
cerebral artery aneurysm
What is Horner’s syndrome
Anisocoria due to damage of the sympathetic innervation to the pupil (i.e. innervation of dilator pupillae)
Horner’s syndrome presentation triad
Miosis
Ptosis
Anhidrosis - loss of sweating on affected side
