Visual system Flashcards
What are the different parts of the extraocular eye?
.
What is the average antero-posterior diameter of the eye in adults?
24mm
What is the sclera?
- tough, opaque tissue that acts as the protective outer coat
- high water content
- protects eye and maintains shape
What is the cornea?
- transparent, dome shaped window covering the front of the eye
- low water content
What is the role of the cornea?
- refracting surface
- provides 2/3 of the eye’s focusing power
What are the 3 layers of the coat of the eye?
- sclera
- uvea
- retina
What is the uvea?
- vascular coat of eyeball
- made up of iris, ciliary body and choroid
Where is the uvea?
between the sclera and retina
What is the choroid?
- the middle, pigmented vascular layer of the coat
- provides circulation to the eye
What is the iris?
- coloured part of the eye
- controls light levels inside the eye
- sphincter and dilator muscles
What is the retina?
the innermost neurosensory layer
What is the role of the retina?
responsible for converting light into neurological impulses, transmitted to the brain by the optic nerve
What is the role of the crystalline lens?
responsible for 1/3 of the refractive power of the eye
What can happen to lens with age?
- opacification
- cataract
What is the retina? DELETE
thin layer of tissue that lines the inner part of the eye
What is the role of the retina? DELETE
responsible for capturing the light that enters the eye, sent to brain, via the optic nerve
What is the role of the optic nerve?
transmits electrical impulses from the retina to the brain
Where is the optic nerve?
- connects to the back of the eye near the macula
- the visible portion of the optic nerve is the optic disc
Where is the blind spot?
on the optic disc
Why is the optic disc a blind spot?
Contains no light sensitive cells
What is the macula?
a small and highly sensitive are in the centre of the retina
What is the role of the macula?
responsible for detailed central vision e.g. reading
Where is the macula?
in the centre of the retina, temporal to the optic nerve
What is the fovea?
- the centre of the macula
- highest concentration of cones
What is central vision?
- detail day vision
- colour
- reading
- facial recognition
How does loss of central vision present?
Problems with reading and recognising faces
What is used to assess central vision?
visual acuity assessment
What is the role of peripheral vision?
- shape
- movement in the environment
- night vision
- navigation
How does loss of peripheral vision present?
problems navigating the world
What is used to assess peripheral vision?
visual field assessment
What are the 3 layers of the retina?
- outer layer of photoreceptors
- middle layer of bipolar cells
- inner layer of retinal ganglion cells
What is the role of the photoreceptors?
detection of light
What is the role of the bipolar cells?
local signal processing to improve contrast sensitivity
What is the role of the retinal ganglion cells?
Transmission of signals from the eyes to the brain
What are the 2 main types of photoreceptors?
Rods and cones
What is the structure of rods?
- Longer outer segment with photo-sensitive pigment
- 100 times more sensitive to light than cones
- Slow response to light
- 120 million rods
- further from fovea
What are rods responsible from?
Responsible for night vision (Scotopic Vision) and peripheral vision
Describe the structure of cones
- shorter outer segment
- less sensitive to light
- faster response
- 6 million cones
- closer to fovea
What are cones responsible for?
Daylight fine vision, colour vision (phototopic vision) and central vision
What are the 2 types of lenses?
- converging (convex)
- diverging (concave)
What is a convex lens?
- converging
- takes light rays and bring them to a point
What is a concave lens?
- diverging
- takes light rays and spreads them outward
What is emmiotropia?
- normal vision
- adequate correlation between axial length and refractive power
- parallel light rays fall on the retina
What is Ametropia?
- mismatch between axial length and refractive power
- light doesn’t fall on the retina
What are the different types of Ametropia?
- near sightedness (myopia)
- far sightedness (hyperopia)
- presbyopia
What is the mechanism of myopia?
parallel rays converge at a focal point anterior to the retina
What are the causes of myopia?
- excessive long globe (axial myopia)
- 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 do you treat myopia?
- Correction with concave lenses (negative lenses)
- Correction with contact lenses
- Correction by removing the lens to reduce refractive power of the eye
What is hyperopia?
Parallel rays converge at a focal point posterior to the retina
What are the causes of hyperopia?
- excessive short globe (axial hyperopia)
- 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
- blurred vision is more noticeable if person is tired, printing is weak or light inadequate
- eyepain
- headache in frontal region
- burning sensation in the eyes
What is the treatment for Hyperopia?
- Correction with concave lens(positive lenses)
- Correction with positive lens + cataract extraction
- Correction with contact lens
- Correction with intraocular lens
What is Presbyopia?
- Naturally occurring loss of accommodation (focus for near objects)
- onset around 40 years old
- distant vision intact
How do you treat Presbyopia?
- reading glasses (convex lenses) to increase refractive power of the eye
- spectacle lens (Monofocal lenses : spherical lenses, cylindrical lenses, Multifocal lenses)
- contact lenses
What is the purpose of Near Response Triad?
adaptation for near vision
What is the Near Response Triad?
- Pupillary Miosis
- Convergence
- Accomodation
What happens in Pupillary Miosis?
sphincter pupillae cause the pupil to constrict, increasing the depth of field
What happens in Convergence?
medial recti contract, adducting the eyes to align both eyes towards a near object
What happens in Accomodation?
circular ciliary muscles increase refractive power of lens by making it thicker for near vision
What is the role of the visual pathway?
transmits signal from eye to the visual cortex
What are the landmarks of the visual pathway?
- 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
- Optic Radiation – 4th order neuron, sends signals from lateral geniculate ganglion to primary visual cortex
- Primary Visual Cortex – within the Occipital Lobe
What relays information to the visual cortex?
Lateral Geniculate Nucleus in Thalamus
What is the impact of a lesion anterior to the optic chiasm?
affects the visual field in only one eye
What is the impact of a lesion posterior to the optic chiasma?
affects the visual field in both eyes
- Right sided lesion: Left Homonymous Hemianopia in Both Eyes
- Left sided lesion: Right Homonymous Hemianopia in Both Eyes
What are the nerves that cross at the optic chiasm responsible for?
temporal visual field
What are the nerves that do not cross at the optic chiasma responsible for?
the nasal visual field
What is the impact of a lesion at the optic chiasma?
temporal field deficit in both eyes - bitemporal hemianopia
What can cause a bitemporal hemianopia?
enlargement of a pituitary gland tumour
What can cause a homonymous hemianopia?
stroke (cerebrovascular accident)
What can damage to the primary visual cortex due to stroke cause?
Homonymous Hemianopia of the contralateral side, with sparing of macula central vision
Why is damage to the primary visual cortex unlikely?
receives dual blood supply from both right and left posterior cerebral arteries
What happens to the pupil in light?
- constricts
- circular muscles contract
- mediated by parasympathetic nerve within CNIII
Why does the pupil constrict under light?
- decreases glare
- increases depth of field
What happens to the pupil in dark?
- dilate
- radial muscles contract
- mediated by sympathetic nerve within CNIII
Why does the pupil dilate in the dark?
increases light sensitivity in the dark by allowing more light in
What happens in the afferent pathway of the pupillary reflex?
- pupil-specific ganglia exit the posterior third of the optic tract
- enter the lateral geniculate nucleus
- synapse at the brainstem pretectal nucleus
- afferent pathways from each eye synapse at the Edinger-Westphal nucleus on both sides in brainstem
What happens in the efferent pathway of the pupillary reflex?
- Edinger-Westphal Nucleus
- Oculomotor Nerve Efferent
- Synapses at Ciliary ganglion upon the Short Posterior Ciliary Nerve
- Pupillary Sphincter
What stimulates the efferent pathway on both eyes?
the afferent pathway for either eye
What is the direct light reflex?
constriction of the pupil of the light-stimulated eye
What is the consensual light reflex?
constriction of the pupil of the other eye (light shone in opposite eye to this one)
What happens if the right afferent pupillary pathway is damaged?
- stimulation of the right eye will elicit weak,
or no pupillary constriction in both eyes. - BUT, Normal pupil constriction in both eyes when left eye is stimulated with light
What happens if the right efferent pupillary pathway is damaged?
no pupillary constriction in the right eye, and normal pupillary constriction in the left eye, whether right eye or left eye is stimulated.
How do you detect a relative afferent pupillary defect?
swinging torch test
What happens in a swinging torch test?
- alternating stimulation of right and left eyes with light
- both pupils constrict when light swings to undamaged side
- both pupils paradoxically dilate when light swings to damaged side
What are the 6 muscles of the eye?
- medial rectus
- lateral rectus
- superior rectus
- inferior rectus
- superior oblique
- inferior oblique
What are the 4 straight muscles of the eye?
- medial rectus
- lateral rectus
- superior rectus
- inferior rectus
What is the role of the superior rectus?
moves the eye up maximally in adducted position
What is the role of the inferior rectus?
moves the eye down maximally in abducted position
Where is the lateral rectus attached?
temporal side of the eye
What is the role of the lateral rectus?
moves the eye towards the outside of the head (towards the temple)
Where is the medial rectus attached?
on the nasal side of the eye
What is the role of the medial rectus?
moves the eye toward the middle of the head (towards the nose)
Where is the superior oblique attached?
- high on the temporal side of the eye
- passes under the superior rectus
- travels through the trochlea
What is the role of the superior oblique?
moves the eye down and out
Where is the inferior oblique attached?
- attached low on the nasal side of the eye
- passes over the inferior rectus
What is the role of the inferior oblique?
moves the eye up and out
What innervates the lateral rectus?
CN 6 - abducens nerve
What innervates the superior oblique?
CN 4 - trochlear nerve
What innervates the superior rectus?
CN 3 - superior branch of the oculomotor nerve
What is innervated by the superior branch of CN3 (oculomotor nerve)?
- levator palpebrae superioris (raises eyelid)
- superior rectus
What is innervated by the inferior branch of C3 (oculomotor nerve)
- inferior rectus
- medial rectus
- inferior oblique
What innervates the inferior rectus?
CN 3 - inferior branch of the oculomotor nerve
What innervates the medial rectus?
CN 3 - inferior branch of the oculomotor nerve
What innervates the inferior oblique?
CN 3 - inferior branch of the oculomotor nerve
What innervates pupil constriction?
CN 3 - the parasympathetic branch of the inferior oculomotor nerve
What is abduction?
moving away from the nose.
What is adduction?
moving towards the nose.
How does third nerve palsy present?
- Affected eye down and out due to unopposed SO and LR
- Droopy eyelid (loss of elevator palpebrae superioris)
- pupil dilated due to loss of parasympathetic action
What can cause third nerve palsy?
aneurysm
What muscles are affected in third nerve palsy?
everything but:
- lateral rectus (abduction)
- superior oblique (depression)
How does sixth nerve palsy present?
- Affected eye unable to abduct and deviates inwards due to unopposed MR
- Double vision worsen on gazing to the side of the affected eye
What can cause sixth nerve pasly?
cranial pressure
How does fourth nerve palsy present?
- affected eye moves up and in
- struggle with actions which invlolve looking down (e.g. reading)
What can cause fourth nerve palsy?
- congenital
- trauma
