Visual System Flashcards
3 layers of eye coat?
Sclera - hard and opaque
Choroid - pigmented and vascular
Retina - neurosensory tissue
Sclera?
White of eyes
Tough, opaque, protective outer coat
High water content
Uvea?
Vascular coat of eyeballs, most vascular part of eye
Between sclera and retina
Iris, ciliary body and choroid
Retina?
Thin layer of tissue, lines inner part of eye
Captures light (which is sent to brain via optic nerve)
Optic nerve?
Transmits electrical impulses from retina to brain
Connects to back of eye near macula
Visible portion called “optic disc”
Macula?
In centre of retina
Small and highly sensitive, responsible for detailed vision (e.g. reading)
Fovea = centre
Blind spot?
Where optic nerve meets retina - no light sensitive cells
Corresponding anatomical landmark for physiological blindspot?
Optic disc
Central vision?
Focus
Detail day vision, colour vision, reading, facial recognition
Fovea has highest conc. of cone photoreceptors
Central vision assessment?
Visual acuity assessment
Loss of fovea vision = poor visual acuity
Peripheral vision?
Motion
Shape, movement, night vision, navigation vision
Peripheral vision assessment?
Visual field assessment
Extensive loss of visual field = unable to navigate in environment, may need white stick
Retinal outer layer?
Photoreceptors - 1st order neuron
Detection of light
Retinal middle layer?
Bipolar cells (2nd order neuron)
Local signal processing to improve contrast sensitivity
Retinal inner layer?
Retinal ganglion cells (3rd order neuron)
Transmission of signal from eye to brain
Photoreceptor classes
Rods - more sensitive to light, slow response, night vision (scotopic vision)
Cones - less sensitive to light, faster response, day light fine vision and colour vision (photopic vision)
2 types of lenses?
Convex - brings light to a point
Concave - spreads light outwards
Emmetropia?
Adequate correlation between axial length and refractive power
Parallel light falls on retina
Ametropia?
Refractive error
Mismatch between axial length and refractive power
Parallel light rays don’t wall on retina
Myopia meaning?
Near-sightedness
Parallel rays converge at focal point anterior to ulna
Myopia causes?
Excessive long globe (axial myopia)
Excessive refractive power (refractive myopia)
Myopia symptoms?
Blurred distant vision
Squint
Headache
Hyperopia meaning?
Far-sightedness
Parallel rays converge at focal point posterior to retina
Hyperopia causes?
Excessive short globe (axial hyperopia)
Insufficient refractive power (refractive hyperopia)
Hyperopia symptoms?
Visual acuity at near tends to blur relatively early (more noticeable when tired)
Eye pain, headaches in frontal region, burning sensation in eyes
Near response triad?
Pupillary miosis
Convergence
Accommodation
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 refractive power of lens for near vision
Presbyopia
Naturally occurring loss of accommodation (focus for near objects)
Onset from age 40
Distant vision intact
Corrected by convex lenses for increased accommodation (refractive power)
In accommodation, which one of the following events does not take place?
A) Relaxation of Circular Ciliary Muscle
B) Relaxation of Zonules
C) Thickening of Lens
D) Increase of Lens Refractive Power
A - relaxation of circular ciliary muscles do NOT relax
Which statement is false for Myopia?
A) May be associated with large globe
B) Light ray converges behind the retina
C) May be associated with increased corneal curvature
D) Unable to see objects clearly at distance without glasses or other optical correction
B is false
Optic chiasm?
Optic nerves from both eyes converge at optic chiasm
53% decussate to contralateral optic tract
Visual pathway?
Eye
Optic nerve
Optic chiasm
Optic tract
Lateral geniculate nucleus
Optic radiation
Primary visual cortex or striate cortex
Lateral geniculate nucleus?
Relay centre within thalamus
Ganglion nerve fibres synapse
Optic radiation?
Forms 4th order neurones
Relay signal form lateral geniculate ganglion to primary visual cortex
Primary visual cortex or striate cortex?
Within occipital lobe
Relays to extra-striate cortex (higher visual processing)
Visual pathway retina order neurones?
First order - rod and cone retinal photoreceptors
Second order - retinal bipolar cells
Third order - retinal ganglion cells
Retinal ganglion cells?
Optic nerve (CN II)
Partial decussation at optic chiasm (53%)
Optic chiasm - crossed fibres?
Originating from nasal retina
Responsible for temporal visual field
Optic chiasm - uncrossed fibres
Originating from temporal retina
Responsible for nasal visual field
Lesions anterior to optic chiasm?
Affect visual field in one eye only
Lesions posterior to optic chiasm?
Right sided lesion - left homonymous hemianopia in both eyes
Left sided lesion - right sided hemianopia in both eyes
often due to stroke
Lesion at optic chiasm?
Bitemporal hemianopia
Typically caused by enlargement of pit. gland tumour
Homonymous hemianopia with macula sparing?
Damage to primary visual cortex (often stroke)
Leads to contralateral homonymous hemianopia with macula sparing
Area representing the Macula receives dual blood supply from posterior cerebral arteries from both sides
Pupils in light?
Constriction
Decreases glare
Increases depth of field
Mediated by parasympathetic nerve (within CN III), causes circular muscles to contract
Pupils in dark?
Dilatation
Increases sensitivity in dark by allowing more light into eye
Mediated by sympathetic nerve, causes radial muscles to contract
Pupillary reflex - afferent pathway
Pupil-specific ganglion cells exit at posterior third of optic tract
Enter lateral geniculate nucleus
Synapse at brain stem pretectal nucleus
Each eye synapses on Edinger-Westphal nuclei on both sides in brain stem
Pupillary reflex - efferent
Edinger-Westphal nucleus -> oculomotor nerve efferent
Synapses at ciliary ganglion
Short posterior ciliary nerve -> pupillary sphincter
Direct pupillary reflex?
Constriction of pupil of the light-stimulated eye
Consensual pupillary reflex?
Constriction of pupil of the other eye
Neurological basis for consensual and direct pupillary reflex?
Afferent pathway on either side alone will stimulate efferent pathway on both sides
Right afferent defect
E.g. damage to optic nerve
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
Right efferent defect?
E.g. damage to right 3rd nerve
No right pupil constriction whether right or left eye is stimulated with light
Left pupil constricts whether right or left eye is stimulated
Relative afferent pupillary defect
Partial pupillary response still present when damaged eye is stimulated, elicited by swinging torch test
E.g. if right eye is damaged…
both pupils constrict when light swings to left undamaged side
both pupils paradoxically dilate when light swings to right damaged side
Cranial nerves that facilitate eye movement?
CN III
CN IV
CN VI
Muscles of eye?
Superior rectus
Inferior rectus
Lateral rectus
Medial rectus
Superior oblique
Inferior oblique
CN III
Innervates all rectus muscles except lateral
Innervates inferior oblique
Upper eyelid muscles?
From CN III and sympathetic
Levator muscles
Superior rectus movement and CN?
Elevates, CN III, superior branch
Inferior rectus movement and CN?
Depresses, CN III, inferior branch
Lateral rectus movement and CN?
Abducts, CN VI
Medial rectus movement and CN?
Adducts, CNIII, inferior branch
Superior oblique movement and CN?
Depresses and abducts, CN IV
Down and out
Inferior oblique movement and CN?
Elevates and abducts, CN III, inferior branch
Up and out
