Visual System Flashcards
Name for the white of the eye
Sclera
Has high water content
name for the corner of the eye where eyelids meet?
lateral/medial canthus
name for the pink bit in the medial corner?
Caruncle
three layers of the eye in order?
sclera (hard and opaque) → choroid (pigmented and vascular) → retina (neurosensory tissue)
name for eyeball vascular coat?
Uvea
What is the uvea made of
choroid, ciliary body, iris (all interconnected)
where is the retina?
Inner part of eye
Retina function
capturing light rays and turning information into optic nerve signalling
visible portion of the optic nerve is called what?
Optic disc
what is the macula responsible for?
spot in centre of retina lateral to optic disc responsible for detailed central vision.
name for the spot in the centre of the macula?
Fovea
Allows appreciation of fine detail and perform tasks that require central vision such as reading
What does the fovea have the highest concentration of
Cone photoreceptors
central vision function vs peripheral vision function?
central: detailed day vision, colour, reading, facial recognition.
lacking = poor visual acuity
peripheral: shape, movement, night vision
lacking = poor visual field (can need visual aids even if perfect acuity)
- layers of the retina → what do they contain?
outer layer: photoreceptors (rods and cones) = 1st order neurons
middle layer: bipolar cells (local signal processing) = 2nd order neurons
inner layer: retinal ganglion cells (transmit info to brain) = 3rd order neuron
differences between rods and cones?
rods more sensitive to light, slow response, more abundant, do night vision (scotopic) 120 million rods
cones less sensitive with faster response, do day light fine vision and colour vision (photopic) 6 million rods
what happens to light when passing from one medium into another?
Velocity changes (refraction)
Two types of lens and their function
convex converges light rays to a point
concave spreads light rays outwards
emmetropia vs ametropia?
emmetropia = adequate correlation bw axial length and refractive power (parallel light rays fall on retina)
ametropia = mismatch bw axial length and refractive power (parallel light rays don’t fall on retina)
Difference types of ametropia
Myopia
Hyperopia
Presbyopia
Myopia
parallel rays converge at a point anterior to retina
can be axial → more common or refractive (excessive long globe or excessive refractive power)
Blurred distance vision,squinting and headache
Hyperopia
parallel rays would converge at a point posterior to retina
excessive short globe (axial, more common) or insufficient refractive power
visual acuity blurred at near, can be more noticeable at times or when tired
eye pain, headache in frontal region, burning in eyes } = asthenopic symptoms
Near response triad
pupillary miosis (contraction, sphincter pupillae) to increase depth of field,convergence (medial recti) to align both eyes towards a near object, accommodation (circular ciliary muscle) → increase lens refractive power and increases lens thickness
- naturally occuring loss of accommodation in old age
what is presbyopia?
Naturally occurring loss of accommodation in old age
What is presbyopia corrected by
reading glasses that increase refractive power
(Convex lenses)
retinal ganglion of the optic nerve reach which landmark next?
Optic chiasm
Approx 53% of fibres decussate to contralateral optic tract
Which fibres decussate at the optic chiasm
those originating from nasal retina (left hand side of left visual field, right hand side of right visual field = temporal visual field)
- one eye only
lesions anterior to optic chiasm affect what?
One eye only
lesions on optic chiasm cause what?
bitemporal hemianopia (temporal visual field fibres from the nasal retina are the ones crossing over)
lesions posterior to optic chiasm cause what?
homonymous hemianopia (left half of both eyes gone or right half of both eyes)
how does homonymous hemianopia with macular sparing occur?
homonymous hemianopia often due to stroke
Central vision preserved as visual cortex area representing macula receives dual blood supply from posterior cerebral arteries of both sides (middle cerebral artery and posterior cerebral artery so adequate perfusion remains if one affected)
Damage to the primary visual cortex
why is pupil constriction part of the near response triad?
Increases depth of field
Decreases glare
What nerves constrict and dilate pupils
Constriction by the parasympathetic nerve in CN III which is the oculomotor
Dilation due to sympathetic nerve not oculomotor (done via dilator pupillae muscle)
pupillary reflex pathway afferent fibres come from where?
Light enters eye and strikes retina (retinal ganglion cells participate)
Axons c8nverge it form optic nerve
Decussation of fibres at optic chiasm
Pupil specific ganglion cells exit at posterior third of optic tract before entering lateral geniculate nucleus
Synapse at pretectal nucleus (brainstem)
Synapse on edinger westphal nuclei on both sides of the brain
afferent pathway synapses where?
Edinger-Westphal nuclei on both sides
efferent pathway synapses where?
Ciliary ganglion
Edinger Westphalia nucleus receives input from pretectal nucleus
Preganglionic parasympathetic fibres travel along oculomotor nerve where they reach cilliary ganglion and synapse
Postganglionic fibres leave and travel via shirt ciliary nerves reaching sphincter pupilae
direct vs consensual light reflex?
constriction of light-stimulated eye vs constriction of other eye
neurological basis of light reflexes
afferent pathway of either single eye stimulates efferent pathway of both eyes
right afferent defect creates what pupil response to light?
right fails to constrict, no consensual reflex by left
right consensual reflex present when left is lit
right efferent defect creates what pupil response to light?
right fails to constrict, consensual reflex by left present
no right consensual reflex when left is lit
what is the swinging torch test used to determine?
partial or relative damage to an afferent pathway
What happens in swinging torch
both pupils constrict when light swings to undamaged side, both paradoxically dilate when swung to damaged side (relative afferent pupillary defect)
six extraocular muscles are called what?
lateral rectus, medial rectus, inferior rectus, superior rectus, superior oblique, inferior oblique
What does superior oblique and inferior oblique do
SO-down and out (attached high to temporal side)
IO-up and out (attached on nasal side)
innervation of the muscles?
superior rectus, inferior rectus, medial rectus, inferior oblique, levator palpebrae superioris (lifts eyelid), pupil constriction = III oculomotor
superior oblique = IV trochlesr
lateral rectus = VI (abducens nerve abducts eye)
What is the antero posterior diameter of the eye in adults
24mm
Visual pathway retina neurones
First order neurones-rod and cone retinal photoreceptors
Second order neurones-retinal bipolar cells where 53% of fibres cross at the midline
Third order neurones-retinal ganglion cells-optic tract synapse where lateral geninucleate tract terminates. Then they extend their axons to the primary visual cortex via optic radiations (4th order neurons)
Optic chiasm
53% of ganglion fibres cross at the optic chiasm
Crossed fibres originating from the nasal retina responsible to temporal visual fields
Uncrossed fibres originating from temporal retina are responsible for nasal visual fields Uncrossed
Why does Bitemporal hemianopia occur
Caused by enlargement of pituitary gland tumour
In dark pupil dilation
Increased light sensitivity by allowing more light into the eye
Mediated by sympathetic nerve
Relative afferent pupillary defect
Partial response still present when damaged eye stimulated
Superior and inferior rectus
Superior attached to eye at 12 o clock and moves eye up
Inferior attached at 6 o clock and moves eye down
Blind spot
Where optic nerve meets retina
Around optic disc
