Unit 8 - Vision Anatomy Flashcards
Where does the pathway for vision travel
Lesion of what vessel affects vision
Length makes it susceptible to demyelination
NB - vascular lesions in posterior cerebral artery
4 cells involved in conduction of visual impulses to visual cortex
Photoreceptor cells - rods and cones
Bipolar neurons - connect rods and cones to ganglion cells
Ganglion cells - whose axons pass to thalamus
Neurons of LATERAL geniculate body in thalamus - axons pass to visual cortex
Layers in retina
Outermost layer = pigment epithelial layer
Single layer of cells containing melanin
Pigment cells absorb light that passes through retina
Difference between rods and cones
110-125 million rods
6-7 million cones
Cones - colour
Rods - low intensity light - contain melanin
What is the fovea
Name the area surrounding it
What is found in the floor of the fovea
the retina is structurally different at fovea
bipolar and ganglion cells lean away from a central pit
Area surrounding it = macula lutea (yellow)
Only cones found in fovea
Inner retinal layers are displaced to allow light rays to reach cones without having to cross other layers - point of most acute vision as it is the visual axis
Blind spot of the eye
Optic disc - papilla
area at which unmyelinated optic nerve fibres exit the retina
Fibres become myelinated as they emerge
What does the retina develop from during development
From CNS
Optic nerve is a CRANIAL nerve but is myelinated by oligodendrocytes - cannot regenerate when damaged
Pathway of visual impulses
Rods and cones → bipolar cells → retinal ganglion cells → optic nerve → optic chiasm → thalamus → visual cortex
What axons make up the optic nerve
Axons of retinal ganglion cells from the optic nerve which passes through the optic canal
Describe binocular vision
Right and left fields of vision are projected on portions of both retina
Visual fields overlap across 2/3rds of total visual fields
right visual cortex sees left visual field
only 1/2 cross in optic chiasm
Image of right field of vision projected onto
Nasal half of right retina
Temporal half of left retina
Right visual field → left visual cortex
How do fibres travel in optic tract
Fibres from nasal/medial retina cross midline and enter optic tract of opposite side
Fibres of temporal/lateral retina pass posteriorly in optic tract of same side
Most fibres terminate in lateral geniculate nucleus
Where is the primary visual cortex found
Walls of calcarine sulcus
What do lateral geniculate neurons give rise to
Optic radiation (loop of meyer)
Broadmans area of visual cortex
BA 17
Visual field quadrants
Visual filed quadrants projected onto retina in reversed and inverted pattern through action of lens
Preserved all the way to visual cortex
Axons and cortex responsible for visual field of right eye - in the gaps
Where is the optic chiasm in close relation to
Stalk of pituitary - pituitary tumour may damage median portion of chiasm
Effect of aneurism in circle of willis
Impacts optic chiasm
Pathway of light reflexes
Some fibres of optic tract pass from lateral geniculate nucleus to pretectal nucleus of midbrain (concerned with light relfexes)
Proportion of fibres that travel to pretectal nucleus
As the optic tract approaches the thalamus it winds around the midbrain - medial and lateral route
Majority of fibres will terminate in lateral geniculate bodies (lateral route)
Medial route - 10% - enters side of midbrain - enter pre-tectal nucleus and serve as papillary light reflex
Direct vs consensual light reflex
How does the impulse travel
Light shone on 1 eye - pupils of both eyes constrict
Constriction of pupil in eye in which light is shone - direct light reflex
Constriction of opposite pupil - consensual light reflex
Impulse travels through optic nerve → optic chiasma → optic tract → synapse on nerves in pretectal nucleus (close to superior colliculus)
Where do light reflexes travel
Pass to PS nuclei of CN III (occulomotor)
Edinger-Westphal nuclei on both sides - posterior commissure
Impulses pass through CN III to ciliary ganglion in orbit - to short ciliary nerves at eyeball - constrictor pupillary muscle of iris
both pupils constrict
Where does the sympathetic nucleus of vision lie
Within tegmentum of midbrain
