Neurology of vision Flashcards
Summarise the visual pathway
Eye-optic nerve, optic chiasm (half fibres switch), optic tract, lateral geniculate nucleus, occipital
What neurons are in the retina?
Photoreceptors-1st order neurons
Second order neurons-bipolar cells (modulate
Retinal ganglial cells-3rd order-back to brain)
What is the receptive field in vision?
its the amount of place a cell can “see”-one photorecetor -its sligthyl around it
But for a bipolar cells, its all the receptors that go into in
and for the retinal ganglia-all the photpreceptors that go to bipolars that go ti it –the receptive fields converge (number of lower order synapsing to higher neuron)
How does convergence of receptive fields vary?
Central convergence (nearly 1:1) is higher than in peripheral convergence
Cone system convergence is higher than rod system convergence
Why more precise in center-each captures make a signal. In periphery-many senses lead to only one signal
What are on and off centre ganglion cells?
Retinal ganglion cells can be divided
On centre ganglion-stimulated by light at center of receptive field, and inhbited by light at edge
Off-centre-inhibited by light at centre, and vice versa
Having both is important for contrast sensitivity and edge detection
How does the optic chiasm work?
53% of the fibres from each eye passes across to the other side-the crossed fibres originate from the nasal retina-responsible for temporal visual field (most lateral)
Non crossing-originating from temporal retina, nasal visual field
Lesions anterior to chiasm affect 1 eye, posterior-both eyes
What visual fields defect originate from the optic chiasm?
Damage at the optic chiasm-bitemporal hemianopia
Damage posterior to chiasm but in one tract-homonymous hemianopia
lots of other lesions exists ofc (partial chiasm damage, anterior, eye damage, occipital cortex damage)
If visual field defect only affect one part of eye (nasal or temporal)-most likely neurological-if not eye damage
What usual causes bitempotal hemianopia?
Pit gland tumour pressing of chiasm
stroke and CVA
Where is the primary visual cortex? how it organised>
Situated along the calcrine sulcus of occipital cortex-macula has a very large area
And the superior part of retina apears on the bottom of the cortex
organised in columns with unique sensitivity to a partical orientation. then right and left dominant colimn intersperes (one then the other) –makes a “cube”
What is macular sparing homonyous hemianopia?
damage to the primary cortex-often due to stroke
ledas to contralateral homonymous hemianopia with macula sparing -because macula area receives dual blood supply from both posterior cerebral ateries
What is the extrastriate cortex? why is it important in vision? what about the ventral/dorsal pathways?
extrastriate -Area around primary visual cortex–converts basic visual informtation into more complex stuff
Dorsal pathway-motion detection, visually guided action, damage results in motion blindness
Ventral pathway-representation, face recognition, detailed fine central and color visual
What happens to the pupil in response to light? Describe neuronal pathway
PSNS mediated-afferent and efferent
Afferent-pass to midbrain where both eye pathways synapse-then go to both eyes (why both eyes contrict when shine light in one –called consensual reflex
the direct reflex is the dilation of the eye with light in
What is the difference between direct vs consesual eye reflex?
If one eye is blind-then when shine light in, no contrict on either side, but if shine in other (not blind)-both contrict (blind eye too)
IF the damage is in the brainstem-
direct efferent defect-shine in one eye-contrict, but not other
Consensual efferent defect-shine in one eye-doest contrict but the other ones
What is a paradoxial reflex to light torch eye test?
Sometimes in some damage, when you shine a light and both contrict, then switch to other eye and it dilates-usually means damage
What is the basic principle of eye movement
Pulling muscle, counteracting one another
