4.1 Visual pathway Flashcards
functions of pigmented layer
main absorption of light
contains melanin to modulate amount of light received
anchors photoreceptor cell, as they’re embedded in pigmented epithelium
functions of horizontal cells
inhibitory interneurones which do lateral inhibition (no signals from photoreceptors either side of the photoreceptor detecting impulse), prevents XS neural impulses to brain
so increases contrast and narrows light signals
functions of bipolar cells
connect a few photoreceptor cells to bipolar neurone to axons of retinal ganglion cells to optic nerve
amaurosis fugal
occlusion of (branch of) central retinal artery/vein or ICA
sudden transient loss of vision due to hypoxia of retina
‘curtain down over vision’
symptom of stroke
papilloedema
swollen optic disc secondary to raised ICP, causing optic nerve compression
retinal detachment: pathophysiology and symptoms
separation of photoreceptors from pigmented layer, fluid can enter space
sudden blurring/loss of vision
seeing stars
benefits of fovea structure
ganglion cell axons are splayed, creating a dip
less distance for light to penetrate to reach cones = impulses easier, central vision
high density of cones = high resolution vision
which lobe do superior optic radiations pass through?
parietal
which lobe do inferior optic radiations pass through?
temporal
how do we have binocular vision?
L+R visual fields overlap
which retinal fibres are responsible for central vision?
temporal (via nasal visual fields)
pattern of vision loss in CN2 lesion
ipsilateral temporal and nasal retinal fibres affected
so lose ipsilateral nasal and temporal visual fields
=MONOCULAR BLINDNESS
pattern of vision loss in optic chasm lesion
ipsilateral and contralateral nasal retinal fibres affected
so both temporal visual fields lost
=BITEMPORAL HEMIANOPIA (tunnel vision)
causes of bitemporal hemianopia
pituitary adenoma
anterior communicating artery aneurysm
pattern of vision loss in optic tract lesion
ipsilateral temporal fibres affected, and contralateral nasal fibres
so loss of ipsilateral nasal visual field, and contralateral temporal visual field
=HOMONOMOUS HEMIANOPIA
which optic radiations are responsible for inferior quadrant field of vision? via projection to which lobe?
superior
parietal lobe
pattern of vision loss for right superior optic radiation lesion
ipsilateral superior temporal fibre + contralateral superior nasal fibre affected
so ipsilateral inferior nasal field and contralateral inferior temporal fields lost
= HOMONOMOUS INFERIOR QUADRANTANOPIA
how to distinguish between a lesion affecting superior and inferior optic radiations e.g. stroke, vs optic tract lesion
history and symptoms
is there cortical involvement in pupillary reflex (1), and accommodation reflex (2)?
- no
- yes due to image processing and analysis
the optic nerve synapses on pre tectal nucleus, where is this?
tectum of midbrain
accommodation reflex pathway
light stimulates afferent optic nerve
synapses in lateral geniculate nucleus
impulses to midbrain from PVC
then to 2 nuclei: edinger Westphal (parasympathetic fibres for sphincter pupillae and ciliary muscle contraction) and CN3 (contraction of medial rectus)
what’s meant by the medial longitudinal fasciclus?
loads of connections between nuclei of CN3,4,6,8 and connections descending to spinal cord to coordinate movements (of head position: CN8)
how does the medial longitudinal fasciclus play a part in looking left?
result if not?
coordinate L lateral rectus (L abducens nuclei) connects to R medial rectus (R oculomotor nuclei)
otherwise = diplopia
internuclear othalmoplegia
paralysis of eyeballs due to loss of connections between cranial nerve nuclei
