neurology of visual system Flashcards
visual pathway anatomy DIAGRAM
signals go from eye to visual cortex from eye, impulses travel along OPTIC NERVE- half of nerve fibres cross at OPTIC CHIASM, and nerve fibres exit as OPTIC TRACT these fibres then synapse at LATERAL GENICULATE NUCLEUS, and emerge as OPTIC RADIATION (4th order neuron), which goes to PRIMARY VISUAL CORTEX in occipital lobe
pathway in retina
first order neurones are rod and cone photoreceptors which transform impulse from photons into electrons- then goes to second order neurones (retinal bipolar cells- then goes to third order neurones (retinal ganglion cells)
receptive field DIAGRAM
space where light hits a specific neurone- this photoreceptor activates a certain bipolar cell, which activates part of a ganglion- other photoreceptors not activated, so are negative
DIAGRAM convergence of receptive fields
number of neurones synapsing on a higher order neurone- cone cells in the CENTRE of the retina have a 1:1:1 convergence (ganglion cell:bipolar: cone cell)= small receptive field (eg like hand), leading to good visual acuity, but poor sensitivity rod cells in the PERIPHERY of the retina have high convergence= large field, high sensitivty but poor acuity
On-centre and off-centre ganglion cells
ganglion cells can be divided into 2 types- on centre ganglion are stimulated by light at CENTRE of receptive field, but inhibited by light on EDGE of receptive field- this is vice versa for off centre ganglion cells important for better contrast sensitivity
DIAGRAMoptic chiasm
half of ganglion fibres cross here- the crossed fibres originate at nasal retina, but needed for TEMPORAL visual field, whereas the uncrossed fibres originate at temporal retina, and needed for NASAL visual field
lesions to optic chiasm anterior and posterior + cause of anterior
anterior affects one eye visual field only (blindess in one eye only), posterior affects both- right sided lesion causes left homonymous hemianopia in both eyes (left part of both eyes affected), left sided lesion cases right hemianopia
cause of anterior- optic nerve compression
lesion at optic chiasm
crossed fibres at both eyes damaged= temporal field issue in both eyes= BITEMPORAL HEMIANOPIA
DIAGRAM other disorders of visual pathway
if lesion further back towards occipital cortex, less of the visual field affected= quadrant anopia however, if injury occurs right at back in primary visual cortex, leads to macula sparing defect ie like homonymous hemianopia, but CENTRE (macula) is spared- this is because macula receives a dual blood supply, hence in a STROKE, macula still receives blood
causes of bitemporal and homonymous hemianopia
bitemporal causes by pituitary tumour, which presses on optic chiasm (gland sits underneath) homonymous due to stroke
primary visual cortex and representation
located along CALCARINE SULCUS in occipital lobe superior visual field goes to below calcarine sulcus, inferior field goes above sulcus
ie if can’t see at top left, means lesion at bottom right
extrastriate cortex
area around visual cortex in occipital lobe- dorsal pathway helps with motion detection, so damage= motion blindness (when object moves, can’t see it well, once still can see it well: ventral pathway= face recognition+ detailed central vision: damage= black and white
afferent and efferent pathway for pupillary reflex
photoreceptors= bipolar cells= retinal ganglion cells= lateral geniculate nucleus, synapsing at brain stem in pretectal nucleus, then edinger westphal nucleus efferent pathway then goes to oculomotor, synapsing at ciliary ganglion, then to pupillary sphincter via short ciliary nerve
swinging torch test with RIGHT afferent defect
torch keeps swinging from left to right- when shone on left, both pupils constrict, but when shone on right, both pupils PARADOXICALLY DILATE
define dunction
eye movement in one eye
