4. The retina & central visual pathways Flashcards
label
label
describe how impulses are spread from photoreceptor cells to the optic nerve
Rods - low light vision; cones - color vision and visual acuity
PhotoRs synapse with bipolar cells which synapse with ganglion cells - these have axons that converse to optic nerve
what is the function of horizontal cells?
lateral inhibition: stop nearby photoRs from firing so that impulses are only generated by photoRs that are most directly stimulated by light
describe 2 functions of the pigmented later of the retina
- contains melanin to prevent excessive refraction when light enters eye
- anchors photoR cells (rods and cones)
label this view of the retina (fundoscopy)
where in the retina is vision the best - explain why
FOVEA:
- highest conc. of cones so highest acuity
- slight dip so light has to travel through fewer cells
where in the retina is vision the least good - explain why
OPTIC DISC:
- no photoRs
describe in general terms the transfer of info. in the central visual pathway
i. light sensed by photoRs in retina…
ii. impulses spread along optic n. …
iii. impulses spread along optic tract to lateral geniculate nucleus (thalamus)…
iv. impulses spread along optic radiation to primary visual cortex (occipital lobe).
what are the 2 types of fibres found in optic ns. and tracts and how are these different
nasal fibres
- medial
- decussate at optic chiasm
- transmit light info. from temporal fields
temporal fibres
- lateral
- don’t decussate
- transmit light info. from nasal fields
what are the 2 types of optic radiation and how are these different
superior radiations
- in parietal lobe
- transmit light info. from inferior visual fields
inferior radiations
- in temporal lobe
- transmit light info. from superior visual fields
does a lesion before the optic chiasm cause uni- or bi-lateral vision loss, and is it ipsi- or contra-lateral
unilateral and ipsilateral
does a lesion at the optic chiasm cause uni- or bi-lateral vision loss
bilateral
does a lesion after the optic chiasm cause uni- or bi-lateral vision loss, and is it ipsi- or contra-lateral
bilateral and contralateral
name this type of vision loss and explain where the lesion has occured
BITEMPORAL HEMIANOPIA
lesion at the optic chiasm (affects both nasal fibres)
(pituitary gland and anterior communicating artery may also be damaged)
name this type of vision loss and explain where the lesion has occured
MONOCULAR BLINDNESS: ipsilateral lesion of optic nerve
name this type of vision loss and explain where the lesion has occured
LEFT HOMONOMOUS HEMIANOPIA: lesion of right optic tract
- affects right temporal and left nasal fibres
name this type of vision loss and explain where the lesion has occured
LEFT LOWER QUADRANTANOPIA: lesion of right superior optic radiation
name this type of vision loss and explain where the lesion has occured
RIGHT HOMONOMOUS HEMIANOPIA: lesion of left optic tract
- affects left temporal and right nasal fibres
name this type of vision loss and explain where the lesion has occured
LEFT UPPER QUADRANTANOPIA: lesion of right inferior optic radiation (Meyer’s loop)
name this type of vision loss and explain where the lesion has occured
HOMONYMOUS HEMIANOPIA WITH MACULAR SPARING: lesion of visual cortex in right occipital lobe
why is a stroke in the visual cortex in occipital lobe often macular sparing
Occipital lobe has a dual blood supply:
- posterior cerebral artery
- middle cerebral artery (supplies occipital pole)
The macula is represented in the occipital pole and has large cortical representation.
So if a stroke affects posterior cerebral artery, most of occipital lobe will be lost (causing homonymous hemianopia) but macular function will be spared as occipital pole still receives blood from middle cerebral artery.
describe the light reflex
i. light stimulates afferent nerve - CN II…
ii. synapses in pretectal area with neurones supplying Edinger Westphal nuclei bilaterally…
iii. synapse with and stimulate both CN III parasympathetic fibres to cause direct and consensual pupillary constriction
name the 3 aspects of the accomodation reflex
- convergence (adduction of medial rectus)
- pupillary constriction (constrictor pupillae)
- convexity of lens to increase refractive power (ciliary muscle)
describe a major difference in the light and accomodation reflex pathway - why does this occur
Cerebral cortex must be involved in accomodation reflex as it relates to image analysis:
i. Stimulation of CN II (afferent branch) which synapses at lateral geniculate nucleus…
ii. Interneurones run from LGN to visual cortex…
iii. Interneurones run from visual cortex to pretectal area and then EWN…
iv. Synapse with and stimulate both CN III (e.g. to medial rectus) to cause accomodation reflex
label this optical coherence tomogram
why are patients with albinism unable to see well in bright light
no melanin in retinal pigment epithelium to refract light so less well absorbed
which neuronal pathway in the brainstem mediates conjugate eye movements?
Medial longitudinal fasiculus: connects the nuclei of the 3 CN that control eye movements - CN III, CN IV and CN VI
