4. The retina & central visual pathways Flashcards Preview

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Flashcards in 4. The retina & central visual pathways Deck (28):
1

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2

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3

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

4

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

5

describe 2 functions of the pigmented later of the retina

1. contains melanin to prevent excessive refraction when light enters eye

2. anchors photoR cells (rods and cones)

6

label this view of the retina (fundoscopy)

7

where in the retina is vision the best - explain why

FOVEA:

1. highest conc. of cones so highest acuity

2. slight dip so light has to travel through fewer cells

8

where in the retina is vision the least good - explain why

OPTIC DISC:

- no photoRs

9

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).

10

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

11

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

12

does a lesion before the optic chiasm cause uni- or bi-lateral vision loss, and is it ipsi- or contra-lateral

unilateral and ipsilateral

13

does a lesion at the optic chiasm cause uni- or bi-lateral vision loss

bilateral

14

does a lesion after the optic chiasm cause uni- or bi-lateral vision loss, and is it ipsi- or contra-lateral

bilateral and contralateral

15

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)

16

name this type of vision loss and explain where the lesion has occured

MONOCULAR BLINDNESS: ipsilateral lesion of optic nerve 

17

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

18

name this type of vision loss and explain where the lesion has occured

LEFT LOWER QUADRANTANOPIA: lesion of right superior optic radiation 

19

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

20

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) 

21

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

22

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. 

23

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

24

name the 3 aspects of the accomodation reflex

1. convergence (adduction of medial rectus)

2. pupillary constriction (constrictor pupillae)

3. convexity of lens to increase refractive power (ciliary muscle)

25

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

26

label this optical coherence tomogram

27

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

28

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