4. The retina & central visual pathways

Question 1

label

Answer 1

Question 2

label

Answer 2

Question 3

describe how impulses are spread from photoreceptor cells to the optic nerve

Answer 3

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

Question 4

what is the function of horizontal cells?

Answer 4

lateral inhibition: stop nearby photoRs from firing so that impulses are only generated by photoRs that are most directly stimulated by light

Question 5

describe 2 functions of the pigmented later of the retina

Answer 5

1. contains melanin to prevent excessive refraction when light enters eye
2. anchors photoR cells (rods and cones)

Question 6

label this view of the retina (fundoscopy)

Answer 6

Question 7

where in the retina is vision the best - explain why

Answer 7

FOVEA:

1. highest conc. of cones so highest acuity
2. slight dip so light has to travel through fewer cells

Question 8

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

Answer 8

OPTIC DISC:

• no photoRs

Question 9

describe in general terms the transfer of info. in the central visual pathway

Answer 9

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).

Question 10

what are the 2 types of fibres found in optic ns. and tracts and how are these different

Answer 10

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

Question 11

what are the 2 types of optic radiation and how are these different

Answer 11

superior radiations

• in parietal lobe
• transmit light info. from inferior visual fields

inferior radiations

• in temporal lobe
• transmit light info. from superior visual fields

Question 12

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

Answer 12

unilateral and ipsilateral

Question 13

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

Answer 13

bilateral

Question 14

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

Answer 14

bilateral and contralateral

Question 15

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

Answer 15

BITEMPORAL HEMIANOPIA

lesion at the optic chiasm (affects both nasal fibres)

(pituitary gland and anterior communicating artery may also be damaged)

Question 16

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

Answer 16

MONOCULAR BLINDNESS: ipsilateral lesion of optic nerve

Question 17

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

Answer 17

LEFT HOMONOMOUS HEMIANOPIA: lesion of right optic tract

• affects right temporal and left nasal fibres

Question 18

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

Answer 18

LEFT LOWER QUADRANTANOPIA: lesion of right superior optic radiation

Question 19

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

Answer 19

RIGHT HOMONOMOUS HEMIANOPIA: lesion of left optic tract

• affects left temporal and right nasal fibres

Question 20

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

Answer 20

LEFT UPPER QUADRANTANOPIA: lesion of right inferior optic radiation (Meyer’s loop)

Question 21

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

Answer 21

HOMONYMOUS HEMIANOPIA WITH MACULAR SPARING: lesion of visual cortex in right occipital lobe

Question 22

why is a stroke in the visual cortex in occipital lobe often macular sparing

Answer 22

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.

Question 23

describe the light reflex

Answer 23

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

Question 24

name the 3 aspects of the accomodation reflex

Answer 24

1. convergence (adduction of medial rectus)
2. pupillary constriction (constrictor pupillae)
3. convexity of lens to increase refractive power (ciliary muscle)

Question 25

describe a major difference in the light and accomodation reflex pathway - why does this occur

Answer 25

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

Question 26

label this optical coherence tomogram

Answer 26

Question 27

why are patients with albinism unable to see well in bright light

Answer 27

no melanin in retinal pigment epithelium to refract light so less well absorbed

Question 28

which neuronal pathway in the brainstem mediates conjugate eye movements?

Answer 28

Medial longitudinal fasiculus: connects the nuclei of the 3 CN that control eye movements - CN III, CN IV and CN VI