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Visual Pathways/magnoceluarl And Parvocellular Pathways Flashcards

1
Q

What are the 7 parts of the visual pathway

A

Retina
Optic nerve

Optic chasm - where you have the crossing of the different fibres - the nasal fibres crossing over to the other side - the nasal fibres crossing over and the temporal fibres stay on the same side

Optic tract - fibres from the optic tract synapse at the LGN

LGN

Optic radiations - two optic radiations one going through the temporal nerve and one going through the parietal lobe - if you have a lesion in one of them - it tends to produce a different pattern of visual field defect

Primary visual cortex - which is a part of your occipital cortex

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2
Q

What visual defects would you expect from lesions at different points of the optic pathway

A
  • optic nerve lesion - total blindness of the Ipsilateral eye - unilateral field deft anything that the eye is seeing is not transmitted to the brain so your lose all the vision in one eye - for an optic nerve lesion it is not affecting the other eye at all the other eye is still working fine its just that one eye that isnt able to transmit info to the brain - you lose vision in that eye
  • optic chasm lesion chiasm all lesions typically result in a - bitemporal heminaopia commonest cause is a pituitary Adenoma - pituitary gland is inferior to pitch chiasm so any lesion on their can compress on the optic chiasm - - nasal fibres cross over - the light hitting the nasal fibres is coming from the temporal visual field hitting your nasal fibres and these fibres cross over at the chias - therefore if you have a lesion at the optic chiasm it means that you cannot see anything from your temporal visual field which means that you end up with a bitemporal hemianopia
  • it is Called a hemanopia because it is only affecting half the visual field in each eye
    • you have a lesion that is only affecting part of the optic chiasm - Rare condition - because your only affecting part of the optic chiasm your only affecting part of the nasal fibres the nasal visual field of one of the eyes - light hitting the temporal retina of your left eye - so you end up with a unilateral nasal hemianopia
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3
Q

What are retrochiasmal lesions

A
  • anything that goes beyond the optic chiasm - will always give you a contra lateral hemianopia /quadranntopia - the reason for that is because at the optic chiasm the fibres have already crossed over so regardless of where the lesion is if it is beyond the chiasm it will always be contralateral - so if the lesion is on the right side of your brain you will be losing the left side of your visual field and if you have a lesion on the right side of your brain you will lose the right visual field
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4
Q

What visual defect would you expect with a optic radiation lesion

A
  • if you gave light from your tempotal visual field hits the nasal retina on the right eye - that’s your nasal fibres which are crossing over - you lose that part of your visual field in your right eye so you lose the temporal visual field
  • in your left eye light coming from the nasal side hits the tempotal retina so they dont cross over at the chiasm - they stay on the Same side so you lose that part of the visual field in that eye so you lose that side

This defect is called a homonymous (because it is on the same side on each eye) and because it is only affecting half the visual field in each eye hemianopia

  • so in this case you have lost the right side of the visual field - even though the lesion is on the left side of your brain - so that is why it is a contralteral defect
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5
Q

Does the parietal lobe carry fibres from the superior or inferior retina

A
  • superior retina - inferior visual field

Remember - light coming from the top hits the bottom of the retina - and light coming from the bottom hits the top of the retina

PITS - this relates to the visual field defect

  • a lesion in the parietal lobe would result in a inferior visual field defect and a lesion in the temporal lobe would result in a superior visual field defect -
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6
Q

What visual field defect would you expect in a parietal lobe lesion

A
  • a contralateral inferior quadtrantopia
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7
Q

What visual field defect would you expect in a temporal lobe lesion

A
  • the temporal carry fibres from your inferior retina - and because they if you have a lesion in your temporal lobe you end up with a superior visual defect
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8
Q

What visual field defect would you expect in a lesion in the occipital lobe

A
  • homonymous hemianopia with macular sparing

Because of a dual blood supply

with these lesions you usually spare the central visual field - you have macular sparing - most common theory as to why this is because the part of the occipital lobe that is responsible for the macula - as got two blood supplies so if ou have a stroke in one of your blood suppplies the other one is still working and you are stilll receiving blood you have maccula sparing -

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9
Q

A lesion through the parietal optic radiation leads to a

A
  • inferior quadrantopia
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10
Q

A lesion through the temporal optic radiation leads to a

A
  • superior quadrantopia
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11
Q

A lesion at the optic nerve will lead to what visual defect?

A

Optic nerve lesions lead to a unilateral, ipsilateral field defect

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12
Q

What visual defect will a lesion at the occipital lobe lead to ?

A
  • a lesion at the level of the occipital lobe usually results in a homonymous hemianopia with macular sparing (which is thought to be due to dual blood supply of the occipital lobe where the macula is represented)
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13
Q

What is the function of the retina

A
  • light is traduced to electrical impulses by photoreceptors
  • complex connections between horizontal , amacrine and bipolar cells
  • converge onto retinal ganglion cells whose axons form the optic nerve
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14
Q

What is the function of photoreceptors

A
  • cells that detect light and transducer light energy into a electrical impulse (action potential )
  • this action potential then travels along your optic nerve and to your visual cortex
  • this is done by your photoreceptors and you have two main types of photoreceptors - rods and cones

Rods are responsible for dark vision

Cones are responsible for high acuity e.g. central vision and colour vision

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15
Q

Why are retinal ganglion cells important

A
  • when retinal ganglion cells send of their axons their axons all come together to form the optic nerve - the optic nerve is essentially the axons of the retinal ganglion cells
  • retina and photoreceptors send action potentials to the retinal ganglion cells and retinal ganglion cells then send these action potentials down their axons their axons all come together to form the optic nerve at the back
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16
Q

Describe the structure of the optic nerve

A
  • formed by axons of the retinal ganglion cells

(Around 1.2 million)

Optic nerve head seen as the disc

4 main parts

Intraocular - 1mm - within the eye - optic nerve head

Intraorbital - 25-30mm - when the eye is moving around you dont want it to be stretching on the optic nerve - if your eye is moving around - or their is any type of swelling and your eye is being pushed around by something your not stretching on the optic nerve to much because you have this extra length

Intracanicular - 4- 10mm - goes through the optic canal -

Intracranial - around 10mm- as soon as it leaves he canal and it goes towards the optic chiasm- you have already reached the inside of the brain

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17
Q

What is the longest part of the optic nerve

A
  • intraorbital part 25-30mm- is within the orbit -is the longest part of the optic nerve
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18
Q

What is the shortest part of the optic nerve

A
  • the intraocular part - 1mm
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19
Q

What are the tow roots that the optic tract terminates at ?

A
  • the optic nerve has now gone to the chiasm - the nasal fibres have crossed over and the temporal fibres have stayed on the same side - they now come together to form the optic tract -
  • the lateral root and the medial root
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20
Q

Describe the lateral root

A
  • (90% of the fibres from the optic tract go here)
  • they terminate in the LGN
  • responsible for conscious visual perception- e.g. visual pathway - that is all the lateral root of the optic tract
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21
Q

Describe the medial root and its 3 main targets

A

(10% of the fibres from the optic tract go here)

  • subconscious visual processes
  • superior colliculus - visual association pathways , automatic scanning
  • pretectal nucleus - pupillary light reflex
  • super chiasmatic nucleus of hypothalamus - photoperiod regulation - circadian rhythms - sleep wake cycle
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22
Q

Describe the structure of the lateral geniculate nucleus

A
  • LGN = lateral root which is concerned with conscious visual perception
  • layered structure located posterolaterally to the dorsal thalamus (LGN lies just posterior to the thalamus) - which is a part of your brain
    • from your LGN that’s where most of your fibres go to your primary visual or text so it is a key visual gateway to the primary visual cortex ( place where you interpret all these conscious images)
  • primary visual cortex = v1
  • receives fibres from the lateral roots of the optic tracts (i.e. after decussation at the optic chiasm )- when the fibres cross over the nasal fibres from one eye and the temporal fibres of one eye are now along the same optic tract
  • implying that each LGN receives fibres from both eyes- because it is receiving fibres from the optic tract which has already crossed over at the optic chiasm
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23
Q

Describe the layers of the LGN

A
  • there are 6 layers of the LGN
  • layers 2, 3 ,5 recieve fibres from the ipslateral eye - (from the eye on the same side -
  • layers 1,4, 6 recieve fibres from the contralteral eye
    e. g. if we are reffering to the right LGN layers 2,3,5 are going to recieve fibres from the right eye , layers 1, 4 ,6 will receive fibres from the left eye
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24
Q

what are magnocellular and parvocelluar pathways

A
  • 2 distinct visual pathways - have been identified by neuroanatomical and physiological studies - they found 2 sets of fibres along the visual pathway
  • magnocellular (M) Pathway - the cells along this pathway are really big
  • parvocellular (P) pathway - have smaller cells

2 completley different pathways with different fucntions - they travel along the same root - but they dont mix with each other

  • these pathways remain sepreated from the retina up to the visual cortex - (i.e. the m and p pathways both travel along the visual pathway but their fibres do not mix)
  • this seperation starts right from the retina - the retinal ganglion cell axons give off the optic nerve
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25
Q
  • how do the cell types differentiate between the magnocelluar and parvocelluar pathways
A
  • depending on which type of retinal ganglion cells is sending off the axons that will define weather it will be the magnocellular or parvocellular pathway
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26
Q

which layers of the LGN carry fibres from the ipsilateral eye

A

2, 3 , 5

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27
Q

which layers of the LGN carry fibres from the contrlateral eye

A
  • layers 1 , 4 , 6
28
Q

which layers of the lgn carry fibres from the magncellular pathway

A

layer 1 and 2

29
Q

which layers of the lgn carry fibres from the parvocellular pathway

A
  • layers , 3, 4 , 5 , 6
30
Q

where do m and p cells lie

A
  • m and p cells all lie within grey matter layers
31
Q

where do m and p fibres terminate

A
  • m and p cells send off fibres that ulitmatley terminate in v1 (primary visual cortex) mostly in layer iv
32
Q

in between each layer of the lgn what is their

A

in between each layer , there lies a konicocellular layer within the white matter

33
Q

what is the optic nerve formed from

A
  • the optic nerve is formed from retinal ganglion cells axons and comprises 4 main portions -introcular , introrbital , intracanicular and intracranial

-

34
Q

what are the two roots of the optic tracts

A
  • optic tracts have a large lateral root and a small medial root
35
Q

what are the 2 main neuroanatomical visual pathways

A

there are 2 main neuroanatomical visual pathways - the magnocellular and parvocellular pathways which travel seperatley from the retina to the visual cortex

36
Q

what are the functions of the magnocelluar pathways

A
  • magno = large
  • rapidly relays imformation to the visual cortes from parasol retinal ganglion cells
  • involved in
  • motion detection
    depth perception
    low acuity / resolution
    high contrast sensitivity - e.g. black and white (monochromatic)
  • because it is very rapid it dosnt allow for high acuity fine vsiion
  • rapid saccadic eye movements rely on m cell imformation
  • vital for binocular vision and rapid eye motor coordination for the body

vital for binocular vision and rapid eye motor coordination for the body

37
Q

if in your retina you have a parsal retinal ganglion cell sending of an axon which pathway will it be

A
  • magnocellular pathway and willl terminate at layers 1 and 2 of your lgn
38
Q

if you have a parsal retinal ganglion cell in your right eye that is sending off an axon to your right lgn whuch layer will this terminate in ?

A
    • because it is ipsilateral and it will be going to the magnocellular pathway pathway it will terminate in layer 2 - parasol retinal ganglion cell will be magnocellular so it will only be layers 1 or 2 but because it is coming from the right eye to the right lgn e.g. ipsilateral so it has to be layer 2
39
Q

describe parasol retinal ganglion cells

A
  • large retinal ganglion cells
  • accpunt for 10% of retinal ganglion cells
  • have large complex dendritic networks
  • multiple photoreceptors (loads of photoreceptors that connect to a single parasol cell ) - thats why you dont have fine detail - because lots of different imformation going to one cell - the advantage of this is that it gives you really fast imformation
  • mostly rod input but no colour imformation

heavily myleinated fibres therefore - fast conduction

  • synapse with m cells
40
Q

describe parvocellular cells

A

small cells

slowly relay imformation to the visual cortex from midget retinal ganglion cells

involved in

  • high acuity/ resolution of vision
  • colour (chomatic processing)
  • low contrast sensitivity
41
Q

describe midget retinal ganglion cells

A
  • small retinal ganglion cells (90%) total
  • smaill, simole dendritic network
  • 1 photoreceptor synapsing onto - 1 rgc
  • less myleinated than parasol cells - slower conduction
  • synapse with p cells
42
Q

what is the difference between magnocellular and parvocellular pathways

A
  • magnocelluar

larger cells

fast conducting

responsible for motion
depth perception
high light/ dark contrast detection

10% of nerve fibres

domninant in the retinotectal pathway

parvocellular

  • smaller cells
  • slower conducting

responsible for

  • colours
  • fine detail
  • low contrast

80% of nerve fibres

dominant in the retinocalcrine pathway

43
Q

what is the retinotectal pathway

A
  • starts from the retina and goes to the pretectal nucleus - medial root - if you shine a light in your pupil you get a very rapid constriction therefore it makes sense that it will be the magnocellular pathway
  • refers to fibres travelling from the retina to the pretectal nucleus which is involved in the pupillary light reflex
  • magnocellular fibres also travel along this pathway to project to pretectal nucleus and superior colliculus
44
Q

describe the function of the superior colliculus

A
  • rapid motor reflexes
  • reflexes involving cranial nerves and spinal motor neuronal pathways
  • also has some fibres from the magnocellular pathway

supeior colliculus is for visual scanning e.g. automatic scanning - it is subconcious and it also plays a role in saccades where your eyes are moving rellay faast from side to side

45
Q

describe the function of the retinocalcrine pathway

A
  • parvocellular fibres travel via the LGN to the calcrine sulcus where the primary visual cortex (v1) lies (aka brodmanns area 17)
    • the calcrine sulcus is at the back of the occipital lobe - above and below it you have your primary visual cortex
  • parvocellular fibres travel via the LGN to the calcarine sulcus where the primary visual cortex (v1) lies ( aka brodmanns area 17)
46
Q

describe the structure of the visual cortex

A
  • like the lgn it is also layered
  • main layer is layer 4 - where all the fibres from the LGN will end up in - main layer is layer 4c
  • main input from LGN goes to layer ivc
  • other layers send fibres to esrastriate area i.e. areas of the visual cortex that do not recieve strong direct input from the LGN - this is mostly concerned with sunconcious visual processing
47
Q

what is retinotopic mapping

A
  • the fibres from the retina to the lgn to the visual cortex remain in the same position
  • the order of retinal neurones is maintained throughou the visual pathway (except at higher cortical areas )
  • neighbouring regions of the image are represented by neighbouring regions of the visual area
    • it is essentially a map of your retina - it has the same pattern in your lgn and in your visual cortex
48
Q

a patient has a left sided homonymous hemianopia what would be the most likely cause

A
  • right optic tract lesion

if someone has a homonymous hemianopia you already know that the lesion has to be retrochiasmal - because any retreochiasmal lesion will lead to a contralteral homnoymous defect

hemianopia means it will most likely be the optic tracts that are affected - rather than the optic radiation

because it is contralteral if it is a left sided lesion - the lesion has to be on the right because it is contralateral - because it is a hemianopia it will be a optic tract - rather than the optic radiations which will most likely give you a quadrantopia

49
Q

the inferotemporal quadrant of the left retina is likely to be represented in

A

left temporal optic radiation- temporal fibres do not cross over at the chiasm - temporal optic radiation lesion gives you a superior quadrantanopia because it carries fibres from the inferior retina

50
Q

what would a optic nerve lesion lead to

A
  • complete blindlenss in that one eye because the eye will not be able to transmit any light imformation - any aps towards the brain - leading to total blindness in that eye
51
Q

what do chiasmal lesions result in

A
  • because the nasal fibres are crossing over you lose the nasal fibres which correspond to your temporal visual field which then leads to a bitemporal hemianopia - the hemianopia means that you lose half of your visual field in one eye - and bitemporal because you are losing your temporal visual feild in both eyes -
52
Q

what does a optic tract lesion result in

A
  • if you have a lesion in your optic tract - or a lesion affecting both optic radiations it will give you a contralateral defect — because the fibres have now already crossed over at the optic chiasm - therefore it will give you a contralateral homnoymous (because it is on the same side) hemianopia
53
Q

what does a parietal optic radiation lesion will lead to ?

A

-a inferior quadrantopia - bevause the parietal optic radiation carries fibres from the superior retina and therefore the inferior visual field

54
Q

what does a temporal optic radiation lesion lead to

A
  • a superior quadrantantopia- you end up with a quadrantopia because it is only a uarter of the visual field that is being affected in each eye
55
Q

what is the temporal optic radiation also called

A

meyers loop passes through the temporal lobe

56
Q

what is the longest section of the optic nerve

A
  • intraorbital portion
57
Q

why is the intraorbital portion so long?

A

-so if the eye is moving around it dosnt stretch the nerve

58
Q

where do the parasympathetic fibres involved in the light reflex originate

A
  • edinger westphal nucleus
59
Q

what are the 2 oculomotor nerve nuclei

A
  • motor nuclei superior rectus , inferior rectus , medial rectus , inferior oblique
  • parasympathetic nucleus (general viscerent efferent) aka the edinger westphal nucelus
60
Q

parasol retinal gagnlion cells of the right nasal retina will send axons to what layer of the left lgn

A
  • layer 1 of the LGN

parasol retinal gagnlion cells are cells of the magnocellular pathway

  • magnocellular pathway goes to layers 1 and 2 of the lgn
  • because it is the right NASAL retina it will cross at the chiasm to the left lateral geniculate nucleus therefore it is contralteral and contralteral fibres go to layers 1, 4 ,6 wheras ipsilateral fibres go to layers 2 , 3 , 5

so it must go to layer 1

61
Q

midget retinal ganglion cells from the left temporal retina can terminate in what layer if the lgn

A

layer 5 of the left lgn

midget retinal ganglion cells are involved in the parvocellular layer of your lgn -

parvocelluar layers will be layers 3, 4 , 5 , 6

temporal fibres dont cross over so it has to be the left lgn

62
Q

parasal retinal ganglion cells of the right nasal retina will send its axons to what layers of the LGN

A

-layers 4 and 6 of the LGN

63
Q

what is not a function of the magnocelluar pathway

A

colour vision

functions of the magnocellular pathway include

motion detection , depth perception , contrast sensnitivity - responsoble for high trasnmission of imformation therefore not high resolution colour vision

64
Q

brodmanns area 17 corresponds to

A
  • primary visual cortex (V1)
65
Q

lgn fibres terminate in what layer of the visual cortex

A
  • layer 4

- main input from the lgn goes to layer 4c

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