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Neurobiology > Visual Pathways > Flashcards

Visual Pathways Flashcards

1
Q

What is the cornea?

A

Clear protective outer layer of the eye

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2
Q

What is the iris?

A

This circular structure that controls the diameter of the pupil and defines eye color

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3
Q

What is the pupil?

A

The hole in the center of the iris that allows light to enter the eye

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4
Q

What is the lens?

A

Structure in the eye that refracts light passed through the cornea to form an image on the retina

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5
Q

What is the vitreous body?

A

clear, gel-like structure between the lens and the retina

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6
Q

What is the retina?

A

Layer of cells lining the back wall of the eye that senses light and sends signals to the brain for vision

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7
Q

What is the fovea?

A

Region of the retina with the highest visual acuity

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8
Q

What is the macula?

A

Oval region that surrounds the fovea that has relatively high visual acuity

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9
Q

What is the optic disc?

A

Region where the axons leaving the retina gather to form the optic nerve, no photoreceptors are in this area (blind spot)

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10
Q

How are images projected onto the retina?

A
  • The lens inverts and reverses the the projection of the visual image on the retina
  • Left and right swapped, up and down swapped
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11
Q

How is the blind spot formed?

A
  • Central fixation point for each eye falls on the fovea
  • Optic discs sits 15 degrees medial to the fovea which creates a blind spot 15 degrees lateral from center of visual field
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12
Q

True or False
You are aware of the blind spots created by the optic disc during daily life

A

False
- No functional deficit caused by blind spots
- Unaware of blind spot even with one eye closed
- Visual analysis pathways “fill in” blind spot

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13
Q

What are the two classes of photoreceptors?

A

Rods and cones

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14
Q

What are rods and what do they do?

A
  • Most numerous type of photoreceptors
  • Responsible for vision in low lighting conditions
  • Does not detect color
  • Poor spatial and temporal resolution
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15
Q

What are cones and what do they do?

A
  • Less numerous type of photoreceptors
  • Highly represented in the fovea for increased visual acuity
  • Detects color: 3 types contain 3 forms of pigment and absorb light from different parts of the spectrum
  • High spatial and temporal awareness
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16
Q

What are the layers of the retina?

A
  • Ganglion cell layer
  • Inner plexiform layer
  • Inner nuclear layer
  • Outer plexiform layer
  • Outer nuclear layer
  • Photoreceptor layer
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17
Q

Why are most layers of the retina absent at the fovea?

A

Allows light to reach receptors without distortion

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18
Q

How do images go from the retina to the optic nerve?

A
  • Photoreceptors activated by light and synapse on bipolar cells
  • Bipolar cells synapse n ganglion cells
  • Axons of ganglion cells sent to the optic nerve
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19
Q

True or False
Photoreceptors communicate with bipolar cells via action potential

A

False
- Neither photoreceptors or bipolar cells fire action potentials
- Information is conveyed via passive electrical conduction
- Neurotransmitters are released in a graded fashion

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20
Q

What are the types of interneurons involved in the visual pathway?

A

Horizontal cells and amacrine cells

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21
Q

How do interneurons influence the visual pathway?

A
  • Interneurons have lateral inhibitory and excitatory connections with nearby photoreceptors, bipolar cells, and ganglion cells
  • Light on the retina can have one effect (excitatory or inhibitory) on bipolar and ganglion cells directly in it’s path
  • The opposite effect (excitatory or inhibitory) will to surrounding bipolar and ganglion cells via interneurons
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22
Q

What are the two classes of center-surround bipolar and ganglion cells?

A

On-center cells and off-center cells

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23
Q

How are on-center cells influenced by light?

A

Cells in the center of receptive field: excited by light
Cells surrounding the area: inhibited by light

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24
Q

How are off-center cells influenced by light?

A

Cells in the center of receptive field: inhibited by light
Cells surrounding the area: excited by light

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25
Q

What are the two types of retinal ganglion cells?

A

Parasol cells and midget cells

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26
Q

What are the characteristics of parasol cells?

A
  • Large cell bodies
  • Large receptive field
  • Responds best to gross stimulus features and movement
  • Large diameter fibers
  • Project to magnocellular layers of the lateral geniculate nucleus of the thalamus
27
Q

What are the characteristics of midget cells?

A
  • Small cell bodies
  • Small receptive fields
  • More numerous
  • Sensitive to fine visual details and colors
  • Smaller diameter fibers
  • Project to parvocellular layers of the lateral geniculate nucleus of the thalamus
28
Q

Describe the optic nerve

A
  • Formed by retinal ganglion cells
  • L and R optic nerves meet at the optic chiasm
29
Q

Describe the optic chiasm

A
  • Area where L and R optic nerves meet
  • located on the ventral surface of the brain beneath the frontal lobes and anterior to the pituitary gland
  • Medial retinal fibers for each eye cross, lateral retinal fibers for each eye remain ipsilateral
30
Q

Describe the left optic tract

A
  • Consists of fibers from the left hemiretina of each eye to convey images from right visual field
  • Medial fibers from right eye, lateral fibers from left eye
31
Q

Describe the right optic tract

A
  • Consists of fibers from the right hemiretina of each eye to convey images from left visual field
  • Medial fibers from left eye, lateral fibers from right eye
32
Q

What two pathways is the optic tract involved in?

A
  • Geniculate pathway
  • Extrageniculate pathway
33
Q

Describe the geniculate pathway

A
  • Optic tract synapses in lateral geniculate nucleus of the thalamus
  • Relays to the primary visual cortex via optic radiations
  • Function: visual discrimination and perception
34
Q

Describe the extrageniculate pathway

A
  • Optic tract synapses in the superior colliculus and pretectal areas of the midbrain
  • Projects to pulvinar and lateral posterior nucleus of the thalamus
  • Projects to lateral parietal cortex and frontal eye fields
  • Function: direct visual attention and eye movement towards stimuli
35
Q

True or False
The lateral geniculate nucleus has 6 layers from ventral to dorsal that keeps information from the left and right eye segregated

A

True
- Dorsal parvocellular layers (3-6): input from retinal ganglion midget cells, relay information for form and color
- Ventral magnocellular layers (1-3): input from retinal ganglion parasol cells, relay information for motion and spatial analysis

36
Q

What are optic radiations?

A
  • Axons that traveling from lateral geniculate nucleus to primary visual cortex
  • Broken up into inferior optic radiations (meyer’s loop) and superior optic radiations
37
Q

Describe the inferior optic radiations (meyer’s loop)

A
  • Information from inferior retina (superior visual field)
  • Arcs into temporal lobe
  • Terminates inferior to calcarine fissure (lingula)
38
Q

Describe the superior optic radiations

A
  • Information from superior retina (inferior visual field)
  • Passes under the parietal lobe
  • Terminates superior to calcarine fissure (cuneus)
39
Q

Where is the fovea and retina in terms of retinotopical organization of the brain

A

Fovea = medial and lateral occipital poles
Peripheral retina = along calcarine fissure

40
Q

Describe visual processing in the neocortex

A
  • Input to the primary visual cortex arrives in layer 4
  • Layer 4 is divided into sublaminae 4A, 4B, 4C alpha, 4C beta
41
Q

Describe layer 4B of the neocortex in the primary visual cortex

A
  • Contains myelinated axon collaterals that results in a pale appearance called the stria of gennari
  • gives the name of striate cortex
42
Q

Describe layer 4C alpha of the neocortex in the primary visual cortex

A
  • Receives input from the magnocellular layers of lateral geniculate nucleus
  • Information about movement and gross spatial features
43
Q

Describe layer 4C beta of the neocortex in the primary visual cortex

A
  • Receives input from the parvocellular layers of lateral geniculate nucleus
  • Information about fine spatial information
44
Q

True or False
The channels for color, form, and motion/spatial analysis cross as they travel to the higher order visual association cortex

A

False
These channels are parallel and never cross

45
Q

Describe the pathway for motion/spatial analysis

A

Parasol cells –> magnocellular layers of LGN –> layer 4C alpha to 4B of the neocortex –> thick strip of visual association cortex –> dorsolateral parieto-occipital cortex

46
Q

Describe the pathway for form

A

Midget cells –> parvocellular layers of LGN –> layer 4C beta to layer 2, interblobs of neocortex –> pale stripe of visual association cortex –> inferior occipitotemporal cortex

47
Q

Describe the pathway for color

A

Midget cells –> parvocellular and interlaminar regions of LGN –> layer 4C beta to layer 2,3 blobs of neocortex –> thin stripe of visual association cortex –> inferior occipitotemporal cortex

48
Q

What are ocular dominance columns?

A

Inputs from each eye terminate in different alternating bands of the cortex

49
Q

What are orientation columns?

A
  • Vertical columns selectively that collectively respond to lines of a specific orientation
  • Detects orientation of light to help detect shape
50
Q

What are hypercolumns?

A

A region that contains a complete sequence of both ocular dominance and orientation columns

51
Q

What are blobs and interblobs sensitive to?

A

Blobs: color
Interblobs: form

52
Q

What is the difference between the dorsolateral parieto-occipital cortex and inferior occipitotemporal cortex?

A

Dorsolateral parieto-occipital cortex: where things are, motion and spatial relationships
Inferior occipitotemporal cortex: what things are, form and color

53
Q

What is monocular scotoma?

A
  • Small region of visual loss in one eye
  • Caused by lesion to retina
54
Q

What is monocular vision loss?

A
  • Loss of vision in one eye
  • Caused by lesion of optic nerve
  • Glaucoma, optic neuritis, elevated ICP, tumor, trauma
55
Q

What is bitemporal hemianopia?

A
  • Loss of vision of the temporal fields (lateral fields) of both eyes
  • Caused by lesion to optic chiasm
  • Tumor located near pituitary gland
56
Q

What are the clinical features of optic neuritis?

A
  • Eye pain, especially with eye movement
  • Monocular vision problems (central scotoma, decreased visual acuity, impaired color vision, complete loss of vision in one eye)
    -50% of individuals will eventually develop MS
  • Recovery begins at 2 weeks, ends at 6 to 8 weeks
  • Some residual loss
57
Q

What is a homonymous defect?

A
  • Visual field defect in the same region for both eyes (left side or right side of both eyes)
  • Caused by retrochiasmal lesion (Structures in the pathway AFTER the chiasm: optic tracts, LGN, optic radiations, visual cortex)
  • Usually contralateral presentation
58
Q

What is contralateral superior quadrantanopia?

A
  • Pie in the sky!
  • Visual defect that effects the same superior quarter region in both eyes
  • Caused by lesion to meyer’s loop or primary visual cortex below calcarine fissure
  • Contralateral presentation
59
Q

What is contralateral inferior quadrantanopia?

A
  • Pie on the floor!
  • Visual defect that effects the same inferior quarter region in both eyes
  • Caused by lesion to superior optic radiations or primary visual cortex above calcarine fissure
  • Contralateral presentation
60
Q

What is macular sparing?

A
  • Visual loss sparing the fovea
  • Can be caused by: chronically elevated ICP, R PCA infarct sparing occipital lobe, or lesion to inferior bank of R calcarine fissure preserving the occipital pole
61
Q

What is monocular altitudinal scotoma?

A

Occlusion to either upper or lower branch of ophthalmic artery causing deficit in upper or lower have of visual field in one eye

62
Q

What is binocular altitudinal scotoma?

A

Bilateral occlusion of the PCA branches supplying the lingular gyri causing deficit in upper or lower visual field in both eyes

63
Q

What is cortical blindness?

A
  • Syndrome of the primary visual cortex
  • Leads to loss of blink to threat, loss of closure to bright light, loss of optokinetic nystagmus, and blind sight

Neurobiology (15 decks)

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