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Neuroscience > Lecture 19 - Vision > Flashcards

Lecture 19 - Vision Flashcards

1
Q

What are two types of photoreceptors?

A
  1. Rods
  2. Cones
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2
Q

List the differences between rods and cones:

A

Rods:

  • Longer outer segment
  • Lower acuity
  • More convergence (more rods per ganglion cells)
  • Night Vision

Cones:

  • Shorter outer segment
  • High acuity (detecting detail)
  • Less convergence (less cones per ganglion cell)
  • Color vision
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3
Q

List the sequence of events for the excitation of rods and cones:

A
  1. In dark, cGMP keeps Na+ channels open
  2. Inner segment pumps Na+ out and K+ in
  3. Light breaks down visual pigments releasing phosphodiesterases
  4. Phosphodiesterase breaks down cGMP
  5. Sodium channels close and cell hyperpolarizes
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4
Q

Are there action potentials in photoreceptors?

A

No, photoreceptors have a short distance to travel that they conduct info without the need of action potentials.

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5
Q

What is usually the transmitter in photoreceptors, and when is it released?

A

The transmitter in photopreceptors is usually glutamate, and its release is actually onging.

It increases with depolarizatoin (dark).

It decreases with hyperpolarization (light).

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6
Q

What is the second cell in the visual pathway, what is the first?

A

First cell in the visual pathway: photoreceptor

Second cell in the visual pathway: bipolar cells

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7
Q

Does the photoreceptor transmitter hyperpolarize bipolar cells or depolarize bipolar cells?

A

Hyperpolarize bipolar cells, this plays an inhibiting effect on bipolar cells.

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8
Q

Based on our knowledge that photoreceptor transmitter hyperpolarizes bipolar cells, what is the sequence of events when the photoreceptor is exposed to light?

A
  1. Light hyperpolarizes photoreceptors
  2. This decreases transmitter release
  3. This releases the inhibitory effects of the transmitter on the bipolar cell and bipolar cell now depolarizes (as opposed to being hyperpolarized).
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9
Q

What type of potentials do we see in bipolar cells?

A

Synaptic potentials

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10
Q

So in essence, what is lights overall effect on the bipolar cell?

A

It depolarizes the bipolar cell (while it hyperpolarizes the photoreceptor)

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11
Q

What is the third cell in the pathway (what were the first and second cells?)

A

First cell in the pathway –> photo receptor

Second cell in the pathway –> bipolar cell

Third cell in the pathway –> Ganglion cell

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12
Q

What does the bipolar cell transmitter do to ganlgion cells (hyperpolarize or depolarize)?

A

Bipolar cell transmitter depolarizes ganglion cells.

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13
Q

Based on our knowledge of the pathway and knowing that the bipolar cell transmitter depolarizes ganglion cells, what is light’s affect on ganglion cells?

A

Since light depolarizes bipolar cell and increases its transmitter release, light also depolarizes gangion cells.

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14
Q

List the two interneurons in the retina:

A
  1. horizontal cells
  2. Amacrine cells
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15
Q

What are horizontal cells?

A
  • They are inhibitory interneurons.
  • Mediate lateral inhibition between photoreceptors (later inhibition increases contrast)
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16
Q

What are amacrince cells?

A
  • Inibitory interneurons
  • Mediate lateral inhibition between bipolar cells
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17
Q

What doe lateral inhibition do?

A

Lateral inhibition increases contrast - the difference between active cells and inactive cells in sensory and motor systems. It is there to focus activity at the center of and not in the periphery. WHen you activate those cells in the center, they inhibit photoreceptors on the outside to increase the contrast in a more lit area and a poorly lit area.

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18
Q

Vision is all about detecting ____.

A

Edges: The differences between objects, between light and dark, between color

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19
Q

Do ganglion cell axons cross at the optic chiasm? (none, some, or all)

A

Some ganglion cell axons cross at the optic chiams (50 -100%)

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20
Q
  • Where is thalamic relay located?
  • And what part of the pathway is this (efferent or afferent)?
A
  • In the lateral geniculate nucleus
  • Afferent pathway
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21
Q

Where do axons from lateral geniculate cells project to?

A

To the cerebral cortex in the internal capsule.

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22
Q

Describe the cortical area for vision:

A
  • Striate cortex in occipital lobe
  • More cortex = greater visual acuity
  • Left visual field is represented on the right side of the brain/visual cortex
  • RIght visual fireld is repsrented in the left side of the brain/visual cortex
23
Q

Is the image produced in the cortical area for vission upright or inverted? What is this due to?

A

Inverted (tunred 180 degrees) due to:

  • Lower visual field on upper retina and vice versa.
  • Lower visual field on upper visual cortex and vice versa
24
Q

Which parts of the retina cross the optic chiasm and which do not?

A

Temporal part of the retina does not cross the optic chiasm

Nasal part of the retina does

25
Q
A

This area is the cross section at 1.

Includes the optic nerve.

4 - optic chiasm

26
Q
A

This are awould be a cross section at 2.

Optic tract

27
Q
A

Cross section taken at 3.

Label the left: optic tract

Label the right: lateral geniculate nucleus

28
Q
A

Cross section at 6.

Rostral colliculus

29
Q

Name what is circled and labeled at #5:

A

Visual (straite) cortex in the occipital lobe

30
Q

Amount of crossing the optic chiasm is related to what?

Give examples in cats, dogs, humans and birds.

A

The frontal positioning of the eyes

  • The more frontally positiioned eyes = less crossing
  • More binocular vision
  • smaller visual field

Example:

Eyes frontal in cat (65% crossed)

Eyes lateral in dog (80-90% crossed)

Eyes frontal in humans (50% crossed)

EYes very lateral in birds (100% crossed)

31
Q

What is the result of crossing?

A
  • Left visual field is represented on the right visual cortex
  • Right visual field is respresented on the left visual cortex
32
Q

What is binocular vision and what are the two things that binocular vision is important for, and which one is more important?

A

Binocular vision is the area where both eyes can see (portion that overlaps), which is bigger in animals that have more frontally positioned eyes. It is important for:

  1. Depth perception
  2. More important for stational objects
33
Q

Why is the visual field represnted on the opposite cortex?

A
  • Nasal retina samples lateral visual field on same side as the eye.
  • Temporal retina samples medial visual field on opposite side as the eye.
  • Nasal half crosses at chiasm.
34
Q

What percentage of fibers cross in the cat or human?

A

50%

35
Q

What percentage of fibers cross in the dog?

A

80 - 90%

36
Q

What percentage of fibers cross in a bird?

A

100%

37
Q

If we cut fibers at site 1 of thd dog, what are teh visual defects in the left and right eye?

A
38
Q

If we cut the nasal hemiretina fibers at the optic chiasm (2) that represent right visual field and left lateral visual field, what are the visual defects in a dog?

A
39
Q

If we cut the fibers just past the optic chiams (3), fibers from the right and left eye, what are the visual defects in a dog?

A
40
Q

If we cut fibers at level 4, this is where image is inverted on the cortex. It is similar to lesion three. What are the visual field defects in the eye of the dog?

A
41
Q

If we cut fibres at level 5, what are the visual defects in the eye of the dog?

A
42
Q

List the two locations of the parallel visual pathways:

A
  1. Pretectal nucleus in midbrain
  2. Rostral colliculus
43
Q

What is the input to the pretectal nucleus in the midbrain?

A

Photoreceptors that don’t go through lateral geniculate to the tocrtex, but rther project to the midbrain and synapse at the pretectum.

  • Retina
  • Cerebral cortex
44
Q

What is the output fromt he pretectal nucleus in the midbrain?

A

Preganglionic parasympathetic neurons in the parasympathetic nucleus of CN IIII (Edinger-Westphal nucleus)

45
Q

What is the function of the pretectal nucleus in the midbrain? Specifically the importance of the retinal input and that from the cerebral cortex.

A
  • Retinal input results in pupillary constriction (pupillary light reflex)
  • Cortical input important in accomodation (thickening lens for near vision)
46
Q

What are the inputs to the rostral colliculus?

A
  • Retina (~20% of ganglion cells go to the rostral collicus and not the lateral geniculus)
  • Visual cortex (involved with head motions, not so much , not so much body turning)
  • Other cortical areas (auditory and somatosensory)
  • Caudal colliculus
  • Spinal Cord
47
Q

What are the outputs form the rostral colliculus?

A
  • Spinal cord - tectospinal tract
    • extends caudally only to upper thoracic spinal cord
  • Brainstem nuclei
    • Nuclei for CNs III, IV, VI, VII
  • Cerebellum
  • Cerebral cortex via thalamus
48
Q

What are the functions of the rostral colliculus?

A
  • Reflexes - pupillary ligh reflex, blink
  • Eye movements - gase shifts in response to:
    • Visual stimuli
    • Auditory stimuli
    • Body movements
    • Movement of stimulus
  • Reflex head movements in resposne to auditory or visual stimuli
49
Q

What contributes to visual acuity?

A

Areas with more photorecepotrs

  • In humans, fovea (macula) contains highest density of cones
  • In dogs, an area near the tapetum lucidum has highest density of photoreceptors (mostly rods)
    • Tapetum lucidum gives photoreceptors a second chance
    • decreases visual acuity in bright light due to reflected light
50
Q

Animals with well-developed color vision see how many visual pigements that absorb light in how many ranges?

What animals does this include?

What pigments are absorbed?

A
  • 3 visual prigments that absorb light in 3 ranges
  • Primates, birds, amphibians, reptiles
  • Red, green, and blue range
51
Q

Animals without well-developed color vission see how many visual pigemnts?

What animals are included?

A
  • 2 visual pigments
  • Most other mammals besides primates
  • Dogs have similar vision to red-green color blind humans
52
Q

What photoreceptors are more important in night vision and what are other factors that help increase night vision?

A
  • rod photoreceptors are more important
  • Better low-light vision in animals with more rods in central part of retina
  • Tapetum lucidum-reflective area - increases vision in low light, but decreases acuity in normal light
  • peripheral vision
53
Q

Why don’t dogs watch TV?

A
  • Not enough animal programming
  • Flicker fusion frequency
54
Q

Where in dog has the highest desnity of photoreceptors (mostly rods)?

A

Tapetum lucidem which gives photoreceptors a second chance.

While it decreases the visual acuity in bright light due to reflected light, it increases visual acuity in dim light.

Important adaptation that lives or used to live a nocturnal life.

Neuroscience (6 decks)

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