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IMED3001 > Physiology of Vision > Flashcards

Physiology of Vision Flashcards

1
Q

What must happen in order to see an object?

A

Amount of light entering the eye must be regulated. (iris control) The object’s pattern must fall on vision receptors. (accomodation) Energy from waves of photons must be transduced into action potentials (phototransduction) Brain must receive and interpret signals (visual pathways)

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2
Q

What process allows the eye to focus on objects of different distances?

A

Accomodation

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3
Q

Which muscles control amount of light entering the eye?

A

Circular iris sphincter muscle is important for constriction. Radial iris dilator muscle is important for dilation.

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4
Q

What nerves stimulate the muscles of accomodation?

A

Circular iris sphincter controlled by parasympathetic innervation. Radial iris dilator is controlled by sympathetic nervous system.

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5
Q

What is the photopupillary reflex?

A

Pupillary constriction in response to light.

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6
Q

What mediates the photopupillary reflex?

A

An autonomic reflex arc. Brighter light signaled to pretectal region of midbrain, connects to Edinger-Westphal nucleus, axons run from this nucleus to both right and left oculomotor nerves. This pathway is limited to parasympathetic nervous system.

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7
Q

What is the near response?

A

Adjustment to close-range vision

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8
Q

What are the 3 processes involved in adjustment to close range stimuli?

A

Convergence of eyes (eyes orient their visual axis towards object) Constriction of pupil (Blocks peripheral light rays and reduces spherical aberration) Accomodation of lens: Change in the curvature of lens that enables focus on nearby objects.

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9
Q

How does lens accomodation take place?

A

Ciliary muscles contract, suspensary ligaments slacken, and lens take more convex shape. Light refracted more strongly and focused onto retina.

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10
Q

What is the near point of vision?

A

The closest an object can be and still come into focus

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11
Q

What happens to near point with age?

A

It lengthens

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12
Q

What is myopia?

A

Image focused too early (before the retina)

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13
Q

What is hyperopia?

A

Opposite of myopia, image focused to far behind the retina

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14
Q

What is astigmatism?

A

The cornia is irregular creating irregular pattern of vision.

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15
Q

What is presbyopia?

A

Stiffening of the lens occurring with aging

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16
Q

What is the ability to judge depth or distance to objects called?

A

Stereopsis

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17
Q

What is required for stereoscopic vision?

A

Requires overlapping visual fields which allows each eye to look at the same object from different angles.

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18
Q

What are the types of photoreceptors?

A

Rods and Cones

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19
Q

Where are cones located?

A

They are densly packed at focea at the center of the macula lutea.

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20
Q

What is the retinal pathway?

A

Photoreceptors -> Bipolar cells -> Ganglion cells -> Optic nerve This allows the eyes to do a bit of processing prior to entry of signal to the brain.

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21
Q

Where is the blindspot located?

A

At the location of convergence of blood vessels and ganglion cells. No photoreceptors at this location so having 2 eyes allows the brain to match it out.

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22
Q

What do horizontal cells and amacrine cells do?

A

They are similar to interneurons which create lateral inhibition and some initial processing (Sharpens image).

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23
Q

What does the pigmented part of the retina do?

A

It absorbs excess energy that hits the retina.

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24
Q

What are the layers of cells of the retina?

A

1st layer contains ganglion cells 2nd layer contains bipolar cells 3rd layer contains the rods and the cones

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25
Q

How are rods and cones organised?

A

Cones are responsible for colour vision and are all located at the fovea and macula lutea directly where light lands. Rods are located more peripherally which means colour can’t be perceived in periphery.

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26
Q

What is the blindspot called?

A

The optic disc

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27
Q

What are the 3 different types of cones?

A

Blue Green Red (RBG)

28
Q

Which photoreceptors are more effective in dim light?

A

Rods

29
Q

Which photoreceptors are more sensitive to light?

A

Rods

30
Q

What does the structure of the photoreceptor look like?

A

Outer segment contains 100s - 1000s of flattened discs. These discs contain visual pigments which absorb photons and initiate photoreception. Inner segment contains organelles and synapses with bipolar cells.

31
Q

What are flattened discs made up of?

A

Rhodopsin: Opsin + retinal

32
Q

What is the difference between content of rods and cones?

A

Retinal is the same in rods and cones. Opsins are different in rods and cones and are responsible for perception of colour.

33
Q

What kind of molecules is opsin and retinal?

A

Opsin is a protein and retinal is a modified vitamin (Vitamin A)

34
Q

What controls opsin and retinal coming together and separating?

A

Light separates them to create a signal An enzyme brings them together

35
Q

How does phototransduction take place?

A

Light energy interacts with rhodopsin breaking it apart and as a result 2nd messengers change membrane potential.

36
Q

What is the default state of photoreceptors?

A

They are depolarized when there is no light shining. They stop firing when light shines on them because light makes them hyperpolarized.

37
Q

How do vision signals begin in photoreceptors?

A

Low energy form of retinal is 11-cis retinal and opsin. When light shines on the 11-cis retinal it gives it energy and converts it into 11-trans retinal which then dissociates from the opsin. *11-trans retinal undergoes phosphorylation with the RPE65 enzyme which regenerates 11-cis retinal and so it is reassembled to form rhodopsin.* Activated opsin activates nearby membrane protein called transducin which activates a phosphodiesterase. Phosphodiesterase decreases levels of cGMP by converting it into GMP. Sodium channels are forced to stay closed making cell hyperpolarized. .Signal transduction stops at the synapse (no glutamate production)

38
Q

What cells do photoreceptors synapse with?

A

Bipolar cells Horizontal cells are also firing at the synapse between photoreceptors and bipolar cells.

39
Q

What kind of signals do horizontal cells commonly produce?

A

Inhibitory signals.

40
Q

What cells do bipolar cells synapse with?

A

Ganglion cells Amacrine cells are also firing at the synapse between bipolar cells and ganglion cells

41
Q

Which cells form the optic nerve?

A

Ganglion cells.

42
Q

What are the types of ganglion cells?

A

M-type (midget) P-type (Parvocellular)

43
Q

What happens in the dark in the retinal cells?

A

It causes photoreceptors to depolarize. This results in glutamate release which switches off excitatroy photosensory pathway (EPSP) and switches on the inhibitory photosensory pathway (IPSP). Horizontal cells inhibit neighbouring bipolar cells with GABA.

44
Q

What happens in the dark in the retinal cells?

A

It causes photoreceptors to depolarize. This results in glutamate release which depolarizes OFF bipolar cells (mGluRs) and hyperpolarises ON bipolar cells (AMPA Rs) Horizontal cells inhibit neighbouring bipolar cells with GABA.

45
Q

Which receptor on bipolar cells is excitatory?

A

mGluRs (a metabotropic receptor)

46
Q

Which receptor on bipolar cells is inhibitory?

A

AMPA Rs (this one is a sodium channel)

47
Q

How do bipolar cells stimulate ganglion cells to fire APs?

A

Bipolar cells generate graded potentials on ganglion cells and summation of this signal triggers ganglion cell AP firing. Amacrine cells further refine the image. (using GABA and glycine)

48
Q

Why is resolution higher in cones than it is in rods?

A

Cones and bipolar cells fire with a 1:1 ratio. No neuronal convergence in cones. Cones are also high resolution due to location within the fovea. Rods are very sensitive but ~600 rods converge onto a single bipolar cell.

49
Q

Do cones or rods have higher sensitivity?

A

Rods due to many rods being able to fire APs at the same time.

50
Q

Which of the retinal cells are first order neurons?

A

Bipolar cells of the retina

51
Q

Which of the retinal cells are second order neurons?

A

Ganglion cells which form optic nerve

52
Q

How do signals from ganglion cells move towards the brain?

A

Right cerebral hemisphere sees objects in left visual field because their images fall on the right half of each retina. Each side of the brain sees what is on side where it has motor control over limbs

53
Q

What happens to nerve fibers at the optic chiasm?

A

Hemidecussation

54
Q

What is the visual projection pathway?

A

The pathway of vision sensation from the eyes to the brain. First order bipolar cells -> second order ganglion cells which go through optic nerve and hemidecussate at optic chiasm and pass laterally to the hypothalamus and ending in the lateral geniculate nucleus of the thalamus. Lateral geniculate nucleus of the thalamus sends 3rd order neurons to the primary visual cortex of occipital lobe where conscious visual sensation occurs. Some optic nerve fibers project to midbrain and terminate in superior colliculi and pretectal nuclei.

55
Q

What kind of vision loss results form lesions in optic nerve?

A

Ipsilateral bind eye

56
Q

What kind of vision loss results from lesions at optic chiasm (pituitary tumours)?

A

Lateral half of both eyes gone

57
Q

What kind of vision loss results from lesions at optic tract?

A

Opposite half of visual field gone

58
Q

What kind of vision loss results from lesions at projection of fibers from lateral geniculate nucleus to the thalamus?

A

Homonymous superior or inferior field defects

59
Q

Lesion visual field test:

A
60
Q

Where does processing of visual information take place?

A

Some processing starts at the retina.

More processing in the lateral geniculate nucleus.

Primary visual cortex is connected by association tracts to visual association areas where more processing takes place.

61
Q

What kind of processing occurs at the retina?

A

Adjustments for contrast, brightness, motion, and stereopsis.

62
Q

What kind of adjustments take place at the lateral geniculate nucleus?

A

Sending distinct visual components via different projection fibers in the optic radiation

63
Q

What kind of processing occurs at the cerebrum following the projections from the lateral geniculate nucleus?

A

V1 - initial conscious perception + retinotopic mapping

V2 - orientation, spatial frequency, size, colour and shape

V3 - motion characteristics

V4 - recognition of form (Capable of LTP memory formation)

V5 - Speed and direction of moving visual stimuli

V6 - Sharpness of boundaries and visual contours

64
Q

Where does integration of colour signals become more complex?

A

In V2. Colour is no longer seen as individual parts.

65
Q

Where does movement perception take place?

A

V3. Characteristics of motion are produced here.

V4 creates visual memories.

V5 - speed and direction of motion is integrated here

66
Q

Where is the sharpness of boundaries and visual contours understood?

A

In V6

IMED3001 (46 decks)

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