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Neuroscience (own deck) > The visual system > Flashcards

The visual system Flashcards

1
Q

why is vision important?

A
  • detect prey/source food
  • detect predators/danger
  • detect mates
  • communicate

more than a third of the human neocortex is involved in analysing the visual

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2
Q

what are the properties of light?

A
  • electromagnetic radiation that is visible
  • has a wavelength (distance between peaks and troughs)
  • has a frequency (number of waves per second)
  • has an amplitude (difference between a peak and a trough)
  • spectrum from 400nm (blue) to 700nm (red)
  • the shorter the wavelength, the higher the frequency
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3
Q

how does light travel?

A

in straight lines, rays until it interacts with molecules

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4
Q

what is reflection?

A
  • light bounces off objects into our eyes
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5
Q

what is absorption of light?

A
  • different materials will absorb different wavelengths of light
  • if something is black, all wavelengths have been absorbed
  • if something is black, all wavelengths have been reflected
  • if something is red, all wavelengths except red have been absorbed
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6
Q

what is refraction of light?

A
  • speed of light differs between mediums
  • light is slower through water and faster through air
  • the greater the difference in speed in the two media, the greater the angle of refraction
  • refraction occurs towards a line that is perpendicular to the border
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7
Q

what are the structures of the eye?

A
  1. pupil - lets light inside the eye
    - appears black due to heavy pigment at back of eye
  2. iris - contains muscles which control the amount of light entering the eye
  3. cornea - transparent covering of the pupil and iris that refracts light
  4. sclera - continuous with cornea and forms protective wall over eyeball to give it its shape
  5. extraocular muscles - move the eyeball, controlled by oculomotor nerve (CN III)
  6. Optic nerve (CN II) - carries axons from retina to brain
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8
Q

how can eye position and pupil shape vary between species?

A

monocular vs binocular vision:

  • monocular = one eye
  • binocular = 2 eyes

predators have circular pupils to focus on prey

prey have horizontal pupils to expand peripheral vision

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9
Q

what does binocular vision enable?

A
  • greater depth perception for predators

- greater peripheral vision for prey

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10
Q

what are the structures inside the eye?

A
  • optic disk: origin of blood vessels and optic nerve, cannot sense light
  • macula: region of retina for central vision, devoid of large blood vessels to improve vision quality
  • fovea: retina is thinnest here, and has highest visual acuity
  • nasal retina: contains lots of blood vessels and the optic disk
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11
Q

what does the retina contain?

A
  • sensory receptor cells

- afferent neurons

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12
Q

how is the lens controlled?

A
  • suspended by zonal fibres/suspensory ligaments
  • ciliary muscle enable stretching of the lens

lens controls refraction

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13
Q

what are the two humors of the eye?

A
  1. aqueous humor
    - clear fluid providing cells of cornea with nutrients
    - allows cornea to be free of blood vessels
  2. vitreous humor
    - provides pressure inside the eyeball to maintain spherical shape
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14
Q

what determines how images formed in the eye?

A
  • light rays are focused onto retina/fovea
  • 80% refraction occurs at cornea, 20% refraction occurs at lens
  • cornea and lens are dense liquids so have higher refractive index than air entering the eye
  • the bigger the distance between air and cornea, the more refraction occurs
  • the angle at which light hits the cornea determines refraction
  • if at an angle, more refraction is needed
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15
Q

what is the refractive index?

A
  • measure of speed of light within it

- light moves quicker through air (1.0003) than the cornea (1.376) due to the increased density of the cornea

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16
Q

how does the cornea refract light?

A
  • light travels more slowly through the cornea than air due to higher density
  • light that hits the cornea directly perpendicular moves straight through to the retina
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17
Q

what is the focal distance?

A
  • distance from the refractive surface (cornea) to convergence of parallel light rays (retina)
  • usually 17-20mm
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18
Q

how does the lens accommodate refraction of distant objects?

A
  • it doesn’t
  • light rays are almost parallel
  • cornea provides sufficient refraction to focus them on the retina
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19
Q

how does the lens accommodate refraction of close objects (<7m away)?

A
  • light rays are not parallel
  • requires additional refraction to focus them on the retina
  • lens becomes fatter to refract the light more
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20
Q

what does rounding of the lens achieve?

A
  • increases refractive power to focus closer objects on the fovea
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21
Q

how does the lens become fattened/rounded for close objects?

A
  • suspensory ligaments relax

- ciliary muscles contract

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22
Q

how does the lens become flattened for distant objects?

A
  • suspensory ligaments contract

- ciliary muscles relax

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23
Q

what is hyperopia? how is it fixed?

A

farsightedness:

  • eye is too short
  • near objects are focused behind the retina as there is too little refraction

fixed with a convex lens (round lens) to increase refraction and make light more parallel when entering the eye

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24
Q

what is myopia? how is it fixed?

A

short-sightedness:

  • eye is too long
  • any distant objects are focused in front of the retina as there is too much refraction

fixed with a concave lens to increase refraction

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25
Q

what is the laminar organisation of the retina?

A
  • light focused on the retina is converted into neural activity
  • light passes through ganglion cells and bipolar cells before it reaches photoreceptors
  • light is absorbed by pigmented epithelium (outer most layer)
  • electrical signals move back from pigmented epithelium to the optic nerve
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26
Q

what are the cells of the retina?

A
  • ganglion cells: output from retina
  • amacrine cells: modulate information transfer between GCs and BCs
  • bipolar cells: connect photoreceptors to ganglion cells
  • horizontal cells: modulate info to transfer between photoreceptors and BCs
  • photoreceptors: sensory transducers, rods and cones
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27
Q

what is the structure of photoreceptors?

A
  • outer segment contains membranous disks which contain photopigments that absorb light (where phototransduction occurs)
  • ciliary stalk connects outer segment to inner segment
  • inner segment contains mitochondria for ATP
  • separate systems for monochrome and colour
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28
Q

what is duplicity theory?

A
  • can’t have high sensitivity and high resolution in a single receptor
29
Q

what is the structure of rods?

A
  • greater number of membranous disks
  • higher photopigment concentration
  • 1000x more sensitive to light than cones
  • 92 million rods in each human retina
30
Q

what do rods do?

A
  • enable vision in low light (scotopic) conditions
  • low visual acuity
  • monochrome
31
Q

what is the structure of cones?

A
  • fewer membranous disks
  • less photopigments
  • 5 million cones in each human retina
32
Q

what do cones do?

A
  • used during daylight (photopic) conditions
  • enable colour vision
  • high visual acuity
  • lower sensitivity
33
Q

how does retinal structure vary with location?

A
  • the fovea contains most of the 5 million cones and no rods
  • within macula, cones are reduced and rods are increased
  • at front of eyeball, number of rods decreases
  • blind spot contains no rods or cones
34
Q

what are the properties of the central retina?

A
  • low convergence
  • low sensitivity
  • high resolution
35
Q

what are the properties of peripheral retina?

A
  • high convergence
  • high sensitivity
  • low resolution
36
Q

what does high convergence enable?

A
  • many rods converge onto one retinal ganglion cell which allows summation of weak GPs
  • just a few photons are needed to produce an AP in the peripheral retina
37
Q

are photoreceptors ionotropic or metabotropic?

A

they are metabotropic glutamate receptors

38
Q

what photopigment do rods contain?

A
  • rhodopsin
39
Q

what wavelength is retina most sensitive to in scotopic conditions (rods)?

A

500nm

40
Q

what photopigments do cones contain?

A
  1. S (short wavelength) - activate maximally with biggest absorbance at 420nm (blue cones)
  2. M - max absorbance of 530nm (yellow cones)
  3. L - max absorbance of 560nm (red cones)
41
Q

what wavelength is the retina most sensitive to in photopic conditions (cones)?

A

560nm due to the combination of different cones and their absorbances found in the retina

42
Q

what is the photopigment of retinal ganglion cells?

A

melanopsin:

  • can absorb light themselves
  • cannot provide a specific picture, but give an idea of where light/dark is
43
Q

what does light do to photoreceptors? hyperpolarise or depolarise?

A

hyperpolarise

44
Q

what is rhodopsin made of and how does it work?

A

retinal and opsin:

  • opsin is a GPCR
  • retinal absorbs photons and changes its shape from cis to trans
  • this leads to a conformtational change in opsin, allowing it to activate transducin G-protein
45
Q

what is the resting membrane potential of photoreceptors in the dark?

A

-30mV

46
Q

how does phototransduction occur in the dark?

A
  1. cGMP-gated non-selective cation channels are open in the dark
  2. allows an Na+ influx into the rod known as the dark current
  3. the dark current depolarises the photoreceptor
47
Q

what re-establishes the membrane potential after phototransduction?

A

sodium-potassium pump

48
Q

how does phototransduction occur in the light?

A
  1. rhodopsin is activated by light
  2. rhodopsin stimulates G-protein transducin to become transducin GTP
  3. alpha-subunit activates enzyme phosphodiesterase (PDE)
  4. PDE cleaves and reduces cGMP levels, causing cGMP-gated cation channels to close
  5. dark current can no longer enter the photoreceptor, so it becomes hyperpolarised
  6. signal amplification can occur as this is an enzyme cascade
49
Q

how is cGMP produced?

A

it is constitutively produced in the dark by enzyme guanylyl cyclase

50
Q

what can 1 photon achieve when acting on photoreceptors?

A
  • hydrolysis of 1400 cGMP molecules

- suppress a current equivalent to 1 million Na+ ions

51
Q

how many photons evoke a sensation of light in humans?

A

5-7

52
Q

what are the differences between rods and cones?

A

rods cannot process bright light as they become easily saturated:

  • rhodopsin is bleached
  • cGMP levels are so low that no additional hyperpolarisation can occur

cones are not saturated so easily, so are used in bright light

53
Q

what is light adaptation?

A

photoreceptors initially hyperpolarise greatly, then gradually depolarise with continued bright light

54
Q

what does light adaptation require?

A

Ca2+

55
Q

how does light adaptation occur in the dark?

A
  • Ca2+ normally enters cells and blocks guanylyl cyclase
  • this reduces cGMP so closes some ion channels
  • this prevents influx of dark current to reduce amount of depolarisation
56
Q

how does light adaptation occur in the light?

A
  • Ca2+ channels are shut, preventing Ca2+ from entering
  • this prevents the block of guanylyl cyclase, meaning more cGMP is produced
  • this allows cGMP-gated cation channels to open, allowing some dark current in
  • cell becomes slightly depolarised
57
Q

what are bipolar cells?

A
  • interact with horizontal cells and respond in different ways to activation of photoreceptors
  • they have complex receptive fields
  • photoreceptors synapse to bipolar cells
58
Q

what are the two types of bipolar cells?

A
  1. on bipolar cell

2. off bipolar cell

59
Q

how are bipolar cells classified?

A

classification is based on their response to glutamate

60
Q

in the dark, do photoreceptors release neurotransmitter?

A

yes, they release glutamate due to depolarisation by dark current
- glutamate acts on bipolar cells

61
Q

in the light, do photoreceptors release glutamate?

A

no, due to hyperpolarisation of the photoreceptor

- no glutamate is released by vesicles, so no glutamate acts on bipolar cell in the light

62
Q

what happens to bipolar cells in the light?

A
  • bipolar cells hyperpolarise due to less glutamate release by the photoreceptor
  • some bipolar cells depolarise despite the reduced glutamate release
63
Q

what is a bipolar cell that hyperpolarises in light?

A

an OFF bipolar cell: it is switched off by light

64
Q

what is a bipolar cell that is depolarised by light?

A

an ON bipolar cell: it is switched on by light

65
Q

what kind of receptor does an OFF bipolar cell use?

A

ionotropic glutamate receptor

  • when glutamate binds to it, ion channel opens, allowing cations into the cell
  • this is why in the dark, the OFF bipolar cells are depolarised, as glutamate is released from the photoreceptor

in the light, photoreceptor is depolarised, so doesn’t release glutamate. no glutamate binds to the bipolar cell, so it is hyperpolarised, hence why it is OFF

66
Q

what kind of receptor does an ON bipolar cell use?

A

metabotropic glutamate receptor
- means they can be inhibitory

in the dark, photoreceptors are depolarised so release glutamate. glutamate binds to the inhibitory metabotropic receptors on the bipolar cell, causing it to hyperpolarise

in the light, on bipolar cells have a reduction in glutamate binding to them, so the bipolar cell is depolarised, as there is no inhibitory glutamate action

67
Q

what is the receptive field of the retina?

A
  • if light is shone on certain part of retina, certain retinal ganglion cells respond to the light
  • retinal ganglion cells will only fire APs when specific areas of the retina are illuminated
68
Q

what is the receptive field of bipolar cells?

A

bipolar cells have centre-surround organisation:

  • bipolar cell directly connect to photoreceptors in the centre
  • bipolar cells connect indirectly, by horizontal cells, to surrounding photoreceptors

Neuroscience (own deck) (14 decks)

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