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Neuroscience > The Visual System 1 & 2 > Flashcards

The Visual System 1 & 2 Flashcards

1
Q

importance of vision

A

detect prey, predators, mates
communicate
more than 1/3 neocortex involved in analysing visual world

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2
Q

light and eye

A

EM radiation that is visible
light has:
a wavelength - distance between peaks and troughs
a frequency - number of waves per second
an amplitude - difference between wave peak and trough

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3
Q

light and environment

A

optics
EM light travels in straight lines - rays - until it interacts with atoms and molecules
interact in 3 ways: reflection, absorption and refraction

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4
Q

pupil

A

lets light inside

black as all light absorbed

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5
Q

iris

A

contains muscles which control amount of light entering eye

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6
Q

cornea

A

glassy

transparent covering of pupil and iris that reflects light

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7
Q

sclera

A

continuous with cornea

forms tough proctective wall of eyeball to give shape

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8
Q

extraocular muscles

A

move eyeball

controlled by oculomotor nerve (CN III)

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9
Q

optic nerve

A

CNII - sensory

carries axons from retina to brain

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10
Q

opthalmoscope

A

to see blood vessels
optic disk - blind spot, origin of blood vessels and optic nerve, cannot sense light
macula - region of retina for central vision, devoid of large blood vessels to improve visual quality
fovea - retina is thinnest here and is area of highest visual acuity

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11
Q

cross section of eye

A

retina - contains sensory receptor cells and afferent receptors
lens - suspended by zonal fibres - ligaments - which are attached to ciliary muscle, enabling stretching of lens
2 solutions in eye:
aqueous humor - provides cornea with nutrients
vitreous humor - provides structure and pressure outwards

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12
Q

image formation

A

light rays must be focussed on retina - ideally fovea

refraction occurs at: cornea - 80%, lens - 20%

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13
Q

degree of refraction determined by

A

difference in refractive indices between the 2 media

angle at which light hits the interface between 2 media

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14
Q

refraction by the cornea

A

light arrives through air but cornea is mainly water
light travels mroe slowly through water than air due to hgiher density = refraction occurs
distance from refractive surface to convergence of parallel light rays = focal distance

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15
Q

accommodation by the lens

A

distant object
- almost parallel light rays
- cornea provides sufficient refraction to focus them on retina
closer objects
- light rays arent parallel
- requires additional refrection to focus them on retina
- provided by the fattening of the lens

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16
Q

rounding of lens:

A

increases refractive power to focus closer objects on fovea

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17
Q

problems with focussing

A

eye is emmetropic when lens is flat and we are focussing a distant object

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18
Q

farsightedness

A

eye is too short
near objects are focussed behind retina
not enough refraction

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19
Q

nearsightedness

A

eye is too long
distant objects are focussed before retina
too much refraction

20
Q

eye summary

A

cornea focus most of light on fovea of retina due to power of refraction
closer objects require additional refraction achieved by accommodation of lens which is regulated by contraction and relaxation of ciliary muscle

21
Q

laminar organisation of retina

A

light focused on retina must now be converted into neural activity
light must pass through ganglion cells and bipolar cells before ti reaches photoreceptors
light that passes all the way through retina is absorbed by the pigmented epithelium

22
Q

layers of retina

A

ganglion cell layer - inner layer, action potentials
inner layers - graded potentials
pigmented epithelium - outer layer

23
Q

cells of retina

ganglion

A

output from retina

24
Q

cells of retina

amacrine

A

modulate info transfer between ganglion cells and bipolar cells

25
cells of retina | bipolar cells
connect photoreceptors to ganglion cells
26
cells of retina | horizontal cells
modulate info transfer between photoreceptors and bipolar cells
27
cells of retina | photoreceptors
sensory transducers, both rods and cones
28
photoreceptors
membranous discs contain light sensitive photoreceptors that absorb light lots of mitochondria - active transport to balance depolarisation
29
phototransduction occurs in:
rod
30
duplicity theory
cant have high sensitivity and high resolution in single receptor separate systems of monochrome and colour
31
rod photoreceptors
``` greater number of discs higher photopigment concentration 1000x more sensitive to light than cones enable vision in low light (scotopic) low visual acuity/resolution ```
32
cone photoreceptors
``` fewer discs used in daylight (photopic) colour vision high visual acuity/resolution lowr sensitivity ```
33
retinal structure varies with region
fovea contains most of 5 million cones and no rods
34
central retina
low convergence, low sensitivity and high resolution
35
peripheral retina
high convergence, high sensitivity and low resolution
36
absorbance spectra in humans
rod photopigment = rhodopsin, in dark conditions (scotopic) optium wavelength of light = 500nm cone photopigment = three varieties of opsin - Short cones, Medium and Long cones, in light conditions optium = 560nm retinal ganglion photopigment = melanopsin
37
photoreceptors are hyperpolarised by light
resting pot = -30mv (in dark) in a rod in the dark - molecule called cGMP - free in cytoplasm, capable of binding to channel in cell membrane and it opens - intracellular ligand = causes pore to open and Na comes in known as dark current. = depolarises photoreceptors K channels further up cells to stop over depolarisation in light - channels are shut, light decrease cGMP levels closing channels and preventing Na influx = hyperpolarisation as K channel doesnt shut
38
phototransduction
5-7 photons can evoke a sensation of light in humans in membrane of disc molecules like rhodopsin live e.g. rhodopsin is made of an opsin (GPCR) and retinal opsin varies in each cones retinal stays the same light comes in and makes retinal change confirmation which changes opsin leads to activation of transducin (G protein mage of a, b, g subunits) in membrane also = phosphodiesterase wehn transducin is activated, alpha subunit moves to actiate phophodiesterase and it becomes activated so cGMP gets broken down to GMP = changes amount of open channels in membrane in dark Na moves in through cGMP channel, but since no GMP - channel closes - in light
39
signal amplification
1 photon being absorbed by rhodopsin can lead to 1,400 molecules of cGMP being broken down = 1,000,000 Na ions not moving in
40
saturation of responses in bright light
rods cannot process bright light as they become saturated easily rhodopsin becomes bleached, cGMP levels are so low that no additional hyperpolarisation can occur cones are not saturated easily, so are used in bright light on a graph - saturation peak around -65 but plateu becomes longer
41
light adaptation
photoreceptors initially hyper polarise greatly | photoreceptors gradually depolarise with continued bright light
42
light adaptation requires calcium
in the dark: Ca normally enters cells and blocks guanylyl cyclase from making more cGMP this reduces cGMP production, so closes some ion channels in the light: channels are blocked due to signalling cascade Ca cant enter cell guanylyl cyclase isnt inhibited so cGMP can start binding to channels and opening them this causes membrane potnetial to return to depolarised level
43
downstream of photoreceptors
there are different types of bipolar cells bipolar cells have complex receptive fields have on and off bipolar cells
44
on and off bipolar cells
classified based on responses to glutamate photoreceptor hyperpolarises to light = reduced glutamate release some bipolar cells hyperpolarises = OFF bipolar cell e.g. switched off by light some depolarise = ON bipolar cell e.g. switched on my light
45
bipolar cells use different receptors
OFF uses ionotropic glutamate receptors, when glutamate binds, pore opens and positive ions move into cell = why they depolarise in dark ON uses metabrotropic cells, in dark release glutamate and binds to inhibitory metabotropic receptor = hyperpolarises, in light = less activation of inhibition
46
the receptive field
retinal ganglion cells will only fire action potentials when specifc areas of the retina are illuminated

Neuroscience (21 decks)

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