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control of human movement > vision and proprioception > Flashcards

vision and proprioception Flashcards

1
Q

what pathway does light take through the eye?

A
  • go through cornea
    -lense
  • aqueous humour
  • retina at back of eye
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2
Q

what does the centre of the retina consist of?

A
  • high concentration of photoreceptors
  • high visual acuity
  • part of fovea
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3
Q

what structures are attached to the lease?

A
  • ciliary muscle attached via zonular fibres (changes shape to accommodate to image)
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4
Q

how is the eye like a camera?

A
  • image is inverted but brain flips image
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5
Q

diopters

A
  • measure lens focussing power = reciprocal of the focal length in metres
  • focal length = distance between lens and image
  • 1D means focal point is 1m away
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6
Q

average human eye diopter strength

A

56-60 diopters

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7
Q

refractive indices

A
  • outside of eye = 1.00
  • cornea = 1.34
  • lense = 1.41
  • victreous humour = 1.34
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8
Q

where does refraction occur?

A

at cornea
refractive index 1.34
48D out of 57D refraction happens here

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9
Q

further refraction where

A

lens
means refraction 1.41

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10
Q

how is focal length changed?

A
  • alter she of Len accommodating by ciliary muscle
  • spherical to oval
  • motion of lense is called accomodation
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11
Q

contraction of ciliary muscle causes what

A

relieves ligamental tension on lens causing lens to squash increasing lens power and shortens local length for closer objects

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12
Q

what is myopia?

A
  • caused by eye ball being too long or overly powerful cornea
  • when someone is nearsighted
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13
Q

what is hypermetropia?

A
  • opposite of myopia
  • far sighter and eyeball too short
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14
Q

presbyopia

A
  • lens seizes up with age - no longer bulges when ciliary muscle contracts and near -point moves further away
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15
Q

the pupil

A
  • diameter is first means of adaptation to changing light levels (2-8mm)
  • alters amount of light captures x16
  • small portion of eyes total light adaptation
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16
Q

pupillary muscles

A
  • sphincter muscles
  • dilator
  • as sphincter contract pupil gets smaller and vice versa
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17
Q

benefits of smaller pupil

A
  • less light reaches retina
  • greater depth of field (high quality image)
    reduces spherical aberration
    reduced glare (scattering of light)
  • why we can see better in light
  • as pupil size reduces it gets closer to a pin hole camera - infinite depth of field (focus)
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18
Q

the retina

A
  • fovea centralis (high level of visual acuity not very sensitive to light)
  • blood vessels
  • signal into brain via optic nerve and leave brain via optic disc
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19
Q

how does info get from retina to brain?

A
  • optic nerve carries info from retina.
  • passes through optic disk (ganglion cells) resulting in a blond spot
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20
Q

retinal cells

A
  • retina transforms image by cone photoreceptors
  • other cells types - rods, cones
  • bipolar cells, ganglion cells, horizontal and amacrine cells
  • once info is transduced within cell needs to be transmitted to ganglion cell forming optic nerve
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21
Q

ganglion cells

A
  • fire action potentials
  • light modulates activity up or down
  • takes single to brain
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22
Q

cones

A
  • don’t have action potential
  • modulates membrane potential
  • translated via bipolar into change of action potential in ganglion cell
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23
Q

what was the structure of the retina

A
  • photoreceptors on outside
  • light passes through cells structures to reach there’s causing scatter degrading image
  • pigment epithelium minimises reflectance and scatter as receptors are adjacent
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24
Q

foveal retina

A
  • taken cell bodies and pushed them aside for light to do straight through without scattering so centre of eye have higher acuity
  • fovea is small bit in middle of eye where cell is teased away for that high visual acuity
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25
Q

rods and cone cells

A
  • photopigment contained within disks of outer segment
  • disks continuously migrate outwards and regenerated
  • certain density of these photoreceptors - point spread function - 2 sources of light and shine them onto retina when is system first capable of distinguishing those 2 bits of light
  • maximum density is well matched to point spread function
26
Q

what chain of events occur during phototransduction

A
  1. photopigment bleaching - retinal and opsin combine to make photopigment = rhodopsin (unbleached)
    light photon interacts with rhodopsin causing change so becomes bleached
  2. cell membrane hyperpolerised via G-protein as released opsin activates enzyme PDE via G protein, PDE converts to cGMP to GMP opining Na+ channels. closing of the channels causes hyper polarisation of cells as K+ continues to leak out
  3. neural output of ganglion cell modified - rod/cone hyper-polarisation results in less neurotransmitter release and modulates membrane potential of bipolar and modulates firing rate of ganglion cell
27
Q

increased light can what

A

increase or decrease fire depending on ganglion cells being excitatory or inhibitory

about modulating cells

28
Q

visible range of luminance

A
  • human vision functions across 10 to power of 15 units of luminance
29
Q

scotopic v photopic vision

A
  • different photoreceptors and different light levels
  • low light = rods as highly sensitive but low acuity (scotopic)
  • brighter light = cones only as low sensitivity and high acuity (photopic) on sunny day rod cells are completely bleached
30
Q

photopic vision

A
  • high luminance
  • cones only
    low sensitivity and high acuity
  • fovea land peripheral
31
Q

scotopic vision

A
  • low light vision
  • rods only
  • high sensitivity and low acuity
  • non-foveal
32
Q

mesopic vision

A
  • intermediate luminance (dusk)
  • rods and cones
  • intermediate sensitivity and acuity
  • foveal and peripheral
33
Q

what 4 mechanisms adapt to luminance

A
  1. pupil size (takes a few seconds)
  2. switch of photoreceptors rods and cones (few seconds)
  3. dark adaptation (bleaching and regeneration of photopigment. takes 15-20 mins to adapt visually)
  4. field adaptation (light adaptation - cellular process in photoreceptors. instantaneous adaptation)
34
Q

rods vs cones

A
  • rods more sensitive than cones
  • bigger convergence in sensitivity (high convergence of rods onto ganglion cells)
  • convergence can be altered in different light
  • mesopic conditions rods and cones could converge together
35
Q

how many of each photoreceptors converge onto ganglion cells?

A

1 cone = 1 ganglion cell
75,000 rods = 1 ganglion cell

36
Q

what is the distribution of rods and cones in the retina?

A
  • foveal vison = cone dependant
  • deviate from fovea - increase of rod cells
  • high density of both in centre
37
Q

dark adaptation - bleaching

A
  • sunlight > pitch black room = regeneration of cones and then rods will continue to regenerate until fully done
38
Q

what processing happens between photoreceptor and ganglion cell

A
  • 130mill photoreceptors in eye - not all info to brain - retina interested in spatial and temporal changes - processes photoreceptor output to identify the changes and send that info to brain through cells (horizontals and amocrines)
39
Q

how do we detect postal change

A
  • detect edges
  • retina detects edges sends that info and loses the rest
40
Q

how does the retina send spatial (edges) info to brain?

A
  • receptive field of ganglion cell = area of retina which activates that cell
  • ganglion cell is the final output of retina, each cell respond to many photoreceptors so shine light on retina to determine ganglion receptive field
41
Q

what is lateral inhibition

A
  • the capacity of an excited neuron to reduce activity of its neighbours
  • mediated by horizontal cells
  • stripe of light best activates ganglion cell - excitatory cells in middle and inhibitory in periphery - light stripe activate so responds to edge (central exiting and lateral inhibiting)
42
Q

how does lateral inhibition create contrast illusions?

A
  • terrible of detecting absolute levels of light
  • surroundings affects acitivity of cell due to retina
43
Q

types of cone cell in colour vision

A
  • red
  • green
  • blue
44
Q

why do we have 3 types of cone

A
  • responds to stimulus intensity
  • if intensity doesn’t change = green
  • a monochrome system doesn’t allow you to distinguish between different light levels
45
Q

what type of system does our colour vision have?

A
  • 3 channel system
  • red, green and blue
46
Q

explain the colour triangle of the 3 channels

A
  • any colour can be dialled up by combination of red green and blue
  • cannot distinguish certain green wavelengths
47
Q

how can we define colour

A
  1. saturation = strength, pure red fully saturated and white is fully unsaturated
  2. hue - colour itself
  • no wavelength can stimulate green cones without red or blue cones
48
Q

explain the neural process of colour (colour opponent)

A
  1. takes place in retina
    2, ganglion cells don’t respond to red, green or red alone but a combination
  2. 3 channels hitting ganglion cell and it responds best to certain combos
    4.
49
Q

colour opponent

A
  • 3 opponent channels in retinal output
  • May explain why yellow is perceived to be a primary colour
  • Explain impossible colours – can have bluish-green (turquoise) but not yellowish-blue or reddish-green
50
Q

colour constancy

A
  • Colours tend to look the same despite large changes in wavelength of illuminating light
  • Multiple mechanisms – one is adaptation we discussed previously
  • High stimulated colour channels will tend to adapt and become less responsive
51
Q

how does colour blindness work?

A
  • deuteranope
  • lost green receptor
  • accept pair of colours as match as long as they have same R and B components
52
Q

colour blindness - rarest

A
  • monochromats = only see black and white
  • dichromats = lost your blue channel
53
Q

define proprioception

A
  • the sense of the relative position of neighbouring parts of the body
  • muscle spindles and Golgi tendon organs
54
Q

how do you get a sense of where your limb is

A
  • joint capsule receptors = Ruffini, paciniform, golgi-type and free nerve endings (historical)
  • modern shows minimal role of joint capsule
55
Q

what does a muscle spindle consist of?

A
  1. bindle of thin muscle fibres contained within capsule
  2. parallel with main extrafusal muscle but generates no useful force
  3. wrapped round air of sensory axons detecting muscle stretch
  4. gamma motor neurons cause active contraction of spindle
56
Q

what do muscle spindles detect?

A
  1. static length of muscle
  2. rate of change of muscle stretch (velocity)
57
Q

characteristics of muscle spindles

A
  • Joint angles calculated from changes in muscle length signalled by spindles
  • Central part of muscle spindle is non-contractile
  • Stretch-sensitive ion channels in sensory axons are activated when muscle is stretched
  • 400 spindles in soleus muscle
  • Primary endings: signal position and velocity. Transmitted via 1a afferents. Sensitive to vibration
  • Secondary endings: position only, via group II afferents
  • Gamma fibres: contractile element of spindle. No useful force output – purely intended to keep spindle taught
58
Q

how does alpha-gamma coactiviation maintain muscle spindle sensitivity?

A
  1. 1a spindle afferent fires signal muscle length
  2. contraction of main muscle cause slackened spindle
  3. causes drop of firing rate and loss of sensitivty
  4. gamma active`tion contracts spindle maintaining sensitivity
59
Q

2 pieces of evidence that support the role of spindles for sensation

A
  1. muscle vibration - 80-100Hz of vibration activates 1a afferents producing muscle stretch illusion
  2. muscle conditioning effects - prior muscle stretch/ condition significantly affects joint position sense due to muscle becoming slack affecting spindle output
60
Q

Golgi tendon organ and proprioception

A
  • contributes to joint rotation sense (signals force and heaviness)
  • important when muscle movement is ambiguous
  • between muscle and tendon
  • when stretched, 1b afferent axon is compressed by collagen fibres so rate of firing increases

control of human movement (10 decks)

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