L19-22: Vision

Question 1

cornea

Answer 1

cornea = first layer, avascular, cells with no pigment, O2 and nutrients via diffusion

Question 2

sclera

Answer 2

white part, has blood vessels, continuous with cornea

Question 3

aqueous humour

Answer 3

fluid in in anterior chamber (basically extracellular fluid)

Question 4

pupil

Answer 4

= a hole through which light passes
- size adjusts aperture
- formed by gap in iris (pigment epithelium in front of muscle)

Question 5

lens

Answer 5

= elastic capsule, avascular, surrounded by circular muscle

Question 6

zonule fibres and ciliary muscle

Answer 6

zonule fibres around the outside of lens, like ligaments that connect to the circular ciliary muscle

Question 7

vitreous humour

Answer 7

fluid in posterior chamber

Question 8

retina

Answer 8

= neural component
is extremely metabolically active → supplied by the othelmic artery (comes in with optic nerve)

Question 9

choroid

Answer 9

= retinal pigment epithelium
- behind retina
- this is the black we see when we look through the pupil
- pigmented black because doesn’t reflect
- light is absorbed here after passing through the neural component

Question 10

optical nerve

Answer 10

= bundle of axons at back of eye, coming from retina to VC

Question 11

optic disk

Answer 11

where axons leave eye = no light sensitive cells here = a blind spot

Question 12

fovea

Answer 12

= part of retina with best 2 point discrimination = acuity, focus point of vision

Question 13

the near response

Answer 13

= ability to change focus
uses accomodation, constriction of pupil and convergence of eyes

Question 14

accomodation

Answer 14

Contraction/relaxation of ciliary muscle to alter lens shape and change refractive power

Question 15

process of accomodation when looking close

Answer 15

parasympathetic activation of ciliary muscle → contracts → tension taken off zonule fibres and lens rounds due to natural elasticity → increases radius of curvature → increases refractive power (goes from 60 to ~75 diopters)

Question 16

effect of pupil constriction on focus

Answer 16

small aperture = better depth of focus, exclude outside edges

Question 17

effect of eye convergence on focus

Answer 17

close objects remain in register on corresponding parts of the two retinae (esp. foveae)

Question 18

myopia

Answer 18

= short sighted, eyeball is elongated
= correction with concave lens to diverge light a bit

Question 19

hyperopia

Answer 19

= eye too short, can focus far away but no refractive power left to focus on something near
= needs convex lens to correct = more refractive power

Question 20

astigmatism

Answer 20

= irregular shaped lens, light from different planes isn’t brought into focus at the same point so lights smear and things that are horizontal/vertical appear to slope
= need irregular lens so that glasses + lens = normal curvature

Question 21

Presbyopia

Answer 21

= age-related loss of accommodation because lens has lost its elasticity, near point gets further away
= convex corrective lens needed

Question 22

cataract

Answer 22

= lens becomes opaque, especially with age
= surgery to replace lens, but fixed radius so no accomodation, need reading glasses

Question 23

photoreceptors

Answer 23

rods and cones

Question 24

components of eye involved in neural transmission of info, in order

Answer 24

photoreceptors (rods and cones) → interneurons (amecrine, bipolar, horizontal cells) → ganglion cells

Question 25

what makes the fovea have the most focused area of vision

Answer 25

fovea = pit = less tissue to pass through = less refraction = most focused image

Question 26

structure of rods

Answer 26

stacked discs of membrane where light sensitive proteins are located → increased SA so really light sensitive in not much space
- functions in low light = night vision

Question 27

structure of cones

Answer 27

series of deep invaginations → lots of SA but not as much as rods
- requires more light to see colour

Question 28

colour vision

Answer 28

• via cones (3 types- most sensitive to red, green and blue (by wavelength))
• perception of colour created by relative activation of 3 cone types.

Question 29

photopigments (function and components)

Answer 29

• give photoreceptors the capacity to respond to light
• 2 components
  
  • retinal = a chromophore → a Vitamin A derivative
  • an opsin = membrane spanning protein
    
    • rods have rhodopsin
    • cones have either S(blue), M(green), or L(red) photopsin

Question 30

phototransduction in the dark

Answer 30

• no light so retinal is inactive (11-cis isoform)
• lots of cGMP (from GTP by guanylyl cyclase) so cGMP-gated Na+ channels open
• Na+ so photoreceptor is depolarised (~ -35mV)
• leads to continuous release of glutamate on to interneurons

Question 31

phototransduction in the light

Answer 31

• light energy activates retinal → conformational change (all trans-isoform) → opsin activates transducin
• transducin = G protein, activates cGMP phosphodiesterase which breaks down cGMP
• less cGMP → cGMP-gated channels close → less Na+ influx so photoreceptor hyperpolarised (~ -60mV) = less likely to fire
• Less glutamate released onto bipolar cells

Question 32

Colourblindness

Answer 32

• can be inherited or acquired (bc of disease)
• R/G colourblindness is X-linked (M & L opsins genes are on the X chromosome), effects 8% of males and 0.5% of females

Question 33

which colour corresponds to which photopsin

Answer 33

S(blue), M(green), or L(red)

Question 34

spatial and temporal summation on the retina

Answer 34

• if pattern varies over time ⇒ movement
• over space ⇒ helps define edges of objects

Question 35

Ganglion cell receptive field

Answer 35

• 2 parts: centre and surround
• needs contrast between the 2 parts for excitability
• 2 types: on centre field (excited by light in the centre, inhibited by light in the surround), and
• off centre field = excited by light in the surround, inhibited by light in the centre

Question 36

Ganglion cell receptive field

Answer 36

• 2 parts: centre and surround
• needs contrast between the 2 parts for excitability
• 2 types:
  
  • on centre field = excited by light in the centre, inhibited by light in the surround
  • off centre field = excited by light in the surround, inhibited by light in the centre

Question 37

receptive field x visual acuity

Answer 37

less photoreceptors per ganglion cell = low convergence on ganglion cell = smaller receptive field = better 2 point discrimination/acuity
- e.g. cones, fovea

Question 38

visual cortex function

Answer 38

• depth perception, colour, form, movement etc. all conveyed with APs
  
  • different groups of neurons for different functions e.g. contrast, edges
  • chain of cells that get more and more info combined → more complex *in higher centres

Question 39

Optic chiasm

Answer 39

where optic nerves meet
- nasal axons project opposite sides (contralateral), temporal axons project to same sides (ipsilateral)
- left half of visual field goes to right side of brain, and visa versa

Question 40

function of binocular vision

Answer 40

2 slightly different images of the same thing brought together for central processing ⇒ important for depth perception

Question 41

ganglion cells project to: (4)

Answer 41

1. LGN = first relay centre to do with sensation of vision
2. Superior colliculus = concerned with eye movements and orientation to visual stimuli = tracking
3. Pretectum = control of pupils → aperture
4. SCN = control of diurnal rhythms, triggered by exposure to daylight