L19-22: Vision Flashcards
cornea
cornea = first layer, avascular, cells with no pigment, O2 and nutrients via diffusion
sclera
white part, has blood vessels, continuous with cornea
aqueous humour
fluid in in anterior chamber (basically extracellular fluid)
pupil
= a hole through which light passes
- size adjusts aperture
- formed by gap in iris (pigment epithelium in front of muscle)
lens
= elastic capsule, avascular, surrounded by circular muscle
zonule fibres and ciliary muscle
zonule fibres around the outside of lens, like ligaments that connect to the circular ciliary muscle
vitreous humour
fluid in posterior chamber
retina
= neural component
is extremely metabolically active → supplied by the othelmic artery (comes in with optic nerve)
choroid
= 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
optical nerve
= bundle of axons at back of eye, coming from retina to VC
optic disk
where axons leave eye = no light sensitive cells here = a blind spot
fovea
= part of retina with best 2 point discrimination = acuity, focus point of vision
the near response
= ability to change focus
uses accomodation, constriction of pupil and convergence of eyes
accomodation
Contraction/relaxation of ciliary muscle to alter lens shape and change refractive power
process of accomodation when looking close
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)
effect of pupil constriction on focus
small aperture = better depth of focus, exclude outside edges
effect of eye convergence on focus
close objects remain in register on corresponding parts of the two retinae (esp. foveae)
myopia
= short sighted, eyeball is elongated
= correction with concave lens to diverge light a bit
hyperopia
= 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
astigmatism
= 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
Presbyopia
= age-related loss of accommodation because lens has lost its elasticity, near point gets further away
= convex corrective lens needed
cataract
= lens becomes opaque, especially with age
= surgery to replace lens, but fixed radius so no accomodation, need reading glasses
photoreceptors
rods and cones
components of eye involved in neural transmission of info, in order
photoreceptors (rods and cones) → interneurons (amecrine, bipolar, horizontal cells) → ganglion cells
what makes the fovea have the most focused area of vision
fovea = pit = less tissue to pass through = less refraction = most focused image
structure of rods
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
structure of cones
series of deep invaginations → lots of SA but not as much as rods
- requires more light to see colour
colour vision
- via cones (3 types- most sensitive to red, green and blue (by wavelength))
- perception of colour created by relative activation of 3 cone types.
photopigments (function and components)
- 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
phototransduction in the dark
- 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
phototransduction in the light
- 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
Colourblindness
- 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
which colour corresponds to which photopsin
S(blue), M(green), or L(red)
spatial and temporal summation on the retina
- if pattern varies over time ⇒ movement
- over space ⇒ helps define edges of objects
Ganglion cell receptive field
- 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
Ganglion cell receptive field
- 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
receptive field x visual acuity
less photoreceptors per ganglion cell = low convergence on ganglion cell = smaller receptive field = better 2 point discrimination/acuity
- e.g. cones, fovea
visual cortex function
- 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
Optic chiasm
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
function of binocular vision
2 slightly different images of the same thing brought together for central processing ⇒ important for depth perception
ganglion cells project to: (4)
- LGN = first relay centre to do with sensation of vision
- Superior colliculus = concerned with eye movements and orientation to visual stimuli = tracking
- Pretectum = control of pupils → aperture
- SCN = control of diurnal rhythms, triggered by exposure to daylight
