The Visual System 1 & 2 Flashcards
importance of vision
detect prey, predators, mates
communicate
more than 1/3 neocortex involved in analysing visual world
light and eye
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
light and environment
optics
EM light travels in straight lines - rays - until it interacts with atoms and molecules
interact in 3 ways: reflection, absorption and refraction
pupil
lets light inside
black as all light absorbed
iris
contains muscles which control amount of light entering eye
cornea
glassy
transparent covering of pupil and iris that reflects light
sclera
continuous with cornea
forms tough proctective wall of eyeball to give shape
extraocular muscles
move eyeball
controlled by oculomotor nerve (CN III)
optic nerve
CNII - sensory
carries axons from retina to brain
opthalmoscope
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
cross section of eye
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
image formation
light rays must be focussed on retina - ideally fovea
refraction occurs at: cornea - 80%, lens - 20%
degree of refraction determined by
difference in refractive indices between the 2 media
angle at which light hits the interface between 2 media
refraction by the cornea
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
accommodation by the lens
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
rounding of lens:
increases refractive power to focus closer objects on fovea
problems with focussing
eye is emmetropic when lens is flat and we are focussing a distant object
farsightedness
eye is too short
near objects are focussed behind retina
not enough refraction
nearsightedness
eye is too long
distant objects are focussed before retina
too much refraction
eye summary
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
laminar organisation of retina
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
layers of retina
ganglion cell layer - inner layer, action potentials
inner layers - graded potentials
pigmented epithelium - outer layer
cells of retina
ganglion
output from retina
cells of retina
amacrine
modulate info transfer between ganglion cells and bipolar cells
cells of retina
bipolar cells
connect photoreceptors to ganglion cells
cells of retina
horizontal cells
modulate info transfer between photoreceptors and bipolar cells
cells of retina
photoreceptors
sensory transducers, both rods and cones
photoreceptors
membranous discs contain light sensitive photoreceptors that absorb light
lots of mitochondria - active transport to balance depolarisation
phototransduction occurs in:
rod
duplicity theory
cant have high sensitivity and high resolution in single receptor
separate systems of monochrome and colour
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
cone photoreceptors
fewer discs used in daylight (photopic) colour vision high visual acuity/resolution lowr sensitivity
retinal structure varies with region
fovea contains most of 5 million cones and no rods
central retina
low convergence, low sensitivity and high resolution
peripheral retina
high convergence, high sensitivity and low resolution
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
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
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
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
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
light adaptation
photoreceptors initially hyper polarise greatly
photoreceptors gradually depolarise with continued bright light
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
downstream of photoreceptors
there are different types of bipolar cells
bipolar cells have complex receptive fields
have on and off bipolar cells
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
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
the receptive field
retinal ganglion cells will only fire action potentials when specifc areas of the retina are illuminated
