visual systems Flashcards
importance of vision (3)
detect prey and predators
detect mates
communication
what is light + features
visible electromagnetic radiation
wavelength = distance between peaks and troughs
frequency = number of waves per second (Hz)
short wavelength = high frequency
amplitude = difference between peak and trough
visible light = 400-700nm wavelength
eye structures and functions:
extraocular muscles
optic nerve
aqueous humor
lens
extraocular muscles = move eyeball, controlled by oculomotor nerve (CN-III) (somatic nervous system moves eyeball and ANS controls dilation and constriction)
optic nerve = CN-II - carries axons from retina to brain, all sensory info out of the eye
aqueous humor = provides substances to cornea so it doesn’t need blood vessels in it
lens = changes how spherical it is for refraction of light and focusing at distances
eye structures and functions:
zonal fibres
ciliary muscles
vitreous humor
zonal fibres = attach lens to ciliary muscles
ciliary muscles = focus light onto retina by adjusting lens
vitreous humor = fluid to maintain spherical pressure in eyeball - lack of this can cause eyeball to shrivel and retina to pull away
eye structures and functions:
pupil
iris
cornea
sclera
pupil = lets light into eye
iris = muscles control amount of light entering eye depending on brightness
cornea = glassy transparent covering of pupil and iris that refracts light, many nerve endings, cannot change for refractions
sclera = continuous with cornea, tough protective wall of eyeball to give it its shape
monocular vs binocular view
alters what can be seen - field of view, depth perception - predator vs prey
retina structures
fovea - most receptors here - focus light on this point
macula - surrounds fovea, many receptors still
optic disk - blind spot - optic nerve and blood vessels here, brain fills in gaps
image formation - cornea refraction
light rays focus on retina
80% of refraction is in cornea and remaining 20% in lens
degree of refraction determined by difference in refractive index between 2 media - big difference between air and cornea
light travels more slowly in fluid of cornea - high density causes refraction
focal distance = distance from refractive surface to convergence of parallel light
accommodation - the lens
close objects (<7 metres) = need additional refraction through accommodation of lens = light rays are not parallel = lens is rounded (ciliary muscles contract, suspensory ligaments are slack) , increasing refractive power
distant objects = almost parallel light rays = cornea provides sufficient refraction to focus them on retina = lens is flattened (ciliary muscle relax, suspensory ligaments are taut), less refractive power
issues with accommodation
emmetropia = normal eye can focus on distant objects when lens is flat
hyperopia = far sightedness
focus on distances, close is blurry (focussed behind the retina) - eyeball is too short so light cannot refract enough
correction = convex lens = less refraction from close objects so they are more similar to distant rays = need less refraction in the eye
myopia = short sightedness
focus on close objects but distances are blurry (focussed in front of retina) - eyeball is too long
light refracts too much before it hits the retina
correction = concave lens = light refracts more before hitting cornea - more similar to rays from closer objects
retina - laminar organisation and cell types (5)
7 layers of cells from ganglion (inner) to pigmented epithelium (outer)
- ganglion cells = output from retina
- amacrine cells = modulate info transfer between GC and BC
- bipolar cells = connect photoreceptors to ganglion cells
- horizontal cells = modulate info transfer between photoreceptors and BCs
- photoreceptors = sensory transducers - rods and cones
photoreceptors
2 types - rods and cones
1 type of rod = monochrome = high sensitivity
3 types of cones = colour = high resolution
duplicity theory = cannot have high sensitivity and high resolution in a single receptor
rods and cones (+ distribution (3 areas))
rods =
greater number of disks
high photopigment concentration
1000x more sensitive to light than cones
enable vision at low light (scotopic)
low visual acuity/resolution
cones =
fewer disks
used in daylight (photopic)
colour vision
high visual acuity/resolution
low sensitivity
distribution:
fovea = 5 million cones and no rods
central retina = low convergence, low sensitivity, high resolution
peripheral retina = high convergence, high sensitivity, low resolution –> not many protons will result in glutamate release and action potential starting in gangion cell –> many photoreceptors to one ganglion cell = threshold reached more easily –> low resolution as it doesn’t matter which photoreceptor is hit, the ganglion cell action potential will give the same signal - no differentiation
phototransduction - rod and cone photopigments
rod = rhodopsin = 500nm
rhodopsin is made of retinal (absorbs photons and changes shape) and opsin (GPCR)
cones: S,M,L opsins
S = blue/violet = 420nm
M = green/yellow = 530nm
L = yellow = 560nm
greater numbers of long wave photoreceptors (M&L opsins)
phototransduction - retinal ganglion photopigment
melanopsin = 475nm = blue/green
responds to big changes in light (night and day differences) - for circadian rhythms
scotopic vs photopic
scotopic = night
photopic = day
photoreceptors in light vs dark
photoreceptors are hyperpolarised by light - not depolarised like axons with action potentials
at rest glutamate is constantly being released from photoreceptors (excitatory)
cGMP (cyclic guanosine monophosphate) levels in rods change depending on light level - bind to ligand gated non selective cation channels in outer membrane of photoreceptor => produced by guanylyl cyclase
dark = high cGMP –> cGMP-gated channels open –> influx of Na+ to depolarise photoreceptor
K+ channels always open so K+ can move out to make sure photoreceptor doesn’t become too depolarised
light = lower levels of cGMP –> cGMP-gated channels close –> Na+ stops moving in –> photoreceptor is hyperpolarised
phototransduction
5-7 photons evoke sensation of light in humans
rhodopsin is activated by light –> retinal absorbs photons and changes shape –> opsin stimulates G-protein transducin –> transducin GTP –> alpha subunit activates phosphodiesterase (PDE) –> reduces cGMP levels, closing Na+ channels
signal is amplified in enzyme cascade
saturation of responses in bright light (rods and cones)
rods cannot process bright light - easily saturated –> rhodopsin is bleached and cGMP levels are low so no additional hyperpolarisation can occur
cones are not saturated easily - used in bright light
photoreceptors gradually depolarise with continued bright light after initial large hyperpolarisation
role of calcium in light adaptation
dark:
Ca2+ enters cell and blocks guanylyl cyclase
reduces cGMP production and closes some ion channels
light:
channels are shut so Ca2+ cannot enter cells
guanylyl cyclase is no longer blocked
more cGMP produced –> more channels open
on and off bipolar cells
classified based on response to glutamate
in light, reduction in glutamate release:
OFF bipolar cells hyperpolarise = have ionotropic glutamate receptors
ON bipolar cells depolarise = have metabotropic glutamate receptors
bipolar cells have centre-surround organisation - receptive field centre surrounded by receptive field surround which all connect to a horizontal cell and then a bipolar cell
bipolar cells receptive fields
retinal ganglion cells only fire action potentials when specific areas of retina are illuminated
can map regions of retina which cause spiking in ganglion cells
