eye overview Flashcards
what are the key brain areas involved in visual information processing?
retina - lateral geniculate nucleus (in thalamus) - optic radiation (geniculocalcarine tract) – primary visual cortex
name and describe 5 features of the eye
Pupil - lets light in, appear black because all light that enters is absorbed on the retina
Iris - muscles controlling amount of light entering eye
Cornea - transparent covering of pupil and iris, refracts light onto retina
Lens = behind the pupil, second part of the refractive system, but the lens can adjust how much it refracts light unlike the cornea
Sclera - outer layer (minus cornea), tough and protective, maintains shape
where is the aqueous humor and what does it do?
Behind the cornea and in front of the lens = aqueous humor - supports cells without the need for blood vessels that would interfere with corneal function
what is the fluid-filled main body of the eye?
vitreous humor = fluid filled, maintains shape
Retina goes all around the sides and back of the eye
what nerves and muscles are in the eye?
Extraocular muscles, controlled by oculomotor nerve (CN III)
Optic nerve -carries axons from retina to brain (CN II)
To flatten and weaken the lens, suspensory ligaments are tight, ciliary muscles relax, and vice versa
how is light refracted in the eye? include the definition of focal length, and why distant objects require less refraction
Cornea refracts 80% of light, occurs due to cornea having a higher density than air
Focal length = distance from refractive surface to point of convergence of parallel light rays
Parallel light rays don’t need as much refraction to focus on the retina. Distant objects have more parallel rays, so the lens is made thinner for distant objects, because the cornea provides adequate refraction
The opposite can be said for nearer objects
describe the features of the retina
The macula is the centre portion of the retina that produces even sharper vision with its rods and cones
The fovea is the pit inside the macula with only cones, so has highest visual acuity
Rest of retina = mostly rods
Light travels to the back of the retina where the photoreceptors are, via muller cells (glia cells of the retina)
macula…
only cones = highest visual acuity
from front to back, what are the layers of the retina?
The ganglion cell layer, closest to vitreous humor, outputs info from retina to brain
Inner plexiform layer - SYNAPSES between ganglion and amacrine cells receiving input from bipolar cells
Inner nuclear layer - amacrine, horizontal and bipolar cell bodies
Outer plexiform layer - (closer to the back of the eye, the sclera) - synapses between bipolar and horizontal cells receiving input from photoreceptor cells
Outer nuclear layer - cell bodies of the photoreceptors
Photoreceptor outer segments
Pigment epithelium - absorbs any light the photoreceptor cells doesn’t
the inner plexiform layer - what can be found there?
its all synapses
Bipolar to amacrine
Amacrine to bipolar
Bipolar to ganglion cells
Amacrine to ganglion cells
the inner nuclear layer - what are the roles of the cells present?
amacrine cells modulate the information between GCs and BCs
horizontal cells modulate information between PRs and BCs
where do you get graded potentials and where do you get action potentials in the eye?
Photoreceptors transduce light to electrical signal, sending it back out to the ganglion cells, whose axons go to the optic nerve
The photoreceptors and bipolar cells produce graded potentials, action potentials only occur at the ganglion cells
function of the pigmented epithelium?
absorbs any light the photoreceptor cells don’t, this increases visual acuity because it ensures photons are not reflected back onto photoreceptors and distort image
give an explanation of the phototransduction cascade in photoreceptors
ligand gated ion channels with ligand binding site on the intracellular side of the ‘outer segment’ (the one furthest back) of PRs
The ligand is cGMP (broken down by a phosphodiesterase activated by the opsin GPCR, which is on in light due to retinal going from cis to trans)
dark = inactive opsin = high levels of cGMP so LGIC is open, allowing an influx of Na+ (and other cations), depolarising the cell in dark
When its light, levels of cGMP drop, these gated channels close (and K+ channels remain open) so there’s no influx of Na+ and the photoreceptors are hyperpolarised in light
rods vs cones?
Fovea = cones only
Rods = rhodopsin
Cones = three different opsins, S, M and L for short wavelength etc…
RODS = many bipolar neurons synapsing with one retinal ganglion cell = convergence of multiple summations mean high sensitivity, but lower resolution/acuity
CONES = one bipolar neuron to one retinal ganglion cell = lower sensitivity as there’s only one neuron trying to trigger an action potential, but higher visual acuity
explain the difference between ON (centre) and OFF (centre) bipolar cells
when dark, photoreceptors are depolarised, so release more glutamate, when it is light they become hyperpolarised, releasing less glutamate
Bipolar cells are named based on how they respond to light - which means how they respond in to less glutamate
ON bipolar cells become depolarised as a result of more light/less glutamate, so they must use metabotropic glutamate receptors (the GPCR) as this results in an inhibitory cascade
light = less glutamate = less inhibition = depolarisation
Off bipolar cells become hyperpolarised in light due to less glutamate, so they use the excitatory ionotropic glutamate receptor AMPA
light = less glutamate = less excitation = fewer positive ions in = hyperpolarisation
knockout experiments of two proteins showed bipolar cells no longer responding to light, what proteins were they?
TRPM1 (ON bipolar cells)
Nyctalopin
photoreceptors — bipolar cells —
then what? include a detail difference between ON and OFF bipolar cells
Bipolar cells release glutamate, synapsing with ganglion and amacrine cells in the inner plexiform layer
Within the IPL the OFF cells only go a little ways into the layer, while the ON cells go all the way into the IPL (closer to ganglion cell layer)
explain the receptive field organisation of bipolar cells
A bipolar cell has a ‘receptive field centre’ where it synapses directly with several photoreceptors
and then a ‘receptive field surround’ where it receives info from the surrounding photoreceptors via a middle man - a horizontal cell
However
The horizontal cell is inhibitory (releases glycine, an inhibitory NT) and so, when the surround is light, PRs of the surround hyperpolarise, so this horizontal cell releases less inhibitory glycine onto the PRs in the ‘centre’ of the receptive field, so these PRs release more glutamate
(for ON cells) = more glutamate = more inhibition = less activation.
light on surround (for ON cells) = hyperpolarisation
basically, the surround being lit up or being dark, causes the surround PRs to release less or more glutamate, causing mess or more inhibition of the central PRs from the horizontal cell, so the surround has the opposite effect to the centre (whether the centre is ON or OFF)
explain the receptive field of retinal ganglion cells
you’ve got ON vs OFF-centre RGCs
looking at ON-centre cells:
When there is no light at all, the on-centre ganglion cell will fire at a baseline rate…
When light is shined on the centre of the RF, firing rate is at max during the stimulus
If light is only on the surround, baseline firing is suppressed
Diffuse light - on the entire receptive field - firing rate is similar to baseline - there is no DIFFERENCE to detect
off-centre RGCs are the opposite way around
why do we end up with ON-centre and OFF-centre retinal ganglion cells?
the RGCs inherit this organisation from the bipolar cells with their whole centre-surround receptive fields
an ON-centre RGC is simply receiving input from an ON-bipolar cell (and vice versa)
what are the features of magnocellular ganglion cells? include the function
larger dendritic tree and therefore larger receptive field
Cannot respond to colour
Transient response - stops responding if the stimulus is prolonged
Faster conduction
High sensitivity
Function = motion detection (low resolution), so good that they have a transient response, and a fast response
what are the features of parvocellular ganglion cells? include the function
smaller dendritic tree and therefore smaller receptive field (so higher resolution)
Most ganglion cells are parvocellular - 80%
Respond to difference in colour
Sustained response - until stimulus is removed
Much slower in conduction
Low sensitivity
Function = form and colour
which kind of ganglion cell sees form and colour? which detects movement?
colour = parvocellular
motion = magnocellular
which ganglion cell is slower in conduction and lower in sensitivity?
parvocellular