describe the components of the eye
pupil - opening where eye enters the eye
sclera - white of eye
iris - gives colour to eye
cornea - glassy transparent external surface of the eye
optic nerve - bundle of axons from the retina. electrical signal passed out of the eye onto visual path to brain. these neurons/axons come from cell type called ganglion cells. they form output of retina and their axons are bundled together called the optic nerve.
structure of eye
has an ophthalmoscopic appearance.
where the optic nerve leaves and circulatory system enter to feed/flush the eye, there is a structure called fovea and it is the centre of the visual eye. lens and cornea are important in focussing light.
describe image formation by the eye
eye collects light, focusses on retina and it forms an image and inverts it.
the ability to focus an image depends on the cornea as it has a refractive index and the lens provides more refractive power and the lens has ability to adjust refractive power.
cornea has refractive index of 42 diopters.
changing the shape of lens allows extra focussing power when focussing on different distances.
errors of refraction
emmetropic - normal eye
presbyopia - our lens harden with age and ciliary muscles weakens, causing decreased ability in accommodation (ability to focus light on retina).
hyperopia - far sightedness; refractive power insufficient; corrected with convex lens.
myopia - near sightedness; refractive power too strong for distant objects; corrected with concave lens.
what are the different muscles around the pupil?
circular (constrictor) muscles act to decrease pupil size under parasympathetic control.
radial (dilator) muscles act to increase the pupil size under sympathetic control.
the pupillary light reflex
- connections between retina and brain stem neurons that control the muscle around pupil.
- continuously adjusting to different ambient light levels
- consensual (both pupils react similarly and simultaneously)
what is the visual field
- amount of space viewed by the retina when the eye is fixed straight ahead.
- image is inverted
what is visual acuity
- ability to distinguish two nearby points e.g. the two sides of the moon
- determined by photoreceptor spacing and refractive power
- visual angle: distances across the retina described in degrees.
how does vision work?
- the pattern of the object must fall on the vision receptors (tods and cones in retina)
- the amount of light entering the eye must be regulated. if there is too much, light will “bleach out” the signal
- the energy from the waves of photons must be transduced into electrical signals.
- the brain must receive and interpret signals
cellular structure of retina
inside out laminar (layered) structure of the retina.
direct (vertical) pathway for electrical signal transmission: photoreceptors -> bipolar cells -> ganglion cells. light direction goes the opposite way.
ganglion cells eventually project to the forebrain along the thalamus and primary visual cortex.
the photoreceptor layer = converts light into electrical signal and is the innermost layer of the retina.
lateral connections influence signal processing: horizontal cells receive input from photoreceptors and project to other photoreceptors and bipolar cells.
amacrine cells receive input fro bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells.
describe photoreceptors
- converts electromagnetic radiation to neural signals (transduction)
- four main regions: outer segment, inner segment, cell body and synaptic terminal
- types of photoreceptor: rods and cones
what is the basis of phototransduction/vision?
- vertebrate photoreceptors have a depolarised rmp.
- with light exposure Vm hyperpolarises. at rest, photoreceptor cells are depolarised and releasing glutamate. as the photoreceptors send signals through a chain to thalamus, there is less glutamate.
photoreceptor cells can have tonic glutamate released and it is diminished when it perceives light stimulus. this is called a dark current. this is caused by a cGMP-gated Na+ channel that is open in the dark and closes in the light.
what is the dark current
in dark, sodium enter, and there is a sodium-potassium pump, and that reconstitutes the homeostasis. the permeability of sodium and potassium are relatively equal.
in response to light, the influx of sodium is decreased, permeability of potassium continues and in response to light the membrane potential tends to be toward equilibrium for potassium so hyperpolarises.
what is the response to light?
- visual pigment molecules called rhodopsin (for rods) = retinal + opsin
- light converts 11-cis-retinal to all-trans-retinal (active form)
- light stimulation causes change in conformation of the opsin molecule which convey this change to a G protein called transducin.
- rhodopsin will activate transducin which will activate cGMP phosphodiesterase and this hydrolyses cGMP to GMP.
- this means theres less cGMP is present in the photoreceptor cell and therefore will be gated so less sodium will enter into photoreceptor cell and therefore will be less depolarised and releasing less glutamate onto bipolar cells.
- in absence of light, transducin is not stimulated.
dark current summary
in dark = more glutamate released. ore depolarised.
in light = less glutamate more hyperpolarised.
what facilitates high acuity?
high acuity is the ability to distinguish between two points.
visual acuity depends on receptor spacing and refractive power.
high density of photoreceptors means image is sharp.
human acuity is circular.
describe the distribution of rods and cones
- centre of retina is the fovea, where cones are in abundance
- the retina has a blind spot where there are no cones, where the optic nerve goes out and blood vessels come in.
- cones are sparesly distributed along retina. rod photoreceptors are priarily excluded from fovea, but present at high levels along the rest of retina.
- rods used to see in dim light, use cones when there are sufficient light levels.
- during day we use fovea to see things under dim light levels.
- affect acuity as rods have high convergence, cones have low. this means the receptor field of one or a few cones will feed into individual ganglion cells. this means individual ganglion cells focus on smaller portion of visual field, so acuity is higher.
- lots of rods will converge into single ganglion cell, so all of their receptor field is a larger area of visual field, diminishing acuity.
- part of the reason we can use rods to see in dim light is because in dim light rods receives all feeds into ganglion cell and the amount of signal that the ganglion receives is enough to stimulate AP. cones may not be stimulated to cause AP.
how do we see colour?
3 cones and 1 rod responsible.
long wavelength (red) cone stimulates 500-700nm light.
medium wavelength (green) cone sees green.
short-wave cone (blue).
different wavelengths overlap and colours mix.
characteristics of rods
achromatic = don’t see colour
peripheral in the retina
high convergence
high light sensitivity
low visual acuity
characteristics of cones
chromatic - see colour
central retina - fovea
low convergence
low light sensitivity
high visual acuity
describe on/off pathways in the retina
- only ganglion produce action potentials, all other cells produce graded changes in membrane potential.
- hyperpolarisation of photoreceptors elicits both hyperpolarisation and depolarisation within bipolar and ganglion cells.
- these graded potentials modulate the discharge rate of ganglion cells.
- on and off bipolar and ganglion cells respectively detect increases and decreases in luminance.
- off pathway bipolar cells have ionotropic glutamate receptors. in light there is less glutamate being released from photoreceptor, causing bipolar cell to be more negative, thus ganglion cell is also negative. in dark, more glutamate is released onto ionotropic bipolar cell causing negative membrane also in ganglion cell.
- on pathway bipolar cells have metabotropic glutamate receptors. in light, photoreceptors release less glutamate onto bipolar cells, causing more positive membrane also in ganglion cell. in dark, more glutamate released onto bipolar cell so bipolar and ganglion are more negative.
describe what is meant by centre-surround organisation in the retina
- lateral inhibition by horizontal cells modifies the receptive fields of ganglion cells to have a centre surround organisation.
- every ganglion is set to have a centre surround organisation and said to be antagonistic in the centre surround.
- if the centre is stimulated by light it is an ‘on’ ganglion cell and the surround of the centre will be off.
- centre field of photoreceptors will be the opposite of receptive field surround.
how does a centre surround organisation transmit signal to optic nerve?
bipolar cell is maximally stimulated when centre in on. less glutamate released and the response of bipolar cell is positive as it contains metabotropic receptor so generates positive signal on ganglion cell. since glutamate is an inhibitory interneuron, GABA is released and inhibits the surround photoreceptors. in the dark they will release lots of glutamate and stimulate an interneuron under surround cone and release lots of glutamate onto central photoreceptor. as GABA inhibits the photoreceptor there will be even less glutamate released. this causes greater depolarisation. horizontal cells release more GABA which further inhibits photoreceptors.
- when surround is dark, horizontal cells will be stimulated the most and they will decrease activity of synapse even more so bipolar cell will receive less glutamate and cause depolarisation of ganglion cell even more.
what is the purpose of centre surround organisation?
central surround/lateral inhibition serves to emphasise areas of difference (contrast).
