Vision Flashcards
where does the optic nerve terminate
LGN lateral geniculate nucleus ( a relay to the visual cortex)
what is the optic chiasm
left and right optic nerve cross at the chiasm, forming a characteristic X shape located in the forebrain directly in front of the hypothalamus
where are the nasal and temporal retinas
nasal (medial) retinas are on the inner half
temporal (lateral) retinas are on the outer halves
the right visual field is detected by ___________________________ and extends to the __________
detected by the left temporal retina and the right nasal retina -
extends to the left side of the brain
the left nasal retina and the right temporal retina take information from the _______________ and project to the _____________
take information form the left visual field and extend to the right side of the brain
the retinal ganglion cells sends information to which two nuclei
pretectal nucleus (reflexive eye movements) & suprachiasmatic nucleus (sleep – wake cycle)
what are the three layers of the eyeball
– Sclera (tough outer layer)
– Choroid (where blood vessels are)
– Retina (at the back)
what is the eyeball filled with
vitreous humour
what are the highly transparent structures of the eye
cornea and lens
where is highest resolution vision detected (high acuity)
in the fovea centralis
because it is densely saturated with cone photoreceptors
where is the blind spot and what causes it
where the optical nerve is – where your optic nerve connects to your retina has no light-sensitive cells, so you can’t see anything there
what structures refract the light.
– Cornea provides most of refraction
– Lens is adjustable => fixed by ligaments that allows for accommodation - changing your optical power to focus near and far, can flatten or rounden
_________ range decreases as you age
accommodation
what are optic gliomas
tumours that can grow within the chiasm or in conjunction with the hypothalamic tumours, they can directly affect optic nerves.
Can cause, buldging eyes or vision loss, squinting or involuntary eye movements, elevated intercranial pressure and loss of appetite/fat reduction
what are refractive errors
caused by irregularity in the size and shape of the eye. vision blurred/ hard to focus because the refracted light isnt focussed on the retina - refractive power doesnt match the length of your eyeball
what is emmentropia
The state of the eyes without refractive errors - your vision is normal at you can see clearly at all distances because a point at an infinite distance from the eye is conjugate to the retina.
give two examples of Ametropia (refractive errors)
myopia - near-sightedness, near objects seen clearly, far objects are blurry because refraction focuses images in front of your retina
hyperopia - far object clear, near object blurry because you eyeball is too short and the light is focussed behind your retina
what is hypertropia
a form of vertical strabismus where one eye is deviated upwards in comparison to the fellow eye. Refraction causes focus behind the retina
what developmental vesicle does the retina form from
the diencephalon (optic vesicle)
what are the 5 neuronal cells types found in the retina
– Photoreceptors – Bipolarcells – Ganglion cells – Amacrine cells (modulate) – Horizontal cells (modulate)
what are the layers of the retina
inner plexiform layer inner nuclear layer outer plexiform layer outer nuclear layer rods and cones pigmented epithelium
the left visual feild is projected to the ___________ lateral geniculate nucelus
contralateral
how much of our cortical area is made up of the visual cortex
40%
why is there overlap of the visual feilds
central part of visual feild (in front of you) is detected by both eyes (peripheral just detected by one)
which retinal ganglion cell axons cross into the contralateral hemisphere
those coming from the nasal part of the retina
where do the cell bodies of the photoreceptors sit
in the outer nuclear layer
where is the pigmented epithelium and why is is specialised in this way
(next to) inner to the photoreceptors
highly pigmented to prevent scatter of light allows us to get a sharper picture and supports the photoreceptors
which layer of the retina are the neuronal synapses of horizontal cells and photoreceptors
outer plexiform layer
where are bipolar cell bodies housed in the retina
inner nuclear layer
what is in the inner plexiform layer
synaptic connections between the axons of bipolar cells and dendrites of ganglion cells
what is the simplest circuit of neurons in the retina
photoreceptor -> bipolar cell -> ganglion cell
what is the order of cells from the surface facing the light to the photoreceptors
light -> ganglion cells -> amacrine -> bipolar -> horizontal -> rods + cones
what do photoreceptors detect
400-700nm of the electromagnetic spectrum
where is the pigment for light detection located
in the disks of the outer segment of photoreceptors
where is photopigment regenerated
in the retinal pigmented epithelium adjacent to the photoreceptors
what clears away the shed disks
the retinal pigmented epithelium
light detection leads to ___________
hyperpolarisation
closing of Na/Ca channels
(intense light big hyperpolarisation, low intensity light little hyperpolarisation)
which cells work with graded responses and which with the classic action potentials
photoreceptors and bipolar cells have graded responses (hyperpolarisation)
ganglion cells have the classic action potential (depolarisation)
what are the Na+/Ca++ channels gated by
cGMP
what are the differences between rods and cones
• low spatial resolution • High sensitivity (1 photon) • don’t contribute much after 0.1luninacelogcd/m-2 (very good in the dark) Cones: • High spatial resolution • Low sensitivity (>100 photons) (more suited to brighter environments) • Less saturation • Rapid recovery mediate colour vision
loss of rod function leads to
night blindness
loss of cone function leads to…
legal blindness
at what luminance do cones begin to contribute
about -4 logcd/m-2 (at about starlight)
at what luminance do rods becomes saturated and stop contributing to vision
in between 0-1 log cd/m-2
what colour are each of the short medium and long wavelength visible light
short = blue medium = green long = red
how do we detect colour
a combination/comparison of activity of difference classes of monochromatic cone cells detecting each of the short (blue) medium (green) and long (red) wavelengths of light
why are males more likely to have different colour vision/ colour blindness
because the medium and long wavelength cones are on the X chromosome
(females have two so can correct)
what happens when light is absorbed by photoreceptors
Upon light adsorption there is a conformational change in the cis to trans retinal that induces a conformation change in the opsin activating transducin, the alpha subunit of transducin dissociates bound to GTP and activates on phosphodiesterase molecule which degrades the cGMP to get GMP. This means the Na+/Ca++ channel closes causing hyperpolarisation.
at what points in the molecular cascade is the signal ampliied
conformational change of opsin activates hundreds of transducins
PDE degradation of 6 cGMPs
- a single photon can close around 200 channels change in -1mV
how is the light signal terminated
intracellular calcium decrease -
=> guanylate cyclase activity increases (cGMP restored)
=> rhodopsin kinase gets activated, phosphorylates rhodopsin => arrestin binding/ transducin displacement
+ all trans retinol dissociates
the biggest signal amplification upon light detection occurs at which stage
the stage of transducin activation
what structure generate 11-cis retinal
the retinal pigmented epithelium
what is the difference between the density of photoreceptors in the perifoveal and foveal zones
the fovea has a very high density of cones and no rods
as you go out to the periphery the cones drastically declines the rods also decline a bit
what is visual resolution
the distance between two points that can be distinguished
how does contract help with resolution
high contact is more easily resolved
square wave grating is more easily distinguished that sine wave grating
most of the information we receive is encoded by _________ rather than
Most info we receive is encoded by light and dark rather than colour.
(we don’t lose that much info in black and white movies)
what cells axons form the optic nerve
the retinal ganglion cells
what do ganglion cells detect
changes in luminance
what are the two types of ganglion cells
on-center GC (a cells that fires when light hits the centre of the receptive feild)
and off-center GC (falls silent when centre is relatively brighter)
what is the receptive feild
the region in which a stimulus elicits an AP
what activity occurs in on centre ganglion cells
when light hits the centre it fires (relative to the outside)
when dark hits centre (outer is relatively brighter than centre) it switches off
when does activity occur in off-centre ganglion cells
when the outer is relatively brighter to the inner
opposite to on centre ganglion cells
what is lateral inhibition in ganglion cells
There is little or no response to a large (full field) spot of light that covers both the center and the surround because excitation in the center cancels the inhibition from the surround
what receptors are associated with on centre ganglion cells
mGluR6
what neurotransmitter receptors are associated with off centre ganglion cells
AMPA/ kainate receptors
which type of ganglion cell is sign inverting and why
on centre ganglion cells - because hyperpolarisation causes them to depolarise in response to light detection in the cone
what do horizontal cells do
sample a broad area of photoreceptors and connect s to other horizontal cells via gap junctions (fast signalling to other horizontal cells). together they allow detection over a large area of the retina (detection of background illumination)
glutamate releases depolarises horizontal cells, releases gaba onto photoreceptor leading to hyperpolarisation (sign inverting) response to whole population over the retina
how many rod cells are in the human retina
90 million
how many cone cells are in the human retina
4.5 million
which cells in the retinal circuit detect background illumination
horizontal cells
ganglion cells in the optic tract project to
pretectum
suprachiasmatic nucleus (hypothalamus)
superior colliculus
dorsal lateral geniculate nucleus
what does the pretectum do in response to light
feeds back to constrictor muscle in iris => pupillary light reflex
what does the suprachiasmatic nucleus do in response to light
circadian (day – night) cycle
what does the superior colliculus do in response to light
coordination of movement in response to visual cues
retinotopic point to point projection is
the organisation of the projections from each eye that terminate in alternating layers
spatial arrangement of the retina is maintain throughout the visual pathway however…
the visual pathway however not in proportion: peripheral field gets a smaller portion of the visual cortex, the macula a relatively small retinal area gets a large proportion of processing area in the cortex
columns in the cortex respond to _________. adjacent columns respond to _________.
columns respond to one direction/orientation.
adjacent columns respond to progressive shifts
which layer of the visual cortex do the lateral geniculate neurons terminate
in alternating ocular dominance columns in cortical layer 4
what allows binocular vision
inputs from both eyes come together (outside layer 4, above and below) in the striate cortex, the relative strength of input varies, the centre of the column input from one eye and at the boundaries there is input from both ayes and everything in between
