vision

Question 1

what is phototransduction

Answer 1

conversion of light into electrical signals

Question 2

what is retinal parallel processing

Answer 2

detection and separation of different features of visual input (feature detection) into multiple parallel streams

integration of these parallel streams of visual information happens in the visual cortex: cortical maps and cortical processing

the simulatenous detection and transmission of multiple types of information from the same image

Question 3

where does sensory input get converted into visual perception

Answer 3

conversion of retina input signals into visual perception in higher visual and other areas of cortex

Question 4

where is the optic nerve

Answer 4

the optic nerve is located towards the nasal side of the eye, the light that touches the optic nerve is the blind spot, the temporal side of the eye is the side on the outside towards the head

fovea is just temporal of the optic nerve

Question 5

what is benefit of binocular vision

Answer 5

binocular vision is importatn in stereopsis (depth perception)

Question 6

what are types of eye movement when looking at a moving target

Answer 6

smooth pursuit eye movements are used to look at a target moving left/right/up/down

vergence eye movements are used to look objects coming closer/further away

Question 7

what are saccades

Answer 7

very fast eye movements that redirect gaze, saccadic movements of different amplitudes occur several times per second

they enable fast foveal sampling of important object features and prevent image stabilisation on the retina, stabilised images fade rapidly

Question 8

what does retina detect

Answer 8

differences but not absolute values of light intensity (its all relative)

Question 9

what features does the visual system detect

Answer 9

wavelength of light corresponds to colour, contrasts in light to edges and spatial frequency to size

an increase in contrast shows edges, a decrease in spatial frequency means larger size

Question 10

describe the structure of the retina

Answer 10

the retina has a layered strucure, it is transparent to light and is 0.5mm thick in humans

at the top it has a pigment epithelium, below this are the rods and cones in a layer called the photoreceptor outer segments, below this is the outer nuclear layer containing cell bodies of rods and cones

below outer nuclear layer is the outer plexiform layer where bipolar cells and horizontal cells synapse onto rods and cones

horizontal cells may form synapses with more than one rods and cones, and allow for lateral information flow across the retina

bipolar cells connect rods and cones to amacrine cells, they only synapse onto one rod or cone but form multiple synapses with each

the inner nuclear layer is below the outer plexiform layer and contains cell bodies of horizonatal and bipolar cells and amacrine cells

the bipolar cells synapse onto amacrine cells in the inner plexiform layer which is below the inner nuclear layer, in the inner plexiform layer amacrine cells also syanpse onto ganglion cells

ganglion cell bodies are in the ganglion cell layer which is below the inner plexiform layer, they lead to the optic nerve

nerve fibre layer is below ganglion layer and is bottom layer of the retina

Question 11

describe the flow of input of cells in the retina

Answer 11

flow of cells: rods+cones to horizontal cells and bipolar cells, horizontal cells to other rods and cones, bipolar cells from rods and cones to amacrine cells, amacrine cells from bipolar cells to ganglion cells, ganglion cells from amacrine cells to optic nerve

Question 12

describe the layers of the retina from top to bottom

Answer 12

layers from top to bottom: pigment epithelium, photoreceptor outersegments, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, nerve fibre layer

Question 13

describe the density of rods in the eye

Answer 13

rods have lowest density right in the centre of the eye (where eccentricity(angle) = 0 degrees), coming outwards density increases a lot and peaks at 20 degrees before it starts to drop

rods have higher density nasally than temporally

Question 14

describe the density of cones in the eye

Answer 14

cones have high density at 0 degrees eccentricity which dramatically decreases as you go outwards, is uniform between temporal and nasal sides

Question 15

where is the optic disk located

Answer 15

optic disk is located between 10 and 20 degrees nasally

Question 16

what are diseases affecting the retina

Answer 16

retinitis pigmentosa: loss of rods followed by progressive loss of cones, causes night blindness and loss of peripheral vision

age-related macular degeneration: cone degeneration in fovea causes loss of central vision

Question 17

describe the acuity in the fovea

Answer 17

acuity (resolution) is highest in the fovea

photoreceptors only detect light that hits them directly, they begin the process of feature extraction since they place a physical limit on the maximal spatial acuity (like pixels of a camera)

in the fovea, densley packed cone photoreceptors transmit signals (via the retinal circuit), from very small areas and can have a one to one relationship with ganglion cells

in the periphery, signals from multiple photoreceptors are pooled onto single ganglion cells so although sensitivity is high, the spatial acuity (resolution lower in comparison to the fovea and responses are slower

Question 18

describe the general structure of photoreceptors

Answer 18

photoreceptor outer segments are tightly packed with membranous discs (in rods) or lamellae (in cones), containing visual pigments, rhodopsin in rods and a single cone opsin in cones, at about 25,000 molecules per square micrometer

Question 19

how do photopigments in photoreceptors act

Answer 19

they are GPCRs

rhodopsin, a GPCR has a transducin binding region on the intracellular side, transducin is a G protein

on the intradiscal side (extracellular side) it has 11-cis-retinal which is converted to all-trans-retinal

isomerisation of retinal causes conformational change in the opsin molecule, unfolding the C-terminal region to reveal G protein (transducin) binding sites

3 cone types are called S,M and L for short medium and long according to pigment content

S detects blue, M green and L red

wavelength sensitivity of retinal is determined by differences in opsin amino acid sequence

yellow is given by a mixture of graded responses from red and green cones, rods have very little sensitivity to this wavelength but much more sensitivity to short wavelengths (blue/green) this is why blue objects appear brighter than yellow ones in moonlight

Question 20

what are the steps of phototransduction

Answer 20

one rhodopsin molecule (rods) absorbs one photon, 500 transducin molecules are activated, 10^5 cGMP molecules are hydrolysed by PDE, 250 ion channels close, rod cell membrane is hyperpolarised by 1mV

(light causes hyperpolarisation of rod cell)

Question 21

what do rods respond to

Answer 21

rods respond to very dim flashes of light;

single photons can generate a measurable electrical response in rods, ocassionally there is no response and sometimes there is a response even when there is no light pulse (a false signal), this is due to spontaneous thermal isomerisation of retinal and makes dark vision a bit noisy (innacurate)

Question 22

how do photoreceptors respond to different light intensities

Answer 22

photoreceptor responses to light are graded with the light intensity, reduction of glutamate release by rods and cones is also graded and depends on the change in membrane potential, linking it to the light intensity

brighter lights give faster responses in rods but they dont saturate at high intensities so rods dont work in bright light

cones start to work at higher light intensities and respond more quickly than rods, they also recover more quickly after light is turned off, cones can adapt to bright light and do not saturate unless exposed to very high light levels

Question 23

how do rods compare to cones

Answer 23

rods: have high sensitivity to light, are specialised for night vision (scotopic vision), more photopigment than cones and so capture more light, higher amplification than cones (can detect single photons), have low time resolution (slow response), only one type of pigment (achromatic), not present in the fovea, highly convergent pathway (low acuity)
cones: have lower sensitivity to light than cones (specialised for day vision), have less photopigment, lower amplification, higher time response (faster), chromatic; three types of cones each with distinct pigment, concentrated in fovea, less convergence (high acuity)

Question 24

what is mesopic vision

Answer 24

combined use of rods and cones in dim light

Question 25

what are types of bipolar cells

Answer 25

3 types of bipolar cells; on, off and rod

many different types of cone bipolar cells, according to shape, which types of photoreceptor they connect to and which part of ganglion cell layer they targer with their synaptic terminals, can also be indentified by their expression of different molecular markers

short, medium and long-wavelength cone bipolar cells sub-divide these into further parallel pathways processing colour

Question 26

what is effect of different types of bipolar cells

Answer 26

on bipolar cells cause increased output, off bipolar cells cause decreased output

cones have glutamatergic synapses with bipolar cells, increase in light causes less glutamate to be released

in on bipolar cells glutamate acts on mGluR6 which is inhibitory, so when light causes less glutamate release there is less inhibition, causing depolarisaiton in light

in off bipolar cells there are AMPA receptors which are excitatory, so less glutamate causes less excitation, causing off BPs to hyperpolarise in light

Question 27

how do bipolar cells interact with ganglion cells

Answer 27

BPs form glutamatergic synapses with ganglion cells, on BPs synapse onto on ganglion cells, off BPs synapse onto off ganglion cells

light causes excitation is on ganglion cells and inhibition in off ganglion cells

Question 28

describe the rod bipolar cell pathway

Answer 28

starts out looking like the on cone pathway, both BP cells depolarise in light using mGluR6, but rod BPs make no direct connections to retinal ganglion cells, instead they signal to ganglion cells via cone bipolar cells through amacrine cells

amacrine cells make lateral connections between rod and cone bipolar cells to transmit rod signals

rod BPs synapse onto amacrine cells which then form gap junctions with on BPs allow excitation to flow through, amacrine cells also synapse onto off BPs via glycine and so inhibit them

Question 29

describe principal biochemical steps in phototransduction

Answer 29

light: activates rhodopsin/opsin via the isomerisation of retinal, transducin is then activated by opsin, G protein then activates cGMP phosphodiesterase which causes decrease in [cGMP] by hydrolysis, causing cGMP gated channels to close causing hyperpolarisation of photoreceptors, causing decreased glutamate release

returning to dark state: activated opsin/rhodopsin phosphorylated by opsin/rhodopsin kinase, inhibiting transducin activation, opsin/rhodopsin is quenched by binding to arrestin, the retinal is recycled in pigment epithelium, phosphodiesterase is inactivated after bound G-protein (transducin) is hydrolysed causing an increase in [cGMP] causing cGMP gated channels to open, causing depolarisation of photoreceptors and increased glutmate release