retinal image processing and receptive fields Flashcards
what does light reflected from surfaces pass through ?
. light reflected from surfaces and objects in our field of view , passes through all the transparent media and crosses all the retinal layers, before the light is captured by visual pigment ( rhodopsin ) in outer segment
what is the direction of information processing?
. information processing direction is opposite to the direction of light
. cell bodies of rods and cones occupy the outer nuclear layer , they have synapses in the inner plexiform layer which they apply into dendrites of bipolar cells
. bipolar cells , cell body resides in the inner nuclear layer
. bipolar cells have synapses , which they form in the inner plexiform layer with the ganglion cells
what is vertical pathway?
- serial processing
. starts from photoreceptors to bipolar cells to ganglion cells
what is lateral pathway?
- lateral inhibition
- mediated by horizontal and amacrine cells
what is the function of horizontal cells?
- modifying the pathways between photoreceptors and bipolar cell
what is the function of amacrine cells?
- modifying the pathways between the bipolar cell and ganglion cell
what is a receptive field?
- all retinal neurons have a receptive field (RF)
. they only respond to specific light stimulus features in a spatially restricted region of the visual field
what are photoreceptors receptive field determined by?
- light sensitivity of their visual pigment and position of their outer segment in the retina
what are all other retinal neurons receptive field determined by?
- synaptic inputs received from photoreceptors and/or other retinal cells in the vertical and lateral pathways
what are the properties of receptive fields of photoreceptors?
- tiny(<0.01 degrees), circular and uniform
- luminance (brightness) and wavelength-dependent responses
- graded changes in membrane potential is their response to capturing light
what are the properties of bipolar and ganglion cells receptive fields ?
- larger, circular and non-uniform( more complex)
- due to convergence and spatial summation of synaptic inputs
- concentric , antagonistic , they have centre and surround regions
- luminance difference (contrast)-dependent responses- interested in intensity falling in centre than surround
- ganglion cells fire action potential
what are the centres of bipolar cells RF mediated by?
. the centres of bipolar cells receptive fields are mediated directly by photoreceptor input
what are the surrounds of bipolar cells RF mediated by?
. surrounds from photoreceptor-horizontal cell ( lateral ) interactions, then relayed to bipolar cells
what is the functional significance of contrast - dependent RF?
. for retinal ganglion cell signalling to the brain
. different types of contrast - sensitivity in midget/parvo compared to parasol/mango ganglion cell types
what is the difference between bipolar cells RF centre and surround?
. RF centre: direct photoreceptor inputs
(vertical pathway)
. RF surround: from photoreceptor-horizontal cell interactions ( lateral pathway)
where is the bipolar cell RF location?
. same region in space as where the photoreceptors are looking
what is the polarisation of photoreceptors ?
. photoreceptors are depolarised in the dark and hyperpolarize in response to light
why are photoreceptors are excited in the dark and inhibited when they catch light ?
. in the dark a current flows through both into the outer segment
. in the dark there are cation channels in the membrane of photoreceptors outer segment which allow sodium ions in the extracellular fluid to enter the cells outer segment ]
. at the same time potassium ions exist into the extracellular fluid
. there is a simultaneous influx of positively charged sodium ions and efflux of positively charged potassium ions - overall resting membrane potential is close to 0
what happens when photoreceptors capture light ?
. in the light , when photoreceptors capture the light through their rhodopsin , this results in closure of sodium channels in their outer segment
. when photoreceptor catches light , it leads to an enzyme cascade which reduces the concentration of cGMP which is converted into GMP
. GMP can’t hold the sodium channels open
. as a consequence in the light, there is no sodium influx into the outer segment
. potassium efflux continues
. cell becomes more negative
what is the difference between the two functional cone bipolar cell types? when responding to light
- OFF-BIPOLAR cell
- in the dark, the cell is depolarised with a membrane potential off -20mV, when light is on the cone hyperpolarizes
- response of OFF-BIPOLAR cell is the same as the cone which is providing its input - ON-BIPOLAR cell
- has a resting membrane potential , when light is one the on-bipolar cell depolarizes , when light is off , it goes back to its resting membrane potential
what is the difference between synapses in the on and off bipolar cells?
. the synapses that the on and off bipolar cells receive from the cone have different receptors associated with them and have different responses to the neurotransmitter
what is an OFF response?
- cells is depolarized = excited by light OFF or darkness in its RF ( like a photoreceptor)
what is an ON response?
- cells is depolarized= excited by light on or brightness in the RF
how can different cone bipolar cells respond in opposite ways to the input from the same cone?
. to do with neurotransmitter release