Sensory Systems Flashcards
Why is the Optic nerve referred to as an information bottleneck?
It has limited capacity, cannot transform everything the eye sees. So the retina has to choose what we see.
What side of the brain does the a) right hemifield b) left hemifield activate?
a) Right hemifield activates left side of brain
b) Left hemifield activates right side of brain
What is the pathway information travels to get from the retina to the visual cortex?
- Retina
- Lateral geniculate nucleus
- Through optic radiations
- Primary visual cortex (back of the brain)
What is the pathway information travels to get from the retina to the visual cortex?
- Retina (Parvocellular Cells)
- Lateral geniculate nucleus
- Through optic radiations
- Primary visual cortex (back of the brain)
What 2 areas process visual information, and which does more?
1) Retina (before visual cortex)
2) Visual cortex (does more)
What are the 2 main visual pathways in the cortex, what information do they process?
1) Ventral stream (“what” pathway)
>Process information about object identity
2) Dorsal stream (“where” pathway)
>Process spatial location and speed (spatial and motion)
What is the function of the a) pupil b) lens
a) Pupil regulate the amount of light that falls on the retina
b) Lens focuses image on the fovea
What is the fovea and how is it different to the rest of the retina?
> Fovea= retina with highest spatial resolution/ visual acuity due to many cones
The rest of the retina has smaller acuity and contains primarily rods
What part of the retina does light travel through to reach the photoreceptors?
Muller cells (glial cells)
What are the 3 neuronal layers of the retina, listed as 1 being further back in the eye?
- Photoreceptors
- Bipolar cells
- Ganglion cells
What are the 3 excitatory neurons in the retina and how do they carry out excitatory feedforward mechanisms?
> photoreceptors, bipolar cells and ganglion cells
- Photoreceptors release glutamate to bipolar cells express glutamate
- Bipolar cells release glutamate to ganglion cells express glutamate
- Ganglion cells release glutamate onto nerves
What are the 2 inhibitory neurons in the retina and how do they carry out inhibitory feedback mechanisms?
> horizontal cells and amacrine cells
- Horizontal cells to Photoreceptors express GABA
- Amacrine cells to Bipolar cells and Ganglion cells, express inhibitory neurotransmitters (GABA)
How do a) horizontal cells b) amacrine cells carry out inhibitory feedforward?
- Horizontal cells release GABA to Bipolar cells
- Amacrine cells release GABA to ganglion cells.
What are the 2 synaptic layers of the retina and what synapses does each contain?
- Inner plexiform layer
>Contains synapses between bipolar cells, amacrine cells and ganglion cells - Outer plexiform layers
>Contains synapses between photoreceptors, bipolar cells, horizontal cells
What are the 4 structures making up photoreceptors (rods and cones) and where does phototransduction occur?
> Outer segment (Phototransduction)
Inner segment
Nucleus
Synapse
How do photoreceptors respond to light flashes in a) vertebraes b) insects?
a) Photoreceptors hyperpolarise
b) Photoreceptors depolarise
When do a) rods b) cones activate?
a) dim lights
>can respond to single photons
b) bright lights
Describe the phototransduction cascade when light hits the retina in 6 steps
- Light changes conformation of Rhodopsin receptor,
- Triggering Gq-protein cascade and G-alpha activates phosphodiesterase
- Phosphodiesterase catalysis cyclicGMP into GMP
- Cyclic GMP conc falls so non-selective ion channel closes
- membrane potential decreases (of outer segment of photoreceptor)/ hyperpolarises photoreceptor
- Hyperpolarisation causes the amount of glutamate released at the synapse to the bipolar cell to decrease.
What takes place in a photoreceptor when in darkness?
> As phosphodiesterase can’t activate without light, In darkness lots of Cyclic GMP present, this activates non-selective cation channels which opens so the membrane depolarises (of outer segment of photoreceptor).
> This leads to a constant release of glutamate from the photoreceptor to the Bipolar cells
When is more glutamate released at the synapse between photoreceptors and bipolar cells?
> Constant release of glutamate in darkness
Less glutamate is released during activation by light
What neurons receive input from photoreceptors in the outer plexiform layer (OPL) of the retina?
Bipolar and horizontal cells receive input from photoreceptors in the outer plexiform layer (OPL)
What neurons receive input from bipolar cells in the inner plexiform layer (IFL) of the retina?
Ganglion cells and amacrine cells receive input from bipolar cell in the inner plexiform layer (IPL)
How do a) ON Bipolar cells b) OFF Bipolar cells respond to increasing light intensity and why?
a) ON cells depolarise when light intensity increases
>As hyperpolarised photoreceptors release less glutamate, less binds to mGluR channels on Bipolar cells which leads to less non-selective cation channels being closed, so positive ions flow in, causing increased ganglion cell firing.
b) OFF cells hyperpolarise
>As hyperpolarised photoreceptors release less glutamate, less ionotropic glutaminergic receptors are activated so less cations enter so the bipolar cells is hyperpolarised, causing decreased ganglion cell firing.
What receptor is present on a) ON bipolar cells b) OFF bipolar cells and the effect of glutamate binding?
a) Metabotropic glutamate receptors (mGluR), glutamate binding causes a signalling cascade where non-selective cation channels close (so in darkness are hyperpolarised due to constant glutamate binding, in light are depolarised as less glutamate binds so less channels are closed).
b) Ionotropic glutamate receptors, glutamate binding allows cations to flow through and depolarise OFF bipolar cells in darkness, hyperpolarise ON bipolar cells in light.
As well as mGluR, what 2 other proteins are required for ON bipolar cells to function?
> TRPM1 channel: expressed in ON but not OFF cells
Nyctalopin is a proteoglycan required for light and glutamate responses in ON cells
How can we label ON and OFF Bipolar cells?
By expressing GFP (green fluorescence protein) in ON bipolar cells and RFP (red fluorescence protein) in OFF bipolar cells
What connections do ON bipolar cells form in the inner (in innermost layer of IPL) plexiform layer?
ON bipolar cells form connections with ON ganglion cells as ON ganglion cell dendrites project into the inner plexiform layer (increase firing rate when light is on, decrease when light is off)
What connections do OFF bipolar cells form in the inner (in outer part of IPL) plexiform layer?
Off bipolar cells form connections with OFF ganglion cells as OFF ganglion dendrites project into the outer plexiform layer (decrease firing rate when light is on, increase when light is off)
What is a receptive field in the retina?
Receptive field is an area in the retina (or space) which when illuminated activates a visual neuron
What does centre-surround organisation of the receptive field mean in the retina?
Illumination of the centre photoreceptors and illumination of surrounding photoreceptors leads to responses in opposite polarities from bipolar and ganglion cells
What 2 neurons have centre-surround organisation of the receptive field in the retina?
Bipolar and Ganglion cells.
What is normally found at the centre of a bipolar cell’s receptive field?
Its cell body (where the synapse is for photoreceptors to bipolar cell)
What happens when stimulating (light) photoreceptors at the a) centre b) outer area of the receptive fields of an OFF Bipolar cell?
a) Stimulate centre of OFF cell hyperpolarises (doesn’t cause OFF-ganglion cells to fire APs to light)
b) Stimulate outer area of OFF cell receptive field causes depolarisation (light on outer area causes increased OFF-ganglion cell AP firing)
Why does activating the photoreceptors at the centre of the receptive field have the polar effect of activating photoreceptors at the outer areas, use light stimulating OFF Bipolar cells in the centre and periphery of the receptive field as an example?
> Photo receptors in centre of receptive field bind directly with bipolar cells (direct synapses), so when an centre photoreceptors are stimulated by light it causes hyperpolarisation of OFF-bipolar cells due to less glutamate being released directly from photoreceptors to OFF-bipolar cells
> Photoreceptors in outer field of receptive field indirectly activate bipolar cells via horizontal cells. Horizontal cells do inhibitory feedback and release GABA to the Bipolar cell causing the opposite response. So if peripheral photoreceptors are hyperpolarised by light, horizontal cells trigger the opposite response in the OFF-bipolar cell so it would be depolarised (increase OFF-ganglion firing rate)
What is the effect of light activating a) centre of RF b) surrounding of RF on OFF-ganglion cells
a) In the centre of receptive field:
>Stop spiking when stimulated by light
>When light goes down, start spiking.
b) In surrounding of receptor field:
>Increase spiking when stimulated by light
>Decrease in spiking by dark
What is the effect of light activating a) centre of RF b) surrounding of RF on ON-ganglion cells
a) In the centre of receptive field:
>Stimulate centre of receptive field of ON ganglion cell causes increased spiking rate.
>When light is off, spiking rate goes down.
b) In the surrounding of receptor field:
>Decrease spiking rate by light
>Increase spiking rate by dark
What inhibitory feedback mechanism allows Bipolar cells to have centre-surround organisation?
centre-surround organisation of the RF of bipolar cells comes from inhibitory feedback from horizontal cells
What inhibitory feedback mechanism allows Ganglion cells to have centre-surround organisation?
RFs of ganglion cells have centre-surround organisation resulting from inhibitory feedback from amacrine cells.
What does the centre-surround organisation of ganglion receptive fields cause and how is this a selective evolutionary advantage?
> Illumination of the whole receptive field does not activate ganglion cells, as usually this means no danger.
So ganglion cells respond to differences in illumination between the centre and surrounding receptive field as this detects a change in environment causing danger.
What are the 2 main classes of ganglion cells and what is their organisation of receptive fields?
> Parvocellular and Magnocellular
Both have centre-surround organisation of receptive fields.
What are 5 differences between Parvocellular and Magnocellular ganglion cells?
- Parvocellular has a small dendritic tree
>Magnocellular has large dendritic tree (usually asymmetrical) - Centre and surround of Parvocellular receptive field respond to different colours
>Centre and surround of Magnocellular Rfs respond to difference in brightness - Parvocellular has sustained response (exposed to light, continuous spiking)
>Magnocellular has transient response (exposed to light, spike quickly but decrease over time) - Parvocellular has slower conduction velocity than Magnocellular
- Parvocellular less sensitive to light than Magnocellular
- Parvocellular processes info to recognise objects
>Magnocellular used for motor detection
What percentage of ganglion cells in the retina are a) Parvocellular b) Magnocellular
a) 80%
b) 20%
What is the difference in function of Parvocellular and Magnocellular ganglion cells?
Parvocellular cells are tuned to process information about shape and colour, while magnocellular cells process information about motion.
If a ganglion cell has a small but dense dendritic tree, what type is it most likely to be and why?
More likely Parvocellular cells as are better for processing fine details of objects.
If a dendritic tree is asymmetric, what type of ganglion cell is it most likely to be and why?
Magnocellular, as will be good for processing motion in a specific direction.
Why is detection in olfactory different from hearing and vision?
As odours don’t have clear dimensions, is a multidimensional coding space.
How can a) Sound b) Light c) Odour be described as?
a) Intensity and frequency (pitch)
b) Location (where in our field), intensity (brightness), wavelength (colour)
c) Recognition of chemicals.
What type of code are odours detected by?
Combinatorial code
What is the effect of signal trasnduction on olfaction?
The second messenger amplifies the sensory signals, so a small change in odour can cause a large effect (increases sensitivity).
Why does signal trasnduction occur in mammal olfactory but not insects?
Insect olfactory receptors are ion channels while in mammals are GPCRs
What does an olfactory receptor respond to?
A specific range of molecules (different olfactory receptor type responds to a different range each).
As we mature what changes in the olfactory system?
As we mature, olfactory neurons express a single olfactory receptor each (makes neuron and receptor odour specific).
Where do olfactory neurons expressing the same receptor converge to?
Despite random distribution, the axons of all the sensory neurons expressing the same olfactory receptor converge on the same glomerulus in the olfactory bulb (is a glomerulus for each olfactory receptor type).
What is the human equivalent of the antennal lobe in Drosophila?
The olfactory bulb
What are the Olfactory second order neurons called in a) Drosophila b) Mammals
a) Projection neurons
b) Mitral cells, or Tufted cells
How is specificity retained from olfactory sensory neurons all the way to second order olfactory neurons?
> Each projection from sensory neurons with the same receptor converge onto the same glomeruli.
> This specific glomeruli inputs onto second order neurons specific to this glomeruli, keeping the specificity.
What are 5 important mechanisms in the early Olfactory processing of odours?
- Adaptation of synapse between sensory and second order neurons
- Sensory neuron convergence reducing noise
- Gain control
- Decorrelation
- Lateral inhibition
What are the advantages of having a relay synapse between sensory olfactory neurons and second order neurons?
- Synaptic adaptation
>As vesicles deplete over time, it allows a synapse to adapt and react to changes in odour intensity. - Many converging sensory neurons onto second-order neurons
>Helps reduce noise
>Allows a weak odour to trigger a strong response in second order neurons.
What are the advantages of having inter-neurons carrying information between glomeruli?
- Gain control
>When a strong odour activates many glomeruli inhibitory inter-neurons suppress output from sensory neurons so the second order neurons (projection) can detect changes in environment at high odour conc. - De-correlation of odour responses
>The response of a neuronal population to different odours is as different as possible.
What allows De-correlation of odour responses to occur?
Lateral inhibition- strongest channels inhibit weaker channels, so population of different odour responses become distinct (means a stronger odour will be smelt over a weaker one)
What are the cells found in the Mushroom Body of a Drosophila called?
Kenyon cells