Vision in Vertebrates

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How does light affect receptor membranes?
It causes graded hyperpolarisation.
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What are some features of rod cells (3)?
Specialised for sensitivity, many receptors on each cell allows absorption of large proportions of light, G-protein cascade produces high amplification.
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What are some features of cone cells (4)?
Specialised for acuity, small and possibly directionally selective, not saturated at high light levels, photoreceptor recovers rapidly from change.
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What two components are light receptors composed of?
Opsin- similar to transmembrane proteins in olfactory receptors. Retinal- absorption of light causes a conformational change in retinal molecule to the activated form.
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What does a change in retinal cause?
A conformational change in rhodopsin, which activates a transducin.
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How does the membrane become more polarised?
Activated rhodopsin activates transducin ---> Breakdown of cGMP ---> Na/ Ca channels close, only K channels remain open.
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What happens to cGMP in a "normal" state (light falling on receptors)?
cGMP is constantly produced, but high levels of activated transducin cause cGMP to be constantly broken down again, so Na/ Ca channels remain closed.
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What does levels of activated rhodopsin depend on?
The balance between the relative levels of light activation and of retinal recycling.
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What happens to activated retinal in the epithelium?
It is constantly transformed back into unactivated retinal.
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What happens to retinal in the absence of light?
Retinal recycling decreases amount of activated rhodopsin, reducing levels of transducin. cGMP levels increase and allow Na/ Ca channels to open, causing influx of ions and depolarisation.
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How do we adapt to ambient light levels?
By ensuring that photo receptors are tuned so that small changes will open or close Na/ Ca channels.
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How does adaptation work in light conditions?
It keeps more channels open.
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How does adaptation work in dark conditions?
Makes it easier for channels to close.
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Which cells are specialised for day vision and night vision?
Cone cells and rod cells respectively.
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What is the purpose of the lens?
Focuses image.
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What is the purpose of the cornea?
Filter to protect lens.
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What is the purpose of the iris?
Aperture to control light entering.
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What is the purpose of the eyelid?
Lens cover for when not in use.
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What is the purpose of photoreceptors?
Pixels to register image.
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What is the purpose of tears?
Cleaning mechanism.
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Which parts of the eye refract light to form an image?
Lens and cornea.
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What is accomodation?
Changing the strength of the lens to form a focussed image.
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What type of cells have a graded response (5)?
Rod, cone, horizontal, bipolar and amacrine.
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What do ganglion cells do?
Produce action potentials and provide afferent pathways from the retina.
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How can processing in the retina be investigated?
By recording spike patterns in ganglion cells.
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What is lateral inhibition?
Producing the surround in the centre-surround patterns.
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What do rod and cone cells do when light levels decrease?
They become depolarised and release more glutamate. This have an excitatory effect on horizontal cells.
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What are horizontal cells?
They receive input from many receptor cells and form a net connected by electrical synapses.
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When is a receptor cell depolarised less?
When all its neighbours are also depolarised, as the cell is inhibited through lateral connections with its neighbours.
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What does glutamate do in off-centre bipolar cells?
Activates AMPA receptors, which opens Na channels.
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What does glutamate do in on-centre bipolar cells?
Activates metabotropic receptors. G-protein cascade closes Na channels.
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When is an on-centre cell depolarised and how does this affect ganglion cells?
If a receptor cell is hyperpolarised. Ganglion cells fire if bipolar cells is sufficiently depolarised. The retinal ganglion cells are specialists for picking out bright or dark spots.
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What mechanisms have developed to help stabilise an image (3)?
The inner ear- sense organs for detecting rotations and accelerations, the cerebellum- processing area to compute the stabilising movement, muscles to move the eye.
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What in the inner ear senses rotational acceleration?
Semicircular canals.
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How is an image kept horizontal?
Sensing linear acceleration using gravity.
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How is directional sensitivity determined?
By alignment of hair bundles.
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What is the purpose of the laternal geniculate nucleus (LGN)?
To make sense of the visual input. 90% of axons from the retina go here.
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What is the purpose of the superior colliculus (SC)?
To orientate the eyes in directions of interest.
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Where does concious control come from?
The frontal eye fields (FEF).
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Where does automatic control come from?
The SC. Also has sensory inputs direct from retinal ganglion cells, auditory and somatosensory systems.
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Where are sensory and motor maps located?
Both in the SC.
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Where does the left half of both visual fields go?
To the right LGN.
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What are P-ganglion cells?
Small, dendritic field, sustained response, slow axons. Respond best to details of stationary objects.
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What are M-ganglion cells?
Large, dendritic field, phasic response, fast axons. Respond best to large, fast-moving objects.
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What are K-ganglion cells?
Function unknown, possibly for colour.
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Where do processing streams from LGN travel to?
Primary visual areas in the cortex.
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Other cards in this set

Card 2

Front

What are some features of rod cells (3)?

Back

Specialised for sensitivity, many receptors on each cell allows absorption of large proportions of light, G-protein cascade produces high amplification.

Card 3

Front

What are some features of cone cells (4)?

Back

Preview of the front of card 3

Card 4

Front

What two components are light receptors composed of?

Back

Preview of the front of card 4

Card 5

Front

What does a change in retinal cause?

Back

Preview of the front of card 5
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