Vision Flashcards
What is the central part of vision?
→ Fovea
Where is aqueous humour produced?
→ Ciliary body
What is the function of aqueous humour?
→ keeps the vitreous humour hydrated
→ maintains enough pressure to keep the eye rigid
What is glaucoma caused by?
→ too much pressure
What do the suspensory ligaments do?
→ suspend the lens
What is the function of the ciliary muscles?
→ They make the lens more convex or concave
What is the function of the iris?
→ Controls the size of the central pupil
→ controls how much light enters the eye
What is the function of the lens?
→ fine focus
What does the pupil do?
→ cut our the light rays that would otherwise go through the edge of the lens
Where is the neural retina generated from?
→ The neural tube
What does the neural retina contain?
→ Neural circuit which links photoreceptors to retinal ganglion cells
What takes the signal to the brain?
→ Retinal ganglions have axons that join with the optic nerve and take the signal to the brain
Describe the pathway to the brain from the eyes
→ Axons project back via the optic nerve
→ the two nerves meet at the optic chiasm and some axons swap over
→ they run into the optic tract which goes up into the brain
→ the main branch goes to the lateral geniculate nucleus
→ they activate relay cells that carry the signal to the primary visual cortex
→they run in the subcortical white matter known as the optic radiation
Where is the visual cortex?
→ occipital lobe
What are rods used for?
→ night vision
What are cones used for?
→ high light levels
What does the synaptic terminal release in cones?
→ Glutamate
What is the outer segment of cones made from?
→ Layers of membranes
What is the ordinary resting potential of cone cells?
→ -45mv
What is the resting potential of nerve cells?
→ -70mv
What channels do cones have?
→ Na+ in the outer segment that leak Na+ inwards
Describe the phototransduction cascade
→ Guanylyl cyclase converts GTP to cGMP
→ cGMP receptors are on Na+ channels which allows Na+ in so the inside of the cell is +ve
→ When light hits rhodopsin cis retinal changes to all trans retinal which frees opsin
→ opsin activates transducin which activates phosphodiesterase
→ PDE breaks down cGMP
→cGMP can’t bind to the channels so they close and the cell hyperpolarises - very little action potentials
→ Less action potentials means less calcium near the synaptic terminal and less glutamate release
→ very little glutamate release stimulates the bipolar neurons
Describe how adaptation works?
→ Phototransduction cascade amplifies the response to each photon so it produces a rapid change in membrane potential
→ when the light is stable for a few seconds the receptor changes its sensitivity and shifts the membrane back down to resting
What are photoreceptors telling us?
→ brightness over a particular area of the retina has changed
What can loss of peripheral vision be due to?
→ Glaucoma
→ Retinitis pigmentosa
What can loss of central vision be related to?
→ Age-related macular degeneration
How do photoreceptors communicate?
→ bipolar cells to ganglion cells
Why is the image blurry?
→ Light rays are going to be diffused as they pass through to get to the end of the photoreceptors
What is convergence?
→ Bipolar cells gather information from multiple photoreceptors before passing it on to ganglions
Where do the nerves converge?
→ The optic nerve head
What is the foveal region?
→ A region where all the retina apart from the photoreceptors have been pushed to one side
What types of photoreceptors are in the foveal region?
→ Only red and green
How do ganglions receive signals in the central part of the retina?
→ They do not converge
→ they receive signals from a single photoreceptor each
What are the 3 properties of peripheral vision?
→ visual image is blurred
→ cone receptors are large and widely spread
→ signals from many cones converge onto single ganglion cells
What are the 4 properties of central vision?
→ Good focus
→ only cone photoreceptors - red and green
→ narrow and closely packed
→ signals from the photoreceptors are kept separate through the primary visual pathway
What is the fovea specialised for?
→ High resolution
Where is information from ganglion cells sent?
→ Back via the optic nerve to the lateral geniculate nucleus
→ to the occipital cortex
What happens to the image as it is passed through the optics?
→ inverted
What sides of the retina crossover?
→ the nasal sides
Where do the right side of both retinae project to?
→ The right side of the two retinae end up on the right side of the visual cortex
What map do axons form?
→ Retinoptic map
→ maps for the two eyes on both sides of the brain
→ right sides of two retinae on right side of the cortex and the left side of the two retinae are on the left side of the cortex
What types of cones are in the fovea and what types of cones are everywhere else (color)?
→ Red and green and blue everywhere else
→ red and green in the fovea
Describe how receptive fields work in an on center off surround situation
→ The center cone is hyperpolarised because it is illuminated
→ it releases very little glutamate
→ the surround cones are depolarised because they are not illuminated
→ they release a lot of glutamate
→ The horizontal cell is stimulated by the glutamate
→ it sends inhibitory feedback to all the cones
→ it causes the center cone to release even less NT
→ which excites the bipolar cell even more
Describe how receptive fields work in an on surround, on center situation
→ All cones are hyperpolarised as all of them are illuminated
→ the horizontal cell is minimally excited and inhibition is reduced
→ they release a lot of NT
→ so the bipolar cell is less excited
Describe how receptive fields work in an on surround, off center situation
→ The outer cones are hyperpolarised as they are illuminated
→ the center cone is depolarised as it is not illuminated
→the horizontal cell is minimally excited and inhibition is reduced
→ the center cone is already depolarised and the inhibition is reduced so it depolarises even more and releases a lot of NT
→ this leads to the bipolar cell being the least excited
What is the relationship between horizontal cells and the number of cones?
→ The more cones send a +ve signal to the horizontal cells
→ the less inhibition the horizontal cell sends
What are parvocellular neurons good at doing?
→ Responding to fast moving things or fine detail
What are magnocellular neurons good at doing?
→ Responding to coarse detail
How can photoreceptors distinguish color?
→ compare inputs from red and green
→ compare inputs from blue and red+green (yellow)
What do laternal geniculate cell inputs look like?
→ same as the retinal inputs
→ faithful relay cells
What happens in primary visual cortical cells?
→ receptor fields converge information from different parts of the retina
→ respond in an orientation specific way
What does the infratemporal area deal with and with what cells?
→ Recognition of the object including color
→ parvocellular areas
What does the parietal cortex deal with and with what cells?
→ Deals with recognising movement and location of things
→ magnocellular cells
