Flashcards in Retina to LGN Deck (16)
What is the cornea and what is its function?
A transparent layer of the eye through which the light reaches the retina. It is curved and made of material, through which light travels slower than through air. Thanks to that if functions as a main lens for the eye, focusing the beam of light on the retina.
What is the iris and what is its function?
Contracts to cause pupil constriction, relaxes to cause pupil dilation. Regulation of the amount of light that enters the eye.
What is the lens and what is its function?
Less light-bending power than the cornea, but adjustable. The lens is held between the zonules of Zinn. Contraction of the circular ring of ciliary muscles relaxes the zonules so the lens gets fatter and we have more refractive power (focus on a close object). Regulation of the focus distance.
What are the myopia and hyperopia?
Myopia - nearsightedness
Hyperopia - farsightedness
How is the retina arranged?
Back-to-front arrangement. Light first travels through exiting nerve fibres and blood vessels, before it reaches photoreceptors. Because of that arrangement we have a blind spot or optic disc, in a place where all exiting nerve fibres bundle together in an optic nerve.
- Contain rhodopsin
- sensitive in low light (scotopic)
- no colour sensitivity
- absent in fovea, distributed across the periphery
- contain red/green/blue-sensitive opsin depending on their type
- subserve daylight (photopic) vision and colour perception
- concentrated in the fovea, reduce in number in periphery
Describe how does an eye adapt to vision in darkness?
During daylight, rods are bleached - low sensitivity. In the dark, rhodopsin replenishes and they become sensitive again, reaching full sensitivity after 20-30 min.
What are the other types of cells present in the retina and what are their functions?
Bipolar cells - connect the receptor cells with the retinal ganglion cells
Horisontal cells - connect laterally at the point where the receptors synapse with bipolar cells
Amacrine cells - connect laterally where the bipolar cells synapse with ganglion cells
Ganglion cells - their axons carry the visual information from their visual field to the lateral geniculate nucleus (LGN).
What are the 2 types of retinal ganglion cells and what are their respective properties?
M cells (magnocellular/parasol) - large receptive fields, low spatial resolution, poor colour selectivity, rapid and transient response, carry information about dynamic aspects of the world, like movements
P cells (parvocellular/midget) - small receptive fields, high spatial resolution, slow sustained response, distinguish between signals from different types of cones (carry colour information)
Each cell type projects to different regions of LGN.
What are the 2 types of receptive fields?
Retinal ganglion cell combine inputs from a small area of visual field. The fields consist of centre and surround, which have different outputs. There can be ON-center, OFF-surround, or OFF-center, ON-surround
What is the optic chiasm?
The point of partial decussation of optic nerves coming from retinal ganglion cells. Left parts of the visual field from both eyes travel to the right hemisphere and right parts of the visual field from both eyes travel to the left hemisphere. After partial decussation the bundles of fibres are called optic tracts.
How are retinal inputs distributed across the LGN? What are the 2 information streams in LGN and what info do they carry?
Both LGN have a retinotopic map with 6 layers (3 from each eye). Layers 1 & 2 are magnocellular layers and layers 3, 4, 5, 6 are parvocellular layers.
M stream carries information about coarse features and movement. P stream carries info about finer features and colour. Projections of adjacent retinal ganglion cells synapse with adjacent cells in the LGN.
What are the Koniocellular cells? Where from do they receive inputs? What type of vision are they involved in?
LGN cells located mainly in between LGN layers. Their inputs come from Bistratified cells in the retina. They are involved in blue colour vision and blue-yellow comparison, whereas P stream carries red-green comparison.
Where from does the LGN receive most inputs? What is currently thought to be its function?
From cortex, not retina. LGN is important in filtering what information gets to the cortex, serving as the spotlight of attention and highlighting the information coming from certain bits of the visual field.