L7 & 8 - Vision Flashcards
Outer layer of eye
Sclera and cornea – strong layer
Orange patch in retinal scan is
Macula – controls central vision: reading text, faces
CASE STUDY: Trouble seeing at night, car accidents, can read ok. What was wrong with Joan?
Retinitis pigmentosa – macula is fine but everything else is not
-rare 1:5000. Genetic defect in rhodopsin or proteins involved in phototransduction -> lose rods -> tunnel vision -> poor peripheral vision
In the light, is the pupil larger or smaller?
Smaller -> clearer vision
3 Layers of retina (superficial to deep)
Rods (Scoptopic), cones (photopic), GCs
Cones vs rods, which one is more sensitive
Rods
Process of phototransduction
DARK:
cGMP gates a Na+ channel, causing influx of Na+ -> depol
LIGHT:
Light activates Rhodopsoin -> Transducin -> Phospho diesterase -> breaks cGMP TO GMP -> hence Na+ channel closed -> no Na+ influx and it hyperpolarises
*11- cis retinal (has a kink) is sitting in middle of Vitamin A -> light hits -> change in conformation to All-trans retinal
What NT is used by photoreceptors?
Glutamate
Photoreceptors respond to light with graded changes in membrane potential or ap?
Membrane potential - as they don’t need to communicate over large distances hence no need for AP
How many types of bipolar cells?
1 Rod bipolar, 9 cone-bipolar cells
They are important for spatial vision and colour vision
What do the ON, OFF, M, P ganglion cells interact with?
ON or OFF – if they depolarise or hyperolarise with light
Magnocellular (5 %)- motion, Parvocellular (90%)– shape and details
Only cell type in retina that can fire AP
GCs
How do ganglion cells respond to light?
Increasing or decreasing their AP firing rate
Receptive field of a ganglion cell is:
The area of retina that when stimulated with light, changes the cells membrane potential
How many types of GCs?
20
BP to GC pathway is parallel or series?
Parallel
Lateral inhibition: horizontal cells
BOTH input and output onto photoreceptors, use NT: GABA, respond to light by hyperpolarizing
Lateral inhibition: amacrine cells
34-5 types, Amarcrine is Greek for Axonless cells, inhibitory cells (release inhibitory NTs: glycine, GABA)
Which lateral inhibition cells are involved in motion?
Amacrine cells
- Ganglion cells are especially tuned for edges
- If you look slight off-center at a star, you’ll see more stars, hence more rods off centre (8 degrees)
- Rods – Rhodopsin
- Cones – Cone-opsins
- Opsins bind to vitamin A (all-trans Retinal)
- Ganglion cells are especially tuned for edges
- If you look slight off-center at a star, you’ll see more stars, hence more rods off centre (8 degrees)
- Rods – Rhodopsin
- Cones – Cone-opsins
- Opsins bind to vitamin A (all-trans Retinal)
CASE STUDY: Why can’t David see properly? (Has a translucent white curtain obscuring the upper part of this vision, motion perception was off, lost a quadrant in his visual field, and doesn’t have facial recognition)
Lesion affecting the fusiform gyrus (area IT). Also caused a visual field defect.
M (parasol) GC
-Magnocellular= large, large receptive fields, M for motion detection, flicker and analysis of gross features, 5% of GC are M cells
P (midget) GC
-Parvocellular = small, visual acuity and colour vision, 90% of GC are O cells
Output target of GCs
Many brain regions, mainly lateral geniculate nucleus aka thalamus
The visual pathway
Retina -> optic nerve -> optic tract -> LGN -> Optic radiations -> Visual cortex
Optic Chiasm – what forms it and where is it located and the clinical significance of the location
Chiasm comes from latin meaning “cross”.
Fibres from left and right optic nerves combine to form it. Lies at the base of the brain, anterior to the pituitary gland, internal carotid arteries found laterally. If diseases affect these two regions, it will compress the optic chiasm and cause issues.
Referring to the motor system, if you have a stroke on your left side of brain – it affects the right side of body. What’s different in the visual system?
Right visual hemifield (right half of both eyes) is viewed by left hemisphere – going to left visual cortex, left visual hemifield is viewed by right hemisphere – going to the right visual cortex
Nasal fibres (found medially in both eyes) cross at optic chiasm. Temporal fibres (found laterally in both eyes) do not
Nasal fibres (found medially in both eyes) cross at optic chiasm. Temporal fibres (found laterally in both eyes) do not
Where do the nasal and temporal fibres synapse?
LGN
How many layers in LGN and what cell types are they?
Six
1,2 = Magnocellular layers (1 layer is for right eye, 1 for left)
3-6 = Parvocellular layers (2 for left, 2 for right)
*There is no communication between both eyes in the LGN, they are in separate layers
Optic radiations and where are they located?
White matter tract formed by axons of LGN that sends info from LGN to primary visual cortex. Located in temporal lobe, partially in parietal lobe.
Where is the primary visual cortex aka Broadmanns area 17 located?
Occipital lobe around the calcarine fissure.
Central vision is located in which area of visual cortex
Most posterior part of visual cortex – it is a retinotopic organization
How many layers in primary visual cortex? Which layer does LGN input go to?
6 (Layer 1 is surface of brain – layer 6 is inside of brain). 4c
Cortical neurons: Orientation selectivity
Neurons respond best to bars moving in particular orientation
Placed electrode into visual cortex from layer 1 to 6, they all responded to the same bar of light in the same orientation, receptive field in the same location as well. NOT square matrix organisation but more fluid
Ocular dominance columns
Input from LGN is segregated into small regions of the primary visual cortex called an ocular dominance column
Segregation of info from different types of LGN cells
M type GC/LGN input to Layer 4C alpha
P-type GC/LGN input to Layer 4C beta
Mixing of info from each eyes occurs in layers
4B and 3
CASE STUDY: Old woman that can’t cross the road and cars appear out of “nowhere. Pouring coffee is very difficult – appears frozen to her until cup overflows. Visual acuity, colour vision and visual fields are normal. Where in the visual pathway is her lesion?
She can’t see motion. Lesion affecting mid temporal lobe (Area MT)
After primary visual cortex, where is the visual info encoded?
After primary visual cortex, where is the visual info encoded?
2 streams. Dorsal pathway (where?) and ventral pathway (what?)
*Dorsal is superior half of brain and ventral is inferior
Area MT
Middle temporal lobe is an area specialised for processing object motion
- Receives retinotopic information from a number of cortical areas including V2 and V3
- Receives input from cells in Layer IVB of the primary visual cortex (M type GCs/LGN)
Ventral stream
V1, V2, V4
- From calcrine fissue, starts from V1 and travels via V2 and V3 down to inferior parts of temporal lobe
- Important for perception of shape and colour, object recognition
Area IT (inferior temporal)
A major output of Area V4
- Neurons respond to a wide variety of abstract shapes and colour
- Important for visual memory and perception
- Important for perception of faces
If only one eye can’t see, area affected is ____ optic chiasm
Between the eye and the optic chiasm, it is before the chiasm
If there is a defect on the same side of both eyes, are affected is ____ optic chiasm
After chiasm
Both eyes but on the opposite sides (medially)
Optic chiasm
