primary visual (V1) cortex: functional organization Flashcards
what is the location of V1 cortex?
- the primary visual cortex is located in the calcarine sulcus which is inside the occipital lobe
- mainly medial, upper and lower banks of the calcarine sulcus in each hemisphere
what is inside the calcarine sulcus?
- gray matter-
- the calcarine sulcus is not a slit: more a U-bend , with cortical tissue in its depth or ‘fundus’
- it is 90 degrees to Pareto-occipital sulcus
what is the precise location of area V1?
- corresponds exactly to brodmann cortical area number 17, which is mainly medial, but extends a bit onto the lateral surface of the occipital lobe, where the fovea is represented
do medial and lateral damage cause different VF losses?
- medial vs lateral damage causes different VF losses
what is the cellular organization of area V1?
- Broadmann area 17, based on ‘cyto-architecture’
- has 6 separate layers of cells ( with very few in layer 1)
- small granule (or stellate) cells concentrated in layer 4
- small granules receive input from the thalamus (LGN) and connect to mainly pyramidal cells in the layers directly above and below to continue processing of the visual image
what are the unique features of area 17?
- layer 4 is very wide and has 3 separate sub-layers (A,B,C)
- with sub-layers 4A and 4B containing pyramidal , not granule cells
- Layer 4C, with granule cells having separate upper (alpha) and lower (beta) sub-layers
what are the three names of the primary visual cortex?
- V1 - functional name
- area 17- anatomical name
- striate
why is primary visual cortex referred to as striate cortex?
- because layer 4c alpha in middle of grey matter to layer 4b contains a band or stripe of myelinated axons
explain V1 cortex: cellular architecture ?
- Grey matter: has 6 major layers ( 2-6 with cell bodies of cortical neurons; layer 1 dendrites and synapses)
- grey matter is 2mm thick
- wide layer 4 subdivided: . 4A . 4B . 4C alpha . 4C beta
. 4C alpha and beta receive input from the LGN and connect to cells in the layers above (2-4B) and below (5-6)
what are the 2 main classes of V1 cortical cells?
- granule or stellate cell in layer 4C
- pyramidal cells in layer 2 and layer 3
why is it named a stellate cell?
- dendrites come out of cell body- looks like a star
what is the function of stellate cell?
- small ‘ local circuit’ neurons, which relay information they receive from the LGN via short axons to pyramidal cells in the layers just above or below them
what is the function of pyramidal cells?
- large projection neurons, which relay information to distant targets via long axons
explain the vertical columnar organization of the V1 cortex?
- granule cells axons run mainly vertically across the layers to make excitatory synapses on the prominent vertically-oriented ( primary ) dendrites and dendritic spines of pyramidal cells, local cortical circuits are organized mainly in vertical columns
. columns are 1mm wide
what type of neurons are granule or stellate cells and pyramidal cells ?
- they are excitatory neurons , they use glutamate as their neurotransmitter
what are the three types of excitatory cortical connections?
cells in different layers
- layer 4C, granule or stellate cells, receive LGN input- have short axons
- pyramidal cells in upper layers 2-4B: connect to other nearby ‘extrastriate’ areas beyond V1 of visual association cortex (e.g. V2)
- pyramidal cells below layer 4c: have descending sub-cortical connections (e.g. layer 5 connections to superior colliculus: layer 6 send axons back to the LGN)- layer 6 gets input from layer 4c
- all these cells use glutamate as their neurotransmitter
which cells in v1 do NOT use excitatory neurotransmitter?
- basket cells
- chandelier cells
. these cells use GABA as their neurotransmitter which is inhibitory
. they mediate lateral inhibitory processes that also influence the RF properties of nearby pyramidal cells
explain the chemical organization of area V1?
- neurons in all cell layers of v1 have mitochondria
- containing the metabolic enzyme, CYTOCHROME OXIDASE (co)
- which can be visualized with special staining techniques
explain the uniques pattern of CYTOCHROME OXIDASE activity in V1?
. columns of intensely CO
stained cells in layers 2-4B, weaker in layers 5-6
. approximately 0.25 mm wide
. separated by columns of less co-stained cells in these layers of similar / wider width
. view horizontally across the layers, the CO-rich cells form a polka-dot arrangement of densely stained cells
. with polka dot-to-dot centre spacing of 1mm- in mitochondria-
what are neurons with a lot of CO called?
cytochrome oxidase blobs
what are neurons with less CO called?
interblob cells which surround the blobs
what is unique about CO in V1?
the cytochrome oxidase ‘blobs’ =surrounded by CO-poor ‘interBlob’ cells
what can we see about CO in a cross section of the visual cortex ?
- all the cells in layer 4c show uniformly strong CO staining - this means highly metabolic neurons
- strong brown staining- indicates a lot of CO in the mitochondria
- alternating strong versus weaker CO staining- 1/2mm which separates
- width of these CO strong cells- 0.25mm separated by each other by cells which contain less CO (interblob zone) - with a dimension of 0.5mm
what can we see with the polka-dot pattern of CO running across layer 3?
- low power=dark dots = blobs
- interblobs surround blobs