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Neocortex
Functional Neuroanatomy of the Neocortex including structures, circuitry, and physiology
29
Anatomy
Undergraduate 3
04/23/2019

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Term
Neocortex
Definition
  • sheet of neurons and glia that make up the surface (not including meninges) of the forebrain
  • relatively conserved structure from lobe to lobe
  • neurons vary by density and size but not by type, general pattern of connectivity, or local projection patterns
  • contains 6 layers indicative of evolutionary adaptation as opposed to the 3-4 layers found in the allocortex
Term
Layers of the Neocortex
Definition
  • organized into 6 layers of varying cell types, densities, and connectivity patterns
  • thickness of layers may differ across lobes/brain areas but there will always be 6 layers
  • capable of projecting locally w/in the general region of the cell body, laterally to other cortical areas or longitudinally to subcortical targets
  • also exhibit columnar organization

[image] 

Term
Superficial (supragranular) Layers
Definition
  • layers 1-3
  • Layer 1 (outermost) contains few cell bodies and mainly consists of axons of passage
    • receives input from intralaminar nuclei of the thalamus
  • Layer 2 projects to association cortices in the ipsilateral hemisphere
  • Layer 3 projects to association cortices in the ipsilateral and contralateral hemispheres
  • General theme of superficial: projection of and passage of axons to other cortices

[image]

Term
L4/Granule Cell Layer
Definition
  • Receives most feedforward ascending projections - sensory input from the thalamus

[image]

Term
Deep (infragranular) layers
Definition
  • layers 5+6 - innermost layers of neocortex
  • layer 5 - projects to non-thalamic structures (e.g. spinal cord)
  • layer 6 - immediately external to white matter
  • projects to thalamus as cortical feedback projections

[image]

Term
Neocortical Neuron Types
Definition
  • Pyramidal cells - excitatory
  • Stellate cells - excitatory
  • GABAergic interneurons
    • basket cells
    • double-bouquet cells
    • chandelier cells
Term
Pyramidal Cells
Definition
  • One of two types of excitatory glutamatergic cells of neocortex
  • Excitatory afferents typically synapse with distal (far from cell body) dendrites
  • Inhibitory afferent typically project to proximal dendrites and the cell body itself
  • Efferent projections go to other areas of cortex and areas outside the brain like spinal cord, thalamus, and basal ganglia
  • Those leaving the forebrain typically exit through the white matter below the neocortex to form corona radiata

[image]

Term
Stellate Cells
Definition
  • excitatory glutamatergic cells
  • Found in layers 2 and 4 
  • Interneurons - only send projections within the neocortex

[image]

Term
GABAergic Interneurons
Definition
  • Inhibitory GABAergic cells
  • Only project locally to other nearby cortical neurons
  • Found in all layers except Layer 1
  • Axon projections can be grouped depending on cell as vertical, local, or horizontal 

[image]

Term
Basket Cells
Definition
  • Horizontally-projecting inhibitory GABAergic interneurons
  • synapse on the soma and proximal dendrites of target neuron resulting in strong inhibition

[image]

Term
Double-bouquet cells
Definition
  • vertically projecting inhibitory GABAergic interneurons
  • Dense, vertical axonal arbor projects to cells w/in multiple layers in a narrow column (weaker inhibition)

[image]

Term
Chandelier Cells
Definition
  • Locally projecting GABAergic interneurons that are not always inhibitory
  • synapse on axon initial segment of pyramidal neurons
  • very powerful control of postsynaptic neuron

[image]

Term
Ascending Projections of Neocortical Circuitry
Definition
  • L4 is the input layer as it receives axons from other brain areas - primarily thalamic sensory input
  • L2/L3 are the output layers as they project to other cortical areas
  • Thalamocortical projections connect thalamus and L4 of a primary sensory area
  • Intracortical projections connect L2/L3 of lower area to L4 of higher area
  • Circuitry:
    • Thalamus → Primary sensory areas → secondary sensory areas → association cortices
Term
Descending Projections
Definition
  • L1 and L6 are input layers while L5 and L6 are output layers
  • Intracortical projections connect axons of L5/L6 of higher areas with L1 and L6 of lower neocortical areas
  • Corticothalamic projections connect L5/L6 of primary sensory areas with the thalamic nucleus
  •  Circuitry
    • association cortices → secondary sensory areas → primary sensory areas → thalamic nucleus
Term
Major Efferent Projection Figure
Definition
[image]
Term
Ascending-Descending Figure
Definition
[image]
Term
Higher-Order Processing
Definition
  • Most information received by the cortex is relatively simple in nature and much of its output is related to motor function - lower-order processing
  • More complex processing involved in recognition of people, planning, and having a sense of self involve input from many sensory association cortices sending more and more complex information up a hierarchy
  • Feedforward occurs as increasingly more complex information rises up the hierarchy but feedback also occurs among these steps going down
Term
Sample sensory-motor hierarchy
Definition
[image]
Term
Dorsal stream of visual pathway
Definition
  • the dorsal or where stream of visual pathway tells you where a particular object is in view 
  • information from V1 and V2 cortices are integrated with other information in association cortices of the parietal lobe
  • Retina → LGN → V1 → V2 → Parietal Cortex for processing of location

[image]

Term
Ventral Stream of Visual Pathway
Definition
  • visual or what stream of visual pathway tells you what a particular object is in view (object recognition)
  • information from V1 and V2 cortices are integrated with other information in association cortices of the temporal lobe
  • Retina → LGN → V1 → V2 → Temporal Lobe Cortices

[image]

Term
Temporal/ Ventral/ What Stream Damage
Definition
  • areas involved mainly recognize and identify stimuli
  • Associated disorders are known as agnosias
  • Agnosias are conditions in which patients are aware of the presence of objects but do not recognize or unable to identify them
  • 3 Important types
    • Associative Visual Agnosia
    • Apperceptive Visual Agnosia
    • Prosopagnosia
Term
Associative visual agnosia
Definition
  • caused by damage to anterior temporal lobe
  • leads to an inability to recognize object or describe what it is/does
  • patients are still keenly aware of its presence and capable of drawing it

[image]

Term
Apperceptive Visual Agnosia
Definition
  • inability to integrate parts of object to perceive it as a whole
  • patient still able to identify parts of the object and can recognize whole object through touch
  • unable to draw the object as they can only really perceive it in parts
  • Associated with damage to occipito-temporal region

[image]

 

Term
Prosopagnosia
Definition
  • otherwise known as face-blindness
  • inability to identify people by their faces even though they can identify parts of their face and identify people by other cues like voice
  • due to damage to fusiform gyrus

[image]

Term
Parietal/ Dorsal/ Where Stream Damage
Definition
  • areas in pathway mainly involved in understanding where object is in space and how it relates to body
  • Disorders involve inability to associate objects w/ location and/or integrate parts of an object
  • 2 important types
    • optic ataxia
    • simultanagnosia
    • contralateral neglect syndrome
Term
Optic ataxia
Definition
  • inability to use visual guidance for accurate reaching to object despite being able to see object
  • able to reach accurately when given auditory cues
  • associated w/ parietal-occipital association cortex damage
    • links vision with higher-level, multi-modal cortices
Term
Simultanagnosia
Definition
  • inability to perceive more than one object at a time
  • due to bilateral lesions of occipital-parietal junction
Term
Contralateral neglect syndrome
Definition
  • also known as hemi-neglect syndrome
  • tendency to ignore sensory information from and perception of side of body contralateral to a lesion
  • due to lesions in right parietal cortex (cut-off of all left sensory info)
Term
Cortical Reorganization
Definition
  • brain is unable to regenerate through new neuron development, but cortical reorganization can result in some regain of function
  • other neurons take over for those that are damaged to maintain function
  • it works very well leading to almost full regain of function in some instances while it may provide an insufficient solution in others
  • Occurs in response to experiences
  • Good example seen in finger usage
    • fingers normally found as separate sections of SI cortex
    • monkeys taught task where two fingers used at once - led to morphing of the two finger segments
    • also can lead to growth of areas for particular control
    • associated w/ timing of activation implying that limb in question is necessary to experience reorganization
  • due to changes in synaptic strength and axon collateral growth
  • limb loss can lead to cortical reorganization in which part of SI involved w/ hand will be remapped to function w/ parts remaining that are close to the hand
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